Chemical modification of the mitochondrial bc1 complex by N,N'-dicyclohexylcarbodiimide inhibits proton translocation

Modulators reducing the efficiency of oxidative ATP synthesis in mitochondria: protonophore uncouplers, cyclic redox agents, and decouplers

This work considers the main indicators of the oxidative phosphorylation efficiency in mitochondria: the ADP/O and H+/O ratios. Three groups of modulators that reduce the efficiency of oxidative phosphorylation are compared: protonophore uncouplers, cyclic redox compounds, and decouplers. It is noted that some of them are considered effective therapeutic agents. The paper analyzes the authors' original data on the mechanism of action of natural decouplers, represented by long-chain α,ω-dioic acids, as antioxidants. In conclusion, we discuss the hypothesis of their participation in the rescue of hepatocytes in various disorders of carbohydrate and lipid metabolism.

MitoQ regulates autophagy by inducing a pseudo-mitochondrial membrane potential

During the process of oxidative phosphorylation, protons are pumped into the mitochondrial intermembrane space to establish a mitochondrial membrane potential (MMP). The electrochemical gradient generated allows protons to return to the matrix through the ATP synthase complex and generates ATP in the process. MitoQ is a lipophilic cationic drug that is adsorbed to the inner mitochondrial membrane; however, the cationic moiety of MitoQ remains in the intermembrane space. We found that the positive charges in MitoQ inhibited the activity of respiratory chain complexes I, III, and IV, reduced proton production, and decreased oxygen consumption. Therefore, a pseudo-MMP (PMMP) was formed via maintenance of exogenous positive charges. Proton backflow was severely impaired, leading to a decrease in ATP production and an increase in AMP production. Excess AMP activates AMP kinase, which inhibits the MTOR (mechanistic target of rapamycin) pathway and induces macroautophagy/autophagy. Therefore, we conclude that MitoQ increases PMMP via proton displacement with exogenous positive charges. In addition, PMMP triggered autophagy in hepatocellular carcinoma HepG2 cells via modification of mitochondrial bioenergetics pathways.

Structural and functional analysis of deficient mutants in subunit I of cytochrome c oxidase from Saccharomyces cerevisiae

Four point mutations in subunit I of cytochrome c oxidase from Saccharomyces cerevisiae that had been selected for respiratory incompetence but still contained spectrally detectable haem aa3 were analysed. The isolated mutant enzymes exhibited minor band shifts in their optical spectra and contained all eleven subunits. However, steady state activities were only a few percent compared to wild type enzyme. Using a comprehensive experimental approach, we first checked the integrity of the enzyme preparations and then identified the specific functional defect. The results are discussed using information from the recently solved structures of cytochrome c oxidase at 2.8 A. Mutation 167N is positioned between haem a and a conserved glutamate residue (E243). It caused a distortion of the EPR signal of haem a and shifted its midpoint potential by 54 mV to the negative. The high-resolution structure suggests that the primary reason for the low activity of the mutant enzyme could be that asparagine in position 67 might form a stable hydrogen bond to E243, which is part of a proposed proton channel. Cytochrome c oxidase isolated from mutant T316K did not meet our criteria for homogeneity and was therefore omitted from further analysis. Mutants G352V and V380M exhibited an impairment of electron transfer from haem a to a3 and ligand binding to the binuclear centre was affected. In mutant V380M also the midpoint potential of CuB was shifted by 65 mV to the positive. The results indicated for these two mutants changes primarily associated with the binuclear centre, possibly associated with an interference in the routes and/or sites of protonation which are required for stable formation of the catalytic intermediates. This interpretation is discussed in the light of the high resolution structure.

Triple inhibitor titrations support the functionality of the dimeric character of mitochondrial ubiquinol-cytochrome c oxidoreductase

The ubiquinol-2 or duroquinol oxidoreductase activity of mitochondrial ubiquinol-cytochrome c oxidoreductase was titrated with combinations of antimycin, myxothiazol and N,N'-dicyclohexylcarbodiimide (DCCD). A statistical model has been developed that can predict the activity of the complex treated with all possible combinations of these inhibitors. On the basis of the measured titration curves the model had to accommodate interaction between the two promoters of the complex. The titrations confirm that treatment with DCCD results in the modification of a certain site in one of the two promoters of the bc1 dimer, thereby blocking one antimycin A binding site without inhibiting electron transfer. Modification of both antimycin A binding sites of the dimer is apparently required for inhibition of electron transfer through the complex, just as modification of both myxothiazol-binding sites is required for full inhibition. The conclusion can be drawn that mitochondrial ubiquinol-cytochrome c oxidoreductase is a functional dimer, consisting of electrically interacting protomers.

Regulation of the proton/electron stoichiometry of mitochondrial ubiquinol:cytochrome c reductase by the membrane potential

The electron transfer reaction catalysed by mitochondrial ubiquinol:cytochrome c reductase is linked to the outwards translocation of protons with an H+ e- stoichiometry of 1 under non-membrane potential condition. The effect of the electrical membrane potential on the H+/e- stoichiometry was investigated. The enzyme was isolated from Neurospora crassa, reconstituted into phospholipid vesicles and electrical membrane potentials of various values were generated across the membranes by means of the valinomycin-induced potassium-diffusion method. Using lithium ions as counterions for the intravesicular potassium, the induced membrane potential was stable for minutes and was not significantly changed by the protons ejected by the working enzyme. This allowed the assay of steady-state reaction rates at pre-given values of electrical membrane potential. The rate ratio between electron transfer and proton translocation declined from 1 to 0.6 with increase of the membrane potential from 0 to 100 mV. The activity of the quinol/cytochrome c redox reaction followed a parabolic dependence, being activated by low (less than 50 mV) potential and inhibited by high (greater than 100 mV) potential. This apparent non-linear dependence was interpreted in terms of a linear flow/force relationship plus a membrane-potential-dependent slip. Evaluation of the parabolic course by means of a modified linear flow/force relation also indicated a decline of the H+/e- stoichiometry from 1 to 0.5 with increase of the membrane potential from 0 to 120 mV. These observations suggest that the membrane potential controls a change of ubiquinol:cytochrome c reductase between two states that have different reaction routes.

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.

DCCD sensitivity of electron and proton transfer by NADH: ubiquinone oxidoreductase in bovine heart submitochondrial particles--a thermodynamic approach

The sensitivity of the H+/2e- ratio of the redox-driven proton pumping by the NADH: ubiquinone reductase (complex I) of the submitochondrial particles to dicyclohexylcarbodiimide (DCCD) was studied by a thermodynamic approach, measuring the membrane potential and delta pH across the membrane and the redox potential difference across the complex I span of the respiratory chain. The delta Gr/delta muH+ ratio did not decrease upon additions of 50 or 100 nmol of DCCD per mg protein in the presence of oligomycin although the H+/2e- ratio has been demonstrated to decrease upon DCCD addition in kinetic experiments with mitochondria. Complex I then becomes reminiscent of the cytochrome bc1 complex, which shows DCCD sensitivity of the kinetically but not thermodynamically determined H+/2e- ratio.

Membrane-potential-dependent changes in the stoichiometry of charge translocation by the mitochondrial electron transport chain

The charge/oxygen (q+/O) stoichiometry of mitochondria respiring on succinate was measured under conditions of high membrane potential (delta psi). The technique used was a variation of the steady-state method of Al-Shawi and Brand [(1981) Biochem. J. 200, 539-546]. We show that q+/O was about 2.7 at high values of delta psi (170 mV). As delta psi was lowered from 170 mV to 85 mV with the respiratory inhibitor malonate the q+/O stoichiometry increased to 6.0. A number of artefacts which could have led to an underestimation of the q+/O stoichiometry were eliminated. These included effects of any rapid change in mitochondrial volume, internal pH, activity of the endogenous K+/H+ exchanger or in H+ conductance due to changes in delta psi after the addition of inhibitor. The experiments presented here are the first direct demonstration that the stoichiometry of proton pumping by the mitochondrial respiratory chain changes as delta psi is varied.

Sensitivity to NN'-dicyclohexylcarbodi-imide of proton translocation by mitochondrial NADH:ubiquinone oxidoreductase

Proton extrusion during ferricyanide reduction by NADH-generating substrates or succinate was studied in isolated rat liver mitochondria with the use of optical indicators. NN'-Dicyclohexylcarbodi-imide (DCCD) caused a decrease of 84% in the H+/e- ratio of NADH:cytochrome c reduction, but a decrease of only 49% in that of succinate:cytochrome c reduction, even though electron transfer was decreased equally in both spans. The data indicate that a DCCD-sensitive channel operates in the NADH:ubiquinone oxidoreductase region of the respiratory chain.

Coenzyme Q analogues reconstitute electron transport and proton ejection but not the antimycin-induced "red shift" in mitochondria from coenzyme Q deficient mutants of the yeast Saccharomyces cerevisiae

Mitochondria isolated from coenzyme Q deficient yeast cells had no detectable NADH:cytochrome c reductase or succinate:cytochrome c reductase activity but contained normal amounts of cytochromes b and c1 by spectral analysis. Addition of the exogenous coenzyme Q derivatives including Q2, Q6, and the decyl analogue (DB) restored the rate of antimycin- and myxothiazole-sensitive cytochrome c reductase with both substrates to that observed with reduced DBH2. Similarly, addition of these coenzyme Q analogues increased 2-3-fold the rate of cytochrome c reduction in mitochondria from wild-type cells, suggesting that the pool of coenzyme Q in the membrane is limiting for electron transport in the respiratory chain. Preincubation of mitochondria from the Q-deficient yeast cells with DBH2 at 25 degrees C restored electrogenic proton ejection, resulting in a H+/2e- ratio of 3.35 as compared to a ratio of 3.22 observed in mitochondria from the wild-type cell. Addition of succinate and either coenzyme Q6 or DB to mitochondria from the Q-deficient yeast cells resulted in the initial reduction of cytochrome b followed by a slow reduction of cytochrome c1 with a reoxidation of cytochrome b. The subsequent addition of antimycin resulted in the oxidant-induced extrareduction of cytochrome b and concomitant oxidation of cytochrome c1 without the "red" shift observed in the wild-type mitochondria. Similarly, addition of antimycin to dithionite-reduced mitochondria from the mutant cells did not result in a red shift in the absorption maximum of cytochrome b as was observed in the wild-type mitochondria in the presence or absence of exogenous coenzyme Q analogues.(ABSTRACT TRUNCATED AT 250 WORDS)

Is there sufficient experimental evidence to consider the mitochondrial cytochrome bc1 complex a proton pump? Probably no

The electron flow through the cytochrome bc1 complex of the mitochondrial respiratory chain is accompanied by vectorial proton translocation, though the mechanism of the latter phenomenon has not yet been clarified. Several proposed hypotheses are briefly presented and discussed here. Recently, a number of papers have appeared claiming the existence of a proton pump in the enzyme mainly on the basis of the interaction of the complex with N,N'-dicyclohexylcarbodiimide. These data are reviewed here with the aim of showing their ability to fit multiple interpretations. This together with some other arguments leads to the conclusion that a proton pump in the mitochondrial bc1 complex has not yet been demonstrated.

Effect of 2,4-dinitrofluorobenzene on the enzymatic properties of the b-c1 complex isolated from beef heart mitochondria

A study is presented on the effect of 2,4-dinitrofluorobenzene (DFNB) on the enzymatic properties of mitochondrial b-c1 complex. The chemical modification by DNFB strongly inhibits the reductase activity of the complex, this being accompanied by labelling by [3H]DNFB of core protein I, the apoprotein of b cytochromes and the 12 kDa subunit. Chemical modification by DNFB appears to alter, in particular, the domain of heme b-562.

Further studies on the binding of DCCD to cytochrome B and subunit VIII of complex III isolated from beef heart mitochondria

Complex III (the cytochrome b-c1 complex) from beef heart mitochondria was incubated with [14C]DCCD for various periods of time. The polypeptide profile of the complex was compared in both stained gels and their autoradiograms when three different methods were used to terminate the reaction. Precipitation with ammonium sulfate resulted in the formation of a new band with an apparent molecular weight of 39,000 in both incubated samples and the zero time controls. Reisolation of the complex by centrifugation through 10% sucrose or by precipitation with trichloroacetic acid did not result in any changes in the appearance of the subunit peptides of the complex. Subunit III (cytochrome b) and subunit VIII were the only bands labeled after termination of the reaction by centrifugation through sucrose, while both ammonium sulfate and trichloroacetic precipitation resulted in nonspecific labeling of several other subunits of the complex and increased labeling of subunit VIII relative to subunit III. Preincubation of the complex with antimycin prior to treatment with [14C]DCCD resulted in a 50% decrease in the binding of DCCD to both cytochrome b and subunit VIII. Furthermore, treatment of the complex III with DCCD resulted in a change in the red shift observed after antimycin or myxothiazol addition to the dithionite-reduced complex resulting in a broad peak with no sharp maximum. These results provide further confirmation that DCCD binds preferentially to cytochrome b and subunit VIII of complex III from beef heart mitochondria and suggest that cytochrome b may play a role in proton translocation.

A clarification of the effects of DCCD on the electron transfer and antimycin binding of the mitochondrial bc1 complex

We have studied in detail the effects of dicyclohexylcarbodiimide (DCCD) on the redox activity of the mitochondrial bc1 complex, and on the binding of its most specific inhibitor antimycin. An inhibitory action of the reagent has been found only at high concentration of the diimide and/or at prolonged times of incubation. Under these conditions, DCCD also displaced antimycin from its specific binding site in the bc1 complex, but did not apparently change the antimycin sensitivity of the ubiquinol-cytochrome c reductase activity. On the other hand, using lower DCCD concentrations and/or short times of incubation, i.e., conditions which usually lead to the specific inhibition of the proton-translocating activity of the bc1 complex, no inhibitory effect of DCCD could be detected in the ubiquinol-cytochrome c reductase activity. However, a clear stimulation of the rate of cytochrome b reduction in parallel to an inhibition of cytochrome b oxidation has been found under these conditions. On the basis of the present work and of previous reports in the literature about the effects of DCCD on the bc1 complex, we propose a clarification of the various effects of the reagent depending on the experimental conditions employed.

Effect of papain digestion on redox-linked proton translocation in b-c1 complex from beef heart reconstituted into liposomes

Papain treatment of the cytochrome b-c1 complex from beef heart results in partial proteolysis of core protein II, the iron-sulphur protein and the 15-kDa subunit. Under these conditions a significant inhibition of electron flow and complete suppression of proton translocation in the complex reconstituted into liposomes are observed. Kinetic experiments indicate a correlation between the digestion of core protein II and 15-kDa subunit and the suppression of proton translocation. The results suggest an active involvement of polypeptides of the complex in stabilizing the semiquinone species and/or providing pathways to exchange protons between bound quinone systems and aqueous phases.

Thermodynamic and steady-state-kinetic investigation of the effect of NN'-dicyclohexylcarbodi-imide on H+ translocation by the mitochondrial cytochrome bc1 complex

Steady-state kinetic measurements showed that NN'-dicyclohexylcarbodi-imide decreased the observed H+/2e ratio of H+ transport by mitochondria respiring on succinate, acting mainly at the cytochrome bc1 complex. Thermodynamic assessment of the H+/2e ratio by measuring the force ratio across the bc1 complex showed that the inhibitor did not affect H+ translocation. Possible explanations of this disagreement between methods are examined; we conclude that the inhibitor does not alter the mechanistic stoichiometry of H+ pumping by the bc1 complex.

Modification of the catalytic function of the mitochondrial cytochrome b-c1 complex by dicyclohexylcarbodiimide

N,N'-Dicyclohexylcarbodiimide (DCCD) induces a complex set of effects on the succinate-cytochrome c span of the mitochondrial respiratory chain. At concentrations below 1000 mol per mol of cytochrome c1, DCCD is able to block the proton-translocating activity associated to succinate or ubiquinol oxidation without inhibiting the steady-state redox activity of the b-c1 complex either in intact mitochondrial particles or in the isolated ubiquinol-cytochrome c reductase reconstituted in phospholipid vesicles. In parallel to this, DCCD modifies the redox responses of the endogenous cytochrome b, which becomes more rapidly reduced by succinate, and more slowly oxidized when previously reduced by substrates. At similar concentrations the inhibitor apparently stimulates the redox activity of the succinate-ubiquinone reductase. Moreover, DCCD, at concentrations about one order of magnitude higher than those blocking proton translocation, produces inactivation of the redox function of the b-c1 complex. The binding of [14C]DCCD to the isolated b-c1 complex has shown that under conditions leading to the inhibition of the proton-translocating activity of the enzyme, a subunit of about 9500 Da, namely Band VIII, is the most heavily labelled polypeptide of the complex. The possible correlations between the various effects of DCCD and its modification of the b-c1 complex are discussed.