Rapid reduction of cytochrome c1 in the presence of antimycin and its implication for the mechanism of electron transfer in the cytochrome b-c1 segment of the mitochondrial respiratory chain

An unexpected protein interaction promotes drug resistance in leukemia

The overall survival of patients with acute myeloid leukemia (AML) is poor and identification of new disease-related therapeutic targets remains a major goal for this disease. Here we show that expression of MPP1, a PDZ-domain-containing protein, highly correlated with ABCC4 in AML, is associated with worse overall survival in AML. Murine hematopoietic progenitor cells overexpressing MPP1 acquired the ability to serially replate in methylcellulose culture, a property crucially dependent upon ABCC4. The highly conserved PDZ-binding motif of ABCC4 is required for ABCC4 and MPP1 to form a protein complex, which increased ABCC4 membrane localization and retention, to enhance drug resistance. Specific disruption of this protein complex, either genetically or chemically, removed ABCC4 from the plasma membrane, increased drug sensitivity, and abrogated MPP1-dependent hematopoietic progenitor cell replating in methylcellulose. High-throughput screening identified Antimycin A as a small molecule that disrupted the ABCC4–MPP1 protein complex and reversed drug resistance in AML cell lines and in primary patient AML cells. In all, targeting the ABCC4–MPP1 protein complex can lead to new therapies to improve treatment outcome of AML, a disease where the long-term prognosis is poor.

ABCC4 is a chemotherapeutic drug exporter highly expressed in acute myeloid leukemia. Here, the authors demonstrate that MPP1 anchors ABCC4 to the outer cell membrane mediating drug resistance in leukemic cells and identify antimycin A as a chemical probe that disrupts such interaction and restores sensitivity.

The antimycin A-sensitive pathway of cyclic electron flow: from 1963 to 2015

Cyclic electron flow has puzzled and divided the field of photosynthesis researchers for decades. This mainly concerns the proportion of its overall contribution to photosynthesis, as well as its components and molecular mechanism. Yet, it is irrefutable that the absence of cyclic electron flow has severe effects on plant growth. One of the two pathways mediating cyclic electron flow can be inhibited by antimycin A, a chemical that has also widely been used to characterize the mitochondrial respiratory chain. For the characterization of cyclic electron flow, antimycin A has been used since 1963, when ferredoxin was found to be the electron donor of the pathway. In 2013, antimycin A was used to identify the PGRL1/PGR5 complex as the ferredoxin:plastoquinone reductase completing the last puzzle piece of this pathway. The controversy has not ended, and here, we review the history of research on this process using the perspective of antimycin A as a crucial chemical for its characterization.

Anti-cooperative Oxidation of Ubiquinol by the Yeast Cytochrome bc1 Complex

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.

Evidence for a concerted mechanism of ubiquinol oxidation by the cytochrome bc1 complex

To better understand the mechanism of divergent electron transfer from ubiquinol to the iron-sulfur protein and cytochrome b(L) within the cytochrome bc(1) complex, we have examined the effects of antimycin on the presteady state reduction kinetics of the bc(1) complex in the presence or absence of endogenous ubiquinone. When ubiquinone is present, antimycin slows the rate of cytochrome c(1) reduction by approximately 10-fold but had no effect upon the rate of cytochrome c(1) reduction in bc(1) complex lacking endogenous ubiquinone. In the absence of endogenous ubiquinone cytochrome c(1), reduction was slower than when ubiquinone was present and was similar to that in the presence of ubiquinone plus antimycin. These results indicate that the low potential redox components, cytochrome b(H) and b(L), exert negative control on the rate of reduction of cytochrome c(1) and the Rieske iron-sulfur protein at center P. If electrons cannot equilibrate from cytochrome b(H) and b(L) to ubiquinone, partial reduction of the low potential components slows reduction of the high potential components. We also examined the effects of decreasing the midpoint potential of the iron-sulfur protein on the rates of cytochrome b reduction. As the midpoint potential decreased, there was a parallel decrease in the rate of b reduction, demonstrating that the rate of b reduction is dependent upon the rate of ubiquinol oxidation by the iron-sulfur protein. Together these results indicate that ubiquinol oxidation is a concerted reaction in which both the low potential and high potential redox components control ubiquinol oxidation at center P, consistent with the protonmotive Q cycle mechanism.

Molecular characterization of two spontaneous antimycin A resistant mutants of Rhodospirillum rubrum

Antimycin A is an inhibitor of cytochrome bc1 complexes acting at the quinone reducing site (Qi) of the cytochrome b subunit. We report here the isolation and molecular characterization of two spontaneous mutants of the purple non-sulfur bacterium Rhodospirillum rubrum resistant to this inhibitor. In the two mutants antimycin A resistance was found to be conferred by replacement of an aspartate residue at position 243 of the cytochrome b polypeptide chain, in one case by histidine and in the other by glutamate. The mutants exhibit cross-resistance to aurachin C but not to aurachin D. The exchange of Asp-243 does not only diminish the antimycin sensitivity of the isolated cytochrome bc1 complexes but also has effects on the function of the quinone reducing site (Qi). Oxidant-induced reduction of cytochrome b, requiring addition of antimycin A in the wild type, is already at a maximum in the absence of antimycin A. This indicates a diminished electron flow between heme b-566 and ubiquinone at the quinone reducing site (Qi) of cytochrome b.

The protonmotive Q cycle in mitochondria and bacteria

The cytochrome bc1 complex is an oligomeric electron transfer enzyme located in the inner membrane of mitochondria and the plasma membrane of bacteria. The cytochrome bc1 complex participates in respiration in eukaryotic cells and also participates in respiration, cyclic photosynthetic electron transfer, denitrification, and nitrogen fixation in a phylogenetically diverse collection of bacteria. In all of these organisms, the cytochrome bc1 complex transfers electrons from ubiquinol to cytochrome c and links this electron transfer to translocation of protons across the membrane in which it resides, thus converting the available free energy of the oxidation-reduction reaction into an electrochemical proton gradient. The mechanism by which the cytochrome bc1 complex achieves this energy transduction is the protonmotive Q cycle. The Q cycle mechanism has been documented by extensive experimentation, and recent investigations have focused on structural features of the three redox subunits of the bc1 complex essential to the protonmotive and electrogenic activities of this membranous enzyme.

Mutational analysis of assembly and function of the iron-sulfur protein of the cytochrome bc1 complex in Saccharomyces cerevisiae

The iron-sulfur protein of the cytochrome bc1 complex oxidizes ubiquinol at center P in the protonmotive Q cycle mechanism, transferring one electron to cytochrome c1 and generating a low-potential ubisemiquinone anion which reduces the low-potential cytochrome b-566 heme group. In order to catalyze this divergent transfer of two reducing equivalents from ubiquinol, the iron-sulfur protein must be structurally integrated into the cytochrome bc1 complex in a manner which facilitates electron transfer from the iron-sulfur cluster to cytochrome c1 and generates a strongly reducing ubisemiquinone anion radical which is proximal to the b-566 heme group. This radical must also be sequestered from spurious reactivities with oxygen and other high-potential oxidants. Experimental approaches are described which are aimed at understanding how the iron-sulfur protein is inserted into center P, and how the iron-sulfur cluster is inserted into the apoprotein.

Comparison between the properties of 3-nitrosalicyl-N-alkylamide and antimycin A acting on QH2:cytochrome c reductase

3-Nitrosalicyl N-alkylamide was found to be an inhibitor different from antimycin A not only in its inhibitory nature but also in many other aspects. This difference indicated that the 11 kDa component, which was identified as the antimycin A (AA) binding factor in the QH2: cytochrome c reductase of Rhodopseudomonas sphaeroides by Wilson et al. ((1985) J. Biol. Chem. 260, 10288-10292) using the radioactive photoaffinity analogue 3-azidosalicyl N-octadecylamide, was not the genuine binding site of AA. Based on the observations that the 3-azidosalicyl N-alkylamide specifically inhibits the reactions of ubiquinone catalyzed by Q-related enzymes of the respiratory chain, the labeled 11 kDa factor might be one of the ubiquinone binding proteins in QH2:cytochrome c reductase.

The antimycin-A-insensitive respiratory pathway of Candida parapsilosis: evidence for a second quinone involved specifically in its functioning

The involvement of a quinone in the antimycin A-insensitive electron transfer from NADH-dehydrogenase to cytochrome c via the alternative respiratory chain of Candida parapsilosis, by-passing complex II, has been studied. After a partial extraction of quinones, the residual respiration was fully antimycin-A-sensitive, but reincorporation of the organic extract partially restored an antimycin A-insensitive respiration. Analysis of quinone content by HPLC, after purification by thin-layer chromatography, evidenced another quinone species in a very low amount. Myxothiazol and stigmatellin were shown to inhibit the alternative pathway but at a higher concentration than required to inhibit the classical pathway. Cytochrome spectra analysis showed that, in the presence of high myxothiazol concentrations, cytochromes c and aa3 were not reduced, while they were in the presence of antimycin A. It is suggested that the secondary pathway of C. parapsilosis involved a specific quinone pool which can be displaced from its binding site by high concentrations of myxothiazol or analogous compounds.

Analysis of inhibitor binding to the mitochondrial cytochrome c reductase by fluorescence quench titration. Evidence for a 'catalytic switch' at the Qo center

The binding characteristics of inhibitors of the mitochondrial cytochrome c reductase were studied by fluorescence quench titration. Based on the standard binding equation, the applied numerical method allowed the online recorded titration curves to be interpreted by fitting the Kd, the number of binding sites, and the specific fluorescence of the free and the bound inhibitor. For the Qi center, 2-n-nonyl-4-hydroxyquinoline N-oxide and for the Qo center (E)-beta-methoxyacrylate-stilbene (MOA-stilbene) were used as fluorescing inhibitors. The experiments could be extended to other, non-fluorescing inhibitors by competition analysis. Using this method we were able to compare the binding behaviour of Qi and Qo center inhibitors under different redox states of the enzyme using the same experimental set up. We studied the competition between inhibitors of the cytochrome c reductase representative for all subgroups and demonstrated that at least three inhibitor binding sites exist, two located in the Qo center, one located in the Qi center. Determination of the dissociation constants of the oxidized, the partially reduced and the fully reduced enzyme showed that inhibitor binding at the Qi center is not redox-dependent. In contrast, the binding of MOA-stilbene to the Qo center is decreased after reduction of the iron-sulfur center and cytochrome c1, whereas this redox change increases the affinity for a Qo center inhibitor of the hydroxynaphthoquinone type, 3-n-undecyl-2-hydroxynaphthoquinone. From these results, aware of the fact that the inhibitory mechanism at the Qo center is a non-competitive one, we made the hypothesis of a 'catalytic switch' to explain both the bifurcation of electron flow and the inhibition at the Qo center. A steric blockage of one of two conformational states could serve as a cogent explanation for the great structural variability of the inhibitors and differential effects on the redox centers exerted by the inhibitors. Moreover, the proposed 'switch' gives some insight into other experimental results which are difficult to explain with the ubiquinone cycle as currently formulated.

Cytochrome bc1 complexes of microorganisms

The cytochrome bc1 complex is the most widely occurring electron transfer complex capable of energy transduction. Cytochrome bc1 complexes are found in the plasma membranes of phylogenetically diverse photosynthetic and respiring bacteria, and in the inner mitochondrial membrane of all eucaryotic cells. In all of these species the bc1 complex transfers electrons from a low-potential quinol to a higher-potential c-type cytochrome and links this electron transfer to proton translocation. Most bacteria also possess alternative pathways of quinol oxidation capable of circumventing the bc1 complex, but these pathways generally lack the energy-transducing, protontranslocating activity of the bc1 complex. All cytochrome bc1 complexes contain three electron transfer proteins which contain four redox prosthetic groups. These are cytochrome b, which contains two b heme groups that differ in their optical and thermodynamic properties; cytochrome c1, which contains a covalently bound c-type heme; and a 2Fe-2S iron-sulfur protein. The mechanism which links proton translocation to electron transfer through these proteins is the proton motive Q cycle, and this mechanism appears to be universal to all bc1 complexes. Experimentation is currently focused on understanding selected structure-function relationships prerequisite for these redox proteins to participate in the Q-cycle mechanism. The cytochrome bc1 complexes of mitochondria differ from those of bacteria, in that the former contain six to eight supernumerary polypeptides, in addition to the three redox proteins common to bacteria and mitochondria. These extra polypeptides are encoded in the nucleus and do not contain redox prosthetic groups. The functions of the supernumerary polypeptides of the mitochondrial bc1 complexes are generally not known and are being actively explored by genetically manipulating these proteins in Saccharomyces cerevisiae.

Organization and function of cytochrome b and ubiquinone in the cristae membrane of beef heart mitochondria

The arrangement and function of the redox centers of the mammalian bc1 complex is described on the basis of structural data derived from amino acid sequence studies and secondary structure predictions and on the basis of functional studies (i.e., EPR data, inhibitor studies, and kinetic experiments). Two ubiquinone reaction centers do exist--a QH2 oxidation center situated at the outer, cytosolic surface of the cristae membrane (Q0 center), and a Q reduction center (Qi center) situated more to the inner surface of the cristae membrane. The Q0 center is formed by the b-566 domain of cytochrome b, the FeS protein, and maybe an additional small subunit, whereas the Qi center is formed by the b-562 domain of cytochrome b and presumably the 13.4 kDa protein ("QP-C"). The "Q binding proteins" are proposed to be protein subunits of the Q reaction centers of various multiprotein complexes. The path of electron flow branches at the Q0 center, half of the electrons flowing via the high-potential cytochrome chain to oxygen and half of the electrons cycling back into the Q pool via the cytochrome b path connecting the two Q reaction centers. During oxidation of QH2, 2H+ are released to the cytosolic space and during reduction of Q, 2H+ are taken up from the matrix side, resulting in a net transport across the membrane of 2H+ per e- flown from QH2 to cytochrome c, the H+ being transported across the membrane as H (H+ + e-) by the mobile carrier Q. The authors correct their earlier view of cytochrome b functioning as a H+ pump, proposing that the redox-linked pK changes of the acidic groups of cytochrome b are involved in the protonation/deprotonation processes taking place during the reduction and oxidation of Q. The reviewers stress that cytochrome b is in equilibrium with the Q pool via the Qi center, but not via the Q0 center. Their view of the mechanisms taking place at the reductase is a Q cycle linked to a Q-pool where cytochrome b is acting as an electron pump.

Experimental observations on the structure and function of mitochondrial complex III that are unresolved by the protonmotive ubiquinone-cycle hypothesis

The current model of the protonmotive ubiquinone cycle as applied to mitochondrial ubiquinol-cytochrome c reductase complex (Complex III) is able to explain a number of previously puzzling observations concerning electron-transfer and proton translocating functions of the complex. However, a number of pertinent experimental observations concerning the structure and function of this complex cannot as yet be incorporated into the present version of the ubiquinone cycle. The yet unresolved problems of electron transfer uncovered by these observations include some kinetic and thermodynamic problems, uncertainties in the binding site(s) and mode of binding of ubiquinol and inhibitors, the observed multiple spectroscopic, electrochemical, and kinetic forms of cytochromes b, iron-sulfur protein, and cytochrome c1, the multiple and overlapping effects of inhibitors, and the functional role of conformational changes in the complex. It is concluded that although the Q cycle is a valuable base for the design of future experiments, its mechanism must be reconciled with the above uncertainties as well as with the accumulated evidence that Complex III can exist in two or more interchangeable forms, exhibiting different properties with respect to electron-transfer pathways, inhibitor binding, and spectral and electrochemical properties of the electron-carrier subunits.

Ubisemiquinone is the electron donor for superoxide formation by complex III of heart mitochondria

Much evidence indicates that superoxide is generated from O2 in a cyanide-sensitive reaction involving a reduced component of complex III of the mitochondrial respiratory chain, particularly when antimycin A is present. Although it is generally believed that ubisemiquinone is the electron donor to O2, little experimental evidence supporting this view has been reported. Experiments with succinate as electron donor in the presence of antimycin A in intact rat heart mitochondria, which contain much superoxide dismutase but little catalase, showed that myxothiazol, which inhibits reduction of the Rieske iron-sulfur center, prevented formation of hydrogen peroxide, determined spectrophotometrically as the H2O2-peroxidase complex. Similarly, depletion of the mitochondria of their cytochrome c also inhibited formation of H2O2, which was restored by addition of cytochrome c. These observations indicate that factors preventing the formation of ubisemiquinone also prevent H2O2 formation. They also exclude ubiquinol, which remains reduced under these conditions, as the reductant of O2. Since cytochrome b also remains fully reduced when myxothiazol is added to succinate- and antimycin A-supplemented mitochondria, reduced cytochrome b may also be excluded as the reductant of O2. These observations, which are consistent with the Q-cycle reactions, by exclusion of other possibilities leave ubisemiquinone as the only reduced electron carrier in complex III capable of reducing O2 to O2-.

Thermodynamic and kinetic considerations of Q-cycle mechanisms and the oxidant-induced reduction of cytochromes b

In coenzyme Q-cycles, it is proposed that one electron from the quinol reduces the Rieske iron sulfur center (Em approximately 280 mV) and the remaining electron on the semiquinone reduces cytochrome br (Em approximately -60 mV). The Em for the two-electron oxidation of the quinol is approximately 60 mV and therefore the reduction of cytochrome bT by quinol is not favorable. As the stability constant for the dismutation of the semiquinone decreases, the calculated Em for the Q/QH. couple is lowered to values below the Em of cytochrome bT. Contemporary coenzyme Q-cycles are based on the belief that the lower value for the Em of the Q/QH. couple compared to the Em for cytochrome bT means that the semiquinone is a spontaneous reducing agent for the b-cytochrome. The analysis in the paper shows that this is not necessarily so and that neither binding sites nor ionization of the semiquinone per se alters this situation. For a Q-cycle mechanism to function, ad hoc provisions must be made to drive the otherwise unfavorable reduction of cytochrome bT by the semiquinone or for the simultaneous transfer of both electrons to cytochrome bT and cytochrome c1 (or the iron sulfur protein). Q-cycle mechanisms with these additional provisions can explain the observation thus far accumulated. A linear path which is functionally altered by conformational changes may also explain the data.

Effects of 2-hydroxy-3-undecyl-1,4-naphthoquinone on respiration of electron transport particles and mitochondria: topographical location of the Rieske iron-sulfur protein and the quinone binding site

2-Hydroxy-3-undecyl-1,4-naphthoquinone is a quinone analogue that inhibits mitochondrial respiration in the cytochrome b-c1 region with an apparent Ki of 2.5 X 10(-7) M. In electron transport particles, it prevents the reduction of cytochrome c1 by succinate but not its oxidation by oxygen and prevents oxidation of cytochrome b but not its reduction by succinate. The analogue increases the amount of steady-state cytochrome b reduction in actively respiring particles. It inhibits oxidant-induced reduction of cytochrome b in the presence of antimycin. Inhibition of succinate oxidase activity in electron transport particles is independent of the pH of the suspending medium while at pH values above 8 with mitochondria, inhibition decreases. Since the apparent pK of the bound quinone is pH 6.6, the pH dependency of the inhibition is likely due to the pK of the Rieske iron-sulfur center (pH 8). The Rieske center and thus the quinone binding site are located on the cytoplasmic face of the inner membrane.

Flash spectroscopic characterization of photosynthetic electron transport in isolated heterocysts

Electron transport was studied in heterocysts of the filamentous cyanobacterium Anabaena 7120 using spectral and kinetic analysis of absorbance transients elicited by single turnover flashes. Consistent photosynthetic turnovers were observed only in the presence of an exogenous source of reductant; therefore measurements were routinely made under a gas phase containing H2. Prominent absorbance changes corresponding to the oxidation of cytochrome c (554 nm) and the reduction of cytochrome b563 (563 nm) were observed. Under the most reducing conditions (99% H2/1% O2) cytochrome b563 was partially reduced between flashes in a slow, dark reaction. At 10-15% O2, the slow, dark reduction of cytochrome b563 was eliminated. Cytochrome turnover ceased entirely at high O2 concentrations (30%) but was restored by the addition of 25 microM KCN, demonstrating an interaction between the photosynthetic and respiratory electron transfer chains. Strobilurin A slowed the re-reduction of cytochrome c and eliminated the appearance of reduced cytochrome b563 by blocking electron transfer between reduced plastoquinone and the cytochrome b/f complex. Inhibition at a second site was apparent with 2-(n-heptyl)-4-hydroxyquinoline N-oxide, which blocked the reoxidation of cytochrome b563 but had little effect on cytochrome c relaxation. In uncoupled heterocysts, the rates of cytochrome c re-reduction and cytochrome b563 reduction were equal. Additional unassigned absorbance changes at 475 nm, 515 nm, and 572 nm were partially characterized. No absorbance change corresponding to an electrochromic shift was observed.

Interaction between isolated cytochrome c1 and cytochrome c

Cytochrome c1 from bovine heart mitochondria was isolated by a modification of the technique of König et al. [(1980) Biochim. Biophys. Acta 621, 283-295] which involved an affinity chromatography step on a gel with yeast cytochrome c as a ligand. Its spectra, electrophoretic pattern in presence of sodium dodecylsulfate, its reducibility by ascorbate and cytochrome c were characteristic of a native cytochrome, with a single polypeptide having an apparent molecular weight of 30 000. By using an arylazido derivative of cytochrome c, having the photoactive group bound to lysine 13, upon illumination a cross-link with the described preparation of cytochrome c1 was obtained. By pepsin digestion of the cross-linked complex a limiting fragment was obtained and partially sequenced. It allowed to identify the site of binding of cytochrome c near the sequence 167-174 of cytochrome c1.

Electron transfer through the isolated mitochondrial cytochrome b-c1 complex

(1) A kinetic analysis of electron donation into and through the cytochrome b-c1 complex isolated from bovine heart mitochondria has been undertaken, using trimethoquinol as the donor. (2) Rate constants of two routes of redox equilibration with quinols have been defined by kinetic measurements and with the use of the inhibitors antimycin A and myxothiazol. (3) A model of electron transfer based upon the original Q-cycle formulation is presented to explain these and related results.

Antimycin inhibition as a probe of mitochondrial function in isolated rat hepatocytes. Effects of chronic ethanol consumption

Previous studies have established that hepatic mitochondria and submitochondrial particles from rats, fed ethanol chronically, display diminished respiratory activities and alterations in the contents of specific electron transfer chain components. The latter include a decrease of about 50% in cytochrome b content. Titrations of respiratory activity in submitochondrial particles with antimycin, a stoichiometric inhibitor of electron flow through the cytochrome b-c1 region of the respiratory chain, indicated a comparable decrease (35%) in the amount of antimycin required to elicit maximal inhibition ('titer') after chronic ethanol treatment. Measurements of antimycin binding to submitochondrial particles by fluorescence quenching demonstrated a similar diminution in the number of tight binding sites per mg protein. By contrast, hepatocytes isolated from control and ethanol-fed rats exhibited nearly identical rates of oxygen utilization under a variety of conditions. However, antimycin titrations of respiratory activity in isolated hepatocytes revealed a 60% decrease in the antimycin titer, but no change in the maximal extent of inhibition after chronic ethanol treatment. Direct measurements of cytochrome b which could be reduced in the presence of antimycin in hepatocytes confirmed a comparable decrease (42%) after chronic ethanol treatment. The results demonstrate that molecular alterations in the cytochrome b region of the respiratory chain caused by ethanol feeding are present in intact liver cells, but suggest that substrate accessibility, rather than the respiratory chain, limits the rate of oxygen utilization in isolated hepatocytes. The data also suggest that mitochondria account for at least 80% of total oxygen utilization by liver cells from both control and ethanol-fed rats.

Multiphasic oxidation-reduction of cytochrome b in the succinate-cytochrome c reductase

The triphasic course previously reported for the reduction of cytochrome b in the succinate-cytochrome c reductase by either succinate or duroquinol has been shown to be dependent on the redox state of the enzyme preparation. Prior reduction with increasing concentrations of ascorbate leads to partial reduction of cytochrome c1, and a gradual decrease in the magnitude of the oxidation phase of cytochrome b. At an ascorbate concentration sufficient to reduce cytochrome c1 almost completely, the reduction of cytochrome b by either succinate or duroquinol becomes monophasic. Owing to the presence of a trace amount of cytochrome oxidase in the reductase preparation employed, the addition of cytochrome c makes electron flow from substrate to oxygen possible. Under such circumstances, the addition of a limited amount of either succinate or duroquinol leads to a multiphasic reduction and oxidation of cytochrome b. After the initial three phases as described previously, cytochrome b becomes oxidized before cytochrome c1 when the limited amount of added substrate is being used up. However, at the end of the reaction when cytochrome c1 is being rapidly oxidized, cytochrome b becomes again reduced. The above observations support a cyclic scheme of electron flow in which the reduction of cytochrome b proceeds by two different routes and its oxidation controlled by the redox state of a component of the respiratory chain.

Reduction of respiratory-chain cytochrome b by lactate in Saccharomyces cerevisiae

Cytochrome b of yeast mitochondria can be reduced by a part of the electrons resulting from the oxidation of lactate enantiomers. 1. The respiration of D-lactate and L-lactate is 30-40% inhibited by antimycin A. 2. Reduction of cytochrome b is observed in submitochondrial particles in the presence of low concentration of D-lactate and L-lactate (half-optimal concentration of 4.7 mM and 2.4 mM respectively) in the presence of different bc1 inhibitors. 3. Reduction of cytochrome b and c1 occurs in purified complex III of yeast in the presence of L-lactate and added L-lactate: NAD+ oxidoreductase. 4. In the particles obtained from yeast grown in lactate the oxidation of L-lactate involves the reduction of a pigment absorbing at 558 nm.

Mitochondrial lipid peroxidation by cumene hydroperoxide and its prevention by succinate

Rat liver mitochondria form lipid hydroperoxides when they are incubated aerobically with cumene hydroperoxide. The rate of reaction is dependent on the initial concentration of the latter and involves the consumption of oxygen. Gradient-separated and cytochrome c-depleted mitochondria, mitoplasts and submitochondrial fractions also undergo this peroxidation. Mitochondrial lipid peroxidation by cumene hydroperoxide is strongly inhibited by SKF52A (an inhibitor of cytochrome P-450), by antioxidants and to a lesser extent by the enzymes superoxide dismutase and catalase. Conversely, rotenone and N-ethylmaleimide stimulate the reaction. Succinate protects against the lipid peroxidation and in some mitochondrial fractions the associated oxygen uptake is also inhibited. This protection by succinate is prevented by malonate but not by N-ethylmaleimide or antimycin. Lipid hydroperoxides present in previously peroxidised mitochondria are partly lost on reincubation with succinate and this reaction is also unaffected by N-ethylmaleimide but inhibited by both malonate and antimycin. The results suggest that reduction of mitochondrial ubiquinone may prevent the generation of lipid hydroperoxides but that their subsequent removal may require reduction at or beyond cytochrome b.

The pathway of electrons through OH2:cytochrome c oxidoreductase studied by pre-steady -state kinetics

The kinetic behaviour of the prosthetic groups and the semiquinones in in QH2:cytochrome c oxidoreductase has been studied using a combination of the freeze-quench technique, low-temperature diffuse-reflectance spectroscopy, EPR and stopped flow. (2) In the absence of antimycin, cytochrome b-562 is reduced in two phases separated by a lag time. The initial very rapid reduction phase, that coincides with the formation of the antimycin-sensitive Qin, is ascribed to high-potential cytochrome b-562 and the slow phase to low-potential cytochrome b-562. the two cytochromes are present in a 1:1 molar ratio. The lag time between the two reduction phases decreases with increasing pH. Both the [2 Fe-2S] clusters and cytochrome c1 are reduced monophasically under these conditions, but at a rate lower than that of the initial rapid reduction of cytochrome b-562. (3) In the presence of antimycin and absence of oxidant, cytochrome b-562 is still reduced biphasically, but there is no lag between the two phases. No Qin is formed and both the Fe-S clusters and cytochrome c1 are reduced biphasically, one-half being reduced at the same rate as in the absence of antimycin and the other half 10-times slower. (4) In the presence of antimycin and oxidant, the recently described antimycin-insensitive species of semiquinone anion, Qout (De Vries, S., Albracht, S.P.J., Berden, J.A. and Slater, E.C. (1982) J. Biol. Chem. 256, 11996-11998) is formed at the same rate as that of the reduction of all species of cytochrome b. In this case cytochrome b is reduced in a single phase. (5) The reversible change of the line shape of the EPR spectrum of the [2Fe-2S] cluster 1 is caused by ubiquinone bound in the vicinity of this cluster. (6) The experimental results are consistent with the basic principles of the Q cycle. Because of the multiplicity, stoicheiometry and heterogeneous kinetics of the prosthetic groups, a Q cycle model describing the pathway of electrons through a dimeric QH2:cytochrome c oxidoreductase is proposed.