Biosynthesis of vitamin B12. Different pathways in some aerobic and anaerobic microorganisms.

Radioactivity from [1'-14C]riboflavin was incorporated into the 5,6-dimethylbenzimidazole moiety of Vitamin B12 in the aerobes Bacillus megaterium, Nocardia rugosa and Streptomyces sp. as well as in the aerotolerant anaerobe Propionibacterium freudenreichii, but not in the anaerobe Eubacterium limosum. As recently published for E. limosum, also in the anaerobe Clostridium barkeri radioactivity from [1-14C]glycine and [2-14C]glycine was found in the 5,6-dimethylbenzimidazole moiety, but not in the corrin moiety. The addition of L-[methyl-14C]methionine to C. barkeri led to the labeling of the corrin moiety and the 5,6-dimethylbenzimidazole moiety, showing that the seven "extra" methyl groups in the corrin ring as well as the two methyl groups of the base part originate from this precursor. In Clostridium thermoaceticum, forming the vitamin B12 analog 5-methoxybenzimidazolylcobamide, [1-14C]glycine and [2-14C]glycine were also incorporated into the 5-methoxybenzimidazole moiety, but not into the corrin ring. In E. limosum L-[U-14C]glutamate led to the labeling of the corrin ring of vitamin B12, but not of its base moiety. These results together with data from the literature indicate that a common biosynthetic pathway might exist for the corrinoid biosynthesis in aerobic microorganisms, and in those aerotolerant anaerobes like the Propionibacteria, which form the 5,6-dimethylbenzimidazole moiety of vitamin B12 only under aerobic conditions. They also show that this pathway differs from the pathway found in anaerobic bacteria.