Ubisemiquinone radicals from the cytochrome b-c₁ complex of the mitochondrial electron transport chain-Demonstration of QP-S radical formation

Stable ubisemiquinone radical(s) in the cytochrome b-c₁-II complex of bovine heart was observed following reduction by succinate in the presence of catalytic amounts of succinate dehydrogenase. The radical was abolished by addition of antimycin A, but a residual radical remained in the presence of excess exogenous Q₂. The radical showed an EPR signal of g = 2.0046 ± .003 at X band (∼9.4 GHz) with no resolved hyperfine structure and had a line width of 8.1 ± .5 Gauss at 23°C. The Q band (35 GHz) spectra showed wellresolved g-anisotropy and had a field separation between derivative extrema of 26 ± 1 Gauss. This radical is evidently from QP-C. These observations substantiate that the radical is immobilized and bound to a protein. The QP-S radical was demonstrated in the cytochrome b-c₁-II complex only in the presence of more than a catalytic amount of succinate dehydrogenase and cytochrome b-c₁. This signal was not antimycin a inhibitory. The signal amplitude paralleled the reconstitutive enzymic activity of succinate-cytochrome c reductase from succinate dehydrogenase and the cytochrome b-c₁-II complex.