Biosynthesis of vitamin B12: nature of the volatile fragment generated during formation of the corrin ring system.

It is shown with the help of radiolabeled substrates that biological formation of the corrin ligand system of cobyrinic acid is linked with the release of an equivalent of acetic acid from a hydroporphinoid precursor in which the C-20 atom has undergone methylation prior to its excision from the macrocycle.