Biosynthesis of porphyrins and related macrocycles. Part 10. Vitamin B12: biochemical derivation of cobyrinic acid from uroporphyrinogen III. Studies with corresponding ring c methyl heptacarboxylic prophyrinogen, and proof of seven intact methyl transfers.

Abstract

A broken-cell enzyme system is developed from Propionibacterium shermanii which converts porphobilinogen into cobyrinic acid in 7–12% yields. This system is used to demonstrate incorporation of unsymmetrically labelled uroporphyrinogen III (7) into cobyrinic acid (5) and retention of specificity of labelling is proved by degradation of (5). Comparative incorporation experiments with the ring c methyl porphyrinogen (13) show that it is 30–50 times less effective than uroporphyrinogen III as a precursor of cobyrinic acid; this finding is discussed.Double labelling with 2H and 13C is used to prove that all seven methionine-derived methyl groups of vitamin B12 are transferred intact without significant exchange of their protons with the medium; the importance of this result for the C-1 methyl group is explained.