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Extinction coefficients for cytochrome b and c1 in the isolated cytochrome bc1 complex from Rhodopseudomonas sphaeroides GA have been determined. They are 25 mM-1 . cm-1 at 561 nm for cytochrome b and 17.4 mM-1 . cm-1 at 553 nM for cytochrome c1, for the difference between the reduced and the oxidized state. Cytochrome b is present in two forms in the complex. One form has an Em7 of 50 mV, an alpha-peak of 557 nm at liquid N2 temperature and of 561 nm at RT, which is red-shifted by antimycin A. The other form has an Em7 of -90 mV, a double alpha-peak of 555 and 561 nm at liquid N2 temperature corresponding to 559 and 566 nm at RT. The absorption at 566 nm is red-shifted by myxothiazol. The two shifts are independent of each other. Both midpoint potentials of cytochromes b are pH-dependent. The redox center compositions of the cytochrome bc1 complexes from Rhodopseudomonas sphaeroides and from mitochondria are identical.

Isolated cytochrome complexes from different sources like beef heart mitochondria, spinach chloroplasts, cyanobacteria, and photosynthetic bacteria were incorporated into liposomes by sonication as revealed by sucrose density gradient centrifugation and electron microscopy. The reconstituted cytochrome complexes show suppressed rates of quinol-cytochrome c/plastocyanin oxidoreduction which can be stimulated by ionophores and uncouplers. In addition, extra proton translocation out of the vesicles and membrane potential generation during electron transport were observed, suggesting a universal mechanism of electron and proton transport through all the tested cytochrome complexes.

A cytochrome b/c1 complex which catalyses the reduction of cytochrome c by ubiquinol has been isolated from Rhodopseudomonas sphaeroides GA. It contains two hemes b and substoichiometric amounts of ubiquinone-10 and of the Rieske Fe-S center per cytochrome c1, and is essentially free of reaction center and bacteriochlorophyll. The complex consists of three major polypeptides with apparent molecular masses of 40, 34 and 25 kDa. The 34-kDa polypeptide carries heme. Cytochrome c1 has a midpoint potential of 285 mV. For cytochrome b two midpoint potentials, at 50 and -60 mV, at pH 7.4, can be derived if one assumes two components of equal amount. Ubiquinol--cytochrome c oxidoreductase activity is specific for ubiquinol and bacterial cytochromes c, and is inhibited by antimycin A and 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole. The complex shows oxidant-induced reduction of cytochrome b.