The interaction of ubisemiquinones with the iron—sulphur centre S-3 of succinate dehydrogenase in plant mitochondria

Abstract

At low temperatures (< 20 K) complex EPR spectra are observed in various plant and mammalian mitochondria which have been attributed to an interaction of quinones in the partially reduced form and oxidized centre S-3 of succinate dehydrogenase [ l-51. From computer simulations [2] and potentiometric titrations [6] it was concluded that the semiquinone signals arise from a dipolar interaction between a pair of ubisemiquinones. In order to explain the low temperatures necessary to observe these signals an additional interaction of the ubisemiquinone pair with centre S-3 was assumed [2,6]. However, the EPR spectrum of the postulated ubisemiquinone pair alone without the presence of centre S-3 could not be observed. In an attempt to resolve such complex spectra we have investigated their temperature dependence and microwave power saturation in mitochondria from mung beans. Since these mitochondria possess a cyanideand antimycin A-insensitive alternative route of substrate oxidation [7] it was also of interest to study the relationship of the complex signal to the alternative oxidation step. Particularly, since recent studies have indicated that the branchpoint of the alternative pathway from the main respiratory chain may be located at the ubiquinone level [8,9]. Small, but detectable differences were observed in the electron spin relaxation behaviour which have allowed us to further characterize the interaction between centre S-3 and the ubisemiquinone species.