Reduction of mitochondrial components by durohydroquinone

Abstract

1. Durohydroquinone (H 2DQ) has been used to provide pulses of reducing equivalents to suspensions of pigeon heart mitochondria and submitochondrial particles. 2. H 2DQ oxidation in the presence of pentachlorophenol was 95% sensitive to antimycin A and 99% sensitive to cyanide. 3. Pulses of 150 μM H 2DQ produced reduction of cytochromes b (60–90% of total absorption), fluorescent flavoprotein (25% of total fluorescence) and pyridine nucleotide (60% of total fluorescence). Reduction of the latter two components was mediated through reversed electron transfer and was energy dependent. 4. Pulses of 10–30 μM H 2DQ brought about redox cycles of ubiquinone, cytochrome(s) b and absorbing flavoprotein (succinate dehydrogenase). Addition of antimycin A altered the kinetics of electron equilibration between ubiquinone and cytochrome(s) b. Antimycin A enhanced the efficiency of reduction of cytochrome(s) b and flavoprotein by 3–20 μM H 2DQ, but diminished the efficiency of ubiquinone reduction under the same experimental conditions. In the absence of inhibitor, ubiquinone was quantitatively the most important acceptor for the electrons arising from H 2DQ. In the presence of antimycin A, cytochrome(s) b was the most important acceptor for H 2DQ. 5. In the presence of antimycin A, ubiquinone did not show redox cycle changes synchronous with the redox cycles of flavoprotein and cytochromes b, and thus seem not to be an obligatory member for electron transfer in the main pathway from succinate dehydrogenase-cytochrome(s) b to oxygen. This antimycin A effect is interpreted as the result of a conformational change of cytochrome(s) b or an alteration of the hydrocarbon core of the membrane that ubiquinone occupies. 6. 1.2–15 μM additions of H 2DQ to cyanide-blocked mitochondria or submitochondrial particles brought about pulses of reducing equivalents which were distributed among the oxidized carriers according to the redox potential of these components. Cytochromes a + a 3 and c, absorbing flavorprotein and ubiquinone titrated in the presence or absence of uncoupler, as homogeneous pools in a sequence according to their reported potential values. Cytochrome(s) b titrated in the absence of uncoupler as a heterogeneous pool and in the presence of uncoupler as a homogeneous pool in the latter case the absorbance reaching 50% of the total. 7. In the aerobic steady state ( plus or minus ATP) a pulse of 250 μM H 2DQ reduces cytochromes b absorbing at 562 and 557 nm. The form absorbing at 557 nm is postulated to be cytochrome b 555.