7-AZABENZIMIDAZOLYLCOBAMIDE AND 5,6-DIMETHYL-7-AZABENZIMIDAZOLYLCOBAMIDE, NEW VITAMIN-B-12 ANALOGS SYNTHESIZED FROM 4(5)-AMINOIMIDAZOLE BY EUBACTERIUM-LIMOSUM

In anaerobic bacteria, glycine, formate, and the amide-N of glutamine are building blocks for the biosynthesis of the imidazole moiety of th e vitamin B-12-base 5,6-dimethylbenzimidazole. These building blocks a re also used for the biosynthesis of the imidazole moiety of purine ba ses. Therefore we tested 4(5)-aminoimidazole, the base moiety of the p urine nucleotide precursor 5-aminoimidazole ribonucleotide, for its pu tative function as precursor of 5,6-dimethylbenzimidazole. The anaerob ic vitamin B-12-producer Eubacterium limosum, grown in the presence of [2-C-13]4(5)-aminoimidazole, synthesized nonlabeled vitamin B-12, but also [2-C-13]7-azabenzimidazolylcobamide and [2-C-13]5,6-dimethyl-7-a zabenzimidazolylcobamide. [2-C-13]lmidazole was used by E. limosum to form [2-C-13]imidazolylcobamlde. Simultaneously nonlabeled vitamin B-1 2 was synthesized. This shows that 4(5)-aminoimidazole and imidazole a re not intermediates in the biosynthesis of 5,6-dimethylbenzimidatole. However, 4(5)-aminoimidazole has obviously a structure similar to the structure of an as yet unknown precursor of the vitamin B-12-base, an d is therefore transformed into the aza analogs. In order to prepare a reference compound 4(5)-azabenzimidazole was added to a culture of Pr opionibacterium shermanii and to a culture of E. limosum. P. shermanii transformed this base mainly to 4-azabenzimidazolylcobamide, as deter mined by H-1 NMR-spectroscopy (NOE experiment). In contrast E. limosum produced mainly 7-azabenzimidazolylcobamide. The reason for this diff erence is discussed.