SUMMARY: Effects of Na on respiratory NADH oxidase were studied in membranes prepared from Gram-negative marine bacteria belonging to the genera and NADH oxidases of 9 out of the 10 strains examined were found to require Na for maximum activity. Without exception, the Na-dependent site was located at the level of the NADH: quinone oxidoreductase region of the respiratory chain and was highly sensitive to 2-heptyl-4-hydroxyquinoline--oxide (HQNO). NADH dehydrogenases in the Na-dependent segment were sensitive to Ag and able to oxidize reduced nicotinamide hypoxanthine dinucleotide as a substrate. Since the NADH dehydrogenase was neither dependent on Na nor sensitive to HQNO, it was suggested that reduction of quinone by the NADH: quinone oxidoreductase involves the formation of an intermediate, presumably semiquinone, and that electron transfer from the intermediate requires Na and is sensitive to HQNO. These results showed a striking similarity to those obtained with the Na-dependent NADH: quinone oxidoreductase, the Na pump, of , indicating that this enzyme is widely distributed among Gram-negative marine bacteria and shares common properties.


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