Biosynthesis of vitamin B12 in anaerobic bacteria. Experiments with Eubacterium limosum on the incorporation of D-[1-13C]erythrose and [13C]formate into the 5,6-dimethylbenzimidazole moiety.

Experiments on the incorporation of erythrose and formate into the 5,6-dimethylbenzimidazole moiety of vitamin B12 are described. In one experiment, a 1:1 mixture of D-[1-13C]erythrose and D-[1-13C]threose was added to a Eubacterium limosum fermentation. The vitamin B12 formed was methylated at N3 of its 5,6-dimethylbenzimidazole part and degraded to 1,5,6-trimethylbenzimidazole. The 13C-NMR spectrum of this compound exhibited a single prominent signal at 109.5 ppm due to 13C labeling in C7. This shows that C1 of erythrose or threose was originally incorporated exclusively into C4 of the 5,6-dimethylbenzimidazole moiety of vitamin B12. In another experiment, sodium [13C]formate was added to a culture of E. limosum. The vitamin B12 isolated was transformed into 1,5,6-trimethylbenzimidazole as before. The 13C-NMR spectrum also showed one prominent signal at 142.8 ppm, evoked by 13C at C2. These results demonstrate that erythrose is incorporated into the base part of vitamin B12 regiospecifically and that formate is the precursor of the C2.