Biological Activity of Pseudovitamin B12on Cobalamin-Dependent Methylmalonyl-CoA Mutase and Methionine Synthase in Mammalian Cultured COS-7 Cells

Adenyl cobamide (commonly known as pseudovitamin B-12) is synthesized by intestinal bacteria or ingested from edible cyanobacteria. The effect of pseudovitamin B(12)on the activities of cobalamin-dependent enzymes in mammalian cells has not been studied well. This study was conducted to investigate the effects of pseudovitamin B(12)on the activities of the mammalian vitamin B-12-dependent enzymes methionine synthase and methylmalonyl-CoA mutase in cultured mammalian COS-7 cells to determine whether pseudovitamin B(12)functions as an inhibitor or a cofactor of these enzymes. Although the hydoroxo form of pseudovitamin B(12)functions as a coenzyme for methionine synthase in cultured cells, pseudovitamin B(12)does not activate the translation of methionine synthase, unlike the hydroxo form of vitamin B(12)does. In the second enzymatic reaction, the adenosyl form of pseudovitamin B(12)did not function as a coenzyme or an inhibitor of methylmalonyl-CoA mutase. Experiments on the cellular uptake were conducted with human transcobalamin II and suggested that treatment with a substantial amount of pseudovitamin B(12)might inhibit transcobalamin II-mediated absorption of a physiological trace concentration of vitamin B(12)present in the medium.