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

Adenyl cobamide (commonly known as pseudovitamin B) is synthesized by intestinal bacteria or ingested from edible cyanobacteria. The effect of pseudovitamin B 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 on the activities of the mammalian vitamin B-dependent enzymes methionine synthase and methylmalonyl-CoA mutase in cultured mammalian COS-7 cells to determine whether pseudovitamin B functions as an inhibitor or a cofactor of these enzymes. Although the hydoroxo form of pseudovitamin B functions as a coenzyme for methionine synthase in cultured cells, pseudovitamin B does not activate the translation of methionine synthase, unlike the hydroxo form of vitamin B does. In the second enzymatic reaction, the adenosyl form of pseudovitamin B 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 might inhibit transcobalamin II-mediated absorption of a physiological trace concentration of vitamin B present in the medium.