MODE OF TRANSFORMATION OF [1-N-15]5-HYDROXYBENZIMIDAZOLE AND [1-N-15]5-HYDROXY-6-METHYLBENZIMIDAZOLE INTO THE 5,6-DIMETHYLBENZIMIDAZOLE MOIETY OF VITAMIN-B-12

The transformation of [1-N-15]5-hydroxybenzimidazole and [1-N-15]5-hyd roxy-6-methylbenzimidazole into the 5,6-dimethylbenzimidazole moiety o f vitamin B-12 by Eubacterium limosum-cultures was studied. The vitami n B-12 obtained was exclusively N-15-labeled in N-1 of the base part, as revealed by NMR-measurements. This indicates that either the unsubs tituted 5,6-dimethylbenzimidazole presumably formed is not released fr om the enzyme until the ribose-5'-phosphate substituent is introduced, or that the precursors are first transformed into their alpha-nucleot ide-5'-phosphates which then react to form lbenzimidazole-alpha-D-ribo furanoside-5'-phosphate (alpha-ribarazole-5'-phosphate).