HYDROGEN-UBIQUINONE OXIDOREDUCTASE ACTIVITY BY THE BRADYRHIZOBIUM-JAPONICUM MEMBRANE-BOUND HYDROGENASE

The Bradyrhizobium japonicum heterodimeric nickel-iron hydrogenase efficiently catalyzed H-2-ubiquinone-1 oxidoreductase activity at rates up to 47% of the maximal rates obtained using the artificial electron acceptor methylene blue. Gel filtration chromatography and SDS-polyacrylamide gel electrophoresis experiments demonstrated that the purified enzyme was a heterodimer containing only the 65 kDa and 33 kDa subunits. Reduced minus oxidized absorption difference spectra demonstrated the absence of detectable cytochromes. The H-2-ubiquinone-1 oxidoreductase activity of both the purified heterodimeric hydrogenase and membranes was significantly inhibited by 2-n-heptyl-4-hydroxyquinoline-N-oxide and antimycin A, inhibitors known to act in the quinone region of electron transport chains. Our results are the first report of H-2-ubiquinone oxidoreductase activity by a purified hydrogenase.