VITAMIN B5

A. History: Vitamin B5 refers to nicotinic acid and was named as pellagra preventive (PP) factor by an Austrian-American physician of the U. S. Public Health Service, Joseph Goldberger (ca 1920) because of its curing action on pellagra (After Goldberger's death, vitamin B5 was sometimes called vitamin G in his honour). The vitamin role on nicotinic acid was first recognized by Conrad Elvehjem and D. Wayne Woolley of Wisconsin University in 1937. As this vitamin has a curing action against blacktongue disease in dogs, it is also called as antiblacktongue factor. It was first isolated by Funk in 1911. Because the name `nicotinic acid' might mislead some people into thinking that tobacco is nutritious, nicotinic acid has been given the alternative official name niacin for public use.
B. Occurrence: Nicotinic acid is widely distributed in nature in plant and animal tissues mainly as its amide called nicotinamide (commercially called niacinamide, to avoid any misassociation with the alkaloid nicotine of tobacco). As dietary tryptophan can be converted, in restricted quantities, to niacin in the body, it can partially substitute for niacin, although other sources of vitamin B5 are necessary. Niacin is most abundantly found in yeast. Liver, lean pork, salmon, poultry and red meat are also good sources, but most cereals contain only small amounts of it. Most vegetables and fruits are poor sources of it. Milk and eggs, which contain very little or practically no niacin, are good pellagra-preventive foods because of their high content of tryptophan. Since a number of stable vegetable articles of diet are not particularly rich in nicotinamide, the vegetarian’s diet may be lacking in this vitamin. Nicotinamide, is present as a constituent in two pyridine nucleotide coenzymes namely NAD and NADP (previously called as DPN and TPN respectively). Since niacin is stable to heating and oxidation, there are only small losses in cooking. Like thiamine, most of vitamin B5 is lost in the milling process.
C. Structure: Niacin (C6H5O2N) is simplest of all the known vitamins. It is a pyridine derivative.
D. Properties: Niacin is a white crystalline substance. It is soluble in ethyl alcohol but is less
soluble in ether and benzene than nicotinamide. It is heat-stable. Nicotinamide, when pure, occurs
as white needle like crystals. It is soluble in water and is stable in air and heat.
E. Metabolism: The conversion of niacin to niacinamide takes place in the kidney and brain slices and also in the liver slices, if glutathione is present. Nicotinamide is synthesized by amidation of nicotinic acid adenine dinucleotide and subsequent degradation of NAD thus formed.
The niacin in man and other animals is derived from the amino acid tryptophan, which also cures pellagra. The conversion of tryptophan to nicotinic acid in the body takes place through a series of intermediate steps,
which are represented below :
Tryptophan → Kynurenine → 3-hydroxykynurenine 3-hydroxyanthranilic acid → ... → Quinolinic acid → Nicotinic acid: Nicotinamide undergoes methylation in mammalian liver to produce N′-methyl nicotinamide which is oxidized to give corresponding 6-pyridone. In many plant seeds nicotinic acid is, however, converted to trigonelline. The nicotinic acid and its amide both are necessary for the growth of various microorganisms. Pyridine-3-sulfonic acid and its amide both prevent such growth which can be resumed by the addition of these vitamins. The relationship of these growth inhibitors to the two vitamins is not much different with the relationship of p-aminobenzoic acid to sulfanilamide .

The two coenzyme forms of this vitamin, NAD and NADP, carry out 2 important functions in the tissues:
(a) Oxidation of alcohols, aldehydes, amino acids and hydroxy-carboxylic acids.
(b) Reduction of the flavin coenzymes.
F. Deficiency: A deficiency of niacin causes pellagra in man and blacktongue in dogs. Pellagra (of Italian origin, pellis = skin; agra = rough) is characterized by 3 “Ds”, namely dermatitis of the exposed parts, diarrhea and dementia. The early symptoms of pellagra are vague. Anorexia, lassitude, fatigue, burning sensations, numbness and dizziness may be prodromal symptoms. Their manifestation in children who have parasites or chronic disorders may be particularly severe. The most characteristic manifestations are the cutaneous ones, which may develop abruptly or insidiously and may be elicitated by irritants, esp., by intense light. They first appear as symmetric erythema of the exposed surfaces that may resemble sunburn. The lesions are usually sharply demarcated from the healthy skin around them, and their distribution may change very often. The lesions on the hands sometimes have the appearance of a glove (pellagrous glove), and similar demarcations are sometimes seen on the foot and leg (pellagrous boot) or around the neck (Casal necklace). The healed parts of the skin may remain pigmented. The cutaneous lesions are sometimes preceded by stomatitis, glossitis, vomiting or diarrhea. Swelling and redness of the tip of the tongue and its lateral margins may be followed by intense redness of the entire tongue and of the papillae and even ulceration. Nervous symptoms include depression, disorientation, insomnia and delirium. The histologic changes in the nervous system occur relatively late in the disease and consist of patchy areas of demyelinization and degeneration of ganglion cells.
The classic symptoms of pellagra are usually not pronounced in infants and children. Anorexia, irritability, anxiety and apathy are common in “pellagra families”. They may also have sore tongues and lips and the skin is usually dry and scaly. Diarrhea and constipation may alternate and a moderate secondary anemia may occur. Pellagral children often have symptoms characteristic of other nutritional deficiency diseases. As coffee (Coffea arabica) is particularly rich in niacin, the heavy coffee drinkers usually do not develop pellagra. Other factors like thiamine-deficiency also seem to be responsible for this disease. Incredible as it may seem, over 600 deaths were attributed to pellagra in 1948. Pellagra is greatly aggravated in persons kept on a corn diet (as natives of Africa) because corn is very much deficient of tryptophan.
The canine blacktongue disease leads to complete loss of appetite. The inner surfaces of the lips and cheeks develop pustules ; the pustules may also develop on the thorax and abdomen. Intensive salivation and bloody diarrhea are other symptoms.
G. Human requirements: The recommended daily allowance of nicotinic acid is between 8 and 15 mg for children, between 15 and 20 mg for men and between 13 and 15 mg for women. Pregnant and lactating mothers may require up to 20 mg daily.
H. Treatment : Children respond quickly to antipellagral therapy. A well-balanced diet should be augmented with 50-300 mg/day of niacin; 100 mg may be given intravenously in acute cases or in cases of poor intestinal absorption. The diet should be supplemented with other vitamins, especially with other members of B complex group. Sun exposure should be avoided during the active phase; the skin lesions may be covered by applying soothers. The diet of the cured pellagrin should be supervised continuously to prevent recurrence.

VITAMIN B6

A. History :The name vitamin B6 was suggested by Albert Szent-Györgyi (1934) to designatesubstances, other than thiamine and riboflavin, which cured a dermatitis (acrodynia) in rats. It was, henceforth, also named as adermin or antidermatitis factor. Vitamin B6 group includes 3 compounds : pyridoxine, pyridoxal and pyridoxamine. Pyridoxine was first isolated, in 1938, from yeast and liver. Later, Snell (1942) discovered the other two compounds.
B. Occurrence. The B6 vitamins are widely distributed in nature in plant and animal tissues. They are especially rich in cereals (wheat, rice), peas, turnip greens, brussels sprouts, carrots,potatoes, sweet potatoes, bananas, avocados, watermelons and yeasts. B6 vitamins are also found in egg yolk, salmon, chicken, fish, beaf, pork and liver. Pyridoxine is adequately available in human and cow’s milk. Pyridoxal (PAL) and pyridoxamine (PAM) also occur in nature as their coenzymes, namely, pyridoxal phosphate (PALP) and pyridoxamine phosphate (PAMP), respectively.
C. Structure. All the 3 forms of vitamin B6 are derivatives of pyridine, C5H5N and differ from each other in the nature of substituent at position 4 of the ring. All the 3 forms are readily interconvertible biologically.
D. Properties. Pyridoxine is a white crystalline substance and is soluble in water and alcohol and slightly so in fat solvents. It is sensitive to light and ultraviolet irradition. It is resistant to heat ( i.e., heat-stable) in both acidic and alkaline solutions but its two allies pyridoxal and pyridoxamine are destroyed at high temperatures ( i.e., heat-labile).
E. Metabolism. The various forms of vitamin B6 serve as growth factors to a number of bacteria. In addition, the 3 forms (pyridoxine, pyridoxal, pyridoxamine) are converted to pyridoxal- 5-phosphate, which acts as a coenzyme in various enzymic reactions involved in amino acid metabolism such as transamination, decarboxylation and racemization and in the metabolism of glycogen and fatty acids. It is also essential in the metabolism of hydroxy amino acids, sulfurcontaining amino acids and also tryptophan.Pyridoxal or its phosphate derivative also possibly acts as a carrier in the active transport of amino acids across cell membranes. Pyridoxine can be converted to either pyridoxal or pyridoxamine but neither of them can be changed to pyridoxine. All these three can be detected in the urine after ingestional though 4-pyridoxic acid is the most important excretion product quantitatively. It is for this reason that when administered in the human body, about 90% of pyridoxine is oxidized to pyridoxic acid and excreted in human urine in this form. B6 vitamins are also essential for the breakdown of kynurenine. When this does not happen, xanthurenic acid appears in the urine. In addition, adequate functioning of the nervous system depends on pyridoxine, deficiency of which leads to seizures and to peripheral neuropathy. Contents Pyridoxal phosphate (PALP) is the coenzyme for both glutamic decarboxylase and γ-aminobutyric acid transaminase; each is essential for normal brain metabolism. It participates in active transport of amino acids across cell membranes, chelates metals, and participates in the synthesis of arachidonic acid from linoleic acid. If it is lacking, glycine metabolism may lead to oxaluria. Normal metabolism of vitamin B6 in higher animals is inhibited by 4-deoxypyridoxine and isonicotinic acid hydrazide (= isoniazid). Isoniazid is noted for its curing properties against tuberculosis.
F. Deficiency. Vitamin B6 deficiency or apyridoxosis in rats leads to the development of acrodynia, a disease of dermatitis on ears, mouth and tail and accompanied by edema and scaliness of these structures. Dogs and chick develop anemia and nervous lesions in apyridoxosis.
In human infants, vitamin B6 deficiency results in convulsions, anemia, dermatitis and gastrointestinal disorders such as nausea and vomiting. However, this deficiency is rare. Moreover, tryptophan metabolism is also disturbed. In adults, the vitamin B6 deficiency is normally not found because the intestinal bacteria are capable of synthesizing vitamin B6. In B6-deficient anemia, the RBCs are microcytic and hyperchromic. There are increased serum iron concentrations, saturation of iron-binding protein, hemosiderin deposits in bone marrow and liver, and failure of iron utilization for hemoglobin synthesis. Diseases with malabsorption, such as celiac syndrome, may contribute to vitamin B6 deficiency. A syndrome resembling vitamin B6 deficiency, as observed in animals, has also been reported in man during the treatment of tuberculosis with high doses of the drug isoniazid . Only 2-3% of patients receiving conventional doses (2-3mg/kg) of isoniazid developed neuritis ; 40% of patients receiving high doses (20 mg/kg) developed neuropathy. The symptoms were alleviated by the administration of pyridoxine. Thus, 50 mg of pyridoxine per day completely prevented the development of neuritis. It is believed that isoniazid forms a hydrazone complex with pyridoxal, resulting in partial activation of the vitamin. Isoniazid, thus, is a potent antagonist of vitamin B6 .
G. Human requirements. The minimum dietary allowance of vitamin B6 is between 0.2 and
1.2 mg for infants and children and around 2.0 mg for men and women per day. During pregnancy and lactation, the recommended daily dose is 2.5 mg. Pyridoxine antagonists, such as isoniazid used in the treatment of tuberculosis, increase the requirements for pyridoxine as do pregnancy and drugs such as penicillamine, hydralazine and the oral progesterone-estrogen contraceptives.
H. Treatement : Balanced diets usually contain enough pyridoxine so that deficiency is rare. For convulsions due to pyridoxine deficiency, 100 mg of vitamin should be given intramuscularly. Excessive intake may cause sensory neuropathy.

VITAMIN B7

A. History: In 1935, Fritz Kögl, a Dutch biochemist, isolated in crystalline form from 250 kg of dried egg yolks about 1 mg of a ‘bios’ factor (growth promoting factor) necessary for yeast
and named it as “biotin”.Four years later, Szent-Györgyi et al conclusively proved that biotin is synonymous to the “antiegg white injury factor” which is responsible for the cure of egg white injury, induced in rats and other animals by feeding them with raw egg white. The raw egg white contains a biotin-antagonist protein, avidin, which combines with biotin in a firm linkage to form a compound that cannot be absorbed by the intestine and is therefore, excreted. It is also called as coenzyme R because it is a growth factor for the nitrogen-fixing bacterium, Rhizobium.

B. Occurrence: Biotin has a wide range of distribution both in the animal and the vegetable kingdoms. Yeast, liver, kidney, milk and molasses are among the richest sources ; peanuts and eggs have lesser amounts. Biotin occurs in nature usually in combined state as biocytin . It is a bound form of biotin, linked as a peptide with the amino acid lysine.
C. Structure: The structure of biotin (C10H16O3N2S) was worked out by Vincent du Vigneaud in 1942. Biotin has an unusual structure and consists of a fused imidazole and thiophene ring with a fatty acid side chain. Two forms of biotin can exist, allobiotin and epibiotin. Biotin is optically active. Only the (+) biotin is active; the DL-biotin is half as active as the naturally occurring biotin. The oxybiotin, in which S atom of biotin is replaced with an O atom, has some activity.Biotin and thiamine are the only sulfur-containing vitamins isolated to date.

D. Properties: Biotin crystallizes as long needles. It is soluble in water and ethyl alcohol but is insoluble in chloroform and ether.It is heat-stable and is resistant to both acids and alkalies. It has a melting point of 230°C.
E. Metabolism: This vitamin serves as a prosthetic group for many enzymes. These biotincontaining enzymes catalyze the fixation of CO2 into organic molecules, thus bringing about carboxylation. The carbon dioxide is carried as a carboxyl group attached to one of the ureidonitrogen atoms of biotin, forming N-carboxybiotin complex .They also bring about synthesis of fatty acids such as oleic acid.

F. Deficiency: In most animals including man, intestinal bacteria synthesize appreciable amounts of biotin. It is because of this reason that biotin-deficiency in human beings, fed on biotinfree diets, cannot be produced. However, biotin-deficiency may be induced by sterilization of intestine and by feeding with raw egg white. Avidin, the egg white protein, inactivates biotin by eliminating it from an otherwise complete diet. Such a deficiency in man leads to dermatitis, loss of hair, decrease in weight and edema. The lesions on skin appear with changes in posture and gait. These disorders may lead to death. Heating egg white destroys the avidin and prevents the so-called egg white injury. Brawny dermatitis, somnolence, hallucinations, and hyperesthesia with accumulation of organic acids are common. Other neurologic signs and defective immunity may occur.

G. Human requirements: The intestinal bacteria synthesize biotin in such appreciable amounts that the amount excreted in urine exceeds the intake. That is why the RDA for this vitamin has not been established. However, about 10 mg per day of biotin is sufficient for an adult.
H. Treatment : Parenteral solutions should contain biotin. Deficient patients respond to oral administration of 10 mg.

VITAMIN B9

VITAMIN B9

A. History Day, for the first time, showed the existence of this nutritional factor by demonstrating that yeast extract could cure cytopenia, a disease experimentally induced in monkeys. The potent factor was obtained from spinach leaf and this led to its nomenclature as folic acid, FA (foliumL = leaf). The official name of this vitamin is folacin. This is also known as liver Lactobacillus casei factor as it was isolated from liver and was shown as necessary for the growth of lactic acid bacteria. Hogan called this as vitamin Bc. However, a number of other compounds or factors , having similar or different biochemical functions but closely related to folic acid, were isolated from different sources. These are fermentation Lactobacillus casei factor, Streptococcus lactis R (SLR) factor, Bc conjugate and citrivorum factor, CF.

B. Occurrence: Folic acid and its derivatives (tri- and hepta-glutamyl peptides) are widely distributed in biological world. A few important sources are liver, kidney, tuna fish, salmon, yeast, wheat, dates and spinach. Root vegetables, sweet potatoes, rice, corn, tomatoes, bananas, pork and lamb contain little folid acid.

With improper cooking, folacin contents are destroyed, like thiamine.

C. Structure: A molecule of folic acid consists of 3 units : glutamic acid, p-aminobenzoic acid and a derivative of the heterocyclic fused-ring compound pterin. Its molecular formula is C19H19O6N7.
The various vitamins of B9 group differ from each other in the number of glutamic acid groups present ; the additional glutamic acid group being conjugated in peptide linkages. For example, folic acid contains one, fermentation Lactobacillus casei factor three and Bc conjugate seven glutamic acid groups. The conjugates (i.e., compounds having more than one glutamic acid groups in the molecule) are ineffective for some species as these species do not possess the enzyme conjugase which is necessary for the release of free vitamin. Citrivorum factor, however, differs from other vitamins of B9 group in the structure of one of the rings of the pterin moiety.
D. Properties: Folic acid is a yellow crystalline substance, slightly soluble in water but insoluble in fat solvents. It is stable to heat in alkaline or neutral solutions only. It is inactivated by sunlight.

E. Metabolism: The reduction products of folic acid act as coenzymes. An enzyme, folic reductase, reduces folic acid to dihydrofolic acid (DHFA or FH2), the latter compound is further reduced by dihydrofolic reductase to 5,6,7,8-tetrahydrofolic acid (THFA or FH4). The formation of FH4 from FA is associated with the oxidation of NADPH or NADH and requires the presence of ascorbic acid.
The vitamins of B9 group are involved in one-carbon metabolism in a way similar to the twocarbon metabolism in which CoA is involved. THFA acts as an acceptor of a one-carbon unit either from formate (in which case 5,6,7,8-tetrahydrofolic acid is formed) or from formaldehyde (in which case 5-hydroxymethyl-5,6,7,8-tetrahydrofolic acid is formed). THFA is also involved in the transfer of the methyl group and in the utilization of single carbons (formate) in the synthesis of serine, methionine, thymine, purines, choline and inosinic acid.
Folic acid, in conjunction with ascorbic acid, also appears to be related to tyrosine metabolism.

Citrivorum factor (CF) is 5 formyl derivative of tetrahydrofolic acid and is so named because it supports the growth of Leuconostoc citrivorum. CF is about one thousand times more potent biologically than folic acid. Its chemical name is folinic acid. During the conversion of FA to CF, vitamin B12 and ascorbic acid are also required. Citrivorum factor ( and also folic acid to a lesser degree) are concerned in the production of an agent that stimulates the formation of normal RBCs. It is interesting to note that the bacteria, which require PABA for growth, also utilize FA with almost equal ease. Recent studies have shown that folic acid provides protection against Alzheimer’s disease. Folic acid is, however, essential for lactation in rats and hatchability of eggs in chicks, turkeys and guinea pigs. Rats, dogs and probably man do not need folic acid because the intestinal bacteria synthesize sufficient quantity of this vitamin.

F. Deficiency: In chicks, a lack of this factor leads to anemia. Rats develop achromotrichia (failure in normal pigmentation of the hair). The monkeys show macrocytic anemia (anemia characterized by the presence of giant RBCs), leukopenia, diarrhea and edema (retention of water by skin tissues).

On a worldwide basis, deficiency of folic acid is believed to be the most common form of vitamin undernutrition. In man, the folic acid deficiency leads to megaloblastic anemia, glossitis and gastrointestinal disorders. Pregnant women and infants are also particularly vulnerable. Folic acid deficiency is a major feature of tropical sprue, in which there is a general deficiency in absorption of many nutrients from the small intestine.
Folic acid has been successfully used in the treatment of certain macrocytic anemias such as those developed in sprue and anemias of pellagra, pregnancy and infancy. However, the long-held hope that it would cure pernicious anemia (caused by avitaminosis B12) has not been held true since it fails to cure the neurological lesions of the disease.
G. Human requirements: The daily dietary allowance of folic acid is 0.1 mg for infants, 0.2 mg for children and 0.4 mg for adult men and women. Pregnant mothers may, however, requireup to 0.8 mg per day.

VITAMIN B12

A. History. In 1926, two American physicians, George Minot and George William Murphy discovered that patients suffering from pernicious anemia could be cured by feeding them with about half a pound of liver a day. This landmark in medicine brought them Nobel Prize in 1934. In 1929, Castle suggested that gastric juice contained a factor (intrinsic factor) that, together with a factor present in the food (extrinsic factor), is responsible for the cure of pernicious anemia. This anti-pernicious anemia factor (APA factor) was, later, isolated in crystalline form in 1948 independently by E. Lester Smith in England and by Edward Rickes and Karl Folkers in the United States. It was then named as vitamin B12 or cyanocobalamin. It is the last B-vitamin to be isolated and is also known as Factor X or L.L.D. factor. The coenzyme form of this vitamin (deoxyadenosyl cobalamin or cobamide coenzyme) was first isolated by Barker of California. Coenzyme B12 has been called a “biologic Grignard reagent”.

B. Occurrence. Vitamin B12 has been found only in animals ; the chief source is liver, although it is also present in milk, meat, eggs, fish, oysters and clams. Animal tissues contain it in varying amounts as shown in Table.
Under certain dietary conditions, vitamin B12 may be synthesized by the intestinal microorganisms. In general, cyanocobalamin is not present in plant foods except in Spirulina, a blue-green alga. However, it occurs in foods bound to proteins and is apparently split off by proteolytic enzymes.
Animals and plants are unable to synthesize this vitamin. Cyanocobalamin is unique in that it appears to be synthesized only by microorganisms especially anaerobic bacteria. However, a process of producing vitamin B12 from waste products has been developed, in 1977, by the department of Chemical Engineering of the Indian Institute of Technology, Chennai.
C. Structure. The structure of vitamin B12 of the most complex known, has been established, in 1957, by Dorothy Crowfoot Hodgkin (Nobel Laureate, 1964). Cyanocobalamin (C63H88O14N14 P Co) is a pigment alike to the tetrapyrrole ring structure of the porphyrins, e.g., chlorophyll and haem. A unique feature of this vitamin (and other related compounds) is the
presence, in its molecule, of an atom of a heavy metal cobalt in the trivalent state. No other cobaltcontaining organic compound has been found in nature. The cobalt atom is centrally-situated andis surrounded by a macrocyclic structure of 4 reduced pyrrole rings (A, B, C and D) collectively called as corrin. It may be noted from the structural formula that the 6 coordinate valences of the cobalt atom (Co2+ )are satisfied by the 4 nitrogens of the reduced tetrapyrrole, a nitrogen atom of 5, 6-dimethylbenzimidazole and a cyanide ion. Two of the pyrrole rings, namely A and D, are directly linked to each other and the corrin has lower degree of unsaturation with only 6 double bonds. The other two pyrrole rings, namely B and C, are joined through a single methene carbon. Another distinct feature of the vitamin B12 molecule is the presence of a loop of the isopropanol, phosphate, ribose and 5,6-dimethyl-benzimidazole in that order, the end of the loop being attached to the central cobalt atom. Many compounds with vitamin B12 activity have been isolated from natural sources. Cyanocobalamin is the most common form and is sometimes also written as vitamin B12a. In other forms, cyanide ion is replaced by other ions, e.g., by hydroxyl ion in hydroxocobalamin (also designated as vitamin B12b), by nitrite ion in nitrocobalamin (also designated as vitamin B12c) etc. The latter two, B12b and B12c can be converted to vitamin B12a by treatment with cyanide.The structure of vitamin B12 coenzyme ( = 5′-deoxyadenosyl cobalamin) is similar to that of cobalamin except that here the CN group is replaced by adenosine and the linking with cobalt atom taking place at 5′ carbon atom of the ribose of adenosine. Vitamin B12 coenzyme is the only known example of a carbon-metal bond in a biomolecule.is surrounded by a macrocyclic structure of 4 reduced pyrrole rings (A, B, C and D) collectively called as corrin. It may be noted from the structural formula that the 6 coordinate valences of the cobalt atom (Co2+ )are satisfied by the 4 nitrogens of the reduced tetrapyrrole, a nitrogen atom of 5, 6-dimethylbenzimidazole and a cyanide ion. Two of the pyrrole rings, namely A and D, are directly linked to each other and the corrin has lower degree of unsaturation with only 6 double bonds. The other two pyrrole rings, namely B and C, are joined through a single methene carbon. Another distinct feature of the vitamin B12 molecule is the presence of a loop of the isopropanol, phosphate, ribose and 5,6-dimethyl-benzimidazole in that order, the end of the loop being attached to the central cobalt atom. Many compounds with vitamin B12 activity have been isolated from natural sources. Cyanocobalamin is the most common form and is sometimes also written as vitamin B12a. In other forms, cyanide ion is replaced by other ions, e.g., by hydroxyl ion in hydroxocobalamin (also designated as vitamin B12b), by nitrite ion in nitrocobalamin (also designated as vitamin B12c) etc.The latter two, B12b and B12c can be converted to vitamin B12a by treatment with cyanide.The structure of vitamin B12 coenzyme ( = 5′-deoxyadenosyl cobalamin) is similar to that ofcobalamin except that here the CN group is replaced by adenosine and the linking with cobalt atom taking place at 5′ carbon atom of the ribose of adenosine . Vitamin B12 coenzyme is the only known example of a carbon-metal bond in a biomolecule.
D. Properties. Vitamin B12 (molecular weight, ca 1,500) is a deep red crystalline substance.It is soluble in water, alcohol and acetone but not in chloroform. It is levorotatory. It is stable to heat in neutral solutions but is destroyed by heat in acidic or alkaline solutions.

E. Metabolism. Vitamin B12 is converted to coenzyme B12 by extracts from microorganisms
supplemented with ATP.
Coenzyme B12 is associated with many
biochemical reactions :
(a) 1,2 shift of a hydrogen atom : Coenzyme B12 catalyzes 1,2 shift of a hydrogen atom from one carbon atom of the substrate to the next with a concomitant 2,1 (reverse) shift of some other group, e.g., hydroxyl, alkyl etc.
(b) Carrier of a methyl group : Coenzyme B12 also serves as a carrier of a methyl group,obtained from N5-methyltetrahydrofolate, to the appropriate acceptor molecule.In the reaction, a methyl group occupies the 5-deoxyadenosyl coordination position of coenzyme B12. Methylation of homocysteine to produce methionine is an example of such reaction.

(c) Isomerization of dicarboxylic acids : Coenzyme B12 is associated with isomerization of dicarboxylic acids, e.g., glutamic acid into β-methyl-aspartic acid.
(d) Dismutation of vicinal diols : Coenzyme 12 also catalyzes dismutation of vicinal diols to the corresponding aldehydes, e.g., propane-1,2-diol into propionaldehyde. Vitamin B12 is also needed for the biosynthesis of methyl groups from 1-carbon precursors and for the synthesis of thymidine and other deoxyribosides. It also functions in protein synthesis and in the activation of SH enzymes. Cyanocobalamin also affects myelin formation.
F. Deficiency: A nutritional deficiency of this vitamin is usually not observed on account of its ubiquitous ( = widespread) nature in foodstuffs. Thus, most cases of deficiency stem from failure to absorb the vitamin. However, deficiency may be observed in individuals who abstain from all animal products including milk and eggs, i.e., those who are strict vegetarians.The rare disease juvenile (or congenital) pernicious anemia springs up due to an inability to secrete gastric intrinsic factors. The symptoms of this disease become prominent at 9 months to 10 years of age. As the anemia becomes severe, irritability, anorexia and listlessness occur. The tongue is smooth, red and painful. Neurologic symptoms include ataxia, paresthesias, hyporeflexia, clonus, Babinski responses and coma. Consanguinity is common in parents of affected children and suggests Mendelian recessive inheritance. The juvenile disease differs from the typical disease in adults in that the stomach secretes acid normally and is histologically normal. This typical deficiency disease, adult pernicious anemia ( = anemia caused by failure of erythrocyte formation), is characterized by R.B.Cs. becoming abnormally large and fewer in number (1–3 million per cubic millimeter instead of the normal 4–5 million). The patient weakens, loses its weight and the nervous system is also gradually affected because there occurs demyelinization of the large nerve fibres of the spinal cord. All these changes ultimately lead to death.

G. Human requirements: The recommended daily allowance of vitamin B12 is 2 to 4 μg for children and 5 μg for men and women. Pregnant and lactating mothers require 8 μg and 6μg daily.
H. Treatment : The excessive secretion of methylmalonic acid in the urine is a reliable andsensitive index of vitamin B12 deficiency. The physiologic need for vitamin B12 is 1-5 μg/24 hr,and hematologic responses have been observed with these small doses. If there is evidence of neurologic involvement, 1 mg should be injected intramuscularly daily for a minimum of 2 weeks.Maintenance therapy is necessary throughout patient’s life; monthly intramuscular administration of 1 mg of vitamin B12 is sufficient.