Vitamin B-12 and methionine synthesis: A critical review. Is nature's most beautiful cofactor(*) misunderstood?

The mechanism by which Vitamin B-12 prevents demyelination of nerve tissue is still not known. The evidence indicates that the critical site of B12 function in nerve tissue is in the enzyme, methionine synthase, in a system which requires S-adenosylmethionine. In recent years it has been recognized that S-adenosylmethionine gives rise to the deoxyadenosyl radical which catalyzes many reactions including the rearrangement of lysine to beta-lysine. Evidence is reviewed which suggests that there is an analogy between the two systems and that S-adenosyl methionine may catalyze a rearrangement of homocysteine on methionine synthase giving rise to iso- or beta-methionine. The rearranged product is readily degraded to CH3-SH, providing a mechanism for removing toxic homocysteine.