Electron Transport in Neurospora Mitochondria

Abstract

Abstract A comparative study has been made of mitochondria isolated from wild type Neurospora and from poky, a cytochrome-deficient mutant. Both types of mitochondria respire with a variety of Krebs cycle acids and with NADH, but State 3 respiration rates are typically 20 to 40% lower in poky than in wild type. The dehydrogenases and flavoproteins of the two strains appear qualitatively similar, as judged by the dependence of respiration rate on substrate concentration and by the effect of pyrrolnitrin, an inhibitor that acts in the flavin region. The major difference between wild type and poky is in their oxidase systems. In wild type, more than 90% of the respiration is mediated by a conventional cytochrome chain (sensitive to antimycin and cyanide), whereas poky uses both a cytochrome chain and an alternate oxidase (resistant to antimycin and cyanide but sensitive to salicyl hydroxamic acid). The alternate oxidase accounts for as much as 70% of the respiration in poky mitochondria. Two lines of evidence suggest that, in poky, the cytochrome chain and the alternate oxidase compete for reducing equivalents on the oxygen side of the dehydrogenases, and thus form a branched electron transport system. (a) Total State 3 respiration rates are significantly lower than would be predicted from the sum of the two oxidase activities measured separately. (b) In addition, it is possible to demonstrate a rapid, energy-dependent reduction of the alternate oxidase by N,N,N' ,N'-tetramethyl-p-phenylenediamine dihydrochloride plus ascorbate (via cytochrome c); the deficiency of Site 1 in poky mitochondria (see next paper) makes it unlikely that electrons pass from cytochrome c through the dehydrogenases in this experiment.