ENZYMATIC CONVERSION OF VITAMIN B12A TO ADENOSLY-B12 - EVIDENCE FOR EXISTENCE OF 2 SEPARATE REDUCING SYSTEMS

The reduction of vitamin B12a, (CoIII) to B12s (CoI) appears to be carried out by two separate enzyme systems in extracts of Clostridium tetanomorphum. The reduction of B12a to B12r (CoII) occurs at a relatively rapid rate and is catalyzed by a DPNH-dependent flavoprotein (B12a reductase) that utilizes PAD better than FMN. Trapping experiments suggest that B12a is not an intermediate in this reaction. The reduction of B12r to B12s, a reaction characterized by an unfavorable equilibrium, requires a second DPNH-dependent flavoprotein (B12r reductase) that is stimulated equally well by FAD or FMN. In addition to their different responses to added flavins, these systems are also distinguished by other characteristics: (a) B12a reductase is considerably more labile than the B12r reductase; and (b) Dithioerythritol can replace DPNH as the reductant for the B12r reductase but not for the B12a reductase. © 1969.