5-Amino[4-13C]laevulinic acid has been synthesised for enzymic conversion into 13C-labelled precorrin-2. This was incubated with an enzyme system from Propionibacterium shermanii in the presence of [4-2H2]NADH and [4-2H2]NADPH to yield cobyrinic acid, shown to carry 2H at C-19 by appropriate 13C-NMR studies. The same reducing cofactors but now stereospecifically labelled at C-4 with tritium were similarly used to biosynthesise cobyrinic acid which was 3H-labelled from the 4(R)-cofactors but carried no 3H when the 4(S)-cofactors were used. Suitable degradation of the cobyrinic acid after conversion into its ester proved 3H-labelling at C-19. These results establish that the biosynthesis of vitamin B12 in the microaerophilic organism Pr. shermanii involves a reductive step in which a reductase enzyme transfers 4-HR of the cofactor to C-19 of the macrocycle.
Department of Chemistry
University of Cambridge