2. Toxicity -- only niacin and pyridoxine are at all toxic (in high conc.). In general, the water soluble vitamins have few toxicities.

Transcription

1 WATER SLUBLE VITAMIS A. Generalities 1. Metabolism and storage -- only B 12 and folate (B 9 ) are appreciably stored. In general, water soluble vitamins are excreted readily and are not stored. As a result, depletion is more of a problem than toxicity. 2. Toxicity -- only niacin and pyridoxine are at all toxic (in high conc.). In general, the water soluble vitamins have few toxicities. The water soluble vitamins are coenzymes for various common biochemical reactions and their status can be readily determined by measuring the appropriate enzyme activities in blood. Typically, the enzyme activity is measured in the absence and the presence of exogenously added coenzyme, to determine whether the patient needs more of the vitamin. B. Thiamin -- (Vitamin B 1 ) 1. Structure thiamin S ATP TMP ATP TPP ATP TTP hemically, the structure includes a pyrimidine ring and a thiazole ring. The thiazole ring has received enormous attention from mechanistic biochemists - they have pondered the reason for the choice by nature of a sulfur atom in the ring. Imidazole rings and oxazole rings would also, in principle, be capable of catalyzing the relevant chemical reactions. Space-filling model of Thiamin 2. Absorption

2 A. Active transport. ccurs at low concentrations. Two thiamin transporter has been identified in intestine: ThTr1 and ThTr2. After cellular uptake, thiamin is converted to TPP. Mutations in ThTr1 gene is associated with thamin-responsive megaloblastic anemia (TRMA). B. Passive diffusion. At high concentration (2.5 mg dose for a human), passive diffusion also occurs. 3. Transport Most thiamin is serum is bound to protein, mainly albumin, non-specifically. About 90% of total thiamin in blood is contained in erythrocytes. ellular intake: by active transport, mediated by ThTr1 and Thtr2. 4. Metabolism Phosphorylation: TMP, TPP, TTP atabolism: TTP and TPP are catabolized by thiamin pyrophosphatase to give TMP. Thiamin has an estimated half-life of days in human so thiamin deficiency states can deplete tissue stores within a couple of weeks. Excretion: Excreted in urine, mainly as free thiamin and TMP, along with over 20 metabolites. 5. Function cofactor for enzymes used to harvest carbohydrates for energy 2

3 a) xidative decarboxylation of α-keto acids e.g. 3 TPP 2 3 lipoic acid AD SoA 2 AD oas pyruvate dehydrogenase complex e.g. α -ketoglutaric acid TPP AD succinyl oa 2 oas lipoic acid α-ketoglutarate dehydrogenase complex The decarboxylation is accomplished by a mitochondrial enzyme complex as shown below. L = lipoic acid, E = enzyme, TPP = thiamin pyrophosphate. Pyruvate dehydrogenase complex in detail (α-hydroxyethyl TPP) 3 AD AD 2 E TPP E L S S E L S S 3 E TPP E L S 3 SoA S 3 oas TA cycle b) Transfer of α-ketols (pentose phosphate pathway) -- 10% of carbohydrate metabolized this way. This pathway provides pentoses for RA and DA synthesis and ADP for the biosynthesis of fatty acids and other endogenous reactions. e.g. 2 2 = transketolase TPP = 2 P P 3 2 xyulose-5-phosphate 2 P 3 2 ribose-5-phosphate glyceraldehyde phosphate 2 P 3 2 sedoheptulose-7-phosphate 3

4 c) on-coenzyme function TTP involved in the control of chloride channels in brain and elsewhere in nerve impulse conduction. 6. Mechanism formation of adduct with 2 of thiazole ring. 3 2 S S 3 S 3 pyruvate S 3 3 S S S 3 3 α-hydroxy-ethyl-tpp lassic experiments support this mechanism, including facile exchange of solvent D 2 at the -2 position. 7. Deficiency thiamin plays a vital role in nerve function as signs of thiamin deprivation are mainly neurologic ones. Thiamin needs are proportional to caloric intake and is essential for carbohydrate metabolism. Usually consider requirement as 0.5 mg/1000 calories plus 0.3 mg during pregnancy and lactation. Studies show laboratory evidence of thiamin deficiency (erythrocyte transketolase assay) in 20-30% of elderly patients and 40-50% of chronic alcoholics. <2% of healthy controls showed evidence of deficiency. a) Early signs of deficiency anorexia, nausea, vomiting, fatigue, weight loss, nystagmus, tachycardia. b) Late signs of deficiency Beriberi cardiac - increased heart size, edema cerebral - depression, irritability, memory loss, lethargy GI tract - vomiting, nausea, weight loss 4

5 neurological - weakness, polyneuritis, convulsions. Signs and symptoms vary with age of patient, rapidity of onset, and severity of deficiency. c) Thiamin deprivation has been shown to cause oxidative stress, alter neurotransmitter metabolism, and cause dysfunction of the BBB, which may account for some of the neurological symptoms. d) Thiamine and the alcoholic (i) intake low and alcohol blocks conversion of thiamin TPP (ii) absorption - active transport (iii) storage (iv) increased fluid intake and urine flow causes thiamin washout (v) involved in fetal alcohol syndrome. e) Wernicke-Korsakoff syndrome seen in some alcoholics; neurological disorder resulting in impaired mental functioning institutionalization for a significant number of patients. Symptoms: confusion, memory loss, confabulation, psychotic behavior; maybe irreversible in part. f) Alzheimer s diseases (AD): In AD patients, 20% reduction of TPP levels was observed in patient brain and associated reduction in TPP-dependent enzyme activities. owever, small clinical trials using thiamin as therapy is inconclusive. g) Risk factors B 1 deficiency (i) carbohydrate intake -- TP, alcoholics (ii) absorption -- ulcerative colitis, etc., alcoholism (iii) intake -- poor diet, geriatrics, breast fed infant from B 1 deficient mother, etc. (iv) alcoholism. 8. Source present in most tissues (as TPP) and plants (as thiamin); rich sources include: lean meat, especially pork, cereal grains, eggs, yeast, nuts. Thiaminase in some fish (raw) and shellfish (raw) and ferns. This enzyme can hydrolyze thiamin. 2 S In the milling or processing or processing of rice and flour, the thiamin is lost. Whole wheat or rice contains ~10 times as much Thiamin as white wheat or rice. Today in the USA, most white flour, rice and pastas are "enriched" to bring thiamin levels to near original levels. "Enriched" products also have added riboflavin, niacin, iron, and folic acid. 5

6 9. Stability labile at p > 4 and when heated (especially at alkaline p values), so prolonged cooking decreases levels especially at p > Diagnosis of deficiency state 11. Uses a) pyruvate and lactate in plasma b) transkelolase activity in RB most important technique. a) deficiency states -- for alcoholics b) thiamin responsive inborn errors of metabolism -- see below c) mosquito repellant research suggests not so much d) acute alcoholism: give 100 mg IM or IV stat. This is a common practice. e) Alzheimer s disease little evidence for benefit (huge doses used). 12. Thiamin responsive inborn errors of metabolism: Disease Wernicke Korsakoff Maple Syrup urine disease Thiamin responsive megaloblastic anemia yperalaninemia yperpyruvic acidemia Defect Transketolase Failure to decarboxylate branched chain amino acids Deficient pyruvate dehydrogenase Deficient pyruvate dehydrogenase 13. Requirement 0.5 mg/1000 cal. DV = 1.5 mg. Minimum intake should be at least 1 mg. 14. Toxicity nontoxic on oral administration; o UL value. Anaphylactic reactions have been observed in patients receiving repetitive parenteral doses. 15. Patient ounseling/ Patient Use Issues a) eeded to drive carbohydrates to energy b) Rarely needed as a single supplement. Use a multivitamin to get needed thiamin. c) Special benefit in alcoholics at higher doses d) Benefit in high doses in rare thiamin-responsive inborn errors of metabolism e) ontoxic. 6

7 . Riboflavin (Vitamin B 2 ) Like Vitamin B 1, Riboflavin is highly water soluble, and it is difficult to achieve toxic levels in the body excess vitamin is efficiently eliminated renally. It used as an additive in many enriched foods and can be produced in huge, industrial scale, quantities by expression of the biosynthetic enzymes in fungi or bacteria. These genetically engineered organisms may appear bright yellowish-orange, which is the color of Riboflavin. 1. Structure FM = riboflavin monophosphate FAD = flavin adenine dinucleotide riboflavin coenzymes gut mucosa riboflavin FM liver FAD ATP ADP ATP ADP 2. Absorption ydrolysis of coenzyme forms: riboflavin in most food is in protein complexes as coenzyme, so need hydrolysis, which occurs by the proteolytic activity of intestinal lumen. Active transport of free riboflavin: an ATP-dependent, carrier-mediated process in the proximal small intestine and colon. 7

8 3. Transport Transported as free riboflavin and FM, both of which are bound to plasma protein in significant amounts. Specific binding proteins (RfBPs): found in plasma ellular uptake: similar to enteric absorption, carrier mediated Tissue distribution: mostly as FM (60-95%) and FAD (5-22%). igh in liver, kidney and heart. 4. Function serve as intermediates in transfers of e - in biological reduction-oxidation (redox) reactions, tissue respiration, transfer as flavin containing enzyme. - oenzymes for > 100 enzymes. - Essential for metabolism of carbohydrates, amino acids, and lipids R 2 R xidized - Yellow -2 Reduced - colorless Examples of enzymes having flavin groups: succinate dehydrogenase (succinate fumerate in TA cycle) fatty acid acyl oa dehydrogenase (β-oxidation of lipids) glutathione reductase important in antioxidant activities ADP FAD 2GS glutathione reductase 2 2 or R glutatione peroxidase ADP FAD 2 GSSG 2 2 or R 2 The following are important flavoproteins (containing FM): cytochrome reductase (electron transport); ADP--cytochrome reductase; cytochrome P-450 reductase (drug metabolism), flavin monooxygenase (drug metabolism). 5. ealth - Protection against oxidative stress (antioxidant enzymes) - Reduce risk of vascular disease: inversely correlated with homocysteine level, which is a risk factor - Anticarcinogenesis: cancer inversely correlated with decreased antioxidant activity (more DA oxidative damage) 8

9 - Deficiency protect against malaria: increase vulnerability of erythrocytes to lipid peroxidation, but malarial parasites are more susceptible than erythrocytes to oxidative stress. 6. Deficiency state a) ot usually seen in isolation but occurs in combination with other B vitamin deficiencies. b) Fatigue, cheilosis, glossitis, vascularization of cornea, dermatitis c) Vegans and teenagers may be low in B 2 if dairy intake is low d) Low B 2 intake may be a risk factor for cataract development e) Alcoholics are at risk due to low intake and low absorption. f) ther risk factors: phototherapy, exercise g) Diagnosis: erythrocyte GS reductase activity is a useful marker for riboflavin status. 7. Source milk, meats, leafy vegetables, eggs, yeast, enriched products. 8. Stability 9. Use a) Usually > 30% destroyed by cooking b) Labile to light c) More stable in acid than alkali in absence of light. a) Deficiency states. Is a component of most multivitamin mixtures. b) ew may help in migraine headache prevention. c) ew high intake associated with lower risk for cataracts and a 3mg supplement reduced risk. 10. Requirements a) DV = 1.7 mg b) Average U.S. diet contains 2 mg for males and 1.5 mg for females c) o UL value 11. Patient ounseling/ Patient Use Issues a) Routine single dose supplementation not needed. Use a multivitamin to get needed riboflavin. b) Possible use in preventing migraine headaches. Use 400 mg/d. c) Will turn urine bright yellow in doses higher than the DV. d) ontoxic. 9

10 D. Vitamin B 6 1. Structure pyridoxine (P) pyridoxal (PL) pyridoxamine (P) Pyridoxine is a commonly used term for this vitamin, but all 3 are equally active so vitamin B 6 is a better term to use. Three phosphorylated forms are present also: P P 4 FM FM PP oenzyme = pyridoxal-5-phosphate 2. Transport: form Schiff base with proteins (lys residue): albumin, hemoglobin 3. Function participates in over 140 enzymatic reactions by forming a Schiff base with the terminal amino group of lysine in the enzyme. It is estimated that this corresponds to ~4% of all enzymatic reactions known. R R 2 2 P 2 P dependent enzyme 3 3 holoenzyme 10

11 a) Transamination (amino acids metabolism) enzyme R R R 2 amino acid #1 3 2 P 3 2 P P 2 2 R α-keto acid # P PP 2 P R α-keto acid #2 3 2 P 2 R amino acid #2 e.g. glutamate-aspartate transaminase aspartic acid pyruvic acid transaminase 2 oxaloacetic acid 2 3 alanine b) Decarboxylation (synthesis of neurotransmitters and histamine) R 2 R 2 R 2 P 2 P 2 P amino acid 3 3 decarboxylase 3 11

12 e.g glutamic acid γ-amino butyric acid (GABA) 2 e.g hydroxytryptophan 5-hydroxytryptamine (serotonin) e.g histidine histamine e.g DPA DPAMIE ote: B 6 contraindicated in levo-dpa therapy because B 6 enhances peripheral decarboxylation of levo-dpa to dopamine which will not cross Blood Brain Barrier; Larobec (Roche) contains no pyridoxine and can be used if multivitamin supplementation is desired for patient on l-dpa. The anti-parkinsons s drug Sinemet contains levo-dpa and carbidopa (A DPA decarboxylase inhibitor) -- therefore, no interaction. c) B 6 and sulfur amino acid metabolism. (ote: elevated homocysteine is an independent risk factor for cardiovascular disease and birth defects.) 12

13 3 S 2 2 methionine S 2 2 TFA 5 -methyl TFA 2 2 homocysteine ATP methyl B 12 hydroxy B 12-3 S adenine 2 S-adenosylmethionine methyl acceptor S adenine 2 S-adenosylhomocysteine serine 3 2 S α-ketobutyrate 2 cystathionine γ-lyase 2 cystathionine S 2 2 cysteine d) B 6 involvement in methionine formation (and S-adenosyl methionine) makes it indirectly involved in methylation. ence B 6 is indirectly involved in lipid metabolism and nucleic acid formation and immune function. 13

14 e) B 6 involved in tryptophan metabolism to serotonin and niacin (B3). tryptophan 2 2 tryp decarboxylase serotonin 2 tryptophan oxygenase 2 -formylkynurenine hydroxykynurenine kynureninase 2 3-hydroxy anthranilic acid iacin f) ther reactions requiring B 6 : (i) (ii) (iii) Glycogen phosphorylase (release of glucose in muscle) eme biosynthesis ucleic acid biosynthesis (via SAM) 14

15 SUMMARY of Pyridoxal/pyridoxamine reactions B: external carbinolamine external imine internal imine internal carbinolamine hydrolysis of Imine leads to incorporation of oxygen atom 2 attacks carbon atom of the imine, that carbon gets an oxygen enzymes control which carbon atom gets the oxygen atom by switching between the external imine and the internal imine. This accounts for their versatility. 4. Deficiency 15

16 a) ot seen usually and, if seen, is associated with other vitamin deficiencies or is iatrogenic; symptoms include rash, peripheral neuritis, anemia and possible seizures. Deficiency diagnosed by low plasma and low transaminase activities (±). b) Iatrogenic B 6 deficiencies (i) Isoniazid also called isonicotinylhydrazide (I), an antituberculosis drug forms Schiff base with B 6. an get neuritis and convulsions mg/d B 6 given to prevent B 6 deficiency Isoniazid (ii) 4-Deoxypyridoxine (experimental only) Symptoms Skin lesions on face, glossitis, stomatitis, convulsive seizures ( GABA?), anemia ( heme synthesis?). (iii) now. ral contraceptives.- older high dose cs can affect B6 but not a problem 5. Source milk, meats, legumes, tuna, whole grains, beans. 6. Stability pyridoxine is stable; some loss on cooking, especially with meats, due to Schiff base formation and decrease of the pyridoxal in the foods. 7. Diagnosis of Deficiency measure erythrocyte transaminases. 8. Use a) Routine use in multivitamin products b) In I therapy 16

17 c) ertain inborn errors of metabolism ame Symptoms Dose of B6 Problem B6--dependent infantile convulsions lonic and tonic seizures mg/day Defective glutamic acid decarboxylase; possible GABA depletion B6--responsive Microcytic, 100 mg/day Defective hemoglobin synthesis anemia hypochromic anemia Xanthurenic acidurea Mental retardation mg/day Defective tryptophan metabolism due to faulty kyureninase, xanthurenic acid spills into urine omocystinurea Mental retardation Early heart disease mg/day Defective cystathionine synthetase homocysteine appears in urine ystathionurea Mental retardation mg/day Defective cystathionase d) PMS ( mg/d) -- evidence is uneven. is known to bind to steroid receptors. e) arpal tunnel syndrome -- evidence is uneven. It seems to work for some. A trial of B mg/d for 6 mos. may be worthwhile. In some trials vitamin B6 combined with lipoic acid worked effects are modest! (numbness, tingling, weakness, and other problems in the hand) f) hinese restaurant syndrome: may be related to large amount of intake of monosodium glutamate (MSG), buy has not been shown in clinical trials. (chest pain, flushing, headache, numbness or burning in or around the mouth, sense of facial pressure or swelling, sweating) g) Use in lowering homocysteine levels (see sulfur amino acid scheme above). igh homocysteine may be an independent risk factor for cardiovascular disease but this is now controversial. ombine with folic acid and B 12 for optimum lowering action. h) ausea and vomiting in pregnancy-elpful in high doses. PremesisRx contains 75mg sustained release B 6 (plus 12ug B 12, 1mg folic acid and 200mg calcium) or 25mg of generic B 6 TID is less expensive. It does not contain Iron. PremesisRx is contraindicated for patients on Fluorouracil FU side effects may be increased by PremesisRx. ot used in the USA any longer. 9. Requirement DV = 2 mg; UL = 100 mg. 10. Toxicity a) > 200 mg/day can decrease prolactin levels 17

18 b) > 1-2 g/day can cause serious neuropathy by an unknown mechanism. Recommendation: avoid long term use in doses above 200 mg. 11. Patient ounseling/ Patient Use onsiderations a) Routine single dose supplementation usually not needed. Use a multivitamin to get needed B 6. b) Sometimes used, with limited evidence, for carpal tunnel syndrome, PMS, and depression on obsessive-compulsive symptoms (s). c) Rare use in high doses for inborn errors. d) Sometimes used to prevent neuropathy with isoniazid. e) For nausea and vomiting of pregnancy, 25 mg TID or use PremensisRx which is FDA approved for this. f) Used with B 12 and folic acid in high homocysteine. g) Keep doses < 200 mg/d to avoid neuritis. 18