Studies with Ubiquinone‐Depleted Submitochondrial Particles

Abstract

1. The incorporation of small amounts of ubiquinone (Q) into pentane‐extracted submitochondrial particles has been studied. A procedure is described which allows nearly quantitative incorporation of as little as 0.2 to 0.3 nmol Q/mg protein, i.e. less than 10% of the Q present in the lyophilized particles prior to extraction.2. It is shown that both NADH oxidase and succinate oxidase activities can be restored to 100% of that in lyophilized particles by incorporation of 6–8 nmol Q/mg protein. The relative activities of both oxidases show a parallel response to an increase of the Q content of the particles between 0.2 and 22 nmol Q/mg protein, 50% reactivation being obtained at about 2 nmol Q/mg protein.3. 50 to 60% of the incorporated Q can be reduced by NADH or succinate, independent of the total concentration of Q in the membrane. Similar reduction values were obtained for lyophilized particles prior to extraction, suggesting that incorporated Q functions similarly to that originally present in the particles.4. Succinate dehydrogenase activity, which is decreased 50% by extraction of Q, can be completely restored upon incorporation of only 1.5 nmol Q/mg protein.5. Extraction of lyophilized particles with pentane +10% acetone results in a more effective removal of Q, but also in a differential Q requirement for NADH oxidase and succinate oxidase. Thin‐layer chromatography shows that extraction with pentane + acetone removes an additional, unidentified, nonpolar lipid.6. The present data do not support the belief that succinate oxidase and NADH oxidase communicate with separate pools of Q.