The mechanism of the ring contraction process during vitamin B12 biosynthesis by the anaerobe Propionibacterium shermanii was investigated under both aerobic and anaerobic conditions by means of feeding experiments with δ‐amino[1‐13C]levulinic acid (a biosynthetic intermediate of tetrapyrrole) and δ‐amino[1‐13C,1,1,4‐18O3]levulinic acid in combination with 13C‐NMR spectroscopy. We showed that the characteristic mechanism of the ring contraction process (the generation of precorrin‐3x from formation of the γ‐lactone from the ring A acetate group at C1 and hydroxylation at C20 by molecular oxygen catalyzed by CobG, and the migration of ring D by cleavage of the carbon–oxygen bond at C1 of precorrin‐3x) in the aerobe Pseudomonas denitrificans was not seen in P. shermanii under aerobic conditions, and the mechanism of the ring contraction process in P. shermanii was the same irrespective of the presence or absence of oxygen.

(5)) with sodium borohydride, and hydrolysis in 33% total yield from the 15 N-source.

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