, Volume 25, Issue 4, pp 385-391

Na+ -translocating NADH-quinone reductase of marine and halophilic bacteria

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The respiratory chain of marine and moderately halophilic bacteria requires Na+ for maximum activity, and the site of Na+-dependent activation is located in the NADH-quinone reductase segment. The Na+-dependent NADH-quinone reductase purified from marine bacteriumVibrio alginolyticus is composed of three subunits, α, β, and γ, with apparentM r of 52, 46, and 32kDa, respectively. The FAD-containing β-subunit reacts with NADH and reduces ubiquinone-1 (Q-1) by a one-electron transfer pathway to produce ubisemiquinones. In the presence of the FMN-containing α-subunit and the γ-subunit, Q-1 is converted to ubiquinol-1 without the accumulation of free radicals. The reaction catalyzed by the α-subunit is strictly dependent on Na+ and is strongly inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), which is tightly coupled to the electrogenic extrusion of Na+. A similar type of Na+-translocating NADH-quinone reductase is widely distributed among marine and moderately halophilic bacteria. The respiratory chain ofV. alginolyticus contains another NADH-quinone reductase which is Na+ independent and has no energy-transducing capacity. These two types of NADH-quinone reductase are quite different with respect to their mode of quinone reduction and their sensitivity toward NADH preincubation.