antimycin and duroquinone

The rates of ATP synthesis and of ATP-driven NAD reduction have been measured in bovine heart submitochondrial particles as a function of the fraction of inhibited redox pumps (in titrations with either antimycin or rotenone) and of the fraction of inhibited ATPases (in titrations with DCCD). The flux control coefficients of the redox and ATPase proton pumps on the rates of ATP synthesis and of ATP-driven NAD reduction have been derived and found to be equal to 1 for both pumps; i.e., both pumps appear to be 'completely rate limiting'. A theoretical analysis of the inhibitor titration approach based on kinetic models of chemiosmotic coupling and on the theory of metabolic control is presented. This analysis (i) shows that the results of the single inhibitor titrations are incompatible with a delocalized chemiosmotic mechanism of energy coupling if the proton conductance of the membrane is sufficiently low with respect to the conductances of the pumps; and (ii) suggests an experimental approach based on the determination of the P/O and the respiratory control ratios at different degrees of inhibition of the proton pumps to establish the origin of the 'loose coupling' of submitochondrial particle preparations. Three independent types of observation show that the 'loose coupling' of the particle preparation is not mainly due to an increased membrane proton conductance. The same and other independent observations are consistent with the view that the loose coupling of submitochondrial particle preparation is due mainly to inhomogeneity, i.e. to the presence of a subpopulation of highly leaky non-phosphorylating vesicles respiring at maximal rate. The results as a whole together with the simulations and analysis presented lead to the conclusion that the mechanism of free-energy coupling in submitochondrial particles is not completely delocalized.

Topics: Adenosine Triphosphatases; Adenosine Triphosphate; Animals; Antimycin A; Benzoquinones; Cattle; Computer Simulation; Dicyclohexylcarbodiimide; Energy Metabolism; Ion Channels; Kinetics; Mitochondria, Heart; Models, Biological; NAD; Oxidation-Reduction; Oxygen Consumption; Phosphorus; Protons; Quinones; Rotenone; Submitochondrial Particles; Thermodynamics

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.

Topics: Antimycin A; Benzoquinones; Cytochrome b Group; Electron Transport; Electron Transport Complex III; Kinetics; Methacrylates; Mitochondria; Oxidation-Reduction; Quinones; Saccharomyces cerevisiae; Succinate Dehydrogenase; Succinates; Succinic Acid; Thiazoles; Ubiquinone

In uncoupled pig-heart mitochondria the rate of the reduction of duroquinone by succinate in the presence of cyanide is inhibited by about 50% by antimycin. This inhibition approaches completion when myxothiazol is also added or British anti-Lewisite-treated (BAL-treated) mitochondria are used. If mitochondria are replaced by isolated succinate:cytochrome c oxidoreductase, the inhibition by antimycin alone is complete. The reduction of a plastoquinone homologue with an isoprenoid side chain (plastoquinone-2) is strongly inhibited by antimycin with either mitochondria or succinate:cytochrome c reductase. The reduction by succinate of plastoquinone analogues with an n-alkyl side chain in the presence of mitochondria is inhibited neither by antimycin nor by myxothiazol, but is sensitive to the combined use of these two inhibitors. On the other hand, the reduction of the ubiquinone homologues Q2, Q4, Q6 and Q10 and an analogue, 2,3-dimethoxyl-5-n-decyl-6-methyl-1,4-benzoquinone, is not sensitive to any inhibitor of QH2:cytochrome c reductase tested or their combined use, either in normal or BAL-treated mitochondria or in isolated succinate:cytochrome c reductase. It is concluded that quinones with a ubiquinone ring can be reduced directly by succinate:Q reductase, whereas those with a plastoquinone ring can not. Reduction of the latter compounds requires participation of either center i or center o (Mitchell, P. (1975) FEBS Lett. 56, 1-6) or both, of QH2:cytochrome c oxidoreductase. It is proposed that a saturated side chain promotes, while an isoprenoid side chain prevents reduction of these compounds at center o.

Topics: Animals; Antimycin A; Benzoquinones; Dimercaprol; Electron Transport; Electron Transport Complex III; Mitochondria; Oxidation-Reduction; Plastoquinone; Quinones; Structure-Activity Relationship; Succinates; Succinic Acid; Swine; Ubiquinone