Water Soluble Vitamins

(Vitamin B _(1,2,3,5,6) )

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Introduction

• Chemically not related to each other
• Grouped together because all of them function in the cells as co-enzymes.
• Vitamin B_1-8
• Others are Choline, Inositol, Lipoic acid and PABA (Para amino benzoic acid).

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Thiamine (Vitamin- B₁)

• Also k/a Aneurine (Can relieve neuritis) or Anti beriberi factor
• Beriberi was seen in patient feeded with polished rice chicken
• Sources:- Aleurone layer of cereals (food grains) is a rich source of thiamine
• Therefore unpolished rice and whole wheat flour are more nutrients.

• Polishing destroys aleurone layer
• Yeast is also a very good source
• Heat labile
• Structure:-
• It contains a substituted pyrimidine ring connected to a substituted thiazole ring by means of methylene bridge.
• The vitamin is then converted to its active coenzyme form by addition of two phosphate groups with the help of ATP.

Thiamine : vitamin form

Thiamine pyrophosphate: coenzyme form by thiamine phosphotransferase

Pyrimidine ring

Thiazole ring

Physiological role of thiamine

• Pyruvate dehydrogenase:-
• Coenzyme form is TPP
• Used in oxidative decarboxylaton of α- keto acids e.g. PD catalyses the breakdown of pyruvate to acetyl Co-A & Co2.
• α-Ketoglutarate dehydrogenase (TCA Cycle)
• Oxidative decarboxylation of α- KG to Succinyl Co-A & Co2.

• Transketolase:-
• Secondary group of enzymes that uses TPP as co-enzyme are transketolases.
• In HMP Shunt pathway of glucose.
• Main role of TPP is Carbohydrate metabolism
• So, Requirement of thiamine is increased along with higher intake of Carbohydrates.

Deficiency

• BeriBeri:- Means weakness
• Anorexia, Dyspepsia, Heaviness, Weakness,

Fatigue & exhaustion

• Wet BeriBeri:-
• CVS manifestations
• Edema of legs, face, trunk, serous cavities
• Palpitations, breathlessness, distended neck

veins

• Dry Beriberi:-
• CNS manifestations, difficulty in walking
• Peripheral neuritis with sensory disturbance

leads to complete paralysis.

• Infantile Beriberi:-
• Seen in infants born to mothers suffering from Thiamine deficiency.
• Restlessness & sleeplessness seen.

• Polyneuritis:- Common in chronic alcoholics
• Alcohol utilization needs large doses of thiamine.
• Alcohol inhibits intestinal absorption of thiamine → causes deficiency.
• Also associated with pregnancy & old age.
• Such thiamine deficiency in alcoholism causes impairment of conversion of pyruvate to acetyl Co-A → Increase plasma conc. of pyruvate & lactate → Causes Lactic Acidosis.

• Wernicke – Korsakoff Syndrome:-
• Name from scientist
• Due to alcoholism, also k/a Cerebral beriberi
• Seen only when nutritional deficiency is

severe.

• C/F :- are of encephalopathy (Ophthalmoplegia , Nystagmus , Cerebellar ataxia)

• In thiamine def. Clinical Lab picture will be:-

Blood B₁- (↓)es,

Pyruvate, α-KG, Lactate- (↑)es

• Test for def. :-

Erythrocyte Transketolase activity is (↓)es.

(It is the earliest manifestation)

• RDA:- Depends on calorie intake

0.5 mg/1000 calories, 1 to 1.5 mg/day

• Used in treatment of Beriberi, Alcoholic polyneuritis, neuritis of pregnancy & old age

Riboflavin (VitaminB₂)

• Lactoflavin (milk), hepatoflavin (liver), and ovoflavin(eggs) are chemically identical to riboflavin.
• First B- complex component to be isolated in pure state.
• Synthesized by green plants & microorganisms

Structure

Dimethyl isoalloxazine ring with D-ribitol

Structure

• Ribitol is alcohol of Ribose sugar
• Heat stable
• Converted to FMN & FAD (active coenzyme) with the help of ATP.
• Sources:-

Rich:- Liver, Dried yeast, Egg, Whole milk

Good:- Fish, Whole cereals, Legumes, Green leafy veg.

• RDA:- 1.5 mg/day, During pregnancy, Lactation additional 0.2 to 0.4mg/day

• Co-enzyme activity Of riboflavin:-
• Riboflavin exists in tissues tightly bound to enzymes (not covalently)
• Enzymes with Riboflavin are k/a flavoproteins:-

FMN (Flavin mononucleotide)

FAD (Flavin adenine dinucleotide)

Enzymes with Riboflavin k/a FLAVOPROTEINS

• FAD accepts hydrogen:- During oxidation process FAD accepts 2H⁺ atoms from substrate.
• Two nitrogen atoms of isoalloxazine nucleus accepts the two H⁺
• FMN dependent enzymes:-
• During amino acid oxidation FMN is reduced , it is reoxidised by molecular O₂ to produce H₂O₂.
• In the respiratory chain (ETC) NADH dehydrogenase contains FMN.
• NAD⁺ → FMN → CoQ
• FADH2 when oxidized in the ETC will generate 1 ½ ATP molecules.

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• Riboflavin Deficiency:-
• Uncommon, because it is synthesized in the intestine by intestinal flora.
• Accompanies other def. disease like:-

Beriberi, Pellagra, Kwashiorkor.

• Manifestations:- Corneal neovascularization

Glossitis, Magenta coloured tongue, Cheilosis,

Angular stomatitis.

• FAD dependent enzymes:-
• Succinate → Fumarate
• Acyl co-A → α-β Unsaturated acyl Co-A
• Xanthine → Uric Acid
• Pyruvate → Acetyl Co-A
• α-KG → Succinyl Co-A

Niacin (Vitamin B₃)

• Also k/a Nicotinic acid, Pellagra preventing factor of Goldberger.
• Nicotin:- Poison from tobacco
• Warburg found the structure of NAD⁺, also k/a Coenzyme-1 , initially designated as DPN (Diphosphopyridine nucleotide), finally NAD⁺

Chemistry

• It is Pyridine 3- carboxylic acid
• Niacinamide is the acid amide.

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• In NAD⁺ & NADP⁺ the reactive site is the C₄ & nitrogen atom of the nicotinamide ring.
• The coenzyme is bound to apoenzyme.
• Coenzyme forms of Niacin:-

Niacin is converted to NAD⁺ & NADP⁺

• NAD⁺ dependent enzymes:-
• Lactate → Pyruvate
• Glyceraldehyde-3-P → 1,3 bisphosphoglycerate
• Pyruvate → Acetyl Co-A
• α-KG → Succinyl Co-A
• β- Hydroxy acyl Co-A → β-Ketoacyl Co-A
• Glutamate → α-KG
• NAD⁺ is a source of ADP-ribose for the ADP- ribosylation of proteins & Poly-ADP- ribosylation of nucleoproteins.

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• One hydrogen One electron:-
• In the oxidised form, nitrogen of nicotinamide residue has +ve charge, so oxidised form written as NAD⁺
• In reduction, NAD⁺ accepts one hydrogen atom fully & other hydrogen is ionized, only the electron is accepted

2H → H + H⁺ + e^-

NAD⁺ + 2H → NADH + H⁺

• NAD⁺ accepts one H and one electron to form NADH
• H⁺ atom is released into the surrounding medium
• During oxidation reaction is reversed.

• One NADH oxidised in respiratory chain to generate 2 ½ ATPs
• But NADPH is used almost exclusively for reductive biosynthetic reactions.

• NADPH generating reactions:-
• Glucose- 6- P → 6- Phosphogluconolactone
• 6-Phosphogluconate → 3-keto 6-phosphogluconate
• Cytoplasmic isocitrate dehydrogenase
• Malic enzyme (Malate to Pyruvate)
• Some enzymes can use any of NAD⁺ & NADP⁺

e.g. Glutamate dehydrogenase

• NADPH utilizing reactions:-
• β keto acyl ACP → β hydroxy acyl ACP
• HMG CoA reductase → Mevalonate
• Met- hemoglobin → Hemoglobin
• Phenylalanine → Tyrosine

• Deficiency:-
• Pellagra:- Italian word means “rough skin”

Due to def. niacin & tryptophan

More in women

• Tryptophan metabolism inhibited by estrogen metabolites.
• Dermatitis, Diarrhoea, Dementia, Death
• Dermatitis:- Bright red erythema on feet, ankles

& face

• Increased pigmentation around neck k/a

Casal’s necklace.

Casal’s necklace

• Diarrhoea:-
• Mild to severe with blood and mucus.
• Wt. loss ,nausea & vomitting
• Dementia:-
• In chronic cases
• Delerium in acute pellagra
• Irritability, inability to concentrate & poor memory seen in mild cases
• Ataxia and spasticity also seen

• Niacin is synthesized from tryptophan.
• Quinolinate phosphoribosyl transferase (QPRT) is the rate limiting enzyme in the conversion of niacin to NAD⁺ .
• About 60 mg of tryptophan =1 mg of niacin.

• Causes of niacin deficiency:-
• Dietary def.:- Seen in people whose staple diet is maize (South & Central America)
• Present as bound form & is unavailable.
• Also seen when staple diet is sorghum (jowar or guinea corn) as in Central & Western India.
• Sorghum contains leucine in high quantities, which inhibits the QPRT enzyme, so niacin cannot be converted to NAD⁺ (Leucine Pellagra)

• Deficient synthesis:- Kynureninase, an important enzyme in the pathway of tryptophan, is pyridoxal phosphate dependent
• So, conversion of tryptophan to niacin is not possible in pyridoxal def..
• INH:- Anti-TB drug, Inhibits PLP formation , so block in conversion of tryptophan to NAD⁺
• Hartnup’s Disease:- Tryptophan absorption is defective from intestine
• Tryptophan is excreted in urine in large quantities, so lack of tryptophan & def. of nicotinamide

• Carcinoid syndrome:- Tumour utilizes major portion of tryptophan for synthesis of serotonin, so tryptophan is unavailable.
• Sources:- Dried yeast, liver, peanut, whole cereals, legumes , meat & fishes
• RDA:- 20 mg/day, Lactation:- additional 5 mg required
• Therapeutic uses of Niacin:-Nicotinic acid inhibits the flux of FFA from adipose tissues, so acetyl CoA pool is reduced, hence Serum cholesterol is lowered.

• Toxicity of Niacin:- When given orally or parenterally produce transient vasodilatation of cutaneous vessels & histamine release
• Itching, burning, tingling
• Nicotinic acid in excess of 50 mg/day leads to liver damage.

Vitamin B₆

• Co- enzyme form:- Related to pyridine derivative. Pyridoxine(Alcohol), Pyridoxal (Aldehyde), Pyridoxamine.
• Active form is PLP
• Synthesized by pyridoxal kinase, utilizing ATP
• Main supply in form of pyridoxine which can be readily converted to pyridoxal & pyridoxine.

• Functions of PLP:-acts as co-enzyme for amino acid metabolism.
• Transamination:- Catalysed by transaminase

Alanine + α-KG → Pyruvate + Glutamic acid

• Decarboxylation:- All decarboxylation of amino acids require PLP as co- enzyme.
• Glutamate → GABA
• Histidine → Histamine
• 5- Hydroxy tryptophan → Serotonin
• Cysteine→ Taurine
• Serine → Etahnol amine

• Sulfur containing amino acid:- PLP plays an important role in methionine & cysteine metabolism

Homocysteine + serine→ Cystathionine

(enzyme cystathionine synthase)

Cystathionine → Homoserine + cysteine

(enzyme cystathionase)

• Both reactions require PLP, so in Vitamin B₆ def. homocysteine increased.
• MI

• Glycogenolysis:-
• Glycogen → Glucose-1 Phosphate (enzyme phosphorylase ) requires PLP.
• More than 70 % of total PLP content of the body is in muscles, where it is a part of the phosphorylase enzyme.

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• Heme synthesis:- ALA synthase require PLP

So in Vitamin B₆ def. anemia may be seen.

• Production of Niacin:- PLP is required for the synthesis of niacin from tryptophan.
• 3- hydroxy kynurenine → 3- hydroxy anthranillic acid (enzyme kynureninase).
• Kynureninase- PLP dependent enzyme.

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• Deficiency:-
• Neurological manifestation:- Serotonin, Epinephrine, Noradrenaline and GABA are not produced properly.
• In children B6 deficiency leads to convulsion due to decrease formation of GABA.
• PLP involved in synthesis of Sphingolipids, so B6 deficiency leads to demyelination of nerves and consequent peripheral neuritis.

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• Dermatological:-
• B6 deficiency affects tryptophan metabolism, since niacin is produced from tryptophan, B6 deficiency leads to niacin deficiency (Pellagra).
• Hematological:-
• Hypochromic microcytic anemia due to inhibion of heme biosynthesis.
• Impaired antibody formation is also reported.

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• Metabolic disorders which respond to B6 therapy are:- Xanthurenic aciduria, homocystinuria.
• Assay of vitamin B6:-
• By the activation of erythrocytes transaminases by addition of PLP in the reaction mixture.

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• Sources:-
• Yeast, wheat germs, cereals, legumes (pulses), oil seeds, egg, milk, meat, fish and green leafy vegetables.
• RDA:- Related to protein intake, not to calorie intake. 1-2 mg/day
• Pregnancy and lactation- 2.5 mg/day.

• Effects of drug on vitamin B6:-
• INH:- Inhibits pyridoxal kinase, reduces formation of PLP and causes B₆ deficiency.
• Cycloserine:- B₆ antagonist.
• Oral contraceptives:- Mild B₆ deficiency seen in women taking OC pills.
• Ethanol:- It is converted to acetaldehyde, which inactivates PLP, so B₆ deficiency neuritis is common in alcoholics.

• Toxicity:- >100 mg/day leads to sensory neuropathy.
• Imbalance, numbness, muscle weakness and nerve damage.
• TPP is involved with carbohydrate metabolism.
• PLP is involved in protein metabolism.

Pantothenic acid

• “Pantos”:- means everywhere, widely distributed in nature.
• Structure:- contains beta alanine and D- pantoic acid in amide likage.
• Co-enzyme activity of Pantothenic acid-
• The beta mercaptoethanol amine contains one thiol or sulfhydryl group.
• It is the active site where acyl groups are carried.

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• Therefore coenzyme A is sometimes abbreviated as CoA-SH to denote this active site.
• The thio ester bond in acyl-CoA is a high energy bond.
• Acetyl CoA + Choline → Acetyl choline + CoA (enzyme- acetyl choline synthase).

• Acyl groups are also accepted by the CoA molecule during the metabolism of other substrate.
• Pyruvate + CoA + NAD⁺ → Acetyl CoA + CO2 + NADH (enzyme pyruvate dehydrogenase)
• Important CoA derivatives:-
• Acetyl CoA, Succinyl CoA, HMG CoA, Acyl CoA.
• CoA is an important component of fatty acid synthase complex.
• ACP (acyl carrier protein) also contains pantothenic acid.

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• Deficiency:-
• Gopalan’s Burning foot syndrome:- manifested as paresthesia in lower extremities, staggering gait due to impaired co-ordination and sleep disturbances.

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Gopalan’s burning foot syndrome

• Sources:-
• Widely distributed in plants and animals.
• Synthesized by normal bacterial intestinal flora, so deficiency is rare.
• Yeast, liver and eggs.
• RDA:- 10 mg/day.

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