Biosynthesis of vitamin B12. Experiments with the anaerobe Eubacterium limosum and some labelled substrates

1. The anaerobic microorganism Eubacterium limosum (DSM 20 402), grown in the presence of L-[methyl-14C]methionine, produces vitamin B12 not only labeled in the corrin ring, but also in the 5,6-dimethylbenzimidazole moiety. The vitamin B12 obtained in a similar experiment with L-[methyl-13C]methionine showed nine signals in the 13CV NMR spectrum. Seven of these signals are due to the seven 'extra' methyl groups of the corrin ring, two signals originate from the two 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 seven corrin methyl groups, and the 5,6-dimethylbenzimidazole is formed from riboflavin. 2. 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. 3. 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.