Biosynthesis of 5,6-dimethylbenzimidazole. Precursor-function of 6,7-dimethyl-8-ribityllumazine.

6,7-Dimethyl-14C-8-ribityllumazine was added to cell homogenates of Propionibacterium shermanii, previously grown anaerobically in the presence of cobaltous sulfate. This homogenates were shaken under aerobic conditions for 48 hours in order to produce vitamin B12 from the corrinoids formed during the anaerobic growth phase. The vitamin B12 was isolated and its 5,6-dimethylbenzimidazole moiety split off by acid hydrolysis. It was shown that the radioactive label of 6,7-dimethyl-14C-8-ribityllumazine was exclusively localized in the 5,6-dimethylbenzimidazole moiety of the vitamin B12 molecule. The specific radioactivity of the 5,6-dimethylbenzimidazole isolated was 18 per cent of the specific radioactivity of the 6,7-dimethyl-14C-8-ribityllumazine added. The radioactive 5,6-dimethylbenzimidazole was further degraded in order to determine the radioactivity in the different carbon atoms. It was found that 45 percent of the total radioactivity was localized in the two methyl carbons (C-10+C-11) and 47 percent in carbon atoms 4 + 7 indicating that 5,6-dimethylbenzimidazole had been formed from two molecules of 6,7-dimethyl-8-ribityllumazine.