Mode of transformation of [1-15N]5-hydroxybenzimidazole and [1-15N]5-hydroxy-6-methylbenzimidazole into the 5,6-dimethylbenzimidazole moiety of vitamin B12.

The transformation of [1-15N]5-hydroxybenzimidazole and [1-15N]5-hydroxy-6-methylbenzimidazole into the 5,6-dimethylbenzimidazole moiety of vitamin B12 by Eubacterium limosum-cultures was studied. The vitamin B12 obtained was exclusively 15N-labeled in N-1 of the base part, as revealed by NMR-measurements. This indicates that either the unsubstituted 5,6-dimethylbenzimidazole presumably formed is not released from the enzyme until the ribose-5'-phosphate substituent is introduced, or that the precursors are first transformed into their alpha-nucleotide-5'-phosphates which then react to form 5,6-dimethylbenzimidazole-alpha-D-ribofuranoside- 5'-phosphate (alpha-ribazole-5'-phosphate).