ubiquinol and 2-3-dimethoxy-5-methyl-6-decyl-1-4-benzoquinone

We have investigated the interaction between monomers of the dimeric yeast cytochrome bc(1) complex by analyzing the pre-steady and steady state activities of the isolated enzyme in the presence of antimycin under conditions that allow the first turnover of ubiquinol oxidation to be observable in cytochrome c(1) reduction. At pH 8.8, where the redox potential of the iron-sulfur protein is approximately 200 mV and in a bc(1) complex with a mutated iron-sulfur protein of equally low redox potential, the amount of cytochrome c(1) reduced by several equivalents of decyl-ubiquinol in the presence of antimycin corresponded to only half of that present in the bc(1) complex. Similar experiments in the presence of several equivalents of cytochrome c also showed only half of the bc(1) complex participating in quinol oxidation. The extent of cytochrome b reduced corresponded to two b(H) hemes undergoing reduction through one center P per dimer, indicating electron transfer between the two cytochrome b subunits. Antimycin stimulated the ubiquinol-cytochrome c reductase activity of the bc(1) complex at low inhibitor/enzyme ratios. This stimulation could only be fitted to a model in which half of the bc(1) dimer is inactive when both center N sites are free, becoming active upon binding of one center N inhibitor molecule per dimer, and there is electron transfer between the cytochrome b subunits of the dimer. These results are consistent with an alternating half-of-the-sites mechanism of ubiquinol oxidation in the bc(1) complex dimer.

Topics: Antimycin A; Cytochromes b; Cytochromes c; Dimerization; Electron Transport Complex III; Fungal Proteins; Heme; Hydrogen-Ion Concentration; Iron-Sulfur Proteins; Kinetics; Mutation; Oxidation-Reduction; Oxygen; Spectrophotometry; Time Factors; Ubiquinone; Ultraviolet Rays

Ubiquinone 0 and decylubiquinone have been reported to inhibit the mitochondrial permeability transition pore (PTP) [Fontaine, E., Ichas, F. and Bernardi, P. (1998) J. Biol. Chem. 273, 25734-257401, offering a new clue to its molecular composition. In patch-clamp experiments on rat liver mitochondria we have observed that these compounds also inhibit the previously described mitochondrial megachannel (MMC), confirming its identification as the PTP. Inhibition can be reversed by increasing [Ca2+], in analogy to the behavior observed with several other disparate PTP/MMC inhibitors. To rationalize the ability of Ca2+ to overcome inhibition by various quite different compounds we propose that it acts via the phospholipid bilayer.

Topics: Animals; Benzoquinones; Calcium; Cations, Divalent; Ion Channels; Membrane Proteins; Mitochondria, Liver; Mitochondrial Membrane Transport Proteins; Mitochondrial Permeability Transition Pore; Rats; Ubiquinone