Exogenous 5-hydroxy-[2-14C]benzimidazole was transformed by Methanosarcina barkeri into 5-hydroxy-[2J4C]benzimidazolylcobamide. Thereby the endogenous biosynthesis of 5-hydroxybenzimidazole was completely blocked.
Benzimidazole and 5,6-dimethylbenzimidazole were used by M. barkeri to form benzimidazolylcobamide respectively 5,6-dimethylbenzimidazolylcobamide (vitamin B 12), but in these cases the endogenous biosynthesis of factor III was not completely suppressed.
With [2-~4C]benzimidazole it was demonstrated that this base as well as the benzimidazolylcobamide formed thereof are no precursors in the biosynthesis of 5-hydroxybenzimidazolylcobamide.
Glycine instead was found to be a building block for the biosynthesis of 5-hydroxybenzimidazole, since radioactivity from [1-1~C] and [2-1r was incorporated into the base moiety of factor III, but not into its corrin moiety. With [1J3C]glycine and 13C-NMR-spectroscopy it was shown that C-1 of glycine gets C-3a of 5-hydroxybenzimidazole.
[1J 3C]glycine also led to a single prominent signal in the 3C-NMR-spectrum of coenzyme F~2o, this was assigned to C-10a.
Thus C-I of glycine was incorporated into the hydroxybenzene part of 5-hydroxybenzimidazole, whereas it was not incorporated into this part of coenzyme F42o, indicating that the hydroxybenzene part of these two compounds is not formed from a common intermediate.
L-[UJ4C]glutamate led to the exclusive labeling of the corrin ring of factor III, showing that the corrin precursor 5-aminolevulinic acid is formed by the C-5 pathway in M.
barkeri.
These experiments indicate that the biosynthesis of factor III in the "archaebacterium" M. barkeri is similar to the corrinoid biosynthesis in the anaerobic "eubacteria" Eubacterium limosum, Clostridium barkeri, and Clostridium thermoaceticum.