Biosynthesis of Vitamin B12. Experiments with the Anaerobe Eubacterium limosum and Some Labelled Substrates

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Abstract
The anaerobic microorganism E. limosum (DSM 20 402), grown in the presence of L-[methyl-14C]methionine, produces vitamin B12 labeled in the corrin ring and in the 5,6-dimethylbenzimidazole moiety. The vitamin B12 obtained in a similar experiment with L-[methyl-13C]methionine showed 9 signals in the 13C-NMR spectrum. Seven of these signals are due to the 7 extra methyl groups of the corrin ring; 2 signals originate from the 2 methyl groups of the 5,6-dimethylbenzimidazole moiety. This result is in contrast to previous experiments with aerobic and aerotolerant organisms, where L-methionine is only the precursor of the 7 corrin methyl groups, and the 5,6-dimethylbenzimidazole is formed from riboflavin. E. limosum transforms exogenously added benzimidazole into benzimidazolylcobamide, but with [2-14C]benzimidazole it was shown that benzimidazole is not a precursor of the 5,6-dimethylbenzimidazole moiety of vitamin B12. Radioactivity from [2-14C]glycine is almost exclusively incorporated into the 5,6-dimethylbenzimidazole moiety of vitamin B12, but not into the corrin moiety. This demonstrates that in E. limosum .delta.-aminolevulinic acid, the precursor of the corrin moiety, is not formed as in many other organisms from succinyl-CoA and glycine.