Sodium-transport NADH-quinone reductase of a marine Vibrio alginolyticus

The respiratory chain of a marine bacterium, Vibrio alginolyticus, required Na+ for maximum activity, and the site of Na+ -dependent activation was localized on the NADH-quinone reductase segment. The Na+ -dependent NADH-quinone reductase extruded Na+ as a direct result of redox reaction. It was composed of three subunits, alpha, beta, and gamma, with apparent Mr of 52, 46, and 32 KDa, respectively. The reduction of ubiquinone-1 to ubiquinol proceeded via ubisemiquinone radicals. The former reaction was catalyzed by the FAD-containing beta subunit. This reaction showed no specific requirement for Na+. For the formation of ubiquinol, the presence of the gamma subunit and the FMN-containing alpha subunit was essential. The latter reaction specifically required Na+ for activity and was strongly inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide. It was assigned to the coupling site for Na+ transport. The mode of energy coupling of redox-driven Na+ pump was compared with those of decarboxylase- and ATP-driven Na+ pumps found in other bacteria.