enzyme cofactors

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Information about enzyme cofactors

Published on March 4, 2008

Author: Samuel

Source: authorstream.com

VITAMINS AND COFACTORS:  VITAMINS AND COFACTORS By Henry Wormser, Ph.D. PSC 3110 – Biochemistry I - Fall 2002 Reading material:  Reading material Principles of Biochemistry with a Human Focus by Garrett and Grisham, First Edition, 2002, pages 453-468 Handbook of NonPrescriptions Drugs, 11th edition, Chapter entitled “Nutritional Products” by Loyd V. Allen, Jr. Vitamins:  Vitamins a group of organic compounds needed in small quantities in the diet for normal activity of tissues between 14 – 20 substances have been identified as vitamins many vitamins act as cofactors, coenzymes or prosthetic groups for enzymes most vitamins are derived from diet no calories are derived from vitamins Vitamins:  Vitamins first vitamin discovered was thiamine or B1 the term vitamin is derived from the fact that the substances are needed for life (vita) and because thiamine happened to be an amine the term was coined as such however, not all vitamins are amines or nitrogen containing compounds Vitamins:  Vitamins vitamin requirements are usually expressed as RDA’s (recommended dietary allowances) guidelines are provided by 2 organizations: the Food and Nutrition Board of the National Academy of Sciences- National Research Council the Food and Drug Administration (FDA) RDAs:  RDAs applications of RDAs include: evaluating the adequacy of the national food supply establishing standards for menu planning establishing nutritional policy for public institutions/organizations and hospitals evaluating diets in food consumption studies establishing labeling regulations setting guidelines for food product formulation developing materials for nutritional education RDAs:  RDAs RDAs have limitations: they are too complex for direct consumer use they do not state ideal or optimal levels of intake the allowances for some categories are based on limited data the data on some nutrients in foods is limited they do not evaluate nutritional status they do not apply to seriously ill or malnourished patients Vitamin deficiencies:  Vitamin deficiencies primary food deficiency crop failure food storage loss food preparation loss diminished food intake poverty anorexia food fadism chronic diseases Vitamin deficiencies:  Vitamin deficiencies diminished absorption absorption defect parasites malignancies increased requirements rapid growth increased physical activity pregnancy hyperthyroidism increased loss drug therapy diuresis lactation Vitamin loss:  Vitamin loss Loss is seen mainly in storage or food preparation Vitamin A: sensitive to oxygen and light Vitamin D: usually little loss Vitamin E: sensitive to oxidation especially when heated or with alkali Vitamin K: sensitive to acids, alkali, light and oxidizing agents Vitamin C: very sensitive to oxidation, especially when heated in contact with metals Vitamin B complex: water solubility results in loss in cooking water Riboflavin is sensitive to light Vitamins:  Vitamins Vitamins are typically divided into 2 groups: The fat soluble vitamins A, D, E, and K The water soluble vitamins The B vitamins (B1, B2, B3, B6, B7, B12 and pantothenic acid) Ascorbic acid (vitamin C) Bogus vitamins:  Bogus vitamins Vitamin B4 adenine Vitamin B10 identical with folic acid Vitamin B11 “ “ “ “ Vitamin B15 pangamic acid Vitamin B13 orotic acid Vitamin B17 laetrile Vitamin B19 wormser’s secret formula Cofactors:  Cofactors provide “chemical teeth” for enzymes sometimes referred to as coenzymes enzymes: proteins with catalytic activity simple enzymes: large protein (polypeptide) that catalyzes a reaction. The enzyme gets all the “tools” (chemical teeth) it needs from the amino acids. However, there are only 20 different amino acids conjugated enzymes : apoenzyme + cofactor = holoenzyme EXAMPLE:Proteases: enzymes that cleave peptide bonds:  EXAMPLE:Proteases: enzymes that cleave peptide bonds Enzymes perform catalytic reactions such as hydrolysis; the side chains of amino acids participate in the reactions example of a simple enzyme:  example of a simple enzyme A serine protease enzyme such as chymotrypsin Usually electron-rich side chains are involved in the catalysis Aliphatic chains are normally involved in hydrophobic interactions Slide16:  HYDROLYTIC CATALYSIS Example of a conjugated enzyme:  Example of a conjugated enzyme Zinc protease such as ACE Cofactors:  Cofactors all water-soluble vitamins with the exception of vitamin C are converted/activated to cofactors only vitamin K of the fat-soluble vitamins is converted to a cofactor not all vitamins are cofactors; i.e., lipoic acid is not a vitamin cofactors may also act as carriers of specific functional groups such as methyl groups and acyl groups The water soluble vitamins:  The water soluble vitamins Pantothenic acid (vitamin B5):  Pantothenic acid (vitamin B5) First recognized in 1933 as a growth factor for yeast (Roger J. Williams) Pantothenic acid:  Pantothenic acid a yellow viscous oil (free acid) stable to moist heat (not to dry heat) and to oxidizing and reducing agents hydrolyzed in acid or alkaline medium sources (numerous): liver, kidney, eggs, lean beef, milk, molasses, cabbage, cauliflower, broccoli, peanuts, sweet potatoes, kale (derive its name from everywhere) Pantothenic acid:  Pantothenic acid serves in its activated form as the cofactor for coenzyme A (CoA) and the acyl carrier protein (ACP) first phosphorylated by ATP to 4’-phosphopantothenate next is the formation of 4’-phosphopantetheine by addition of cysteine and decarboxylation adenylation by ATP forms dephospho-CoA phosphorylation to the 3’-OH of the ribose generates CoA (coenzyme A) Coenzyme A:  Coenzyme A performs a vital role by transporting acetyl groups from one substrate to another the key to this action is the reactive thioester bond in the acetyl form of CoA the thioester bond is stable enough that it can survive inside the cell, but unstable enough that acetyl-CoA can readily transfer the acetyl group to another molecule Example of an acetylation reaction:  Example of an acetylation reaction Acetylcholine is an important neurotransmitter in the autonomic nervous system (cholinergic) and in the brain Pantothenic acid:  Pantothenic acid Deficiency: rats graying of hair/fur in black rats dermatitis inflammation of nasal mucosa hemorrhage of adrenal cortex humans has not been encountered or extremely rare difficult to induce with either synthetic diets and/or with antagonists (omega-methylpantothenic acid Pantothenic acid:  Pantothenic acid vague symptoms in human deficiency: numbness and tingling in feet “burning foot” fatigue GIT disturbances available pharmaceutically as calcium pantothenate (d-isomer) and as racemic mixture 5 - 7 mg/day appear to prevent signs of deficiency appears to be non-toxic (up to 10-20 gm have been tolerated) Thiamine:  Thiamine Vitamin B1; antiberi-beri vitamin; antineuritic factor was the first water soluble vitamin discovered (Eijkman) Thiamine:  Thiamine has the odor and flavor of yeast slowly destroyed by moist heat; more rapidly destroyed in a basic medium than in an acid one source: whole cereals and grains; yeast; organ meat pharmaceutical products use the hydrochloride or mononitrate salts Thiamine:  Thiamine active form is thiamine pyrophosphate (formed by the action of thiamine diphosphotransferase) involved in the oxidative decarboxylation of pyruvic acid and a-ketoglutaric acid involved in the transketolase reactions of the triose phosphate pathway also required for nerve function (unrelated to coenzyme activity) Conversion of thiamine to TPP:  Conversion of thiamine to TPP Typical reactions catalyzed by TPP:  Typical reactions catalyzed by TPP Reactions in which thiamine pyrophosphate is a cofactor:  Reactions in which thiamine pyrophosphate is a cofactor Pyruvate decarboxylase Alcohol fermentation – pyruvate to acetaldehyde Pyruvate dehydrogenase Synthesis of acetyl-CoA Alpha-ketoglutarate dehydrogenase Citric acid cycle Transketolase reaction Carbon-fixation reactions of photosynthesis Acetolactase synthetase Valine, leucine biosynthesis Thiamine pyrophosphate:  Thiamine pyrophosphate the key portion of this cofactor is the thiazolium ring with its acidic hydrogen the hydrogen is removed by the enzyme forming an ylid (anion next to cation) the anion can then react with carbonyl groups in such molecules as pyruvate the pyrophosphate functionality acts as a chemical handle which holds the cofactor in place within the enzyme Slide36:  Chemical mechanism for action of B1 in pyruvate dehydrogenase Transketolase reaction:  Transketolase reaction Transketolase reaction:  Transketolase reaction These reactions provide a link between the pentose phosphate pathway and glycolysis Activity of erythrocyte transketolase is commonly used as an index of thiamine deficiency Thiamine deficiency:  Thiamine deficiency earliest symptoms of thiamine deficiency include: constipation appetite suppression nausea mental depression peripheral neuropathy fatigue Thiamine deficiency (severe):  Thiamine deficiency (severe) beri-beri (once associated with white polished rice diets and with highly milled wheat diets) 2 clinical types dry beri beri or neuritic beriberi associated with polyneuropathy (depressed peripheral nerve function, sensory disturbance, loss of reflexes and motor control and muscle wasting wet beri beri or cardiovacular beriberi edema, congestive heart failure Slide41:  These 2 compounds are potent antithiamine agents which may be used to induce symptoms of vitamin B1 deficiency in selected animals. Oxythiamine competitively inhibits thiamine pyrophosphate and becomes active after phosphorylation; neopyrithiamine prevents the conversion of thiamine to thiamine pyrophosphate Other clinical applications:  Other clinical applications Alcohol neuritis (peripheral neuropathy) Sharp burning pain in the feet Deep muscle tenderness with numbness Coarse tremors, foot drop Wernicke’s encephalopathy Results from degeneration of basal ganglia due to chronic/heavy use of alcohol Rigidity of extremities Complete or partial ophthalmoplegia Sleep disturbances Nausea and vomiting Other clinical applications:  Other clinical applications Korsakoff’s syndrome or psychosis Also a complication of chronic/heavy use of alcohol Usually follows DT’s (delirium tremens) Memory loss Delusions Disorientation Ocular palsies Combined Wenicke-Korsakoff syndrome Pregnancy neuritis Certain gastrointestinal disorders Requirement for thiamine:  Requirement for thiamine Based on energy needs 0.3 – 0.6 mg/1000 calories Increased requirements: Pregnancy and lactation Eating large amounts of raw sea food (clams) – contain thiaminase Stress situations (high level of exercise, fever, hyperthyroidism) Drinking large quantities of tea (contains antagonist) Thiamine assay:  Thiamine assay biologic assay – in animals – time consuming and costly (curative or protective) microbiologic using bacteria which require thiamine for growth chemical/fluorescent assay – conversion of thiamine to thiochrome by alkaline ferricyanide Lipoic acid:  Lipoic acid lipoic acid is a co-factor found in pyruvate dehydrogenase and a-ketoglutarate dehydrogenase, two multienzymes involved in a-keto acid oxidation lipoic acid functions to couple acyl group transfer and electron transfer during oxidation and decarboxylation of a-ketoacids no evidence exists of a dietary lipoic acid requirement in humans; therefore it is not considered a vitamin Slide47:  Lipoic acid exists in 2 forms: a closed-ring disulfide form and an open-chain reduced form; oxidation-reduction cycles interconvert these 2 species; lipoic acid exists covalently attached in an amide linkage with lysine residues on enzymes Riboflavin:  Riboflavin vitamin B2, lactoflavin (ovo, hepato, verdo), vitamin G a heterocyclic flavin linked to ribose analogous to the nucleosides in RNA orange-yellow fluorescent compound found in significant quantities in green leafy vegetables, milk and meats heat stable, but easily destroyed by light recommended intake is related to energy intake (kcal) – RDA 1 – 2 mg/day Slide49:  dimethylisoalloxazine ring system – confers some degree of planarity to the molecule and also color (yellow) Slide50:  Decomposition of riboflavin Riboflavin:  Riboflavin 2 cofactors are involved: riboflavin phosphate (flavin mononucleotide, FMN) flavin adenine dinucleotide (FAD) involved in the metabolism of carbohydrates, fats and proteins (flavin dehydrogenases/flavoproteins) hydrogen carriers in the respiratory chain Riboflavin:  Riboflavin Riboflavin:  Riboflavin Riboflavin:  Riboflavin Enzymes utilizing riboflavin cofactors: NADH dehydrogenase succinate dehydrogenase d and l-amino acid oxidases pyridoxine-5-phosphate oxidase glutathione reductase xanthine oxidase In some enzymes, the cofactor is covalently bonded to an amino acid (dehydrogenases) Dehydrogenase reaction:  Dehydrogenase reaction Amino acid oxidases:  Amino acid oxidases Xanthine oxidase:  Xanthine oxidase Xanthine oxidase is a flavoprotein which also contains Fe and Mo Fatty acyl-CoA desaturase:  Fatty acyl-CoA desaturase Important step in the biosynthesis of unsaturated fats; this reaction is actually more complex than shown here and involves other cofactors, but FAD is a key cofactor for the enzyme Riboflavin deficiency:  Riboflavin deficiency seldom seen in industrialized societies deficiency when seen: cheilosis (vertical fissure in the lips) angular stomatitis (craks in the corner of the mouth) glossitis photophobia seborrheic dermatitis normochromic normocytic anemia usually encountered along with pellagra (niacin deficiency) newborns treated for hyperbilirubinemia by phototherapy (riboflavin is unstable to light) Biotin:  Biotin Biotin:  Biotin an imidazole sulfur containing compound sometimes referred to as vitamin B7 or vitamin H widely distributed in foods (liver, kidney, milk, molasses) a large portion of the daily need of biotin is met by synthesis by intestinal bacteria deficiency is usually the result of a defect in utilization rather than simple dietary deficiency Biotin:  Biotin like lipoic acid, biotin is converted to its coenzyme form (called biotinyllysine or biocytin) by formation of a covalent amide bond to the nitrogen of a lysine residue like lipoic acid it performs a highly specialized function : adds a carboxyl group to substrates Biotin:  Biotin biochemical role: carbon dioxide fixation two step process: Binding of CO2 to biotin – N-carboxybiotin Transfer of CO2 to a substrate Activation of biotin requires enzyme, CO2, ATP and Mg++ Biotin:  Biotin Biotin-dependent enzymes: Pyruvate carboxylase (synthesis of oxaloacetate for gluconeogenesis and replenishment of the citric acid cycle) Acetyl CoA carboxylase (fatty acid biosynthesis) Propionyl-CoA carboxylase b-methylcrotonyl-CoA carboxylase holocarboxylase synthase (multiple carboxylase) Reactions involving biotin enzymes:  Reactions involving biotin enzymes Biotin:  Biotin deficiency: quite uncommon can be induced by feeding raw egg white (avidin) avidin is a protein which binds tighly with biotin (MW 70,000) symptoms are: anorexia, nausea, muscle pain, fine scaly desquamation of the skin requirements: 150 – 200 mcg/day therapeutic use: in babies with infantile seborrhea (cradle cap) and Leiner’s disease Pyridoxine (vitamin B6):  Pyridoxine (vitamin B6) A pyridine derivative Other forms of B-6:  Other forms of B-6 Collectively, pyridoxine, pyridoxal and pyridoxamine are known as vitamin B6 Pyridoxine:  Pyridoxine vitamin B6, rat “acrodynia factor”, antidermatitis factor widespread occurrence pyridoxine: mostly in vegetable products pyridoxal and pyridoxamine: mostly in animal products pyridoxine is stable in acid solution, but unstable in neutral or alkaline solutions (destroyed by light) Pyridoxal phosphate:  Pyridoxal phosphate pyridoxine is converted to pyridoxal phophate by phosphorylation and oxidation to the aldehyde pyridoxal phosphate is then attached to the holoenzyme via a covalent bond to a lysine residue (a Schiff’s base) the Schiff’s base bond is readily broken and reformed this reversibility is very important in the biochemical action of this cofactor Biochemical functions:  Biochemical functions Able to catalyze the breakdown of amino acids Pyridoxal phosphate:  Pyridoxal phosphate Biochemical functions: Decarboxylation of amino acids Transaminase reactions Racemization reactions Aldol cleavage reactions Transulfuration reactions Conversion of tryptophan to niacin Conversion of linoleic acid into arachidonic acid (prostaglandin precursor) Formation of sphingolipids Slide77:  Decarboxylation of amino acids Important transaminases:  Important transaminases ALT ( alanine aminotransferase) formerly known as SGPT (serum glutamate pyruvate transaminase) alanine + alpha-ketoglutarate = pyruvate + glutamate increased serum level in liver injury Important transaminases:  Important transaminases AST (aspartate aminotransferase) formerly known as SGOT (serum glutamate oxaloacetate transaminase) aspartate + alpha-ketoglutarate = oxaloacetate + glutamate elevated when heart and/or liver are damaged Important decarboxylases:  Important decarboxylases Mechanism for transamination reaction:  Mechanism for transamination reaction Pyridoxine:  Pyridoxine deficiency: difficult to produce in humans may be accomplished artificially with a pyridoxine antagonist (deoxypyridoxine) symptoms include: nausea and vomiting, seborrheic dermatitis, depression and confusion, mucous membrane lesions, peripheral neuritis, anemia Pyridoxine antagonists:  Pyridoxine antagonists Slide84:  Pyridoxine can antagonize the antiparkinsonian use of L-DOPA Pyridoxine deficiency:  Pyridoxine deficiency can be monitored by measuring the level of xanthurenic acid in the urine this is related to a decrease in kynureninase activity (pyridoxal phosphate is the coenzyme) kynurenine, a breakdown product of tryptophan is normally converted to kynurenic acid – but in B6 deficiency it is shunted to form xanthurenic acid XANTHURENIC ACID:  XANTHURENIC ACID Pyridoxine:  Pyridoxine requirements: children: 0.5 – 1.2 mg adults: 2.0 mg pregnancy: 2.5 mg Requirement for B6 is proportional to the level of protein consumption therapeutic uses: deficiency to counterract the effects of antagonists certain rare forms of anemia in women taking oral contraceptives (estrogen shifts tryptophan metabolism Slide88:  Discovered in 1913 from yeast; also known as vitamin B3 1915 – 1920: Irving Golberg demonstrated that lack of niacin causes pellagra one of the simplest vitamin; like B6 also a pyridine derivative Oxidation of nicotine yields nicotinic acid:  Oxidation of nicotine yields nicotinic acid This reaction does not occur in vivo – strictly a laboratory reaction Nicotinic acid:  Nicotinic acid niacin, vitamin B3, niacinamide, antipellagra vitamin both form are active: the free acid and the amide sources: organ meat (largest source), fish, yeast, dried fruit, nuts, cereal grains, some vegetables pellagra-inducing diets: corn meal, corn starch, sweet potatoes, rice, syrup, pork fat (once a common diet in southern states among sharecroppers) Coenzyme forms:  Coenzyme forms Two cofactor forms of niacin: NAD and NADP; these cofactors are not tightly held by the enzyme and may be reused for reaction after reaction Biochemical function:  Biochemical function In the older literature NAD+ is referred to as DPN or coenzyme I NADP+ is referred to as TPN or coenzyme II Oxidized and reduced forms:  Oxidized and reduced forms Sparing action of tryptophan:  Sparing action of tryptophan Tryptophan can substitute for niacin: 60 mg of tryptophan is equivalent to 1 mg of niacin; 60 gm of protein contains 600 mg of tryptophan which then represent 10 mg of niacin Pellagra:  Pellagra Early stages: Anorexia Indigestion Muscle weakness Reddened skin Rough skin Advanced stages 3 D’s of pellagra: dermatitis, diarrhea, dementia Clinical uses of nicotinic acid:  Clinical uses of nicotinic acid pellagra symptoms from: gastric ulcer or carcinoma diarrhea isoniazid therapy carcinoid syndrome Hartnup disease (impairment of tryptophan absorption) peripheral vasodilator (nicotinic acid or nicotinyl alcohol) hypolipidemic agent (only nicotinic acid in large doses – lowers both triglycerides and cholesterol (Niaspan, Nicobid) Carcinoid syndrome:  Carcinoid syndrome a slow growing neoplasm of enterochromaffin cells (ileum, stomach, bronchus) tryptophan metabolism is altered resulting in excess serotonin synthesis symptoms include: facial flushing edema of head and neck abdomina cramps and diarrhea asthmatic symptoms cardiac insufficiency urinary 5-HIAA (5-hydroxyindole acetic acid) is high (5-HIAA is a metabolite of serotonin; serotonin is derived from tryptophan) Cautions concerning the use of nicotinic acid in large doses:  Cautions concerning the use of nicotinic acid in large doses as an acid, it can erode gastrointestinal mucosa leading to ulceration it also causes a depletion of glycogen stores and fat reserves in skeletal and cardiac muscle additionally, there is an elevation in blood glucose and uric acid production for these reasons, nicotinic therapy is not recommended for diabetics or persons who suffer from gout Ascorbic acid:  Ascorbic acid vitamin C; anti-scorbutic vitamin (scurvy) structure is reminiscent of glucose produced in plants from glucose via the uronic pathway the enzyme gulonolactone oxidase converts gulonolactone to ascorbic acid exists in the enolic and ketonic forms sources: citrus fruits, tomatoes, green peppers, strawberries, cantaloupe, cabbage, turnips, peas, lettuce and aspargus ASCORBIC ACID AND DEHYDROASCOBIC ACID:  ASCORBIC ACID AND DEHYDROASCOBIC ACID Ascorbic acid:  Ascorbic acid Biochemical functions: Production and maintenance of collagen Proline --------hydroxyproline Lysine -------- hydroxylysine Mitochondrial electron-transport chain (cytochrome C) Metabolism of tyrosine Tyrosine ----- p-hydroxyphenylpyruvic acid---- 2,5-dihydroxyphenylacetic acid (homogentisic acid) Slide104:  Proline hydoxylase: (collagen formation) Dopamine-beta hydroxylase ( neurotransmitter formation) Slide105:  Anti-oxidant properties of vitamin C: helps prevent damage to cellular proteins and DNA Normal metabolic processes in the cell lead to the generation of reactive oxidizing agents such as superoxide Superoxide can react with and damage protein and DNA, leading to cellular changes that can lead to premature aging and cancer Vitamin C reacts with superoxide, thus preventing this damage Ascorbic acid:  Ascorbic acid conversion of folic acid to THFA hydroxylation reactions of cholesterol to cholic acid hydroxylation of tryptophan to 5-hydroxytryptophan regulation of cholesterol biosynthesis in the adrenal gland aids in the absorption and utilization of iron antioxidant properties may inhibit formation of nitrosamines during digestion of protein Ascorbic acid:  Ascorbic acid defiency: scurvy hemorrhage from mucous membranes, mouth and GIT, skin and muscles gingivitis: swelling, tenderness, redness and ulceration of gums loosening or loss of teeth swelling of joints rarefaction of bones and dentine Ascorbic acid:  Ascorbic acid requirements: children: 30 mg adults: 40 –80 mg pregnancy: 100 mg therapeutic uses scurvy idiopathic methemoglobinemia questionable use: common cold Vitamin B12:  Vitamin B12 Vitamin B12:  Vitamin B12 cyanocobalamin (Redisol) hydroxocobalamin (Alpha redisol) function deficiency hematological sequelae neurological sequelae Vitamin B12:  Vitamin B12 synthesized by bacteria only red in color, levorotatory and stable to heat commercially available either as cyano or hydroxocobalamin stored in the liver as the coenzyme absorbed only in the presence of the intrinsic factor (a glycoprotein released by parietal cells) transported to tissues via transcobalamin II present in foods such as liver, fish, eggs, milk absent in vegetables and fruits Vitamin B12:  Vitamin B12 by far the most complex vitamin in structure made up of a planar corrin ring (4 pyrroles) the only vitamin that possesses a metal ion (cobalt) as part of its structure the major cofactor form of B12 is adenosylcobalamin or 5’-deoxyadenosylcobalamin small amounts of methylcobalamin also occur (intermediate in methyl transfer reactions) Vitamin B12:  Vitamin B12 the corrin ring is similar to the porphyrin ring system found in hemoglobin except that in corrin 2 of the pyrroles are linked directly (without methylene bridges) the cobalt is coordinated to the 4 pyrrole nitrogens one of the axial cobalt ligands is a nitrogen of the dimethylbenzimidazole group the other axial ligand may be CN, OH, CH3 or the 5’-carbon of a 5’-deoxyadenosyl group Slide115:  corin nucleus benzylimidazole cobalt coordinated Vitamin B12:  Vitamin B12 biochemical functions (mediated by coenzymes) mutase reaction (rearrangement reaction methylmalonyl CoA to succinyl CoA (lipid metabolism) methylation reactions uracil to thymine homocysteine to methionine aminoethanol to choline activation of amino acids for protein synthesis ribonucleotides to deoxyribonucleotides for DNA synthesis in certain bacteria Causes of B12 deficiency:  Causes of B12 deficiency Pernicious anemia (autoimmune gastritis against parietal cells - loss of intrinsic factor) rarely due dietary deficiency N2O/oral contaceptive drugs intestinal parasite gastrectomy chronic gastritis Schilling test Diagnosis of B12 deficiency:  Diagnosis of B12 deficiency Schilling test distinguishes deficiency caused by pernicious anemia with that caused by malabsorption compares absorption in radiolabeled B12 with intrinsic factor and radiolabeled B12 without intrinsic factor in pernicious anemia the B12 with intrinsic factor will be absorbed while the B12 by itself will not in malabsorption neither will be absorbed Manifestation of B12 deficiency:  Manifestation of B12 deficiency macrocytic megaloblastic anemia megaloblasts are abnormal erythroid precursors in bone marrow (most cells die in the bone marrow) reticulocyte index is low hyperchromic macrocytes appear in blood anemia reflects impaired DNA synthesis other cells may be involved (leukopenia, thrombocytopenia spinal cord degeneration (irreversible) swelling, demyelination, cell death neurological disease results from deficient methylmalonyl-CoA mutase this cannot be treated with folic acid!! Treatment of B12 deficiency:  Treatment of B12 deficiency use IM cyanocobalamin or hydroxocobalamin administer daily for 2 - 3 weeks, then every 2 - 4 weeks for life monitor reticulocytosis early to assure treatment is working (reticulocyte count should go up) monitor potassium levels to ensure hypokalemia does not occur due to excessive RBC synthesis Folic acid:  Folic acid MOA deficiency use drug interactions with folic acid Slide124:  Also known as folacin, vitamin M and pteroylglutamic acid Widely distributed in leaves (foliage) of plants Chemically composed of pteroic acid (pteridine and PABA) and glutamic acid FOLIC ACID:  FOLIC ACID absorbed by both active and passive transport on the average we absorb 50 -200ug per day (about 10 -25% of dietary intake) storage is in the form of 5-methyl THF (5 -20 mg) found in green vegetable, dietary yeasts, liver, kidney bacteria synthesize their own folic acid (dihydropteroate synthetase) Folic acid:  Folic acid Biochemical functions one carbon fragment transfer (formyl, methyl, hydroxymethyl) conversion of homocysteine to methionine conversion of serine to glycine synthesis of thymidylic acid synthesis of purines (de novo) histdine metabolism synthesis of glycine PURINE CARBONS DERIVED via FOLATE:  PURINE CARBONS DERIVED via FOLATE Deficiency of folic acid:  Deficiency of folic acid Inadequate intake defective absorption (most common) sprue gastric resection and intestinal disorders acute and chronic alcoholism drugs (anticonvulsants and oral contraceptives) pregnancy pellagra Deficiency of folic acid:  Deficiency of folic acid abnormal metabolism of folates folic acid antagonists (dihydrofolate reductase inhibibitors - methotrexate, pyrimethamine, trimethoprim) enzyme deficiency vitamin B12 deficiency oral contraceptives increased requirement pregnancy, infancy METHOTREXATE:  METHOTREXATE Inhibits enzyme dihydrofolate reductase (DHFR) which is necessary for maintaining pool of reduced folates required for DNA synthesis METHOTREXATE:  METHOTREXATE also known as amethopterin or MTX a potent inhibitor of dihydrofolate reductase which catalyzes the conversion of folic acid to tetrahydrofolic acid (THFA) THFA acts as an acceptor of a one-carbon unit from either formate or formaldehyde 5-formyl THFA is also known as folinic acid or the citrovorum factor (leucovorin) THFA one-carbon carriers are important in the synthesis of purines, thymine, choline, and other important cellular constituents MTX is used in treating acute lymphocytic leukemia in children, choriocarcinoma, osteogenic sarcoma, carcinomas of the head, neck, bladder and testis in lower doses: treatment of psoriasis and rheumatoid arthritis Slide134:  diaminopyrimidines inhibitors of dihydrofolate reductase have activity in both bacterial and protozoal organisms more effective if used in combination with another drug pyrimethamine is more selective for protozoal enzyme than trimethoprim used in treatment of malaria and PCP The fat soluble vitamins :  The fat soluble vitamins By Henry Wormser Professor of Medicinal Chemistry Fat soluble vitamins:  Fat soluble vitamins Vitamins A, D, K and E are the fat-soluble vitamins excessive use of vitamins A and K can lead to toxicities fat soluble vitamin tend to be stored in fatty tissues of the body and in the liver Vitamin A:  Vitamin A Exits in 3 forms: all trans-retinol long chain fatty acyl ester of retinol (main storage form) retinal (the active form in the retina) retinoic acid is also considered to be physiologically active provitamin A or carotene can be converted to retinol in vivo Vitamin A:  Vitamin A recommended intakes are expressed in retinol equivalents (RE) 1 RE = 1 mcg of retinol = 6 mcg of b-carorene = 12 mcg other carotenes older usage expressed activity in USP units or International units (IU). These were based on biological activity in the vitamin a-deficient rat (1 IU = 0.3 mcg of retinol) Slide139:  Vitamin A contains 5 conjugated double bonds which are key to some biological actions Isolated in impure form by McCollum in 1915 RDA: 0.7 mg Vitamin A:  Vitamin A Diseases of deficiency: Nigh blindness and xerophthalmia (dry eye) Skin disorders Lack of growth Hypervitaminosis: A serious potential problem (CNS disorders; birth defects) Vision and the role of vitamin A:  Vision and the role of vitamin A photoreception is the function of 2 specialized cell types: rods and cones both types of cells contain a photosensitive compound called opsin in rod cells opsin is called scotopsin and the receptor is called rhodopsin or visual purple rhodopsin is a serpentine receptor imbedded in the membrane of the rod cell; it is a complex between scotopsin and 11-cis retinal Vision and the role of vitamin A:  Vision and the role of vitamin A intracellularly, rhodopsin is coupled to a G-protein called transducin when rhodopsin is exposed to light, it is bleached releasing the 11-cis-retinal from opsin absorption of photons by 11-cis-retinal triggers the conversion to all-trans-retinal (one important conformational intermediate is metarhodopsin II); also there is a change in conformation of the photoreceptor Vision and the role of vitamin A:  Vision and the role of vitamin A these transformations activate a phosphodiesterase (which hydrolyzes c-GMP to GMP) c-GMP is necessary to maintain the Na+ channels in the rods in the open conformation with a decrease in c-GMP, there occurs a closure of the Na+ channels, which leads to hyperpolarization of the rod cells with concomittant propagation of nerve impulses to the brain Additional role of retinol:  Additional role of retinol retinol also functions in the synthesis of certain glycoproteins and mucopolysaccharides necessary for mucous production and normal growth regulation this is accomplished by phosphorylation of retinol to retinyl phosphate which then functions similarly to dolichol phosphate Slide148:  Retinoic acid (Retin-A) is important for cellular differentiation; It controls cellular growth – particularly cell growth Used in the treatment of acne; also used as an anti-wrinkle agent (Retin A, Retin A micro, Avita, Renova) Also used orally to treat acute promyelocytic leukemia (APL) Product used is Vesanoid (10 mg capsules) Slide149:  Isotretinoin or accutane is a modification of retinoic acid; it contains a 13-cis double bond and is orally effective Used in the treatment of severe acne Slide150:  An aromatic analog of retinoic acid; orally effective and used in the management and treatment of psoriasis Etretinate (Tegison):  Etretinate (Tegison) Esterified form of acitretin; also used orally in the treatment of recalcitrant psoriasis; 10 and 25 mg capsules Alitretinoin (Panretin):  Alitretinoin (Panretin) Currently used as a 0.1% gel for the topical treatment of cutaneous lesions in patients with AIDS-related Kaposi sarcoma BEXAROTENE (Targretin):  BEXAROTENE (Targretin) Bexarotene (Targretin) :  Bexarotene (Targretin) indicated for the treatment of cutaneous manifestations of cutaneous T-cell lymphoma usually the patients receiving this drug have failed to respond to other treatment protocols pregnancy (Category X drug) Adapalene (Differin):  Adapalene (Differin) Used as a 0.1% gel in the treatment of acne vulgaris Tazarotene (Tazorac):  Tazarotene (Tazorac) Topical treatment of patient with facial acne vulgaris of mild to moderate severity; gel 0.05%, 0.1% Vitamin A toxicity:  Vitamin A toxicity vitamin A is higly toxic when taken in large amounts either acutely or chronically may occur with 200 mg (666,000 IU) in adults or half this amount in children signs include headache, nausea and vomiting, increased cerebrospinal fluid pressure, blurred vision and bulging of the fontanelle in infants Vitamin D:  Vitamin D There are 2 major precursor forms: 7-dehydrocholesterol ergosterol UV irradiation affords cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) Discovery: 1890 – sunlight prevents rickets 1924 – Steanbock and Hess found that irradiating certain foods produced vitamin D2 1970 – hormonally active form of vitamin D discovered Vitamin D:  Vitamin D RDA – 20 mg (required in minute amounts) disease of deficiency: rickets Malformation of bones – due to improper bone mineralization Hypervitaminosis Toxic dose only 10X higher than the RDA Causes hypercalcemia – can lead to cardiac arrest vitamin D is not a vitamin (or a cofactor) – it is a steroid hormone Biological functions:  Biological functions Calcium homeostasis – it is critical for the body to maintain the proper calcium level in the blood stream Intestinal calcium absorption: acts as a signal to tell intestinal cells to take up more calcium from the gut Bone calcium mobilization Signals osteoclast (bone cells) to release calcium into the blood stream in response to low calcium levels Biological functions:  Biological functions Cellular differentiation – much less well understood – signal to bone marrow cells to change into other cells Problem: 1a,25(OH)2-D3 causes hypercalcemia Various analogs of vitamin D:  Various analogs of vitamin D Potential use: -anti-cancer agent -immunosuppressive Doxercalciferol (Hectorol):  Doxercalciferol (Hectorol) a synthetic vitamin D analog that undergoes in vivo metabolic activation to 1-a,25-dihydroxyvitamin D2 Activation does not require involvement of the kidneys Used in hyperparathyroidism in patients undergoing chronic renal dialysis Initial dose 10 mcg orally 3 times per week PARICALCITOL (Zemplar):  PARICALCITOL (Zemplar) A synthetic vitamin D analog indicated for the prevention and treatment of secondary hyperparathyroidism associated with chronic renal failure Calcipotriol (Dovonex):  Calcipotriol (Dovonex) a vitamin D derivative approved for the treatment of psoriasis. Mechanism of action is unknown. Receptor affinity is similar to that of calcitriol, but is less than 1% as active in regulating calcium metabolism Calcipotriene:  Calcipotriene An analog of vitamin D3 with a modified side-chain containing a 24-OH group and a cyclopropyl group binds strongly to the D3 receptor on keratinocytes in skin and it suppresses their proliferation (used in psoriasis) has only about 0.5% of the activity of D3 on calcium and phosphorus metabolism Dihydrotachysterol (DHT):  Dihydrotachysterol (DHT) A reduction product of vitamin D-2 Used in the management of hypoparathyroidism has only 1/450th the antirachidic activity of vitamin D-2 Vitamin K:  Vitamin K the koagulation vitamin exists in 2 forms: plant origin: phylloquinone or vit K1 bacterial origin: menaquinones or vit K2 also certain synthetic quinones have vitamin K activity menadione (vitamin K3) menadiol sodium phosphate (vitamin K4) Vitamin E:  Vitamin E alpha (E1), beta (E2) and gamma(E3) tocopherol sources: plant oils (corn, peanut, wheat germ), green leafy vegetables, meat, eggs value resides in the antioxidant properties of vitamin E (may prevent the formation of peroxides) ALPHA TOCOPHEROL:  ALPHA TOCOPHEROL Found in a variey of different sources (primarily vegetable fats) Vitamin E:  Vitamin E Estimated requirements: 5 mg/day + 0.6 mg/day of unstaurated fat Biological function – antioxidant for fatty acids Acts like vitamin C; prevents lipid peroxidation and/or damage to cells by lipid hydroperoxides Uses for vitamin E:  Uses for vitamin E hemolytic anemia in premature infants, unresponsive to B12, Fe and folic acid macrocytic megaloblastic anemia seen in children with severe protein-calorie malnutrition Other coenzymes:  Other coenzymes Serves as entry into the electron- transport chain Involved in the conversion of phenylalanine to tyrosine Slide187:  09/12/02

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