naphthoquinones and antimycin

Flash-induced redox changes of b-type and c-type cytochromes have been studied in chromatophores from the aerobic photosynthetic bacterium Roseobacter denitrificans under redox-controlled conditions. The flash-oxidized primary donor P+ of the reaction center (RC) is rapidly re-reduced by heme H1 (Em,7 = 290 mV), heme H2 (Em,7 = 240 mV) or low-potential hemes L1/L2 (Em,7 = 90 mV) of the RC-bound tetraheme, depending on their redox state before photoexcitation. By titrating the extent of flash-induced low-potential heme oxidation, a midpoint potential equal to -50 mV has been determined for the primary quinone acceptor QA. Only the photo-oxidized heme H2 is re-reduced in tens of milliseconds, in a reaction sensitive to inhibitors of the bc1 complex, leading to the concomitant oxidation of a cytochrome c spectrally distinct from the RC-bound hemes. This reaction involves cytochrome c551 in a diffusional process. Participation of the bc1 complex in a cyclic electron transfer chain has been demonstrated by detection of flash-induced reduction of cytochrome b561, stimulated by antimycin and inhibited by myxothiazol. Cytochrome b561, reduced upon flash excitation, is re-oxidized slowly even in the absence of antimycin. The rate of reduction of cytochrome b561 in the presence of antimycin increases upon lowering the ambient redox potential, most likely reflecting the progressive prereduction of the ubiquinone pool. Chromatophores contain approximately 20 ubiquinone-10 molecules per RC. At the optimal redox poise, approximately 0.3 cytochrome b molecules per RC are reduced following flash excitation. Cytochrome b reduction titrates out at Eh < 100 mV, when low-potential heme(s) rapidly re-reduce P+ preventing cyclic electron transfer. Results can be rationalized in the framework of a Q-cycle-type model.

Topics: Antimycin A; Bacteria; Bacterial Physiological Phenomena; Benzoquinones; Cytochrome b Group; Cytochrome c Group; Electron Transport Complex III; Electrons; Enzyme Inhibitors; Ferricyanides; Kinetics; Light; Methacrylates; Naphthoquinones; Oxidation-Reduction; Phenylenediamines; Photosynthesis; Photosynthetic Reaction Center Complex Proteins; Proteobacteria; Thiazoles; Time Factors; Titrimetry

A combination of atovaquone and proguanil has been found to be quite effective in treating malaria, with little evidence of the emergence of resistance when atovaquone was used as a single agent. We have examined possible mechanisms for the synergy between these two drugs. While proguanil by itself had no effect on electron transport or mitochondrial membrane potential (DeltaPsim), it significantly enhanced the ability of atovaquone to collapse DeltaPsim when used in combination. This enhancement was observed at pharmacologically achievable doses. Proguanil acted as a biguanide rather than as its metabolite cycloguanil (a parasite dihydrofolate reductase [DHFR] inhibitor) to enhance the atovaquone effect; another DHFR inhibitor, pyrimethamine, also had no enhancing effect. Proguanil-mediated enhancement was specific for atovaquone, since the effects of other mitochondrial electron transport inhibitors, such as myxothiazole and antimycin, were not altered by inclusion of proguanil. Surprisingly, proguanil did not enhance the ability of atovaquone to inhibit mitochondrial electron transport in malaria parasites. These results suggest that proguanil in its prodrug form acts in synergy with atovaquone by lowering the effective concentration at which atovaquone collapses DeltaPsim in malaria parasites. This could explain the paradoxical success of the atovaquone-proguanil combination even in regions where proguanil alone is ineffective due to resistance. The results also suggest that the atovaquone-proguanil combination may act as a site-specific uncoupler of parasite mitochondria in a selective manner.

Topics: Animals; Antimalarials; Antimycin A; Atovaquone; Drug Synergism; Female; Folic Acid Antagonists; Male; Membrane Potentials; Mice; Mice, Inbred BALB C; Mitochondria; Naphthoquinones; Oxygen Consumption; Plasmodium yoelii; Proguanil

The activity of a particulate succinate cytochrome c reductase is inhibited by antimycin, 2-heptyl-4-hydroxyquinoline-N-oxide, 2-(9-cyclohexyl-n-nonyl)-3-hydroxy-1,4naphthoquinone and thenoyltrifluoroacetone. The ratio of antimycin A (required for complete inhibition) to the molar content of the cytochrome b of the reductase is approximately 0.5 in contrast to the reported value of 1.0 or higher for succinate oxidase preparations. However, the degreeof inhibition by antimycin is dependent on the exogenous coenzyme Q (ubiquinone) present. Indeed, the inhibition from any of these compounds is competitively reversed by exogenous coenzyme Q in the system.

Topics: Antimycin A; Electron Transport Complex II; Electron Transport Complex IV; Enzyme Inhibitors; Hydroxyquinolines; Indophenol; Lipotropic Agents; Naphthoquinones; Oxidoreductases; Pharmacology; Quinolines; Research; Succinate Cytochrome c Oxidoreductase; Succinate Dehydrogenase; Ubiquinone