VITAMIN B12 METABOLISM IN ATAXIC NEUROPATHY

There are clinical similarities between W. African ataxic neuropathy and subacute combined degeneration but absolute 

Nevertheless, abnormalities of vitamin Bj2 metabolism have been sought because of experimental evidence that vitamin Bj2 and cyanide are linked metabolically, and because of clinical evidence of these metabolic abnormalities in other amblyopic syndromes [47, 48, 49, 50, 51, 52, 53, 54]. The evidence relating cyanide and vitamin Bj2 can be summarized as follows  

The cyano-radical is an integral but labile component of the cyanocobalamin molecule, and studies using C-labelled cyanide suggest that vitamin Bj2 may serve as an intermediary whereby the cyanide carbon can be incorporated into the 1-C metabolic pool [47]. The precise mechanism of this incorporation is unknown, but decyanase activity has been identified in liver [55] suggesting that the cyano-radical is removed in its entirety, with formation of hydroxocobalamin. On the other hand, it is possible that cyanocobalamin may be converted to methylcobalamin although this has never been specifically tested. 

In normal subjects, there has been demonstrated an inverse relationship between the concentrations of cyanide and total vitamin Bj2 inviting speculation on conditions that might prevail in vitamin B 12-deficient states [49]. 

With the refined chromatographic techniques now available for the identification of plasma cobalamins, it has been shown that two-thirds of normal subjects do not have any cyanocobalamin in plasma, and the remainder have only traces, most of the vitamin B12 being in the forms of hydroxocobdamin, methylcobalamin and 5 -deoxyadenosyl i2 ( co-enzyme B12 ). The low cyanocobalamin levels may reflect equilibrating mechanisms which prevent the toted conversion of vitamin B12 to cyanocobalamin which would be expected from the exposure to large eimounts of cyanide by tobacco smokers [56]. 

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