Iron-sulphur proteins in the succinate oxidase system

Abstract

1. Succinate reduces at least three different species of iron-sulphur protein, detectable by EPR spectroscopy at 82° K, in sub-mitochondrial particles or preparations of Complex I + II + III. Two of these species (symbolized as Fe-S S and Fe-S P 1 ) are rapidly reduced by succinate in non-phosphorylating particles, the third (Fe-S P 2 ) is slowly reduced. 2. The reduction of Fe-S S is unaffected by addition of 2-thenoyltrifluoroacetone (TTFA), or by extraction of the dried particles with pentane. This species probably belongs to succinate dehydrogenase. 3. Reduction of the other rapidly reduced species (Fe-S P 1 ) is inhibited by addition of TTFA or by pentane extraction. Re-incorporation of Q-10 or ‘P’ ( Albracht, Van Heerikhuizen and Slater, FEBS Lett., 13 (1971) 265) restores the rate of reduction. 4. Antimycin blocks about one-half of the EPR signal of the slowly reduced species (Fe-S P 2 ). 5. All three iron-sulphur species reducible by succinate have similar EPR spectra with g z = 2.02, g y = 1.94 and g x = 1.92. 6. NADH also reduces these iron-sulphur species as well as those associated specifically with NADH dehydrogenase. 7. The bleaching of a pigment absorbing at 460 nm brought about by addition of succinate to pentane-extracted particles is largely (62 %) due to reduction of succinate dehydrogenase (flavin and Fe-S S). Fe-S P 1 , Fe-S P 2 and cytochrome b contribute to the extent of 15 %. The component responsible for the remainder (23 %) has not been identified. Its reduction is inhibited by TTFA. 8. A model of the respiratory chain is proposed that takes account of the recently discovered electron carriers.