Properties and physiological roles of Na^+-motive respiratory chain in marine bacteria.

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The Marine bacterium Vibrio alginolyticus possesses a Na^+ pump that generates an lectrochemical potential of Na^+ as a direct result of respiration. Examinations of Na^+ pump-defective mutants, Napl and Nap2, revealed that Na^+ is extruded at the NADH:quinone oxidoreductase segment of NADH oxidase. In the wild type, two kinds of NADH:quinone oxidoreductases (NQR) are present; one is dependent on Na^+-dependent NQR. The Na^+ and another is not. Both Nap1 and Nap2 lack the activity of Na^+-dependent NQR. The Na^+-dependent NQR of the wild type was purified to near homogeneity and found to be composed of three subunits, <Alpha>, <beta> and <gamma>. Moreover, it was shown that Nap2 has a point mutation in <beta> subunit whereas Napl lacks all the subunits. Mechanism of electron transfer by the Na^+-dependent NQR were examined in detail using purified subunits and NQR complex. The subunit <beta> catalysed the reduction of ubiquinone to ubisemiquinone. The subunit <alpha> and <gamma> were e … More ssential for the reduction of ubiquinone to ubiquinol. NQR complex reconstituted in liposomes generated a membrane potential, which was dependent on Na^+.Besudes V, akgubikttucysm gakiogukuc V, cistucoka aksi retaubs the BA^+-motive NADH oxidase. which requires Na^+ for maximum activity. In order to investigate the distribution of Na^+-motive NADH oxidase in halophiles, Na^+-requiremet of NADH oxidase was exanubed ub narube bacterua. Out of 10 strains examined, 9 strains belonging to Alteromonas, Alcaligenes or Vibrio retained Na^+-dependent NADH oxidases. Moreover,Na^+-dependent site of all NADH oxidases existed at the NADH:quinone oxidoreductase and was inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), a specific inhibitor of the Na^+ pump in V. alginolyticus and V. costicola. Na^+-dependent NADH oxidases in all the strajns including V. alginolyticus were able to oxidize deamino-NADH whereas NADH oxidase in Nap1 and Nap2 showed little activity to deamino-NADH. These results indicated that the Na^+-dependent (Na^+-motive) NADH oxidase is a general mechanism to generate energy in marine bacteria and shares some commmon properties. Less