Biosynthesis of vitamin B-12 in anaerobic bacteria. Experiments with Eubacterium limosum and D-erythrose 14C-labeled in different positions.

In anaerobic microorganisms the origin of C atoms 2 and 4-7 of the 5,6-dimethylbenzimidazole moiety of vitamin B-12 is still unknown. In order to tackle this problem we added several 14C-labeled putative precursors to Eubacterium limosum fermentations. The degradation of the isolated vitamin B-12 revealed that only D-erythrose, 14C-labeled in different positions, was efficiently incorporated into the 5,6-dimethylbenzimidazole part. The 5,6-dimethylbenzimidazole obtained from an experiment with D-[U-14C]erythrose was further degraded. It was found that C-2 was unlabeled, whereas half of the label was located in C-5 plus C-6, and the other half in C-4 plus C-7. These results demonstrate that in E. limosum D-erythrose is a precursor of C-atoms 4, 5, 6 and 7 of the 5,6-dimethylbenzimidazole part of vitamin B-12.