Cell extracts of Methanobacterium thermoautotrophicum catalyzed the reduction of methyl viologen (apparent K m=0.1 mM) with formylmethanofuran (apparent K m=10 μM) at a specific rate of 4 μmol · min−1 · mg protein−1. Coenzyme F420 (apparent K m=25 μM) rather than NAD, NADP, FAD, or FMN was used as physiological electron acceptor. With coenzyme F420 the specific activity was 0.4 μmol · min−1 · mg−1. More than 60% of the formylmethanofuran dehydrogenase activity was associated with the pellet fraction, which was obtained by centrifugation at 160 000 × g. An enzyme system with very similar properties was also found to be present in cell extracts of Methanosarcina barkeri grown on methanol. In both organisms the formylmethanofuran dehydrogenase activity was rapidly inactivated by cyanide.
