Oxidative phosphorylation supported by an alternative respiratory pathway in mitochondria from Euglena

The effect of antimycin, myxothiazol, 2-heytyl-4-hydroxyquinoline-N-oxide, stigmatellin and cyanide on respiration, ATP synthesis, cytochrome c reduct ase, and membrane potential in mitochondria isolated from dark-grown Euglen a cells was determined. With L-lactate as substrate, ATP synthesis was part ially inhibited by antimycin, but the other four inhibitors completely abol ished the process. Cyanide also inhibited the antimycin-resistant ATP synth esis. Membrane potential was collapsed (< 60 mV) by cyanide and stigmatelli n. However, in the presence of antimycin, a H+ gradient (> 60 mV) that suff iced to drive ATP synthesis remained. Cytochrome c reductase, with L-lactat e as donor, was diminished by antimycin and myxothiazol. Cytochrome bc(1) c omplex activity was fully inhibited by antimycin. but it was resistant to m yxothiazol. Stigmatellin inhibited both L-lactate-dependent cytochrome c re ductase and cytochrome bc(1) complex activities. Respiration was partially inhibited by the five inhibitors. The cyanide-resistant respiration was str ongly inhibited by diphenylamine, n-propyl-gallate, salicylhydroxamic acid and disulfiram. Based on these results, a model of the respiratory chain of Euglena mitochondria is proposed, in which a quinol-cytochrome c oxidoredu ctase resistant to antimycin, and a quinol oxidase resistant to antimycin a nd cyanide are included. (C) 2000 Elsevier Science B.V. All rights reserved .