On the Efficiency of Oxidative Phosphorylation in Membrane Vesicles of Azotobacter vinelandii and of Rhizobium leguminosarum Bacteroids

The effect of O 2 on the rate of ATP synthesis and the P/O ratio has been studied in membrane vesicles of Azotobacter vinelandii and Rhizobium leguminosarum bacteroids (strain PRE). It was observed that the P/O ratio rises to a maximum value when the O 2 input is increased, but declines when O 2 becomes detectable in the medium. Evidence is presented, that in A. vinelandii H 2 donates its electrons at a fourth proton‐translocating site. The presence and the efficiency of this site is independent of the growth conditions and it appeared that no flavoproteins are directly involved in the oxidation of H 2 . The unidirectional hydrogenase in membrane vesicles of A. vinelandii is not inhibited by acetylene. The function of hydrogenase in the process of nitrogen fixation by A. vinelandii is discussed. Two types of membrane vesicles of R. leguminosarum bacteroids were prepared. Membranes isolated in a low salt medium lose the ability to couple succinate oxidation to oxidative phosphorylation; upon isolation in a high salt medium this ability is preserved. In contrast the efficiency of oxidative phosphorylation with NADH is not affected by the isolation procedure. The efficiency of energy coupling in membranes vesicles of R. leguminosarum bacteroids can be enhanced by the addition of fatty‐acid‐free bovine serum albumin. No active membrane‐bound hydrogenase could be detected in membrane vesicles of R. leguminosarum bacteroids.