William George Nelson, V, M.D., Ph.D.

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Under normal circumstances muscle relaxant gi tract purchase generic sumatriptan, acetaminophen is mostly metabolized to nontoxic end products through glucuronidation and sulfation spasms lower left abdomen purchase generic sumatriptan online. N-acetylcysteine spasms with broken ribs best order sumatriptan, the antidote spasms near ribs discount 50 mg sumatriptan overnight delivery, rapidly enters the hepatocytes and provides the substrate cysteine for glutathione synthesis muscle relaxant 5859 order sumatriptan uk, and rescues hepatic injury (refer to Chapters 7 and 15). N-acetylcysteine is also used as an inhalation therapy to convert thick mucus, which occurs in the airways of certain lung diseases. Watt, Pain management in the cirrhotic patient: the clinical challenge, Mayo Clin. Younossi, When and how to evaluate mildly elevated liver enzymes in apparently healthy patients, Clev. Goetze, B-type natriuretic peptide: from posttranslational processing to clinical measurement, Clin. Janjwal, Troponin elevation in patients with various tachycardias and normal epicardial coronaries, Indian Pacing Electrophysiol. Cervellin, Laboratory diagnosis of acute pancreatitis: in search of the Holy Grail, Crit. Younossi, When and how to evaluate mildly elevated liver enzymes in apparently healthy patients, Cleve. Monosaccharides are polyhydroxy compounds that also contain carbonyl functional groups, namely an aldehyde or ketone. The structure of carbohydrates, with the exception of dihydroxyacetone, contains at least one asymmetrical or chiral carbon atom giving rise to stereoisomers. The number of stereoisomers or enantiomers for a given carbohydrate is determined by 2n, where n is the number of chiral carbons. D- and L- designation of a monosaccharide is based on the chiral carbon located farthest from the carbonyl. In the Fischer projection, the D-monosaccharide has a hydroxyl group attached to the chiral carbon, on the right-hand side. Aldoses and pentoses are the most abundant monosaccharides, and in humans, they are D-stereoisomers. Stereoisomers that are not mirror images of each other are known as diastereoisomers; examples are D-ribose and D-arabinose. Diastereoisomers that differ by only a single chiral carbon atom are known as epimers; examples are D-glucose and D-galactose. In aqueous solutions, stable forms of monosaccharides of five or more carbon atoms exist as cyclic structures, which are formed due to reversible chemical reactions between a hydroxyl group and carbonyl group. In the process of cyclization, the carbonyl carbon becomes a chiral atom and is known as an anomeric carbon atom. The resulting two diastereoisomers are known as anomers, designated as - or -structures. In aqueous solutions, the - and -forms undergo interconversion and reach a stable ratio of anomers; this process is known as mutarotation. The conformational structural formulae of ring structures of monosaccharides are accurate representations in aqueous solutions. The physiologically important monosaccharide derivatives include sugar alcohols, sugar acids, amino sugars, sugar phosphates, and deoxysugars. Glycosides are formed when the hydroxyl group linked to an anomeric carbon atom condenses with the hydroxyl group of a second molecule with the elimination of water. Nutritionally and physiologically important disaccharides include sucrose, maltose, and lactose. A therapeutically useful nonabsorbable disaccharide is lactulose, which is used in the management of hepatic encephalopathy due to ammonia toxicity to the central nervous system. The chemosensory perception of the sweet taste of sucrose and other nonsucrose molecules is mediated by G-proteincoupled receptors located on the sensory cells of the tongue, ionotropic channels, and generation of electrical impulses. The nonsucrose synthetic sweeteners are varied in structure and have therapeutic applications in the management of diabetes and obesity. If the monosaccharides are all the same, they are known as homopolysaccharides; and if they are different, they are known as heteropolysaccharides. The three most important homopolysaccharides are glycogen, starch, and cellulose, and they contain glucose units. In both glycogen and starch, which is derived from plants, the glycosidic linkages are (1-4) and (1-6). In cellulose, a nondigestible carbohydrate in humans, the linkages between glucose units are (1-4). Cellulose and other indigestible polysaccharides and a nonpolysaccharide component, lignin, constitute dietary fiber and provide fecal mass. Dietary fiber is an essential component in the maintenance of optimal health and nutrition. They are the primary source of energy in animal cells; carbohydrates are synthesized in green plants from carbon dioxide, water, and solar energy. They provide the skeletal framework for tissues and organs of the human body, and serve as lubricants and support elements of N. They confer biological specificity and provide recognition elements on cell membranes. In addition, they are components of nucleic acids and are found covalently linked with lipids and proteins. However, a large number of compounds are classified as carbohydrates even though they do not have this empirical formula; these compounds are derivatives of simple sugars. Carbohydrates may be classified as monosaccharides, oligosaccharides, or polysaccharides; the term saccharide is derived from the Greek word for sugar. Sucrose and lactose are disaccharides, since they are each made up of two monosaccharide units. Polysaccharides, also known as glycans, are polymers that may contain many hundreds of monosaccharide units. The designation of D or L, given to a monosaccharide with two or more asymmetrical centers, is based on the configuration of the asymmetrical carbon atom located farthest from the carbonyl functional group. Thus, if the configuration at that carbon is the same as that of D-glyceraldehyde (with the hydroxyl group on the right-hand side), it belongs to the D series. A similar relationship exists between L-glyceraldehyde (with the hydroxyl group on the lefthand side) and the L series of monosaccharides. The optical rotation of a monosaccharide with multiple asymmetrical centers is the net result of contributions from the rotations of each optically active center. Thus, the prefix D or L provides no information with regard to optical rotation; it indicates only the configuration around the asymmetrical carbon atom located farthest from the carbonyl carbon. The numbering system for monosaccharides depends on the location of the carbonyl carbon (or carbon atom in the most oxidized state), which is assigned the lowest possible number. For glucose (an aldohexose), carbon C1 bears the carbonyl group and the farthest asymmetrical carbon atom is C5 (the penultimate carbon), the configuration which determines the D and L series. For fructose (a ketohexose), C2 bears the carbonyl group, and C5 is the highest numbered asymmetrical carbon atom. The simplest monosaccharides are the two trioses: glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose). Four-, five-, six-, and seven-carbon-containing monosaccharides are called tetroses, pentoses, hexoses, and heptoses, respectively. All monosaccharides, with the exception of dihydroxyacetone, contain at least one asymmetrical or chiral carbon atom, and therefore two or more stereoisomers are possible for each monosaccharide depending on the number of asymmetrical (chiral) centers it contains. In general, the total number of possible isomers with a compound of n asymmetrical centers is 2n. Thus, for aldohexoses having four asymmetrical centers, 16 isomers are possible, 8 of which are mirror images of the other 8 (enantiomers). Most of the physiologically important isomers belong to the D series, although a few L-isomers are also found. In later discussions, the designation of D and L is omitted, and it is assumed that a monosaccharide belongs to the D series unless it is specifically designated an L-isomer. Of the D series of aldohexoses, three are physiologically important: D-glucose, D-galactose, and D-mannose. D-galactose and D-mannose are not epimers, since they differ in configurations around both C2 and C4. D-fructose, one of eight 2-ketohexoses, is the physiologically important ketohexose. Cyclic forms of D-glucose are formed by the hemiacetal linkage between the C1 aldehyde group and the C4 or C5 alcohol group. The thick line of the structure projects out toward the observer and the upper edge (thin line) projects behind the plane of the paper. Aldohexoses exist in solutions mainly as sixmembered pyranose ring forms, since these forms are thermodynamically more stable than furanose ring forms. Cyclization of a monosaccharide results in the formation of an additional asymmetrical center, known as the anomeric carbon, when the carbon of the carbonyl group reacts with the C5 hydroxyl group. Aldohexoses in their cyclic forms have five asymmetrical centers and, therefore, 32 stereoisomers. In other words, each of the 16 isomers that belong to the D or L series has two anomeric forms. The systematic names for these two anomers are -D-glucopyranose and -D-glucopyranose. Carbon atoms of the ring are not explicitly shown but occur at junctions of lines representing bonds. Sometimes the hydrogen atoms are also omitted and are presumed to exist wherever a bond line ends without a specified group. Interconversion of - and -forms can be followed in a polarimeter by measuring the optical rotation of D-glucopyranose (D-glucose) in aqueous solutions. However, over a period of a few hours at room temperature, the specific rotation of both forms in aqueous solution changes and attains a stable value of 152. This change in optical rotation, known as mutarotation, is characteristic of sugars that form cyclic structures. Thus, the change in structure can occur in solution and attain equilibrium, which favors the formation of more stable (lowest energy) forms. D-fructose, a ketohexose, can potentially form either a five-membered (furanose) or a six-membered (pyranose) ring involving formation of an internal hemiketal linkage between C2 (the anomeric carbon atom) and the C5 or C6 hydroxyl group, respectively. In aqueous solution at equilibrium, fructose is present predominantly in the -fructopyranose form. However, when fructose is linked with itself or with other sugars, or when it is phosphorylated, it assumes the furanose form. Fructose 1,6-bisphosphate is present in the -fructofuranose form, with a 4:1 ratio of - to -anomeric forms. Fructose is a major constituent (38%) of honey; the other constituents are glucose (31%), water (17%), maltose (a glucose disaccharide, 7%), sucrose (a glucoseructose disaccharide, 1%), and polysaccharide (1%). The individual monosaccharides are better quantitated by specific procedures, such as an enzymatic procedure. A reaction frequently used in the determination of carbohydrate structure, and for its identification in tissue preparations, is the periodate reaction. The vicinal glycols on periodate oxidation yield a dialdehyde; this reaction is quantitative. Periodate cleavage of glycogen (a polyglucose) yields a polyaldehyde that can be coupled to a visible dye reaction. Some physiologically important monosaccharide derivatives include sugar alcohols, sugar acids, amino sugars, sugar phosphates, deoxy sugars, and sugar glycosides. Sialic Acids Sialic acids are derivatives of a nine-carbon-containing monosaccharide, ketonanose, known as neuraminic acid. They typically occur as the terminating units of oligosaccharide side chains of some glycoproteins and glycolipids of mammalian cell membranes, as well as on secreted glycoproteins (Chapter 9). In humans, due to deletion of the gene for the hydroxylase enzyme, N-glycolylneuraminic acid (Neu5Gc) is absent. However, Neu5Gc is found in foods derived from poultry, fish, red meat, and milk products, and has been shown to be metabolically incorporated into cell surface membranes of humans. Recent studies have revealed that the incorporation of this nonhuman glycan provides highaffinity receptors for a cytotoxic protein secreted by Shiga toxigenic Escherichia coli. The gastrointestinal disease (see Chapter 11 for mechanism) and hemolytic uremic syndrome caused by the infection of this toxigenic E. Maltose is composed of two glucose residues joined by an -glycosidic linkage between C1 of one residue and C4 of the other residue [designated (1-4)]. In maltose, the second sugar residue has an unsubstituted anomeric carbon atom and therefore can function as a reducing agent, as well as exhibit mutarotation. In trehalose, two glucose residues are joined by an -linkage through both anomeric carbon atoms; therefore, the disaccharide is not a reducing sugar, nor does it exhibit mutarotation. Lactose, synthesized only by secretory cells of the mammary gland during lactation, is a disaccharide consisting of galactose and glucose. Lactose is a reducing sugar and exhibits mutarotation by virtue of the anomeric C1 of the glucose residue. Lactulose is a synthetic disaccharide consisting of galactose and fructose linked through a -linkage between C1 of galactose and C4 of fructose. It is used in the treatment of some forms of chronic liver disease (such as hepatic encephalopathy) in which the ammonia content in the blood is elevated (hyperammonemia). Normally, ammonia produced in the gastrointestinal tract, principally in the colon by microbial action, is transported to the liver via the portal circulation and inactivated by conversion to urea (Chapter 15). Oral administration of lactulose relieves hyperammonemia by microfloral conversion in the colon to a variety of organic acids. Reduction of luminal pH may additionally promote a microflora that causes a decrease in the production of ammonia, as well as an increase in its utilization. The osmotic activity of the disaccharide and its metabolites causes an osmotic diarrhea, which is useful in eliminating toxic waste products.

Reddish-yellow pigments infantile spasms 4 months buy generic sumatriptan 50 mg, particularly carotene and lycopene back spasms 39 weeks pregnant sumatriptan 100 mg for sale, may give a yellowish tinge to the skin quad spasms after acl surgery discount sumatriptan 25 mg buy on-line, but they do not usually produce scleral coloration skeletal muscle relaxant quizlet buy discount sumatriptan. Hyperbilirubinemia may result from elevation of unconjugated or conjugated bilirubin levels spasms pancreas order genuine sumatriptan on line. If a greater increase occurs, some degree of liver dysfunction probably also occurs. These disorders are usually due to decreased uptake of pigment by hepatocytes or to failure of these cells to store, transport, or conjugate bilirubin. Except in infancy, or when pigment gallstones form, unconjugated hyperbilirubinemias are benign. Serum bilirubin concentration rarely exceeds 5 mg/dL and usually fluctuates between 1. The syndrome is usually asymptomatic and is detected during routine laboratory testing or examination for other diseases. The disease is apparent shortly after birth, kernicterus develops, and death commonly occurs during the neonatal period. Orthotopic liver transplantation is the definitive treatment, and it normalizes serum bilirubin levels. Unconjugated Hyperbilirubinemias Unconjugated hyperbilirubinemias result from imbalance between the rates of production of pigment and of its uptake or conjugation in the liver. Because of the large reserve capacity of the liver for conjugation and Conjugated Hyperbilirubinemias Conjugated hyperbilirubinemias are due to intra- or extrahepatic reduction in bile flow (cholestasis) with spillage of conjugated bilirubin into the bloodstream, which may occur from injury to the endothelial cells lining bile 526 Essentials of Medical Biochemistry ductules or from reverse pinocytosis by the hepatocytes. Since the serum bilirubin is mostly the water-soluble glucuronide, bilirubinuria is usually present. Abdominal tumors, gallstones, strictures, hepatitis, and cirrhosis can mechanically block the biliary canaliculi or ducts. If obstruction affects only intrahepatic bile flow, hyperbilirubinemia occurs when 50% or more of the liver is involved. Nonmechanical cholestasis can be caused by bacterial infection, pregnancy, sex steroids and other drugs, or it may be genetically determined. In cholestasis, bile salts and bile pigments are retained and appear in the circulation, and steatorrhea and deficiencies of fat-soluble vitamins may occur. These deficiencies are often manifested as hypoprothrombinemia (from lack of vitamin K) and osteomalacia (from lack of vitamin D). If blockage is complete, urinary urobilinogen will be absent, and the stools will have a pale, clay-like color. Neonatal Hyperbilirubinemia Normal neonates are frequently hyperbilirubinemic [12,13]. Birth interrupts normal placental elimination of pigment, and the "immature" liver of the neonate must take over. Normally, serum bilirubin levels rise on the first day of life, reaching a maximum (rarely greater than 10 mg/dL) by the third or fourth day. If jaundice is present at birth, a cause other than hepatic immaturity must be sought. The primary blocks to bilirubin metabolism are low activity of bilirubin glucuronyltransferase and a low concentration of ligandin in the liver at birth. Hepatic immaturity may be partly due to diversion in utero of blood from the liver by the ductus venosus. When this channel closes shortly after birth and normal hepatic blood flow is established, concentrations of a number of substances rise within the hepatocytes and may induce enzymes needed for their metabolism. Accumulation of bilirubin in plasma may play an important role in hastening the maturation. Although the liver normally matures within 1 weeks after birth, hypothyroidism can prolong this process for weeks or months. The neonate is at risk for kernicterus if the serum unconjugated bilirubin level is higher than 17 mg/dL. Kernicterus is characterized by yellow staining of clusters of neuronal cell bodies in the basal ganglia, cerebellum, and brain stem, leading to motor and cognitive deficits or death. Immaturity and perhaps hypoxia make the bloodrain barrier permeable to bilirubin and contribute to the likelihood of kernicterus. A major complicating factor can be hemolytic anemia such as that of erythroblastosis fetalis caused by Rh incompatibility between mother and child. The hemolysis increases the rate of bilirubin formation, which soon overwhelms the liver and produces severe jaundice and kernicterus. A decrease in bilirubin production in the neonatal period can also be achieved by inhibiting the rate-limiting enzyme of bilirubin formation from heme, namely, the heme oxygenase [14]. A potent competitive inhibitor of heme oxygenase is the synthetic heme analogue tin (Sn41) protoporphyrin (see Clinical Case Study 27. When administered parenterally, tin protoporphyrin safely decreases bilirubin formation. Sahani, Case 33-2006: a 43-year-old man with diabetes, hypogonadism, cirrhosis, arthralgias, and fatigue, N. Synopsis A 43-year-old Caucasian male presented with a chief complaint of fatigue, decreased libido, and erectile dysfunction. Comprehensive imaging and laboratory testing showed that the subject had hypogonadotropic hypogonadism, recent onset diabetes type 2, arthralgias, and hemochromatosis. The initial diagnosis of hemochromatosis was based on highly elevated serum iron, percent iron saturation and ferritin, and aminotransferase levels. The patient was placed on a therapeutic phlebotomy schedule to reduce iron overload, treatment with insulin for diabetes, and testosterone supplementation for decreased libido. Lifestyle modification of avoidance of ethanol intake, iron and vitamin C supplements, and decreased consumption of red meat were implemented. The most common nutritional disorder is due to iron deficiency, which leads to hypochromic, microcytic anemia. The accumulation of iron in the body, either due to frequent blood transfusions. The toxicity of iron is due to the production of superoxide anions and hydroxyl radicals, which inactivate proteins, lipids, and nucleic acids. Iron absorption, storage, and utilization are orchestrated by gastrointestinal duodenal cells, macrophages of liver and spleen, and hepatocytes. Hepcidin is a 25-amino acid peptide consisting of four intra-disulfide bonds; it attenuates (inhibits) iron absorption and iron release from macrophages. Thus, mutation in iron regulatory proteins that result in decreased hepcidin synthesis can lead to all forms of currently known genetic hemochromatosis. Clinical penetrance of the homozygous C282Y mutation is incomplete and is probably affected by modifier genes. Timely diagnosis of genetic hemochromatosis in subjects with high serum iron, percent iron saturation, and ferritin levels, and unexplained elevated serum aminotransferase levels is vital to prevent multiorgan iron damage. Initiation of iron removal by regular therapeutic phlebotomy can ameliorate symptoms. Iron removal by chelation therapy is utilized in some forms of genetic hemochromatosis associated with anemia and in transfusion-dependent secondary hemochromatosis. Anemia of chronic disease that occurs in acute and chronic inflammatory immune disorders is caused by a cytokinemediated increase of hepcidin synthesis, leading to decreased availability of iron required for heme biosynthesis. In addition to changes in iron homeostasis, anemia of chronic disease affects erythropoietin synthesis and proliferation of erythroid precursor cells. In anemia due to chronic renal disease, the primary cause has been attributed to decreased erythropoietin production in the kidneys. Thus, administration of recombinant erythropoietin is employed in correcting anemia of chronic renal disease. Badizadegan, Case 21-2005: a fourweek-old male infant with jaundice and thrombocytopenia, N. Flamm, Hemochromatosis: a new look at a familiar disease, Cortland Forum 20 (2007) 357. Enns, Iron homeostasis: recently identified proteins provide insight into novel control mechanisms, J. Marx, Hereditary hemochromatosis: genetic complexity and new diagnostic approaches, Clin. Sohani, Case 12-2014: a 59-year-old man with fatigue, abdominal pain, anemia, and abnormal liver function, N. Synopsis A 59-year-old man presented to the clinic with a 3-day history of fatigue, epigastric pain, nausea, and ankle swelling. His physical examination was unremarkable, but laboratory studies revealed anemia and elevated liver enzymes. Review of a peripheral blood smear showed microcytic anemia, polychromasia (red cell enlargement with a purplish hue), and basophilic stippling (punctate basophilic inclusions that are evenly distributed throughout the cytoplasm). He was sent home with omeprazole for the treatment of peptic-ulcer disease and bleeding ulcer. However, within one week his abdominal pain worsened, and he developed an unusual constellation of symptoms raising concerns for lead poisoning, such as behavioral changes and dysgeusia (altered sense of taste). The most common source of lead poisoning in the United States is from workplace exposures. Basophilic stippling is a hallmark feature of lead poisoning and sideroblastic anemia, but it may not be seen in all cases of lead poisoning. Synopsis Two unrelated newborn infants at the same hospital were found to have progressive hyperbilirubinemia despite phototherapy. Exchange transfusion therapy was considered when their plasma unconjugated bilirubin had reached a level of 19. However, the families of both infants refused exchange transfusion therapy due to religious concerns. Bilirubin is a catabolic product of heme synthesized in the macrophages in two enzymatic steps. Elimination of bilirubin requires albumin-bound transport to the liver, conjugation with glucuronic acid, and eventual removal via the biliary-gastrointestinal tract. Newborn unconjugated hyperbilirubinemia, a common condition, is usually treated with phototherapy. Note that the photoisomers of bilirubin are water-soluble, do not require glucuronidation, and are eliminated in the urine. Severe hyperbilirubinemia can occur due to prematurity, isoimmune hemolytic disease, glucose-6-phosphate dehydrogenase deficiency, asphyxia, acidosis, and hypoalbuminemia. The precise plasma levels of bilirubin that cause abnormal neurologic manifestations are not understood (see references [2] and [3]). A determining factor for the management of hyperbilirubinemia is its progressive increase, unresponsiveness to phototherapy, and the presence of risk factors. In the treatment of neonatal hyperbilirubinemia, exchange blood transfusion therapy is effective for rapid elimination of bilirubin. While the primary management consideration for neonatal jaundice requires incorporation of methods that decrease plasma bilirubin levels, the management of adult jaundice requires the diagnosis of diseases of the hepatic-biliary system. Biggs, Control of iron deficiency anemia in low- and middle-income countries, Blood 121 (2013) 2607617. Mast, Iron deficiency: what are the future trends in diagnostics and therapeutics Vercellotti, Hemolysis and free hemoglobin revisited: exploring hemoglobin and hemin scavengers as a novel class of therapeutic proteins, Blood 121 (2013) 1276284. Desnick, the porphyrias: advances in diagnosis and treatment, Blood 120 (2012) 4496504. Ingelfinger, Bilirubin-induced neurologic damage-mechanisms and management approaches, N. Chapter 28 Endocrine Metabolism I: Introduction and Signal Transduction1 Key Points 1. Hormones and neurotransmitters are integrated, and they coordinate cellular functions in the body. Hormones can be amino acid-derived amines, peptides, proteins or glycoproteins, steroids, or eicosanoids. The nervous and endocrine systems function in a coordinated manner to promote growth, homeostasis, and reproductive competence. Feedback regulation of an endocrine system (usually negative) involves both simple feedback loops. Steroid hormones and thyroid hormones (T3 and T4) require transport proteins in the blood to reach the target sites. The physiological response to a hormone (ligand) is determined by the presence of a specific receptor at the target cell. The receptors may be located on the plasma cell membrane, in the cytosol, or in the nucleus. Hormone recognition and binding at their specific receptor binding site initiate the signal transduction amplification pathways, which culminates in an appropriate biological response. Nuclear receptors are responsible for the action of the thyroid hormone tetraiodothyronine (T3). T3 binding to the nonhistone receptor proteins stimulates transcription at the 12. Amine, polypeptide, and protein hormones (either growth promoting or inhibiting) initiate their action by binding to plasma membrane receptors on the cell surface. Monomeric G-proteins anchored to the inner cytoplasmic membrane also participate in the normal cellular functions when activated by external stimuli. Some of these are proto-oncogenes and, when mutated, become oncogenes that promote cancer. The first type contains an extracellular receptor domain and an intracellular domain with tyrosine kinase activity. Nonreceptor tyrosine kinases are found in the cytosol and are involved in the signal transduction pathways of normal cellular processes. Specific inhibitions at the receptor sites using monoclonal antibodies or tyrosine kinase inhibitors are therapeutic targets used in the management of some cancers. Endocrine topics not discussed in Chapters 28 through 32 that are covered elsewhere in the text are as follows: gastrointestinal hormones, Chapter 11; eicosanoids, Chapter 16; pancreatic hormones, Chapter 20; parathyroid hormone and vitamin D, Chapter 35; reninngiotensin system and antidiuretic hormone, Chapters 30 and 37.

Acute stressful physiological conditions such as trauma spasms calf muscles buy sumatriptan 50 mg otc, burn gas spasms in stomach order sumatriptan with a visa, or sepsis can also precipitate protein energy malnutrition due to hypermetabolism caused by the neuroendocrine system muscle relaxant erowid best buy for sumatriptan. Prompt diagnosis and appropriate nutritional intervention are required in the management of patients with protein energy malnutrition muscle relaxant video 100 mg sumatriptan order with visa. Measurements of the levels of serum proteins spasms 1983 dvd 25 mg sumatriptan buy visa, such as albumin, transthyretin (also known as prealbumin), transferrin, and retinol-binding protein are used as biochemical parameters in the assessment of protein energy malnutrition. An ideal protein marker should have rapid turnover and be present in sufficiently high concentrations in serum to be measured accurately. Transthyretin has these properties; it is a sensitive indicator of protein deficiency and is effective in assessing improvement with refeeding. The plasma half-life of transthyretin is about 1 days, whereas albumin has a half-life of 159 days. Transport of Amino Acids into Cells the intracellular metabolism of amino acids requires their transport across the cell membrane. Transport of L-amino acids occurs against a concentration gradient and is an active process usually coupled to Na1-dependent carrier systems, such as for transport of glucose across the intestinal mucosa (Chapter 11). At least five transport systems for amino acids (with overlapping specificities) have been identified in kidney and intestine. They transport neutral amino acids, acidic amino acids, basic amino acids, ornithine and cystine, and glycine and proline, respectively. Na1-independent transport carriers for neutral and lipophilic amino acids have also been described. D-amino acids are transported by simple diffusion favored by a concentration gradient. Inherited defects in amino acid transport affect epithelial cells of the gastrointestinal tract and renal tubules. Some affect transport of neutral amino acids (Hartnup disease); others that of basic amino acids and ornithine and cystine (cystinuria), or of glycine and proline (Chapter 11). Cystinosis is an intracellular transport defect characterized by high intralysosomal content of free cystine in the reticuloendothelial system, bone marrow, kidney, and eye. After degradation of endocytosed protein to amino acids within lysosomes, the amino acids are normally transported to the cytosol. The conjugation reaction is catalyzed by specific glutathione S-transferases and the product is eventually converted to mercapturic acids and excreted. The enzyme is present in all tissues, but the highest level is in the kidney; however, the serum enzyme originates primarily from the hepatobiliary system. General Reactions of Amino Acids Some general reactions that involve degradation or interconversion of amino acids provide for the synthesis of nonessential amino acids from -keto acid precursors derived from carbohydrate intermediates. Deamination Removal of the -amino group is the first step in catabolism of amino acids. Oxidative deamination is stereospecific and is catalyzed by L- or D-amino acid oxidase. The initial step is removal of two hydrogen atoms by the flavin coenzyme, with formation of an unstable -amino acid intermediate. This intermediate undergoes decomposition by addition of water and forms the ammonium ion and the corresponding -keto acid: L-amino acid oxidase occurs in the liver and kidney only. Conversion of D-amino acids to the corresponding -keto acids removes the asymmetry at the -carbon atom. The initial step probably involves formation of -iminoglutarate by dehydrogenation. Glutamate dehydrogenase is the only amino acid dehydrogenase present in most cells. Glutamine synthetase is a cytoplasmic enzyme; it is both developmentally regulated and allosterically regulated by metabolites and hormones. Glutamine synthetase participates in the detoxification of ammonia, interorgan nitrogen transport, and acidase regulation. In the central nervous system, glutamine and glutamate cycle between the astrocytes and neurons, and serve a vital role in excitatory neurotransmission. Astrocytes are located in proximity to blood vessels, and their cellular processes surround the neurons. Aminotransferases occur in cytosol and mitochondria, but their activity is much higher in cytosol. Since glutamate dehydrogenase is restricted to mitochondria, transport of glutamate (generated by various transaminases) into mitochondria by a specific carrier becomes of central importance in amino acid metabolism. Deficiency of glutamine synthetase in the astrocytes results in deprivation of glutamate and leads to neurological disease. Deficiency in uptake of excess glutamate from the synaptic cleft is thought to result in continued excito-toxicity, resulting in neuronal death as it may occur in amyotrophic lateral sclerosis. Transaminationminotransfer Reactions Transamination reactions combine reversible amination and deamination, and they mediate redistribution of amino groups among amino acids. Transaminases (aminotransferases) are widely distributed in human tissues and are particularly active in heart muscle, liver, skeletal muscle, and kidney. The general reaction of transamination is Formation of Glutamine and the Glutamatelutamine Cycle Glutamine is the most abundant amino acid, and it participates in many essential metabolic reactions. The specificity of a particular transaminase is for the amino group other than the glutamate. Isonicotinic acid hydrazide (used in the treatment of tuberculosis) and hydralazine (a hypertensive agent) react with the aldehyde group of pyridoxal (free or bound) to form pyridoxal hydrazones, which are eliminated in the urine. Isonicotinic acid hydrazide is normally inactivated in the liver by acetylation; some individuals are "slow acetylators" (an inherited trait) and may be susceptible to pyridoxal deficiency from accumulation of the drug. Cycloserine (an amino acid analogue and broad-spectrum antibiotic) also combines with pyridoxal phosphate. All of the amino acids except lysine, threonine, proline, and hydroxyproline participate in transamination reactions. Transaminases exist for histidine, serine, phenylalanine, and methionine, but the major pathways of their metabolism do not involve transamination. Thus, transfer of a -amino group of ornithine to -ketoglutarate converts ornithine to glutamate-semialdehyde. All transaminase reactions have the same mechanism and use pyridoxal phosphate (a derivative of vitamin B6; Chapter 36). During catalysis, the amino acid substrate displaces the lysyl -amino group of the enzyme in the Schiff base. An electron pair is removed from the -carbon of the substrate and transferred to the positively charged pyridine ring but is subsequently returned to the second substrate, the -keto acid. In the Role of Organs in Amino Acid Metabolism In the postabsorptive state, maintenance of steady-state concentrations of plasma amino acids depends on release of amino acids from tissue protein. After a meal, dietary amino acids enter the plasma, replenish the tissues, and are metabolized during fasting. Liver plays a major role, since it can oxidize all amino acids except leucine, isoleucine, and valine. It also produces the nonessential amino acids from the appropriate carbon precursors. Ammonia formed in the gastrointestinal tract or from various deaminations in the liver is converted to urea and excreted in urine (discussed later). Skeletal muscle tissue constitutes a large portion of the body weight and accounts for a significant portion of nonhepatic amino acid metabolism. It takes up the amino acids required to meet its needs for protein synthesis, and metabolizes alanine, aspartate, glutamate, and the branched-chain amino acids. Alanine and glutamine constitute more than 50% of the -amino acid nitrogen released. Excitatory neurotransmitter glutamate required for glutamatergic neurons is provided by the astrocytes in the form of glutamine. Astrocyte glutamine synthetase converts the excess neurotransmitter glutamate taken up from the synaptic cleft and glutamate obtained from the capillaries to glutamine. Either deficiencies of astrocyte glutamine synthetase or reuptake of glutamate by the Na1-dependent glutamate transporters from the synaptic cleft by the astrocytes can lead to neurological and neurodegenerative diseases. The carbonyl carbon reacts with the -amino group of the lysyl residue near the active site to yield a Schiff base. Ionic interactions involve its positively charged pyridinium ion and negatively charged phosphate group. The second phase occurs by the reversal of the first phase reactions and is initiated by formation of a Schiff base with the -keto acid substrate and pyridoxamine phosphate. The transamination cycle is completed with formation of the corresponding -amino acid and pyridoxal phosphate. However, the amino acid composition of these proteins does not account for the large amount of alanine and glutamine released. For example, aspartate can be converted to alanine as follows: AspartateOxaloacetate Given the appropriate apoenzyme, any atom or group on the carbon atom proximal to the Schiff base can be cleaved. Isonicotinic acid hydrazide (antituberculosis drug) Hydralazine (hypertensive agent) Cycloserine (antibiotic) inactivated by oxidation of fatty acids and ketone bodies (Chapters 12 and 16). All except leucine and lysine (which are oxidized solely to acetylCoA) can be used in net synthesis of -ketoglutarate to enhance glutamate synthesis. These intermediates and the unmetabolized dietary amino acids are transferred to the portal blood and then to the liver for further metabolism. In the fasting state, the intestinal mucosa depends on other tissues for metabolites to provide energy and precursors for protein and nucleotide synthesis to maintain the rapid cell division characteristic of that tissue. Glutamine, released from liver and muscle, is utilized for purine nucleotide synthesis (Chapter 25), is oxidized to provide energy, and can be converted to aspartate for pyrimidine nucleotide synthesis (Chapter 25). Intestine can also oxidize glucose, fatty acids, and ketone bodies to provide energy. Kidney releases serine and small (but significant) quantities of alanine into the blood, and takes up glutamine, proline, and glycine. It can provide two ammonia molecules, by glutaminase and glutamate dehydrogenase, respectively, in renal tubular mitochondria. Its carbon skeleton can be oxidized or converted to glucose, since renal tissue is capable of gluconeogenesis (Chapter 14). Brain takes up significant quantities of valine and may be a major (if not primary) site of utilization of branched-chain amino acids. The role of the glutamateglutamine cycle between neuronstrocyte in the excitation glutamatergic pathway was discussed previously. Glutamate is a precursor of -aminobutyrate; tyrosine of dopamine, norepinephrine, and epinephrine; and tryptophan of serotonin, all of which are neurotransmitters. N-acetylaspartate occurs in high levels in the brain but its function is not completely understood. It may provide acetate for myelin lipid synthesis and participate in the synthesis of neuronal dipeptide Nacetylaspartylglutamate. It is synthesized from acetylCoA and aspartic acid catalyzed by acetyl-CoA aspartate N-acetyl transferase. Aspartoacylase catalyzes the hydrolysis of N-acetylaspartate to acetate and aspartic acid. The deficiency of aspartoacylase, which is inherited as an autosomal recessive trait, is associated with degenerative brain changes. Patients with this disorder, also known as Canavan dystrophy, are usually of Eastern European Jewish heritage. Some products of these reactions are utilized for other purposes (thus salvaging a portion of the amino nitrogen), while others are excreted. In humans, ammonia is excreted mostly as urea, which is highly water-soluble, is distributed throughout extracellular and intracellular body water, is nontoxic and metabolically inert, has a high nitrogen content (47%), and is excreted via the kidneys. Ammonia is produced by deamination of glutamine, glutamate, other amino acids, and adenylate. The urea comes from body fluids that diffuse into the intestine, and the other nitrogenous products are derived from intestinal metabolism. The ammonia diffuses across the intestinal mucosa to the portal blood and is converted to urea in the liver. Ammonia is particularly toxic to brain but not to other tissues, even though levels in those tissues may increase under normal physiological conditions. In brain mitochondria, excess ammonia may drive the reductive amination of -ketoglutarate by glutamate dehydrogenase. This hypothesis does not explain why the same result does not occur in tissues that are not affected by ammonia. A more plausible hypothesis is depletion of glutamate, which is an excitatory neurotransmitter. Glutamine, synthesized and stored in the astrocytes and glial cells, is the precursor of glutamate. A third hypothesis invokes neuronal membrane dysfunction, since elevated levels of ammonia produce increased permeability to K1 and Cl2 ions, while glycolysis increases H1 ion concentration (stimulates 6phosphofructokinase; Chapter 12). Encephalopathy of hyperammonemia is characterized by brain edema and astrocyte swelling. Edema and swelling have been attributed to intracellular accumulation of glutamine, which causes osmotic shifts of water into the cell. Behavioral disorders such as anorexia, sleep disturbances, and pain insensitivity associated with hyperammonemia have been attributed to increased tryptophan transport across the bloodrain barrier and the accumulation of its metabolites. Two of the tryptophan-derived metabolites are serotonin and quinolinic acid (discussed later).

Glucose binds to the receptor spasms muscle pain generic 100 mg sumatriptan amex, facilitated by the simultaneous binding of two Na1 ions at separate sites muscle relaxant migraine 50 mg sumatriptan order free shipping. The glucose and Na1 are released in the cytosol as the receptor affinity for them decreases muscle relaxant for sciatica 100 mg sumatriptan overnight delivery. Glucose is transported out of the cell into the intercellular space and hence to portal capillaries muscle relaxant injection for back pain order 100 mg sumatriptan fast delivery, both by a serosal carrier and by diffusion muscle relaxant cyclobenzaprine dosage order sumatriptan online now. Thus, glucose and Na1 are transported by a common carrier, and energy is provided by the transport of Na1 down the concentration and electrical gradient. Although this mode of glucose transport is the most significant, passive diffusion along a concentration gradient may also operate if the luminal concentration of glucose exceeds the intracellular concentration. The intracellular glucose is transferred to the portal capillary blood by passive diffusion and by a carriermediated system. Intracellular glucose can be converted to lactate (Chapter 12), which is transported via the portal blood system to the liver, where it is reconverted to glucose (gluconeogenesis; Chapter 14). The quantitative significance of this mode of glucose transport is probably minimal. Gastrointestinal Digestion and Absorption Chapter 11 151 Fructose transport is distinct from glucosealactose transport and requires a specific saturable membrane carrier (facilitated diffusion). Disorders of Carbohydrate Digestion and Absorption Carbohydrate malabsorption can occur in a number of diseases that cause mucosal damage or dysfunction. Lactose Intolerance Lactose intolerance (also known as milk intolerance) is the most common disorder of carbohydrate absorption, leading to diarrhea [15]. Lactase deficiency occurs in the majority of human adults throughout the world and appears to be genetically determined. The prevalence is high in persons of African and Asian ancestry ($65%) and low in persons of Northern European ancestry. Lactase deficiency in which mucosal lactase levels are low or absent at birth is rare and is transmitted as an autosomal recessive trait. Since hydrolysis of lactose by lactase is rate-limiting, any mucosal damage will cause lactose intolerance. In full-term human infants, lactase activity attains peak values at birth and remains high throughout infancy. As milk intake decreases, lactase levels drop and lactose intolerance may develop. The extent of the decrease of lactase activity distinguishes lactose-tolerant from -intolerant populations. Ingestion of milk (or lactose) by individuals who have lactose intolerance leads to a variety of symptoms (bloating, cramps, flatulence, and loose stools). Severity of the symptoms depends on the amount of lactose consumed and the enzyme activity. Lactose-depleted milk or fermented milk products with negligible amounts of lactose are good substitutes for milk. The intestinal problems are due primarily to osmotic effects of lactose and its metabolites in the colon. The lactose not absorbed in the small intestine increases the osmolarity and causes water to be retained in the lumen. In the colon, it is metabolized by bacterial enzymes to a number of short-chain acids, further increasing osmolarity and aggravating fluid reabsorption. These disorders are rare autosomal recessive traits; clinical problems can be corrected by removing the offending sugars from the diet. Lactulose, a synthetic disaccharide consisting of galactose and fructose with a (1-4) linkage, is hydrolyzed not in the small intestine but in the colon, and is converted to products similar to those derived from lactose fermentation. Proteins Protein is an essential nutrient for human growth, development, and homeostasis. The nutritive value of dietary proteins depends on its amino acid composition and digestibility. Dietary proteins supply essential amino acids, which are not synthesized in the body. Nonessential amino acids can be synthesized from appropriate precursor substances (Chapter 15). In human adults, essential amino acids are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine. Histidine (and possibly arginine) appears to also be required for support of normal growth in children. In the absence from the diet of an essential amino acid, cellular protein synthesis does not occur. Thus, quality and quantity of dietary protein consumption and adequate intake of energy (carbohydrates and lipids) are essential. Animal proteins, with the exception of collagen (which lacks tryptophan), provide all of the essential amino acids. Vegetable proteins differ in their content of essential amino acids, but a mixture of these proteins will satisfy the essential amino acid requirement. This combination also corrects for the methionine (which is supplied in corn) deficiency of legumes. The recommended allowance for mixed proteins in an adult in the United States is 0. Intolerance of Other Carbohydrates Intolerance to sucrose and -limit dextrins may be due to deficiency of sucrase-dextrinase or to a defect in 152 Essentials of Medical Biochemistry Besides dietary protein, a large amount of endogenous protein undergoes digestion and absorption. Enzymes, glycoproteins, and mucins secreted from the salivary glands, stomach, intestine, biliary tract, and pancreas, which together constitute about 200 g/day; 2. Rapid turnover of the gastrointestinal epithelium, which contributes about 30 g/day; and 3. Plasma proteins that normally diffuse into the intestinal tract at a rate of 1 g/day. It cannot degrade proline-containing dipeptides, which are largely hydrolyzed intracellularly. Dipeptidases and tripeptidases are associated with the brush-border membranes, but their functions are not clearly understood. The major products of intraluminal digestion of protein are mixtures of amino acids (30%40%) and small peptides (60%0%). Absorption of Amino Acids and Oligopeptides Dipeptides and tripeptides that escape brush-border membrane peptidases are actively transported against a concentration gradient by Na1-dependent mechanisms. Free amino acids are transported into enterocytes by four active, carrier-mediated, Na1-dependent transport systems remarkably similar to the system for glucose. These systems transport, respectively, neutral amino acids; basic amino acids (Lys, Arg, His) and cystine; aspartic and glutamic acids; and glycine and imino acids. Entry of amino acids into cell compartments elsewhere in the body may require different transport systems. Glutamate, glutamine, aspartate, and asparagine are metabolized in the enterocyte (Chapter 15). Absorption of food proteins (or their partially digested antigenic peptides) can cause allergic manifestations, whereas bacterial and viral antigens stimulate immunity by production and secretion of secretory IgA (Chapter 33). Digestion Protein digestion begins in the stomach, where protein is denatured by the low pH and is exposed to the action of pepsin. This endopeptidase hydrolyzes peptide bonds that involve the carboxyl group of aromatic amino acid residues, leucine, methionine, and acidic residues. Chyme contains potent secretagogues for various endocrine cells in the intestinal mucosa. Activation begins with the conversion of trypsinogen to trypsin by enteropeptidase (previously called enterokinase) present in the brushborder membranes of the duodenum. Enteropeptidase cleaves between Lys-6 and Ile-7 to release a hexapeptide from the N-terminus of trypsinogen. The importance of the initial activation of trypsinogen to trypsin by enteropeptidase is manifested by children with congenital enteropeptidase deficiency who exhibit hypoproteinemia, anemia, failure to thrive, vomiting, and diarrhea. The combined action of these enzymes produces oligopeptides having two to six amino acid residues and free amino acids. Hydrolysis of oligopeptides by the brush-border aminopeptidases releases amino acids. Leucine aminopeptidase, a Zn21-containing enzyme, is an integral transmembrane glycoprotein with a carbohydrate-rich Disorders of Protein Digestion and Absorption the principal causes of protein maldigestion and malabsorption are diseases of the exocrine pancreas and small intestine. Primary isolated deficiency of pepsinogen or pepsin, affecting protein assimilation, has not been described. Gastrointestinal Digestion and Absorption Chapter 11 153 Defects in neutral amino acid transport (Hartnup disease), in basic amino acids and cystine (cystinuria), dicarboxylic aminoaciduria, and aminoglycinuria have been reported. The clinical severity of these disorders is usually minimal and relates to the loss of amino acids or relative insolubility of certain amino acids in the urine. In cystinuria, for example, cystine can precipitate in acidic urine to form stones. In Hartnup disease, severe nutritional deficiencies are uncommon, since the essential amino acids are absorbed as dipeptides or oligopeptides. Skin and neuropsychiatric manifestations characteristic of nicotinamide deficiency that occur in Hartnup disease respond to oral nicotinamide supplementation. Lipids Dietary fat provides energy in a highly concentrated form and accounts for 40%5% of the total daily energy intake (100 g/day in the average Western diet). Lipids contain more than twice the energy per unit mass than carbohydrates and proteins (Chapter 5). The efficiency of fat absorption is very high; under normal conditions, almost all ingested fat is absorbed, with less than 5% appearing in the feces. The predominant dietary lipid is triacylglycerol (a triglyceride), which contains three long-chain (l6-carbon or longer) fatty acids (Chapter 16). The dietary lipids include essential fatty acids (Chapter 16) and the lipid-soluble vitamins A, D, E, and K (Chapters 35 and 36). Digestion and absorption of lipids involves the coordinated function of several organs but can be divided into three phases: luminal, intracellular, and secretory. However, lipases secreted by lingual glands at the base of the tongue are active at acid pH and initiate the hydrolysis of triacylglycerol without a requirement for bile acids. The free fatty acids also stabilize the surface of triacylglycerol particles and promote binding of pancreatic colipase. This phase aids in the optimal action of pancreatic lipase and is particularly important in disorders of pancreatic function or secretion. The major functions of the stomach in fat digestion are to store a fatty meal, to contribute to emulsification by the shearing actions of the pylorus, and to gradually transfer the partially digested emulsified fat to the duodenum by controlling the rate of delivery. The hydrolysis of triacylglycerol in the duodenum and jejunum requires bile and pancreatic juice. Bile acids are powerful detergents that, together with monoacylglycerol and phosphatidylcholine, promote the emulsification of lipids. The products of digestion are relatively insoluble in water but are solubilized in micelles. Micelles also contain lipid-soluble vitamins, cholesterol, and phosphatidylcholine. Pancreatic lipase functions at the lipidater interface, its activity being facilitated by colipase (M. B10,000), also secreted by the pancreas as procolipase activated by tryptic hydrolysis of an Argly bond in the N-terminal region. Colipase anchors the lipase to the triacylglycerol emulsion in the presence of bile salts by forming a 1:1 complex with lipase, and protects lipase against denaturation. Colipase deficiency (with normal lipase) is accompanied by significant lipid malabsorption, as is pancreatic lipase deficiency. Pancreatic juice contains esterases that act on short-chain triacylglycerols and do not require bile salts, as well as cholesteryl esterase. Phosphatidylcholine in the diet (4 g/day) and in bile secretions (172 g/day) is hydrolyzed to lysophosphatidylcholine and fatty acid by phospholipase A2, a pancreatic enzyme with an absolute requirement for Ca1 ions and for bile acids. The secreted form, pro2 phospholipase A2, is activated by tryptic hydrolysis of an Argla bond in the N-terminal region. Phospholipase A2 also hydrolyzes fatty acids esterified at the 2-position of phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine, phosphatidylinositol, and cardiolipin, but has no effect on sphingolipids. Lipid absorption in the duodenum and jejunum appears to be a passive diffusion process. Lipid-laden micelles migrate to the microvilli, and the fatty acids, monoacylglycerols, and lysophosphoglycerols are transferred across the membrane according to their solubility within the lipid bilayer of the cell surfaces. Bile acids are not absorbed into the enterocyte but migrate to the ileum, where they are actively absorbed and transferred to the liver via the portal venous system. The bile acid pool is recycled several times daily (enterohepatic circulation) to meet the demands of lipid digestion, and disorders that interfere with this process lead to malabsorption of lipids. A cytoplasmic fatty acid-binding protein with high affinity for long-chain fatty acids transports fatty acids to the smooth endoplasmic reticulum for resynthesis of triacylglycerol. Medium-chain triacylglycerols are partly water-soluble, are rapidly hydrolyzed by lingual and pancreatic lipases, and do not require the participation of bile acids. Thus, medium-chain triacylglycerols can be digested and absorbed in the presence of minimal amounts of pancreatic lipase and in the absence of bile salts. For this reason, they are used to supplement energy intake in patients with malabsorption syndromes. Coconut oil is rich in trioctanoylglycerol (8-carbon) and tridecanoylglycerol (10-carbon). Intracellular (Mucosal) Phase Fatty acids (long-chain) are activated, and monoacylglycerols are converted to triacylglycerols at the smooth endoplasmic reticulum. Esterification of monoacylglycerol to diacylglycerol and triacylglycerol catalyzed by monoacylglycerol transacylase and diacylglycerol transacylase, respectively. In a minor alternative pathway, triacylglycerol is synthesized from glycerol-3-phosphate and acyl-CoA by esterification at the 1,2-positions of glycerol, removal of the phosphate group, removal of the phosphate group, and esterification at C3 (Chapter 17). Mg21;K1 the triacylglycerols are incorporated into a heterogeneous population of spherical lipoprotein particles known as chylomicrons (diameter 7500 nm) that contain about 89% triacylglycerol, 8% phospholipid, 2% cholesterol, and 1% protein.

Estrogen has many actions muscle spasms 9 weeks pregnant sumatriptan 100 mg order on-line, including both reproductive and nonreproductive functions muscle relaxant 751 generic 25 mg sumatriptan with mastercard. Its action is mediated by the presence of the isoforms of the estrogen receptors ( spasms pelvic floor 50 mg sumatriptan order free shipping, or both) in target tissue and differences in estrogen receptor conformations upon ligand binding and interaction with transcriptional coregulatory proteins spasms when falling asleep sumatriptan 100 mg mastercard. In postmenopausal women with estrogen-positive breast cancers muscle relaxant veterinary purchase sumatriptan mastercard, inhibition of estrogen formation from testosterone by aromatase via specific inhibitors has provided therapeutic benefits. Selective estrogen receptor modulators demonstrate both tissue-specific agonistic and antagonistic effects on estrogen action and also have been used therapeutically in the management of breast cancer. Both genetic and hormonal determinants normally operate at only two phases of life: during fetal development and at puberty. In females, one of the X-chromosomes is randomly inactivated permanently during early embryonic life when the embryo consists of fewer than 200 cells. Thus, both X-chromosomes, although one of them is mostly inactivated, participate in the female phenotype. This phenomenon is illustrated in Turner syndrome, in which the reduced complement of genes that are normally expressed from both X-chromosomes gives rise to abnormal phenotypes. In both genotypes, the embryonal gonads develop from the epithelium and stroma of the urogenital ridge, a thickening of the celomic (ventromedial) aspect of the mesonephros that emerges at about the second or third week of pregnancy. Both the epithelium and stroma of the urogenital ridge are derived from the intermediate mesoderm of the embryo; however, invading this structure at about week 4 are primordial germ cells from the yolk sac, which take residence in association with the epithelial cells of the developing gonad and replicate. During the germ cell invasion, the epithelial cells of the gonads undergo proliferation and begin entering the stromal spaces as cord-like projections, called primary sex cords. Until about week 6, the gonads are undifferentiated and uncommitted; that is, the structure has the potential to develop into either ovaries or testes. The latter is a severe form of skeletal dysplasia associated with dysmorphic features and cardiac defects. Testosterone promotes the development of the Wolffian duct into male internal genitalia, including epididymis, vas deferens, and seminal vesicles. The effects of testosterone on the Wolffian duct and urogenital sinus are dependent on the presence of androgen receptors and 5-reductase, which are expressed by genes on the X-chromosome and chromosome 2, respectively, and are present in both genotypic sexes. This explains why exposure of the fetus to androgens during the critical period may lead to masculinization of the internal and external genitalia of genotypically female fetuses. It also explains why lack of expression of either gene can lead to feminization in genotypically male fetuses. Gametogenesis depends on gonadal hormone production, which is influenced by gametogenesis. In addition, gametogenesis is regulated by the paracrine actions of gonadal hormones. The obvious difference between the sexes in gametogenesis is the formation of ova (ootids) in the female and of sperm (spermatozoa) in the male. Spermatogenesis becomes operational from about the time of puberty and continues throughout life. The initial phase involves proliferation of the stem cells (oogonia) and occurs only in fetal life. The second phase involves the first maturational division (formation of the secondary oocyte) and occurs about the time of ovulation (see later). The third phase involves the second and final maturational division (formation of ova) and occurs at fertilization. Unlike the continuous generation of sperm in the testes, the ovaries generally produce only one secondary oocyte every 228 days. Ovaries and testes produce identical steroid hormones, but the amounts and their patterns of secretion are different Table 32. Although testosterone is present in both males and females, its level in the male is about 18 times that in the female; conversely, circulating levels of estradiol in the female are about 35 times those in the male. In either sex, the major sex steroid originates in the gonads, whereas in the opposite sex this steroid is generated in substantial amounts by the adrenal cortex or by peripheral conversion of another steroid. For example, almost all of the circulating estradiol in the female comes from the ovaries, whereas in the male only about one-third comes from the testis, the rest being generated by peripheral conversion of androgenic precursors. In the male, the secretion of gonadal steroids is fairly constant, although minor fluctuations occur as a result of circadian rhythms. In the adult female, gonadal steroid secretion undergoes dramatic, cyclic changes at about monthly intervals. These cyclic changes, which are dictated by processes regulating oogenesis, are referred to as the menstrual cycle. Regulation of Spermatogenesis: Sertolieuroendocrine Axis Sertoli cells are epithelial cells that line the seminiferous tubules of the testes. At their basal aspects, these cells form the basement membrane and tight junctions that make up the highly selective "bloodestes barrier," which normally prevents entry of immune cells into the lumen. Their function is to provide nutritional and hormonal support for cells undergoing spermatogenic transformation. Environment Drugs Age Brain centers +/testosterone, maintains high levels of testosterone in the seminiferous tubules and thereby helps maintain spermatogenesis. The major steroid product of the Leydig cells is testosterone, which accounts for most of the steroid output by the adult testes. Secretion of testosterone is fairly constant and consistent in adult men, although higher levels occur in the morning and lower levels in late evening. Testosterone levels decline by 105% between the ages of 30 and 70 years, accompanied by a reduction in tissue responsive to androgenic stimulation. These and other polar compounds account for about 30%0% of the testosterone metabolites excreted daily. Unlike the androgenic steroids, the only major site of estrogen inactivation appears to be the liver; therefore, estrogen theoretically can be recycled until it is transported to the liver, which itself is an estrogenresponsive tissue. Testosterone is known to exert some important biological effects in the liver and kidney, which are major testosterone-inactivating tissues in the body. About 50% of the testosterone is removed from plasma with each passage through the liver. Epitestosterone is biologically inactive, but is not a metabolite, and is believed to be produced only by the gonads; thus, it is used as a gonadal steroid marker. Urinary T:epiThis useful in monitoring abuse of anabolic steroids by athletes because the ratio increases when any exogenous testosterone derivative is used. These effects are mediated by testosterone, dihydrotestosterone, and estradiol (E2). Endocrine Metabolism V: Reproductive System Chapter 32 595 Biological Effects of Androgens the biological effects of androgenic hormones can be of two types: (1) reproductive (androgenic), i. However, under physiological conditions, a critical factor that determines which hormone is active in a given tissue is the presence or absence of 5-reductase. During the follicular phase, the uterine endometrium is stimulated by estrogen to proliferate and to synthesize cytosolic receptors for progesterone. The follicle that ruptures at ovulation becomes the corpus luteum, which produces progesterone and estradiol. During this period, the uterine endometrium becomes secretory under the influence of progesterone. About 5 days into the luteal phase, the endometrium is ready to accept a blastocyst for implantation; in the absence of fertilization, however, the corpus luteum degenerates after about 12 days, steroid production ends, and the endometrium deteriorates (menstruation). Endocrine Control of Folliculogenesis Two types of endocrine cells are associated with the ovarian follicle. One is the granulosa cell, which resides within the follicle and is encased by the basal lamina. Unlike Sertoli cells, however, granulosa cells proliferate in response to estrogen, and this proliferation is inhibited by androgens. The nonendocrine function of granulosa cells is to promote growth of oocytes by conditioning the follicular fluid. The other type of follicular cell with endocrine function is the theca interna cell. Such cells are positioned along the outer border of the basal lamina; thus, they are located outside the follicle and are perfused by blood. Unlike Leydig cells, they produce mainly androstenedione, although some testosterone is also produced. Menstrual Cycle the menstrual cycle consists of the follicular phase and the luteal phase, each lasting about 2 weeks. During the second half of the follicular phase, the follicle accumulates fluid, which leads to formation of an antrum (antral follicle). This causes the follicle to grow and to accumulate fluid, and leads to the formation of an antrum. Although the intrafollicular concentration of estradiol is sufficient to stimulate proliferation of granulosa, it is not high enough to enter the general circulation. Thus, the formation of estradiol by aromatization of androgens derived from the theca interna is augmented by estradiol synthesized de novo. The increased estradiol pool causes marked acceleration in follicle growth and spillage of estradiol into the general circulation. This rise in estradiol levels is a critical cue for the neuroendocrine system because it indicates that the ovarian follicle is ready for ovulation. In contrast to the negative feedback effect that estradiol normally exerts on the release of gonadotropins, the very high levels of estradiol presented over 2 to 3 days exert a positive feedback effect. These luteal cells are steroidogenic and produce large amounts of progesterone and moderate amounts of estradiol. Morphogenesis of the corpus luteum is not complete until about 1 to 4 days after ovulation, and luteal production of progesterone and estradiol gradually increases to a maximum about 6 to 7 days after ovulation. Exposure to high levels of estrogen during this interval would lead to expulsion of the ovum or to blockage of ovum transport. The rise in levels of progesterone and of estradiol during the first week of the luteal phase is required for the endometrium to become secretory in preparation for implantation and pregnancy. The corpus luteum has a lifespan of about 12 days; it can synthesize steroids autonomously without extra-ovarian hormonal stimulation. Withdrawal of progesterone and estradiol during luteolysis results in deterioration of the endometrium and its shedding (menstruation). The regularity of the menstrual cycle in women of reproductive age can be affected by anatomical defects of the uterus or vagina, or by functional or structural defects in the hypothalamicituitaryvarian axis that affect hormonal secretions. Complete cessation of menses (for more than 6 months) is known as amenorrhea, and a reduction in the frequency is known as oligomenorrhea. Physiologic states of amenorrhea include prepuberty, pregnancy, lactation, and postmenopause. These enzymes are critical for the maintenance of pregnancy because the placenta assumes the role of the ovaries as the major generator of progesterone and estrogen after the sixth week of pregnancy. The implantation of the fertilized ovum at a site other than the endometrium is known as ectopic pregnancy. Patients with ectopic pregnancy frequently exhibit abdominal pain with amenorrhea. The tubal pathology 598 Essentials of Medical Biochemistry can result from pelvic infection, endometriosis (occurrence of endometrial tissue outside the uterus), or previous surgery. The treatment for ectopic pregnancy consists of either surgery to remove the embryonic tissue or medical treatment with methotrexate, a folic acid antagonist. Small amounts of estrogen are required for the maintenance of pregnancy because estrogen maintains tissue responsiveness to progesterone. In the fetus, it promotes the formation of insulin-like growth factor and growth factors believed to promote growth of most, if not all, fetal tissues. Estrogens enter the maternal circulation and appear in maternal urine as conjugated estrogens. Placental Steroids the absence of progesterone is incompatible with the gravid pregnant state. It maintains placental viability, thus ensuring adequate exchange of substances between maternal and fetal compartments; 2. It maintains perfusion of the decidua basalis (maternal placenta), presumably by inhibiting the formation of vasoconstrictive prostaglandins; and 3. It diminishes myometrial contractility, possibly by increasing the resting membrane potential or by inhibiting the formation of prostaglandins E1 and F2 (Chapter 16). The placenta is the major site of conversion of cholesterol to progesterone after the sixth week; however, it is not capable of synthesizing cholesterol from acetate. Although the placenta cannot process cholesterol beyond progesterone, it contains some Parturition During late gestation, rising levels of estrogen are thought to increase the synthesis of oxytocin in hypothalamic magnocellular neurons to induce oxytocin receptors in the myometrium and to increase myometrial contractility by lowering the membrane potential. During this period, relaxin from the decidua softens the cervix for impending delivery. Dilation of the cervix by the infant stimulates the release of oxytocin by neuroendocrine Endocrine Metabolism V: Reproductive System Chapter 32 599 reflex, further stimulating uterine contractions, which, in turn, causes more cervical stretching and a positive feedback to oxytocin release, until delivery is complete. Estrogen also stimulates production of lactogenic receptors in mammary ductal cells and acts on the anterior pituitary to stimulate prolactin secretion. Lactogenesis begins during the third trimester of pregnancy and involves synthesis of the milk-specific proteins casein, lactalbumin, and lactoglobulin. The primary regulator of lactogenesis is prolactin, although the participation of additional hormones is needed. Lactation is inhibited by estrogen and, thus, is held in check by the high levels of estrogen during pregnancy; the withdrawal of estrogen after birth triggers lactation. Exemestane can be used in chemoprevention in postmenopausal women with high-risk breast cancer. Estrogen is the main determinant of female reproductive function, bone maintenance, and cardioprotection.

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