[Transformation of [5'-3H]riboflavin into 5,6-dimethylbenzimidazole].

The 5,6-dimethylbenzimidazole unit of vitamin B12 is formed from riboflavin via FMN in aerobic and some aerotolerant bacteria. Thereby C-1' of the ribityl side chain gets C-2 of 5,6-dimethylbenzimidazole. Experiments with homogenates of Propionibacterium shermanii on the fate of C-atoms 2' to 5' of the ribityl side chain of riboflavin in this transformation are reported. It was found that [5'-3H]riboflavin leads to radioactive sugar phosphates. These were isolated, dephosphorylated, and separated. Thus 3H-D-fructose and 3H-D-glucose were detected. The degradation of 3H-D-glucose revealed that 14 per cent of the total radioactivity was located in C-1, and 86 per cent in C-6. This indicates that during 5,6-dimethylbenzimidazole biosynthesis a three carbon unit is formed from the ribityl side chain of riboflavin.