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Membranes prepared from Vibrio natriegens oxidized both NADH and deamino-NADH as substrates. The maximum activity of the membrane-bound NADH oxidase was obtained at about pH 8.5 in the presence of 0.2 M NaCl, whereas that of the NADH:ubiquinone oxidoreductase was obtained at about pH 7.5 in the presence of 0.2 M NaCl. Electron transfer from NADH or deamino-NADH to ubiquinone- 1 or oxygen generated a considerable membrane potential (?ψ) which occurred even in the presence of 20 μM carbonylcyanide m-chlorophenylhydrazone (CCCP). However, the ?ψ was completely collapsed by the combined addition of 10 μM CCCP and 20 μM monensin. On the other hand, the activity of the NADH oxidase and the ?ψ generated by the NADH oxidase system were inhibited by about 90% with 10 μM HQNO, whereas the activity of the NADH:ubiquinone oxidoreductase and the ?ψ generated at the NADH:ubiquinone oxidoreductase segment were inhibited by about 60%. Interestingly, the activity of the NADH:ubiquinone oxidoreductase and the ?ψ generated at the NADH:ubiquinone oxidoreductase segment were resistant to the respiratory chain inhibitors such as rotenone, capsaicin, and AgNO3 and the activity of the NADH oxidase and the ?ψ generated by the NADH oxidase system were very sensitive only to AgNO3. It was concluded, therefore, that V. natriegens cells possess a AgNO3-resistant respiratory Na+ pump that is different from the AgNO3-sensitive respiratory Na+ pump of a marine bacterium, Vibrio alginolyticus.
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