Vitramins_and_coenzymes.docx

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

VITAMINS Vitamins: Vitamins are naturally occurring organic micronutrients required for normal growth and development of living organisms. They are accessory, indispensable food factors and their deficiency leads to disorders. Some of the vitamins are lipophilic in nature and are usually present in the lipids of natural foods. They are called fat soluble vitamins and include vitamins A, D, E and K. Hydrophilic vitamins, called water soluble vitamins include vitamins of B complex group and (vitamin C). Classification: Vitamins are classified into two groups based on their solubility. A. Fat soluble vitamins: These are the lipophilic vitamins usually found in the lipids of natural foods. It includes; 1. 2. 3. 4.

Vitamin A – Retinol and dehydroretinol Vitamin D – Calciferol, Ergocalciferol and Cholecalciferol Vitamin E – Tocopherols Vitamin K – Phylloquinone, Menaquinone etc.

B. Water soluble vitamins: These are the hydrophilic vitamins and include vitamins of B complex group and vitamin C (Ascorbic acid). B complex group includes; 1. Thiamine (Vitamin B1) 2. Riboflavin (Vitamin B2) 3. Niacin (Vitamin B3) 4. Pyridoxin (Vitamin B6) 5. Pantothenic acid (Vitamin B5) 6. Cyanocobalamine (Vitamin B12) 7. Lipoic acid 8. Biotin 9. Folic acid 10. Inositol 11. Para amino benzoic acid 12. Choline Vitamin A: Synonyms: Antixerophthalmic factor Dietary sources: Fish liver oils, liver, milk, egg and cheese contain preformed vitamin A. The plant sources of vitamin A contain precursors of vitamin A called carotenes. β carotene is a dimer of vitamin A and gives two molecules of vitamin A on cleavage. Fruits like carrot, sweet potato, apricot, yellow peaches and green leafy vegetables are rich in carotenes. Chemistry: Vitamin A is a complex primary alcohol. It is a polyisoprenoid compound with a β inone (cyclohexenyl) ring. It occurs in two forms ie. Retinol called vitamin A1 and dehydroretinol called vitamin A2. H3C

CH3

CH3

CH3

H3C

CH3

CH3

CH 2OH

CH3

CH 2OH

CH3

Retinol (Vitamin A 1)

CH3

Dehydroretinol (Vitamin A 2)

Dehydroretinol contains an additional double bond in the β inone ring. The aldehyde form of retinol is called retinal while its acid form is called retinoic acid. 1

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

Biological role: 1. 2. 3. 4. 5. 6.

Vitamin A is involved in formation of certain pigments required for dim light vision. It plays important role in biosynthesis of certain glycoproteins and muco-polysaccharides. It is also involved in regulation of growth and development of cells. It is also involved in mobilization of iron from liver to form hemoglobin. It protects cells from infections and cancer. It plays important role in normal development of epithelial tissue of salivary gland, tongue, respiratory tract, genitourinary tract, eyes etc.

Deficiency disorder: Depending on age, the daily dietary requirement of vitamin A varies between 1500 to 5000 IU (1 IU of vitamin A is equivalent to 0.3 μg of retinol). Inadequate availability of vitamin A in diet leads to following deficiency states. 1. Deficiency of vitamin A leads to Nyctalopia (Night blindness), Xerophthalmia and Keratomalacia. 2. It causes follicular hyperkeratosis of skin and mucous membrane (follicular conjunctivitis). 3. Prolonged deficiency of vitamin A leads to keratinizing metaplasia of epithelium of nose, respiratory mucosa, esophagus and genitourinary tract. 4. It also leads to formation of urinary calculi (Urolithiasis) Xerophthalmia: Xeropthalmia is a disorder of deficiency of vitamin A. It is characterized by thickening of bulbar conjunctiva and loss of its transparency with yellowish pigmentation. The keratinisation of ocular tissue ultimately leads to blindness. Keratomalacia: Keratomalacia is a disorder of deficiency of vitamin A. It is characterized by softening of the cornea with ulceration and necrosis. Due to keratinisation of the epithelium of cornea, the vision becomes defective. Nyctalopia (Night blindness): Nyctalopia is a disorder of deficiency of vitamin A. It is one of the earliest signals of deficiency of vitamin A. It is characterized by inability to see in dim light. Continued supply of retinol is essential for formation of rhodopsin which is involved in dim light vision. The deficiency of vitamin A impairs biosynthesis of rhodopsin and thus results in night blindness. Vision cycle and role of vitamin A: Retina of eye contains two types of cells ie. Cone cells which are responsible for bright light vision and rod cells which are responsible for dim light vision. The rod cells contain a pigment – Rhodopsin which contains 11- cis retinal (retinene) coupled with a protein opsin. This pigment is essential for dim light vision. When light falls on rhodopsin, it causes isomerisation and splitting of rhodopsin into opsin and all trans retinal. This photolysis of rhodopsin initiates vision signal from rod cells. The all trans retinal formed is inactive. It is reduced to all trans retinol by the enzyme Retinene reductase and NADPH. The all trans retinol enters into blood stream and in liver it is isomerized to 11-cis retinol by the enzyme Retinol isomerase. The 11-cis retinol from blood stream enters in to retina of eye where it is oxidized to 11-cis retinal by the enzyme Retinene reductase and NAD+. The 11-cis retinal combines with protein opsin to form rhodopsin. Thus the cycle is repeated to help normal dim light vision.

2

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

Vitamin D: Synonyms: Antirachetic factor, calciferol Dietary sources: Richest source of vitamin D include liver oils of fishes like cod fish, halibut, eggs, cheese, etc. Chemistry: Vitamin D is a group of closely related steroids (D1 to D5) of which Vitamin D2 and D3 are physiologically active. 7-dehydrocalciferol in animals (skin) and ergosterol in plants are provitamins of vitamin D3 and D2 respectively which produce the vitamins on exposure to UV light. H3C CH3

H3C

CH3

CH3 CH3

CH3

CH2

CH2

HO

CH3

Cholecalciferol (Vitamin D 2)

Ergocalciferol (Vitamin D 3) HO

Biological role: 1. 2. 3. 4. 5.

Vitamin D plays important role in intestinal absorption of calcium and phosphate ions. It promotes mineralisation of bone and helps in growth and development of bones. It plays important role in citrate metabolism in body. It increases renal excretion of phosphate ions. It increases activity of Phytase enzyme in intestine.

Deficiency disorders: Deficiency of vitamin D in children leads to rickets and osteomalacia in adult. Rickets: Rickets is a disorder of deficiency of vitamin D in children. It is characterized by bow legs, knock knee, swollen joints and a pigeon breast appearance. It is due to decreased calcification of growing bones. It may also be due to calcium deficiency. For treatment of Rickets external supplementation of vitamin D and calcium compounds is done. Osteomalacia: Osteomalacia is a disorder of deficiency of vitamin D in adult. It is characterized by softening and deformation of bones of pelvic girdle, ribs and femora. It is painful condition 3

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

generally found in women during pregnancy and lactation. For treatment of Osteomalacia vitamin D and calcium compounds are given as external supplements. Vitamin E: Synonyms: Antisterility factor Dietary sources: Good dietary source of vitamin E include eggs, milk, meat, corn oil, liver, fish etc. Chemistry: Vitamin E refers to a group of isoprenoid substituted 6 hydroxy chroman derivatives called tocopherols. Four tocopherols ie α, β, γ and δ have been isolated of which α tocopherol is the most abundant and potent tocopherol present in nature. CH3 H3C

O

CH3

CH3

(CH 2)3

CH

CH3 (CH 2)3

CH

CH3 (CH 2)3

CH

CH3

HO CH3

a Tocopherol

Biological role: 1. Vitamin E acts as natural antioxidant in body. It is the first line of defence against peroxidation of polyunsaturated fatty acids present in phospholipids of cellular and subcellular membranes. 2. Vitamin E and selenium act synergistically and reduce each others requirement in body. 3. Vitamin E prevents hepatic necrosis produced by lack of sulphur containing amino acids in dietary proteins. 4. It acts as a co factor in electron transport system between cytochrome B and C. Deficiency disorders: Daily dietary requirement of vitamin E is 25 to 30 mg. Inadequate supply of vitamin E or malnutrition lead to deficiency of vitamin E. Deficiency of vitamin E causes;         

Muscle weakness Muscular dystrophy Fragility of erythrocytes Mild anemia Hepatic necrosis Neurological disorders Nocturnal muscle cramps Fibrocytic breast disease and Atherosclerosis

Vitamin K: Synonym: Coagulation vitamin Dietary sources: Green leafy vegetables like spinach, cabbage, cauliflower, and cereals are rich sources of vitamin K. Animal foods like liver, milk, egg and fish also contain vitamin K. The bacterial flora of intestine also produce vitamin K2. Chemistry: Vitamin K refers to a group of naturally occurring naphthaquinones and their synthetic analogues. Two natural naphthaquinones Phylloquinone called vitamin K1 and Menaquinone called vitamin K2 have been isolated. A synthetic analogue menadione called vitamin K3 is also used clinically.

4

Biochemistry and Clinical Pathology

Vitamins and Coenzymes O

CH3

CH3 CH2

CH2 (CH 2 CH2

CH C

CH CH 2)3

H

Phylloquinone (Vitamin K1 )

CH3 O

O O

CH3

CH3 CH2 (CH C

CH

CH 2)5

C

CH3 CH3

Menaquinone (Vitamin K 2)

CH3

O Menadione (Vitamin K 3)

O

Biological role: 1. 2. 3. 4. 5.

Vitamin K catalyses biosynthesis of prothrombin in liver. It regulates biosynthesis of clotting factors VII, IX and X. It reduces the prothrombin time It is also involved in oxidative phosphorylation in animal tissues. It is also involved in maintenance of normal levels of clotting factors II, VII, IX and X which are synthesised as inactive precursors in liver.

Deficiency disorders: Pancreatic dysfunction, biliary diseases or intestinal disorders causing fat malabsorption. Chemosterlization of bowel lead to deficiency of vitamin K. Deficiency of vitamin K leads to lowering of prothrombin level and increase in clotting time leading to hemorrhagic conditions. In newborns, deficiency of vitamin K causes hemorrhagic diseases. Thiamine (Vitamin B1) Synonyms: Aneurine, Anti Beri Beri Factor Dietary sources: Rice polishings, Wheat germ, cereals, pulses, milk, eggs and meat etc. are good dietary sources of thiamin. Chemistry: Thiamin is a water soluble, basic derivative of amino pyrimidine. It contains a substituted amino pyrimidine ring linked to a substituted thiazole ring through a methylene bridge. NH2 Cl

CH3

+

N H3C

-

N

CH 2CH 2OH S

N

Biological role: 1. Thiamine plays important role in normal growth and development of body. 2. It is essential for normal functioning of nerves. 3. The active form of Thiamin is thiamine pyrophosphate (TPP) which is a co enzyme of various aldehyde transfer reactions. 4. TPP is involved in oxidative decarboxylation of certain important intermediates in carbohydrate metabolism. 5. TPP acts as a coenzyme for Pyruvate dehydrogenase enzyme which converts pyruvic acid to acetyl Coenztme A (Acetyl CoA) 6. TPP also acts as coenzyme for Transketolase enzyme in HMP shunt pathway. Deficiency disorders: Daily dietary requirement of thiamin is between 1.2 to 1.7 mg for adults. It is 5

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

obtained from diet and also produced by the bacterial flora of intestine. Deficiency of thiamine leads to Beri Beri. Beri Beri: Beri Beri is a disorder of deficiency of thiamin. There are three types of Beri Beri. 1. Wet Beri Beri: It is characterized by edema of legs and face with anorexia and dyspepsia. The calf muscles are tense, swollen and tender. there is palpitation and precordial pain and there is decrease in volume of urine excreted. 2. Dry Beri Beri: The main characteristic of dry Beri Beri is polyneuropathy. The muscles become weak and the person becomes unable to walk. 3. Infantile Beri Beri: It is acute and fatal thiamin deficiency state occuring in breast fed infants. The symptoms are restlessness, excessive crying, puffiness and cyanosis and decreased urine excretion. The infant generally dies within 24 to 48 hrs. Riboflavin (Vitamin B2) synonyms: Lactoflavin Dietary sources: Milk, liver, kidney, hear, fish, cereals, roots and germinating barley are good dietary sources of riboflavin. Chemistry: Riboflavin is an isoxazoline derivative substituted with sugar alcohol ribitol. It occurs as a water soluble yellow green pigment stable to heat but affected by light. H2C

OH

OH

OH

C

C

C

H

H H3C

N

H3C

N

N

CH 2OH

H O

NH O

Biological role: 1. It is involved in regulation of some hormones of carbohydrate metabolism. 2. In retina, it is involved in stimulation of optic nerve. 3. Active forms of riboflavin are Flavin Mono Neucleotide (FMN) and Flavin Adenine Dinucleotide (FAD). They serve as prosthetic groups of oxidation reduction enzymes. FMN is a constituent of cytochrome c reductasea and L amino acid dehydrogenase while FAD is constituent of Xanthine oxidase, D amino acid dehydrogenase, glycine oxidase, acetyl CoA dehydrogenase etc. 4. FMN and FAD serve as components of respiratory chain. Deficiency disorders: Dietary requirement of riboflavin is 1.2 to 1.7mg daily in adults. Its deficiency leads to following disorders.      

Angular stomatitis (Fissures at angles of mouth) Cheilosis (red epithelium at line of closure of lips) Glossitis (inflammation of toungue) Dyssebacia (appearance of enlarged follicles around the nose, plugged with dry sebacious material scrotal dermatitis vascularization of cornea of eye causing lacrimation of photophobia.

Niacin (Vitamin B3)) Synonyms: Nicotinic acid, pellagra preventive factor 6

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

Dietary sources: Niacin is abundantly present in nature. Rich sources of niacin are rice polishing, yeast etc. milk, meat, eggs, tomato and green leafy vegetables also contain niacin. Chemistry: Niacin is pyridine 3 - carboxylic acid. In tissues it occurs as its amide ie. nicotnamide. COOH

CONH 2

N Nicotinic acid

N Nicotinamide

Biological role: 1. The biologically active forms of niacin, ie. NIcotinamide Adenine Dinucleotide (NAD) and NIcotinamide Adenine Dinucleotide Phosphate (NADP), act as coenzyme for several Oxidoreductase enzymes. 2. NAD is a component of respiratory chain. 3. Niacin is involved in regulation of biosynthesis of cholesterol containing lipoproteins. Deficiency disorders: Niacin requirement in adults is 10-20mg per day. Deficiency of niacin leads to Pellagra. Pellagra: Pellagra is a disorder of deficiency of niacin. It is characterized by; Dermatitis: Skin of face shows erythema similar to severe sunburn. Skin of neck, face, knees and breast shows thickening, roughening and dryness. Diarrhea: The person passes watery stools with nausea and vomiting. Dementia: forgetfulness with insomnia, depression, anxiety, headache etc. Pyridoxine (Vitamin B6) Dietary sources: Yeast and rice polishing are rich sources of pyridoxine. Milk, eggs, meat, liver and green leafy vegetables also provide pyridoxine. Chemistry: Vitamin B6 consists of three closely related naturally occurring pyridine derivatives and their phosphates. They are pyridoxine, pyridoxal and pyridoxamine. They have equal vitamin activity. CHO

CH 2OH CH 2OH

HO

H3C

N Pyridoxine

CH 2NH 2 CH 2OH

HO

H3C

N Pyridoxal

HO

H3C

CH 2OH

N Pyridoxamine

Biological functions: 1. Pyridoxine is required for normal growth of infants. 2. It is important role in heme biosynthesis. 3. Active form of pyridoxine is pyridoxal phosphate which is the major coenzyme for activity of Decarboxylase, Deaminase, Phosphorylase and Transulphurase enzymes. 4. It is required as conenzyme for the formation of neurotransmitters γ – amino butyric acid (GABA) and serotonin in brain. Deficiency disorders: Deficiency of pyridoxine leads to;   

Impaired growth Acrodynia (typical dermatitis) Edema of connective tissue layer of skin 7

Biochemistry and Clinical Pathology

  

Vitamins and Coenzymes

Convulsions Irritability Hypochromic microcytic anemia

Pantothenic acid (Vitamin B5) Dietary sources: Pantothenic acid is abundantly present in yeast, liver, rice polishing, milk, eggs and green leafy vegetables etc. Chemistry: Pantothenic acid consists of β alanine and pantoic acid connected by a peptide bond. O HO

C

O

OH

CH3

C

CH

C

CH2 CH2 NH

CH2OH

CH3

Pantothenic acid

Biological role: 1. Pantothenic acid is required for growth of infants. 2. It is a constituent of coenzyme A (CoA) which is required for; a. Activation of acetyl to acetyl CoA b. Formation of succinyl CoA in TCA cycle c. Activation of fatty acids for their oxidation d. For synthesis of fatty acids and cholesterol Deficiency disorders: Deficiency of Pantothenic acid leads to;       

Inadequate growth Nausea and vomiting Certain gastro intestinal disorders Anemia Fatty liver Loss of hair Burning foot syndrome etc.

Folic acid (Vitamin Bc) Synonyms: Folacin, Pteroyl glutamic acid Dietary sources: Yeast, liver and leafy vegetables like cauliflower, milk and fruits are good dietary sources of folic acid. Chemistry: Folic acid consists of pteridine, para amino benzoic acid (PABA) and glutamic acid. In liver it is present as pentaglutamate. O

COOH CH

OH

NH N

N H3C

NH N

N

CH2 H2C

COOH

Folic acid

Biological role: 1. Folic acid is essential for growth and cell division. 2. The active forms of folic acid, dihydro folic acid (DHF) and tetrahydro folic acid (THF) are involved in transfer of single carbon moieties (formyl, formate, methyl, hydroxymethyl etc.). 3. It is involved in metabolism of histidine. 4. It plays important role in formation of; 8

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

a. methionine from homocystein. b. serine from glycine c. thymine from uracil Deficiency disorders: Deficiency of folic acid leads to;     

Megaloblastic anemia Leucopenia Glossitis Gastrointstinal tract disorders Infertilty and inadequate lactation in females

Cyanocobalamine (Vitamin B12) Synonyms: Antipernicious anaemia factor Dietary sources: Liver, kidney, meat, fish and eggs are rich sources of cyanocobalamine. Chemistry: Cyanocobalamine consists of a central portion made up of four reduced and extensively substituted pyrrol rings connected through methylidine bridge. This central portion is called corrin ring system. A cobalt atom is present at the centre of the corrin ring system. Below the corrin ring system there is a substituted benzimidazole riboside residue connected at one end to the central cobalt atom and at the other end through riboside moiety to the side chain of corrin ring system. A cyano group is coordinately bound to the central cobalt atom. Biological role: The active form of cyanocobalamine is cobamide which acts as co enzyme in isomerization of methyl malonyl CoA and in the combined conversion of homocystein to methionine and methyl tetrahydrofolate to tetrahydrofolate. By this reaction stores of methionine are maintained in the body and tetrahydrofolate is made available to participate in purine and pyrimidine synthesis. Deficiency disorders: Deficiency of Vitamin B121eads to Prenicious anaemia. Biotin Synonyms: Anti egg white injury factor Dietary sources: Eggs, liver, kidney, yeast, milk, fruits and vegetables are good dietary sources of biotin. Chemistry: Biotin is sulphur containing imidazole derivative. O HN

NH

S

(CH 2)4

COOH

Biotin

Biological role: 1. Biotin acts as co carboxylase. 2. It is required in the fixation of CO2 in the conversion of pyruvate to oxalo acetate, acetyl CoA to malonyl CoA, propionate to succinate and in purine synthesis. Deficiency disorders: Biotin deficiency leads to egg white injury. 9

Biochemistry and Clinical Pathology

Vitamins and Coenzymes

Egg white injury: Consumption of raw egg causes deficiency of biotin. Egg white contains a protein Avidin which combines very tightly with biotin and prevents its absorption. The symptoms of egg white injury are;      

Retarded growth Loss of hair Depression Hallucination Muscle pain Dermatitis etc.

Lipoic acid Synonyms: Thioctic acid Dietary sources: Lipoic acid is widely distributed in natural foods. Chemistry: Lipoic acid is a water soluble sulphur containing fatty acid. Chemically, it is 6, 8 dithio octanoic acid. It exists in oxidized and reduced forms.

Biological role: Lipoic acid acts as coenzyme in oxidative decarboxylation of α keto acids like pyruvate and α keto glutarate. Deficiency disorders: No deficiency symptoms have been established for lipoic acid. Vitamin C Synonyms: L - Ascorbic acid, Antiscorbutic agent Dietary sources: Amla is the richest source of vitamin C while citrous fruits, tomato, lemon, adrenal cortex etc. are also rich in vitamin C. grapes, apple, banana and green leafy vegetables also provide vitamin C. Chemistry: Ascorbic acid is an enediol lactone resembling hexose sugar L-gulose. It is a strong reducing agent and readily gets oxidized to dehydro ascorbic acid (ascorbone). Both the reduced and oxidized forms are biologically active. On heating (cooking) it gets decomposed. OH

OH O

HO

O

HC

O

O

CH 2OH

O

OH Ascorbic acid

HC

CH 2OH

O

Dehydro ascorbic acid

Biological role: 1. Ascorbic acid is a strong reducing agent. It is involved in biochemical oxidation reduction reactions of cells. 2. It is involved in biosynthesis of hydroxyproline from proline. Hydroxy proline is an important amino acid of collagen. 3. In adrenal cortex, it is involved in hydroxylation of steroids. 10

Biochemistry and Clinical Pathology

4. 5. 6. 7. 8.

Vitamins and Coenzymes

It is also involved in biosynthesis of bile salts. It is also involved in metabolism of tyrosine, phenylalanine and tryptophan. It is also involved in conversion of folic acid to folinic acid. It plays important role in absorption and mobilisation of iron. It plays important role in maintaining normal intracellular material of cartilage, dentine and bone.

Deficiency disorders: Daily dietary requirement of ascorbic acid is 50 – 70 mg. Prolonged deficiency of ascorbic acid leads to scurvy. Scurvy: Scurvy is a deficiency disorder of vitamin C. Daily dietary requirement of ascorbic acid is 50– 70 mg. Prolonged deficiency of ascorbic acid leads to scurvy. Scurvy is characterised by;         

Internal hemorrhage Swelling, sponginess, tenderness and bleeding of gums Loosening of teeth Poor healing of wounds Swelling of ends of long bones and pain on movement Easy fracturability of bones Anemia Susceptibility to infection General weakness.

11