THE CRUCIAL CONTRIBUTION OF STARVED RESTING CELLS TO THE ELUCIDATION OF THE PATHWAY OF VITAMIN-B-12 BIOSYNTHESIS

Resting cells were crucial in elucidating the steps of vitamin B-12 bi osynthesis, and their use led to the following conclusions: (1) 5-amin olevulinic acid is a common precursor of vitamin B-12 and porphyrins i n various groups of microorganisms. (2) Glutamine is the donor of the amide groups in the biosynthesis of the amidated corrin structure of v itamin B-12. (3) Methylated derivatives of uroporphyrinogen III, calle d corriphyrins, are precursors of the corrin ring of vitamin B-12. The y are formed in the initial steps of uroporphyrinogen III transformati on to cobyrinic acid. (4) The initial common steps of heme and vitamin B-12 synthesis are controlled by heme, while the biosynthetic pathway starting with methylation of uroporphyrinogen III and leading to corr inoids is controlled by vitamin B-12. Aeration interferes with vitamin B-12 formation but not that of porphyrins. The mechanism is that of o xygen suppression of methylation of uroporphyrinogen III. These conclu sions were completely supported by subsequent studies of others. This shows that suspensions of resting microbial cells can be profitably em ployed in solving important problems of microbial biosynthesis of phys iologically active compounds.