THE EFFECTS OF NITRIC-OXIDE ON ELECTRON-TRANSPORT COMPLEXES

The effect of nitric oxide on mitochondrial electron transfer complexe s was studied by comparing the activities of nitric oxide-treated and untreated, deoxygenated samples of purified beef heart succinate-cytoc hrome c reductase, succinate-ubiquinone reductase, and ubiquinol-cytoc hrome c reductase. More than 90% of succinate-cytochrome c reductase a ctivity is lost during nitric oxide treatment. The activity of the suc cinate-ubiquinone reductase component of succinate-cytochrome c reduct ase decreases 95%, while the ubiquinol-cytochrome c reductase componen t is unaffected by nitric oxide. This inactivation is due primarily to the destruction of iron-sulfur clusters from succinate-ubiquinone red uctase. When purified beef heart succinate-ubiquinone reductase was tr eated with nitric oxide, virtually all activity was irreversibly lost. The electron paramagnetic resonance (EPR) spectra of the treated comp lex showed typical iron-nitric oxide complex signals, confirming that inactivation is due to destruction of the iron-sulfur clusters. Simila r results were obtained with purified Escherichia coli succinate-ubiqu inone reductase, Pure beef heart ubiquinol-cytochrome c reductase trea ted with nitric oxide loses 40% of its initial activity, but regains m ost of it (90-100%) after 24 h of incubation at 0 degrees C in the abs ence of nitric oxide. This suggests that ubiquinol-cytochrome c reduct ase is protected from nitric oxide when complexed with succinate-ubiqu inone reductase or that when split from succinate-ubiquinone reductase , ubiquinol-cytochrome c reductase undergoes a conformational change w hich allows access of nitric oxide to the Rieske iron-sulfur center. S uch access is not possible when ubiquinol-cytochrome c reductase is co mplexed with succinate-ubiquinone reductase. The loss of ubiquinol-cyt ochrome 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 incubati on in the absence of nitric oxide. (C) 1996 Academic Press, Inc.