Cobalamins hydroxycobalamin

B03B A Vitamin B 2 (Cyanocobalamin) The naturally occurring cobalamin has attached to the Cobalt-atom a desoxyadenyl residue, which is substituted by a cyanide group (by addition of KCN-solution), thus giving the modified natural product cyanocobalamin. Hydroxycobalamin occurs in the liver and can be produced by the effect of light on a slightly acidic solution of cyanocobalamin. 

Figure 12.1 Chemical structure of cobalamins. R group may be CN (cyanocobala-min), OH (hydroxycobalamin), H2O (aquacobalamin), CH3 (methylcobalamin) and 5 -deoxyadenosyl (adenosylcobalamin).

Humans receive cobalamins from food— mainly from meat and meat products, and to a lesser extent from milk and milk products. Cobalamins are found in food in five forms hydroxycobalamin, AdoCbl, MeCbl, suphitoco-balamin and cyanocobalamin. AdoCbl and hydroxycobalamin are the most frequent types, followed by MeCbl (Farquharson and Adams 1976). Nevertheless both AdoCbl and MeCbl are light-sensitive, with light evoking a photodissociation of the Co C bond. The naturally found hydroxycobalamin is the product of photolysis of the light-sensitive cobalamins (Farquharson and Adams 1976 Seetharam and Alpers 1982). 

There are five naturally occurring forms of cobalamins with methylcobalamin, adenosylcobalamin and hydroxycobalamin being the most frequently found forms. 

Inborn errors of cobalamin metabolism are rare cobalamin C/D deficiency impairs the synthesis of methyl or adenosyl cobalamin, and can give rise to combined homocysteinuria and methylmalonic aciduria causing severe neurologic symptoms and epilepsy. Therapy with hydroxycobalamine and betaine is rarely successful. 

Initial confusion in the nomenclature of the cyanocobalamin reflected the fact that the vitamin exists as hydroxycobalamin, methylcobalamin, and adenosylcobalamin. Given the fact that all of the above compounds have the biologic activity formally attributed to vitamin B12, the general term cobalamin is now used. 

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