The enzyme, which participates in the anaerobic (early cobalt insertion) pathway of adenosylcobalamin biosynthesis, catalyses the reduction of the double bond between C-18 and C-19 of cobalt-precorrin-6A. The enzyme from the bacterium Bacillus megaterium has no activity with NADPH. See EC
1.3.1.54, precorrin-6A reductase, for the corresponding enzyme that participates in the aerobic cobalamin biosynthesis pathway.