The Effects of Nitric Oxide on Electron Transport Complexes

Abstract

The effect of nitric oxide on mitochondrial electron transfer complexes was studied by comparing the activities of nitric oxide-treated and untreated, deoxygenated samples of purified beef heart succinate–cytochrome c reductase, succinate–ubiquinone reductase, and ubiquinol–cytochrome c reductase. More than 90% of succinate–cytochrome c reductase activity is lost during nitric oxide treatment. The activity of the succinate–ubiquinone reductase component of succinate–cytochrome c reductase decreases 95%, while the ubiquinol–cytochrome c reductase component is unaffected by nitric oxide. This inactivation is due primarily to the destruction of iron–sulfur clusters from succinate–ubiquinone reductase. When purified beef heart succinate–ubiquinone reductase was treated with nitric oxide, virtually all activity was irreversibly lost. The electron paramagnetic resonance (EPR) spectra of the treated complex showed typical iron–nitric oxide complex signals, confirming that inactivation is due to destruction of the iron–sulfur clusters. Similar results were obtained with purifiedEscherichia colisuccinate–ubiquinone reductase. Pure beef heart ubiquinol–cytochrome c reductase treated with nitric oxide loses 40% of its initial activity, but regains most of it (90–100%) after 24 h of incubation at 0°C in the absence of nitric oxide. This suggests that ubiquinol–cytochrome c reductase is protected from nitric oxide when complexed with succinate–ubiquinone reductase or that when split from succinate–ubiquinone reductase, ubiquinol–cytochrome c reductase undergoes a conformational change which allows access of nitric oxide to the Rieske iron–sulfur center. Such access is not possible when ubiquinol–cytochrome c reductase is complexed with succinate–ubiquinone reductase. The loss of ubiquinol–cytochrome c reductase activity correlates with a decrease in the Rieske protein EPR signal intensity without formation of any new EPR signal. The Rieske iron–sulfur cluster signal is recovered after 24 h incubation in the absence of nitric oxide.