Reduction of cytochrome b in mitochondria from yeast lacking coenzyme Q

Ubiquinol-cytochrome c oxidoreductase. The redox reactions of the bis-heme cytochrome b in ubiquinone-sufficient and ubiquinone-deficient systems

Antimycin and myxothiazol are stoichiometric inhibitors of complex III (ubiquinol-cytochrome c oxidoreductase), exerting their highest degree of inhibition at I mol each/mol of complex III monomer. Phenomenologically, however, they each inhibit three steps in the redox reaction of the bis-heme cytochrome b in submitochondrial particles (SMP), and all three inhibitions are incomplete to various extents. (i) In SMP, reduction of hemes bH and bL by NADH or succinate is inhibited when the particles are treated with both antimycin and myxothiazol. Each inhibitor alone allows reduced bH and bL to accumulate, indicating that each inhibits the reoxidation of these hemes. (E)-Methyl-3-methoxy-2-(4')-trans-stilbenyl)acrylatc in combination with antimycin or 2-n-heptyl-4-hydroxyquinoline-N-oxide in combination with myxothiazol causes less inhibition of b reduction than the combination of antimycin and myxothiazol. (ii) Reoxidation of reduced b, is inhibited by either antimycin or myxothiazol (or 2-n-heptyl-4-hydroxyquinoline-N-oxide, (E)-methyl-3-methoxy-2-(4'-trans-stilbenyl)acrylate, or stigmatellin). (iii) Reoxidation of reduced bH is also inhibited by any one of these reagents. These inhibitions are also incomplete, and reduced bL is oxidized through the leaks allowed by these inhibitors at least 10 times faster than reduced bH. Heme bH can be reduced in SMP via cytochrome c, and the Rieske iron-sulfur protein by ascorbate and faster by ascorbate + TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine). Energization of SMP by the addition of ATP affords reduction of bL as well. Reverse electron transfer to bH and bL is inhibited partially by myxothiazol, much more by antimycin. Ascorbate + TMPD also reduce bH in ubiquinone-extracted SMP in which the molar ratio of ubiquinone to cytochrome b has been reduced 200-fold from 12.5 to aproximately 0.06. Reconstitution of the extracted particles with ubiquinone-10 restores substrate oxidation but does not improve the rate or the extent of b, reduction by ascorbate + TMPD. These reagents also partially reduce cytochrome b in SMP from a ubiquinone-deficient yeast mutant. The above results are discussed in relation to the Q-cycle hypothesis.

Reduction of exogenous quinones and 2,6-dichlorophenol indophenol in cytochrome b-deficient yeast mitochondria: a differential effect on center i and center o of the cytochrome b-c1 complex

The reduction of duroquinone (DQ), 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone (DB), and dichlorophenol indophenol (DCIP) by succinate and NADH was investigated in yeast mitochondria which have no spectrally detectable cytochrome b. Succinate reduces DB in the cytochrome b-deficient mitochondria at rates comparable to that observed in wild-type mitochondria, suggesting that succinate:ubiquinone oxidoreductase is unaffected by the lack of cytochrome b. In the mutant mitochondria, succinate does not reduce DQ or DCIP at significant rates; however, NADH reduces both DQ and DCIP at rates similar to that of the wild-type mitochondria in a myxothiazol, but not antimycin, sensitive reaction. The Ki for myxothiazol in this reaction is close to that for electron transfer through the cytochrome b-c1 complex. In addition, myxothiazol does not inhibit NADH:ubiquinone oxidoreductase. These results confirm our previous suggestion that the cytochrome b-c1 complex is involved in electron transfer from the primary dehydrogenases to DQ and DCIP and suggest that cytochrome b is not the binding site for myxothiazol.

The interaction of quinone analogues with wild-type and ubiquinone-deficient yeast mitochondria

The interaction of the exogenous quinones, duroquinone (DQ) and the decyl analogue of ubiquinone (DB) with the mitochondrial respiratory chain was studied in both wild-type and a ubiquinone-deficient mutant of yeast. DQ can be reduced directly by NADH dehydrogenase, but cannot be reduced by succinate dehydrogenase in the absence of endogenous ubiquinone. The succinate-driven reduction of DQ can be stimulated by DB in a reaction inhibited 50% by antimycin and 70-80% by the combined use of antimycin and myxothiazol, suggesting that electron transfer occurs via the cytochrome b-c1 complex. Both DQ and DB can effectively mediate the reduction of cytochrome b by the primary dehydrogenases through center o, but their ability to mediate the reduction of cytochrome b through center i is negligible. Two reaction sites for ubiquinol seem to be present at center o: one is independent of endogenous Q6 with a high reaction rate and a high Km; the other is affected by endogenous Q6 and has a low reaction rate and a low Km. By contrast, only one ubiquinol reaction site was observed at center i, where DB appears to compete with endogenous Q6. DB can oxidize most of the pre-reduced cytochrome b, while DQ can oxidize only 50%. On the basis of these data, the possible binding patterns of DB on different Q-reaction sites and the requirement for ubiquinone in the continuous oxidation of DQH are discussed.