There are only two forms of vitamin B12 that have biological activity as cofactors in enzyme reactions. These are adenosyl-B12 (Ado-B12) and methyl-B12 (Me-B12). In mammalian cells, B12 is required for only two key enzymatic reactions. On the one hand, Ado-B12 is a cofactor for L-methylmalonyl CoA mutase (MCM) that is involved in the isomerization of L-methylmalonyl-CoA (MM-CoA) to succinyl-CoA (1). The excess of MM-CoA is converted into methylmalonic acid (MMA). Vitamin B12 deficiency leads to enhanced conversion of MM-CoA into MMA and elevation of the latter compound in the blood. On the other hand, Me-B12 is a cofactor for methionine synthase (MS) that transfers a methyl group from 5-methylTHF to Hcy during the synthesis of methionine. This reaction occurs in the cytosol while the Ado-B12-mediated pathway takes place in the mitochondria. In case of B12 deficiency this reaction is inhibited and plasma Hcy will increase. Therefore, concentrations of MMA and total Hcy (tHcy) in plasma are markers for B12 status.
The synthetic forms of B12 are cyano-B12 (CN-B12) and hydroxyl-B12.
1. Stroinsky A, Schneider Z. Cobamide dependant enzymes. In: Schneider Z, Stroinsky A, eds. Comprehensive B-12. Printing house de Gruyter, Berlin, 1987:225-66.