Water-Soluble Vitamins. Libin Xu, Ph.D. Department of Medicinal Chemistry

Transcription

1 Water-Soluble Vitamins Libin Xu, Ph.D. Department of Medicinal Chemistry

2 Learning bjectives At the end of the course, you should be able to: Recognize the structures of vitamins Describe the main biochemical and biological functions of each vitamin utline and classify main metabolic reactions that each vitamin participates in Identify the symptoms and common disease name related to a particular vitamin deficiency Apply chemical or biochemical methods to diagnose deficiency of a particular vitamin Evaluate the risk factors and/or populations for each vitamin deficiency Describe and apply ways to treat a particular vitamin deficiency Recommend the use of certain vitamin on relevant diseases or conditions Recognize and explain interactions of certain drugs with vitamins Identify the toxicities of vitamins when overdosed (rare in water-soluble vitamins, but common in fat-soluble ones) Explain the interactions between some vitamins

3 Related to Their igh Water Solubility 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. Toxicity -- only niacin (b 3 ) and pyridoxine (b 6 ) are at all toxic (in high conc.). In general, the water soluble vitamins have few toxicities.

4 Common Functions The water soluble vitamins are coenzymes for various common biochemical reactions 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.

5 Thiamin (B1) - Structure Pyrimidine ring 2 + S ATP ATP ATP TMP TPP TTP Thiamin pka = Thiazole ring

6 Absorption and Transport 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). Passive diffusion. At high concentration (2.5 mg dose for a human), passive diffusion also occurs. Serum. Most thiamin is serum is bound to protein, mainly albumin, non-specifically. About 90% of total thiamin in blood is contained in erythrocytes.

7 Metabolism Phosphorylation: TMP, TPP, TTP Catabolism: 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.

8 Function Cofactor for Enzymes Used to arvest Carbohydrates for Energy

9 Enzyme cofactor - xidative decarboxylation of a- keto acids e.g. C 3 TPP C C 2 C 3 C lipoic acid AD+ SCoA + C 2 + AD CoAS pyruvate dehydrogenase complex e.g. α -ketoglutaric acid TPP AD+ succinyl CoA + C 2 CoAS lipoic acid α-ketoglutarate dehydrogenase complex The decarboxylation is accomplished by a mitochondrial enzyme complex as shown in next slide.

10 Pyruvate dehydrogenase complex (α-hydroxyethyl TPP) C 3 AD AD + C 2 E C TPP E L S S E L S S C 3 C C E TPP E L S C 3 CSCoA S C C 3 CoAS TCA cycle L = lipoic acid, E = enzyme, TPP = thiamin pyrophosphate.

11 Transfer of a-ketols (pentose phosphate pathway) e.g. C 2 C C 2 C = C C + C C C transketolase TPP C C = C + C C C 2 P 3 2 C 2 P 3 2 xyulose-5-phosphate C 2 P 3 2 ribose-5-phosphate C C glyceraldehyde phosphate C 2 P 3 2 sedoheptulose-7-phosphate 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.

12 Catalytic Mechanism formation of adduct with C2 of thiazole ring 3 2 S S + C C + C 3 C C C S C 3 C C pyruvate + S C 3 C C C 3 C S + + S S + C + C 3 C + C C 3 C α-hydroxy-ethyl-tpp

13 Deficiency Signs of deprivation are mainly neurologic. Thiamin needs are proportional to caloric intake because it is essential for carbohydrate metabolism. 0.5 mg/1000 calories plus 0.3 mg during pregnancy and lactation. Deficient in 20-30% of elderly and 40-50% of chronic alcoholics. <2% healthy controls showed evidence of deficiency. Thiamin and alcoholism Alcohol blocks conversion of thiamin à TPP Alcohol decrease absorption, active transport, and storage Alcohol increased fluid intake and urine flow à thiamin washout Thiamin deficiency also involved in fetal alcohol syndrome

14 Deficiency Signs Early signs: anorexia, nausea, vomiting, fatigue, weight loss, nystagmus, tachycardia Late signs: Beriberi Cardiac - increased heart size, edema (wet beriberi) Cerebral - depression, irritability, memory loss, lethargy (dry beriberi) GI tract - vomiting, nausea, weight loss neurological - weakness, polyneuritis, convulsions. Vary with age of patient, rapidity of onset, and severity of deficiency. 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. Beriberi (from Wikipedia) Edema From aibolita.com

15 Wernicke-Korsakoff Syndrome Wernicke encephalopathy (dilirium) and Korsakoff dementia. eurological disorder resulting in impaired mental functioning. Seen in some alcoholics. Also seen in an inborn error of metabolism in transketolase Symptoms: confusion, memory loss, confabulation, psychotic behavior. verlap with symptoms of dry beriberi. Thinning of Corpus callosum and enlargement of ventricles

16 Risk Factors Increased carbohydrate intake: total parenteral nutrition (IV for GI disorder), alcoholics Decreased absorption: ulcerative colitis, alcoholism Decreased intake: poor diet, geriatrics, breast fed infant from B1 deficient mother Alcoholism

17 Diagnosis of Deficiency State Increased levels of pyruvate and lactate in plasma Transketolase activity in RBC most important technique e.g. C 2 C C 2 C = C C + C C C transketolase TPP C C = C + C C C 2 P 3 2 C 2 P 3 2 xyulose-5-phosphate C 2 P 3 2 ribose-5-phosphate C C glyceraldehyde phosphate C 2 P 3 2 sedoheptulose-7-phosphate

18 Use Deficiency states alcoholics Acute alcoholism: give 100 mg IM or IV stat. This is a common practice. Thiamin responsive inborn errors of metabolism (next slide) Alzheimer s disease little evidence for benefit (huge doses used) (although 20% reduction of TPP levels was observed in patient brain association, but may not be causative). Requirement: 0.5 mg/1000 cal. DV = 1.5 mg. Minimum intake 1 mg. Rarely needed as a single supplement; can be taken in a multivitamin. ontoxic on oral administration; anaphylactic reactions have been observed in patients receiving repetitive parenteral doses. Stability: labile to alkaline p and heat.

19 Thiamin Responsive Inborn Errors of Metabolism Disease Wernicke-Korsakoff Maple syrup urine disease Thiamin responsive megaloblastic anemia yperalaninemia (elevated alanine in serum) yperpyruvic acidemia Defect Transketolase Failure to decarboxylate branched chain amino acids Thtr1 (transport) Pyruvate dehydrogenase Pyruvate dehydrogenase AD TPP CoAS SCoA Pyruvate dehydrogenase Pyruvate Acetyl CoA Transaminase (B6-dependent) 3 Alanine

20 Dietary Sources Cereal grains, lean meat (pork), eggs, yeast, nuts Today, most white flour, rice and pastas are enriched to bring thamin levels up (recall thiamin in polishings); other water soluble vitamins also enriched. Image from vkool.com owever, thiaminase exists in some raw fish and shellfish and ferns, which can break down thiamin. + 2 S

21 Riboflavin (B2) - Structure FM = riboflavin monophosphate FAD = flavin adenine dinucleotide riboflavin coenzymes gut mucosa riboflavin FM liver FAD ATP ADP ATP ADP

22 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 ATPdependent, carrier-mediated process in the proximal small intestine and colon.

23 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. Cellular uptake: similar to enteric absorption, carrier mediated. Tissue distribution: mostly as FM (60-95%) and FAD (5-22%). igh in liver, kidney and heart.

24 Function Serve as intermediates in transfers of e - in biological reduction-oxidation (redox) reactions, tissue respiration, transfer as flavin containing enzyme. Coenzymes for > 100 enzymes. Essential for metabolism of carbohydrates, amino acids, and lipids R +2 R -2 xidized - Yellow Reduced - colorless

25 Examples of Flavin-containing Enzymes Succinate dehydrogenase (succinate à fumerate in TCA cycle) Fatty acid acyl CoA dehydrogenase (β-oxidation of lipids) n n Cytochrome C reductase ADP-cytochrome C reductase Cytochrome P450 reductase (drug metabolism) Flavin-containing monooxygenase (drug metabolism) Glutathione reductase important in antioxidant activities ADP ADP FAD FAD 2 succinate 2GS glutathione reductase GSSG SCoA fumerate 2 2 or R glutatione SCoA peroxidase 2 2 or R + 2

26 ealth Related 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) Deficiency protect against malaria: increase vulnerability of erythrocytes to lipid peroxidation, but malarial parasites are more susceptible than erythrocytes to oxidative stress.

27 Deficiency State ot usually seen in isolation but occurs in combination with other B vitamin deficiencies. Fatigue, cheilosis, glossitis, vascularization of cornea, dermatitis Vegans and teenagers may be low in B2 if dairy intake is low Low B2 intake may be a risk factor for cataract development Alcoholics are at risk due to low intake and low absorption. ther risk factors: phototherapy, exercise Diagnosis: erythrocyte GS reductase activity is a useful marker for riboflavin status.

28 Use For deficiency states. Is a component of most multivitamin mixtures. May help in migraine headache prevention. eed 400 mg/day. igh intake associated with lower risk for cataracts and a 3mg supplement reduced risk. DV = 1.7 mg; no UL value (excess vitamin is efficiently eliminated renally); nontoxic. Average US diet contains 2 mg for males and 1.5 mg for females. Will turn urine bright yellow in doses higher than DV. Supplemented as multivitamin. Labile to heat, light, and base.

29 Sources Milk, leafy veggies, meats, eggs, yeast Image from medlineplus.gov

30 B6 - Structures pyridoxine (P) pyridoxal (PL) pyridoxamine (P) C 2 C C 2 2 C 2 C 2 C 2 C 3 C 3 C 3 Pyridoxine is a commonly used term for this vitamin, but all 3 are equally active, so vitamin B6 is a better term to use. Three phosphorylated forms are also present: P P PLP 4 FM FM PP Coenzyme = pyridoxal-5-phosphate PLP

31 Transport Form Schiff base with lysine residue of proteins: albumin, hemoglobin 3 + C C 2 P R C C 2 P 3

32 Function Participates in over 140 enzymatic reactions by forming a Schiff base with the terminal amino group of lysine in the enzyme. Corresponds to ~4% of all enzymatic reactions known. R C C R 2 + C 2 P C 2 P PLP dependent enzyme C 3 C 3 PLP holoenzyme

33 Function - Transamination (amino acids metabolism) enzyme R C C R C C R C C 2 amino acid #1 + C 3 C C 2 P C 3 C C 2 P C 3 C2 C 2 P 2 2 R C C α-keto acid #1 + C 3 C2 C 2 P C 3 2 C 2 PP C 2 P + R C C α-keto acid #2 e.g. glutamate-aspartate transaminase 2 C C 2 C C + C3 C C aspartic acid pyruvic acid PLP C 3 transaminase C C 2 P 2 + R C C amino acid #2 PLP C C 2 C C + oxaloacetic acid 2 C 3 C C alanine

34 Function - Decarboxylation (synthesis of neurotransmitters and histamine from amino acids) R C 2 R C C + 2 C C C R C C C 2 P C 2 P C 2 P C 2 amino acid C 3 C 3 decarboxylase C 3 e.g. C C 2 C 2 C C 2 glutamic acid C C PLP C 2 C C2 C2 C C 2 γ-amino butyric acid (GABA) 2 C 2 2 PLP 2 5-hydroxytryptophan 5-hydroxytryptamine (serotonin) C 2 C C 2 C 2 C 2 C 2 histidine PLP histamine

35 Levo-DPA Chirality B6 and Anti-Parkinson s Drug (Levo-DPA) DPA C 2 C 2 C PLP C 2 C2 C 2 B6 contraindicated in Levo-DPA therapy because B6 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. DPAMIE Carbidopa 2

36 B6 and sulfur amino acid metabolism Elevated homocysteine is an independent risk factor for cardiovascular disease and birth defects C 3 S C 2 C 2 methionine S PLP C 2 TFA 5 -methyl B12 TFA C 2 Methyl B12 C 2 C C 2 homocysteine serine ATP methyl B 12 5-methyl hydroxy TFAB 12 C TFA Cystathionine synthase C 3 S C 2 C 2 C C - C 2 adenine 2 S-adenosylmethionine methyl acceptor S C 2 C 2 C C - C 2 adenine 2 S-adenosylhomocysteine 2 3 C C C C 2 C C C S C 2 C 2 C C 2 2 PLP α-ketobutyrate C 2 C C cystathionine + γ-lyase 2 cystathionine S C 2 C C 2 cysteine B6 involvement in methionine formation (and S-adenosyl methionine) makes it indirectly involved in methylation (other involved vitamins folate B9 and B12): hence involvement in lipid metabolism and nucleic acid formation.

37 Function - Tryptophan Metabolism to Serotonin and iacin tryptophan 2 C C PLP 2 tryp decarboxylase serotonin 2 tryptophan oxygenase C C 2 -formylkynurenine C hydroxykynurenine PLP kynureninase C C 2 3-hydroxy anthranilic acid iacin Vitamin B3

38 Mechanisms of Pyridoxal/Pyridoxamine Reactions - - C C C C 2 C external carbinolamine C 2 external imine B: C internal imine C internal carbinolamine ydrolysis of imine leads to incorporation of oxygen atom PLP enzymes control which carbon atom gets the oxygen atom by switching between the external imine and the internal imine. This accounts for their versatility. 2 2 C C 2

39 Deficiency Symptoms: rash, peripheral neuritis, anemia and possible seizures. ot usually seen but could be induced by some treatment (iatrogenic) Isoniazid also called isonicotinylhydrazide (I), an antituberculosis drug, forms Schiff base with B6, leading to deficiency. Symptoms: neuritis and convulsion. Use mg/d B6 for prevention. C Isoniazid 2 + C 3 C C 2 C C C 3 C 2 2 Deficiency diagnosis: measure erythrocyte transaminase activities and plasma PLP levels ydralazine (vasodilator)

40 Use In isoniazid (I) therapy PMS ( mg/d) - evidence is uneven. PLP is known to bind to steroid receptors. Certain inborn errors of metabolism (next slide) Carpal 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)

41 Use Cont d Chinese restaurant syndrome: may be related to large amount of intake of monosodium glutamate (MSG), buy has not been shown in clinical trials. Studies suggest B6 deficiency renders the patients unable to metabolize MSG (to GABA neurotransmitter) Symptoms: chest pain, flushing, headache, numbness or burning in or around the mouth, sense of facial pressure or swelling, sweating MSG GABA Lower homocysteine levels (combined with B9 and B12) cardiovascular disease risk factor ausea and vomiting in pregnancy. PremesisRx contains 75mg sustained release B6 (plus 12ug B12, 1mg folic acid and 200mg calcium) or 25mg of generic B6 TID is less expensive. PremesisRx is contraindicated for patients on Fluorouracil (thymidylate synthase inhibitor, cancer drug) FU side effects may be increased by PremesisRx. ot used in the USA any longer.

42 B6-Responsive Inborn Errors of Metabolism ame Symptoms Dose of B6 Problem B6-dependent infantile convulsions Clonic and tonic seizures mg/day Defective glutamic acid decarboxylase; possible GABA depletion B6-responsive anemia Microcytic, hypochromic anemia 100 mg/day Defective hemoglobin synthesis 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 à homocysteine appears in urine Cystathionurea Mental retardation mg/day Defective cystathionase omocysteine Cystathionine Synthetase Cystathionine Cystathionase Cysteine + -ketobutyrate

43 Requirement, Toxicity, Sources DV = 2 mg; UL = 100 mg. > 200 mg/day can decrease prolactin levels; > 1-2 g/day can cause serious neuropathy by an unknown mechanism. Recommendation: avoid long term use in doses above 200 mg. Sources: milk, meats, legumes, tuna, whole grains, beans Pyridoxine is stable; pyridoxal could be lost during cooking (Schiff base formation and oxidation) Image from vkool.com