Cobalamin-folate interrelations: a critical review.

. cobalamin . + homocysteine p H4PteGlu + methionine. In the absence of cobalamin the reaction is impaired, the methyl group remains on tetrahydrofolate (H4PteGlu), and a critical lack of free tetrahydrofolate develops. As a result, single-carbon units such as formate and methylene cannot be taken up because of lack of a folate acceptor and synthesis of purines requiring formate, and that of thymidine requiring methylene, is impaired. The thermodynamics of the reaction by which methylfolate is formed (reaction 2) strongly favors the formation of methylfolate and oxidation of the methyl group to methylene is considered unlikely.3