Hematoporphyrin-promoted photoinactivation of mitochondrial ubiquinol-cytochrome c reductase: selective destruction of the histidine ligands of the iron-sulfur cluster and protective effect of ubiquinone.
Purified ubiquinol-cytochrome c reductase of beef heart mitochondria is very stable in aqueous solution; it suffers little damage upon illumination with visible light under aerobic or anaerobic conditions. However, it is rapidly inactivated when the photosensitizer hematoporphyrin is present during illumination. The hematoporphyrin-promoted photoactivation is dependent on sensitizer dose, illumination time, and oxygen. Singlet oxygen is shown to be the destructive agent in this system. The photoinactivation of ubiquinol-cytochrome c reductase is prevented by excess exogenous ubiquinone, regardless of its redox state. This protective effect is not due to protein-ubiquinone interactions but to the singlet oxygen scavenger property of ubiquinone. Ubiquinone also protects against hematoporphyrin-promoted photoinactivation of succinate-ubiquinone reductase and cytochrome c oxidase. The photoinactivation site in ubiquinol-cytochrome c reductase is the iron-sulfur cluster of Rieske's protein. Two histidine residues, presumably serving as two ligands for the iron-sulfur cluster of Rieske's protein, are destroyed. No polypeptide bond cleavage is detected. Photoinactivation has little effect on the spectral properties of cytochromes b and c1 but alters their reduction rates substantially. this photoinactivation also causes the formation of proton-leaking channels in the complex. When the photoinactivated reductase is co-inlaid with intact ubiquinol-cytochrome c reductase or cytochrome c oxidase in a phospholipid vesicle, no proton ejection can be detected during the oxidation of their corresponding substrates.
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Department of Biochemistry, OAES, Oklahoma State University, Stillwater 74078.
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D000818:Animals; D002417:Cattle; D004578:Electron Spin Resonance Spectroscopy; D014450:Electron Transport Complex III / Q000037:antagonists & inhibitors* / Q000528:radiation effects; D006415:Hematoporphyrins / Q000494:pharmacology*; D006639:Histidine; D007506:Iron-Sulfur Proteins / Q000037:antagonists & inhibitors* / Q000528:radiation effects; D007700:Kinetics; D008027:Light; D008929:Mitochondria, Heart / Q000201:enzymology*; D013053:Spectrophotometry; D013482:Superoxide Dismutase / Q000494:pharmacology; D013997:Time Factors; D014451:Ubiquinone / Q000494:pharmacology* / Q000031:analogs & derivatives
D006415:Hematoporphyrins; D007506:Iron-Sulfur Proteins; D014451:Ubiquinone; D006639:Histidine; D013482:Superoxide Dismutase; D014450:Electron Transport Complex III; C003741:ubiquinol