PROTON-PUMPING OF MITOCHONDRIAL COMPLEX-I - DIFFERENTIAL ACTIVATION BY ANALOGS OF UBIQUINONE

As part of the ongoing studies aimed at elucidating the mechanism of t he energy conserving function of mitochondrial complex I, NADH: ubiqui none (Q) reductase, we have investigated how short-chain Q analogs act ivate the proton pumping function of this complex. Using a pH-sensitiv e fluorescent dye we have monitored both the extent and initial veloci ty of proton pumping of complex I in submitochondrial particles. The r esults are consistent with two sites of interaction of Q analogs with complex I, each having different proton pumping capacity. One is the p hysiological site which leads to a rapid proton pumping and a stoichio metric consumption of NADH associated with the reduction of the most h ydrophobic Q analogs. Of these, heptyl-Q appears to be the most effici ent substrate in the assay of proton pumping. Q analogs with a short-c hain of less than six carbons interact with a second site which drives a slow proton pumping activity associated with NADH oxidation that is overstoichiometric to the reduced quinone acceptor. This activity is also nonphysiological, since hydrophilic Q analogs show little or no r espiratory control ratio of their NADH:Q reductase activity, contrary to hydrophobic Q analogs.