Na+-dependent NADH : quinone oxidoreductase in the respiratory chain of the marine bacterium Marinomonas vaga

The Gram-negative marine bacterium Marinomonas vaga, which requires 0.5 M NaCl concentration for optimal growth, is slightly halophilic. The growth of M. vaga was highly resistant to the proton conductor, carbonyl cyanide m-chlorophenylhydrazone (CCCP) under alkaline pH conditions (pH 8.5) but very sensitive to CCCP under acidic pH conditions (pH 6.5). These results suggest that the respiratory chain-linked NADH oxidase system of M. vaga may lead to generation of a Na+ electrochemical gradient. In order to examine the existence of Na+-stimulated NADH oxidase in M. vaga, membrane fractions were prepared by the osmotic lysis method. The membrane-bound NADH oxidase oxidized both NADH and deamino-NADH as substrates and required Na+ for maximum activity. The maximum activity of NADH oxidase was obtained at about pH 8.5 in the presence of 0.2 M NaCl. The site of Na+-dependent activation in the NADH oxidase system was at the NADH:quinone oxidoreductase segment. The NADH oxidase and NADH:quinone oxidoreductase were very sensitive to the respiratory chain inhibitor, 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO) in the presence of 0.2 M NaCl but highly resistant to another respiratory inhibitor, rotenone. Based on these findings, we conclude that M. vaga possesses the Na+-dependent NADH:quinone oxidoreductase that may function as an electrogenic Na+ pump.ope