Proton translocation coupled to quinol oxidation in ox heart mitochondria

Abstract

The suitability of ubiquinol1 and duroquinol as pulse reductants for initiating respirationdriven proton translocation by aerobic ox heart mitochondria was investigated. At 25°C the Vmax. for oxidation was close to 280nmol of quinol oxidized/min per mg of protein, and the Km values were 8μm for ubiquinol1 and 28μm for duroquinol. Pulses of ubiquinol1 and duroquinol were rapidly and completely oxidized by aerobic mitochondria with a simultaneous acidification of the suspending medium as detected with a glass electrode. The →H+/2e−ratios (Mitchell, 1966) calculated from the observed extent of acidification and the amount of quinol added were 3.62 for ubiquinol1 and 2.98 for duroquinol. These values are underestimates of the true value owing to proton back-flow across the membrane. An analogue computer model was used to correct the observed extent of respirationdriven acidification for proton back-flow. The corrected →H+/2e−values were 4.01 for ubiquinol and 3.86 for duroquinol oxidation. Attempts to measure the rate of proton translocation with a pH-measuring system with a response time of 0.4s were not entirely satisfactory, owing to the relative slowness of the electrode response. Nevertheless the maximal rate of proton generation during ubiquinol1 oxidation was about 1200ng-ions of H+/min per mg of mitochondrial protein. It is concluded, contrarily to Chance & Mela (1967), that mitochondria exhibit a proton-translocating ubiquinol oxidase activity with a →H+/2e−ratio of 4.0.