The mechanism of mitochondrial superoxide production by the cytochrome bc1 complex

Rip1 enhances methylmercury toxicity through production of reactive oxygen species (ROS) in budding yeast

Reactive oxygen species (ROS) produced by mitochondria are potentially involved in the manifestation of methylmercury toxicity. However, the molecular mechanism underlying methylmercury toxicity remains poorly understood. We examined susceptibility to methylmercury in yeast strains that each lacked one of components of the mitochondrial electron transport system. Resistance to methylmercury was exhibited only by yeast that lacked Rip1, a component of electron transport system complex III. Resistance to methylmercury in Rip1-deficient yeast was independent of the activity of electron transport system complex III. Also, ROS levels induced by methylmercury in Rip1-deficient yeast were significantly lower than in wild-type yeast. Thus, Rip1 is potentially involved in ROS production through an as-yet unknown mechanism that is independent of the activity of electron transport system complex III, thereby enhancing methylmercury toxicity.

What are the sources of hydrogen peroxide production by heart mitochondria?

Coupled rat heart mitochondria produce externally hydrogen peroxide at the rates which correspond to about 0.8 and 0.3% of the total oxygen consumption at State 4 with succinate and glutamate plus malate as the respiratory substrates, respectively. Stimulation of the respiratory activities by ADP (State 4-State 3 transition) decreases the succinate- and glutamate plus malate-supported H2O2 production 8- and 1.3-times, respectively. NH4+ strongly stimulates hydrogen peroxide formation with either substrate without any effect on State 4 and/or State 3 respiration. Rotenone-treated, alamethicin-permeabilized mitochondria catalyze NADH-supported H2O2 production at a rate about 10-fold higher than that seen in intact mitochondria under optimal (State 4 succinate-supported respiration in the presence of ammonium chloride) conditions. NADH-supported hydrogen peroxide production by the rotenone-treated mitochondria devoid of a permeability barrier for H2O2 diffusion by alamethicin treatment are only partially (approximately 50%) sensitive to the Complex I NADH binding site-specific inhibitor, NADH-OH. The residual activity is strongly (approximately 6-fold) stimulated by ammonium chloride. NAD+ inhibits both Complex I-mediated and ammonium-stimulated H2O2 production. In the absence of stimulatory ammonium about half of the total NADH-supported hydrogen peroxide production is catalyzed by Complex I. In the presence of ammonium about 90% of the total hydrogen peroxide production is catalyzed by matrix located, ammonium-dependent enzyme(s).

Electron transfer in Paracoccus denitrificans with the modified fbc operon

Membrane fragments of two mutant strains of Paracoccus denitrificans genetically modified in the bc(1) complex have been studied for comparison of enzymic activities of succinate-cytochrome-c reductase and its components, viz. succinate dehydrogenase (Complex II) and ubiquinol-cytochrome-c reductase (Complex III) and their response to changes in concentration of succinate, cytochrome c, ionic strength, pH, temperature and sensitivity to antimycin A. The mutants synthesized and assembled the b and c hemes in the ratio characteristic for the wild type strain. The mutant strain M 71 expressing the truncated copy of cytochrome c(1) (devoid of a stretch of 150 mainly acidic amino acids) was less sensitive to increasing concentration of cytochrome c and changes in ionic strength of the medium, but maintained the original affinity to succinate and sensitivity to antimycin A. The mutant strain M 36 with an overexpressed bc(1) content showed the highest response to changes in ionic strength and physical parameters, exhibited the lowest turnover number values with succinate-cytochrome-c reductase, but positively affected the succinate dehydrogenase. In view of the interaction of the redox components in native membranes the functional analyses of separated Complexes II and III should be regarded with caution.

Terpestacin inhibits tumor angiogenesis by targeting UQCRB of mitochondrial complex III and suppressing hypoxia-induced reactive oxygen species production and cellular oxygen sensing

Cellular oxygen sensing is required for hypoxia-inducible factor-1alpha stabilization, which is important for tumor cell survival, proliferation, and angiogenesis. Here we find that terpestacin, a small molecule previously identified in a screen of microbial extracts, binds to the 13.4-kDa subunit (UQCRB) of mitochondrial Complex III, resulting in inhibition of hypoxia-induced reactive oxygen species generation. Consequently, such inhibition blocks hypoxia-inducible factor activation and tumor angiogenesis in vivo, without inhibiting mitochondrial respiration. Overexpression of UQCRB or its suppression using RNA interference demonstrates that it plays a crucial role in the oxygen sensing mechanism that regulates responses to hypoxia. These findings provide a novel molecular basis of terpestacin targeting UQCRB of Complex III in selective suppression of tumor progression.

Inactivation of sestrin 2 induces TGF-beta signaling and partially rescues pulmonary emphysema in a mouse model of COPD

Chronic obstructive pulmonary disease (COPD) is a leading cause of morbidity and mortality worldwide. Cigarette smoking has been identified as one of the major risk factors and several predisposing genetic factors have been implicated in the pathogenesis of COPD, including a single nucleotide polymorphism (SNP) in the latent transforming growth factor (TGF)-beta binding protein 4 (Ltbp4)-encoding gene. Consistent with this finding, mice with a null mutation of the short splice variant of Ltbp4 (Ltbp4S) develop pulmonary emphysema that is reminiscent of COPD. Here, we report that the mutational inactivation of the antioxidant protein sestrin 2 (sesn2) partially rescues the emphysema phenotype of Ltbp4S mice and is associated with activation of the TGF-beta and mammalian target of rapamycin (mTOR) signal transduction pathways. The results suggest that sesn2 could be clinically relevant to patients with COPD who might benefit from antagonists of sestrin function.

Closure of VDAC causes oxidative stress and accelerates the Ca(2+)-induced mitochondrial permeability transition in rat liver mitochondria

The electron transport chain of mitochondria is a major source of reactive oxygen species (ROS), which play a critical role in augmenting the Ca(2+)-induced mitochondrial permeability transition (MPT). Mitochondrial release of superoxide anions (O(2)(-)) from the intermembrane space (IMS) to the cytosol is mediated by voltage dependent anion channels (VDAC) in the outer membrane. Here, we examined whether closure of VDAC increases intramitochondrial oxidative stress by blocking efflux of O(2)(-) from the IMS and sensitizing to the Ca(2+)-induced MPT. Treatment of isolated rat liver mitochondria with 5microM G3139, an 18-mer phosphorothioate blocker of VDAC, accelerated onset of the MPT by 6.8+/-1.4min within a range of 100-250microM Ca(2+). G3139-mediated acceleration of the MPT was reversed by 20microM butylated hydroxytoluene, a water soluble antioxidant. Pre-treatment of mitochondria with G3139 also increased accumulation of O(2)(-) in mitochondria, as monitored by dihydroethidium fluorescence, and permeabilization of the mitochondrial outer membrane with digitonin reversed the effect of G3139 on O(2)(-) accumulation. Mathematical modeling of generation and turnover of O(2)(-) within the IMS indicated that closure of VDAC produces a 1.55-fold increase in the steady-state level of mitochondrial O(2)(-). In conclusion, closure of VDAC appears to impede the efflux of superoxide anions from the IMS, resulting in an increased steady-state level of O(2)(-), which causes an internal oxidative stress and sensitizes mitochondria toward the Ca(2+)-induced MPT.

The sites and topology of mitochondrial superoxide production

Mitochondrial superoxide production is an important source of reactive oxygen species in cells, and may cause or contribute to ageing and the diseases of ageing. Seven major sites of superoxide production in mammalian mitochondria are known and widely accepted. In descending order of maximum capacity they are the ubiquinone-binding sites in complex I (site IQ) and complex III (site IIIQo), glycerol 3-phosphate dehydrogenase, the flavin in complex I (site IF), the electron transferring flavoprotein:Q oxidoreductase (ETFQOR) of fatty acid beta-oxidation, and pyruvate and 2-oxoglutarate dehydrogenases. None of these sites is fully characterized and for some we only have sketchy information. The topology of the sites is important because it determines whether or not a site will produce superoxide in the mitochondrial matrix and be able to damage mitochondrial DNA. All sites produce superoxide in the matrix; site IIIQo and glycerol 3-phosphate dehydrogenase also produce superoxide to the intermembrane space. The relative contribution of each site to mitochondrial reactive oxygen species generation in the absence of electron transport inhibitors is unknown in isolated mitochondria, in cells or in vivo, and may vary considerably with species, tissue, substrate, energy demand and oxygen tension.

Ascochlorin is a novel, specific inhibitor of the mitochondrial cytochrome bc1 complex

Ascochlorin is an isoprenoid antibiotic that is produced by the phytopathogenic fungus Ascochyta viciae. Similar to ascofuranone, which specifically inhibits trypanosome alternative oxidase by acting at the ubiquinol binding domain, ascochlorin is also structurally related to ubiquinol. When added to the mitochondrial preparations isolated from rat liver, or the yeast Pichia (Hansenula) anomala, ascochlorin inhibited the electron transport via CoQ in a fashion comparable to antimycin A and stigmatellin, indicating that this antibiotic acted on the cytochrome bc(1) complex. In contrast to ascochlorin, ascofuranone had much less inhibition on the same activities. On the one hand, like the Q(i) site inhibitors antimycin A and funiculosin, ascochlorin induced in H. anomala the expression of nuclear-encoded alternative oxidase gene much more strongly than the Q(o) site inhibitors tested. On the other hand, it suppressed the reduction of cytochrome b and the generation of superoxide anion in the presence of antimycin A(3) in a fashion similar to the Q(o) site inhibitor myxothiazol. These results suggested that ascochlorin might act at both the Q(i) and the Q(o) sites of the fungal cytochrome bc(1) complex. Indeed, the altered electron paramagnetic resonance (EPR) lineshape of the Rieske iron-sulfur protein, and the light-induced, time-resolved cytochrome b and c reduction kinetics of Rhodobacter capsulatus cytochrome bc(1) complex in the presence of ascochlorin demonstrated that this inhibitor can bind to both the Q(o) and Q(i) sites of the bacterial enzyme. Additional experiments using purified bovine cytochrome bc(1) complex showed that ascochlorin inhibits reduction of cytochrome b by ubiquinone through both Q(i) and Q(o) sites. Moreover, crystal structure of chicken cytochrome bc(1) complex treated with excess ascochlorin revealed clear electron densities that could be attributed to ascochlorin bound at both the Q(i) and Q(o) sites. Overall findings clearly show that ascochlorin is an unusual cytochrome bc(1) inhibitor that acts at both of the active sites of this enzyme.

Cytochrome c oxidase isoform IV-2 is involved in 3-nitropropionic acid-induced toxicity in striatal astrocytes

Astrocyte mitochondria play an important role for energy supply and neuronal survival in the brain. Toxic and degenerative processes are largely associated with mitochondrial dysfunction. We, therefore, investigated the effect of 3-nitropropionic acid (NPA), a mitochondrial toxin and in vitro model of Huntington's disease (HD), on mitochondrial function and viability of primary striatal astrocytes. Although NPA is known as an irreversible inhibitor of succinate dehydrogenase, we observed an increase of astrocyte ATP levels after NPA treatment. This effect could be explained by NPA-mediated alterations of cytochrome c oxidase subunit IV isoform (COX IV) expression. The up-regulation of COX isoform IV-2 caused an increased enzyme activity at the expense of elevated mitochondrial peroxide production causing increased cell death. The application of a small interfering RNA against COX IV-2 revealed the causal implication of COX isoform IV-2 in NPA-mediated elevation of oxidative stress and necrotic cell death. Thus, we propose a novel, additional mechanism of NPA-induced cell stress and death which is based on structural and functional changes of astrocyte COX and which could indirectly impair neuronal survival.

Conditional knockout of Mn-SOD targeted to type IIB skeletal muscle fibers increases oxidative stress and is sufficient to alter aerobic exercise capacity

In vitro studies of isolated skeletal muscle have shown that oxidative stress is limiting with respect to contractile function. Mitochondria are a potential source of muscle function-limiting oxidants. To test the hypothesis that skeletal muscle-specific mitochondrial oxidative stress is sufficient to limit muscle function, we bred mice expressing Cre recombinase driven by the promoter for the inhibitory subunit of troponin (TnIFast-iCre) with mice containing a floxed Sod2 (Sod2(fl/fl)) allele. Mn-SOD activity was reduced by 82% in glycolytic (mainly type II) muscle fiber homogenates from young TnIFastCreSod2(fl/fl) mice. Furthermore, Mn-SOD content was reduced by 70% only in type IIB muscle fibers. Aconitase activity was decreased by 56%, which suggests an increase in mitochondrial matrix superoxide. Mitochondrial superoxide release was elevated more than twofold by mitochondria isolated from glycolytic skeletal muscle in TnIFastCreSod2(fl/fl) mice. In contrast, the rate of mitochondrial H(2)O(2) production was reduced by 33%, and only during respiration with complex II substrate. F(2)-isoprostanes were increased by 36% in tibialis anterior muscles isolated from TnIFastCreSod2(fl/fl) mice. Elevated glycolytic muscle-specific mitochondrial oxidative stress and damage in TnIFastCreSod2(fl/fl) mice were associated with a decreased ability of the extensor digitorum longus and gastrocnemius muscles to produce contractile force as a function of time, whereas force production by the soleus muscle was unaffected. TnIFastCreSod2(fl/fl) mice ran 55% less distance on a treadmill than wild-type mice. Collectively, these data suggest that elevated mitochondrial oxidative stress and damage in glycolytic muscle fibers are sufficient to reduce contractile muscle function and aerobic exercise capacity.

Substrate redox potential controls superoxide production kinetics in the cytochrome bc complex

The Q-cycle mechanism of the cytochrome bc(1) complex maximizes energy conversion during the transport of electrons from ubiquinol to cytochrome c (or alternate physiological acceptors), yet important steps in the Q-cycle are still hotly debated, including bifurcated electron transport, the high yield and specificity of the Q-cycle despite possible short-circuits and bypass reactions, and the rarity of observable intermediates in the oxidation of quinol. Mounting evidence shows that some bypass reactions producing superoxide during oxidation of quinol at the Q(o) site diverge from the Q-cycle rather late in the bifurcated reaction and provide an additional means of studying initial reactions of the Q-cycle. Bypass reactions offer more scope for controlling and manipulating reaction conditions, e.g., redox potential, because they effectively isolate or decouple the Q-cycle initial reactions from later steps, preventing many complications and interactions. We examine the dependence of oxidation rate on substrate redox potential in the yeast cytochrome bc(1) complex and find that the rate limitation occurs at the level of direct one-electron oxidation of quinol to semiquinone by the Rieske protein. Oxidation of semiquinone and reduction of cyt b or O(2) are subsequent, distinct steps. These experimental results are incompatible with models in which the transfer of electrons to the Rieske protein is not a distinct step preceding transfer of electrons to cytochrome b, and with conformational gating models that produce superoxide by different rate-limiting reactions from the normal Q-cycle.

Visualizing changes in electron distribution in coupled chains of cytochrome bc(1) by modifying barrier for electron transfer between the FeS cluster and heme c(1)

Cytochrome c₁ of Rhodobacter (Rba.) species provides a series of mutants which change barriers for electron transfer through the cofactor chains of cytochrome bc₁ by modifying heme c₁ redox midpoint potential. Analysis of post-flash electron distribution in such systems can provide useful information about the contribution of individual reactions to the overall electron flow. In Rba. capsulatus, the non-functional low-potential forms of cytochrome c₁ which are devoid of the disulfide bond naturally present in this protein revert spontaneously by introducing a second-site suppression (mutation A181T) that brings the potential of heme c₁ back to the functionally high levels, yet maintains it some 100 mV lower from the native value. Here we report that the disulfide and the mutation A181T can coexist in one protein but the mutation exerts a dominant effect on the redox properties of heme c₁ and the potential remains at the same lower value as in the disulfide-free form. This establishes effective means to modify a barrier for electron transfer between the FeS cluster and heme c₁ without breaking disulfide. A comparison of the flash-induced electron transfers in native and mutated cytochrome bc₁ revealed significant differences in the post-flash equilibrium distribution of electrons only when the connection of the chains with the quinone pool was interrupted at the level of either of the catalytic sites by the use of specific inhibitors, antimycin or myxothiazol. In the non-inhibited system no such differences were observed. We explain the results using a kinetic model in which a shift in the equilibrium of one reaction influences the equilibrium of all remaining reactions in the cofactor chains. It follows a rather simple description in which the direction of electron flow through the coupled chains of cytochrome bc₁ exclusively depends on the rates of all reversible partial reactions, including the Q/QH₂ exchange rate to/from the catalytic sites.

Mitochondria and reactive oxygen species

Mitochondria are a quantitatively relevant source of reactive oxygen species (ROS) in the majority of cell types. Here we review the sources and metabolism of ROS in this organelle, including the conditions that regulate the production of these species, such as mild uncoupling, oxygen tension, respiratory inhibition, Ca2+ and K+ transport, and mitochondrial content and morphology. We discuss substrate-, tissue-, and organism-specific characteristics of mitochondrial oxidant generation. Several aspects of the physiological and pathological roles of mitochondrial ROS production are also addressed.

Across membrane communication between the Q(o) and Q(i) active sites of cytochrome bc(1)

The ubihydroquinone:cytochrome c oxidoreductase (cyt bc(1)) contains two catalytically active domains, termed the hydroquinone oxidation (Q(o)) and quinone reduction (Q(i)) sites, which are distant from each other by over 30 A. Previously, we have reported that binding of inhibitors to the Q(i) site on one (n) side of the energy-transducing membrane changes the local environment of the iron-sulfur (Fe/S) protein subunit residing in the Q(o) site on the other (p) side of the lipid bilayer [Cooley, J. W., Ohnishi, T., and Daldal, F. (2005) Biochemistry 44, 10520-10532]. These findings best fit a model whereby the Q(o) and Q(i) sites of the cyt bc(1) are actively coupled in spite of their distant locations. Because the Fe/S protein of the cyt bc(1) undergoes a large-scale (macro) domain movement during catalysis, we examined various macromobility-defective Fe/S subunit mutants to assess the role of this motion on the coupling of the active sites and also during the multiple turnovers of the enzyme. By monitoring the changing environments of the Fe/S protein [2Fe-2S] cluster upon addition of Q(i) site inhibitors in selected mutants, we found that the Q(o)-Q(i) site interactions manifest differently depending on the ability of the Fe/S protein to move between the cytochrome b and cytochrome c(1) subunits of the enzyme. In the presence of antimycin A, an immobile Fe/S protein mutant exhibited no changes in its EPR spectra. In contrast, mobility-restricted mutants showed striking alterations in the EPR line shapes and revealed two discrete subpopulations in respect to the [2Fe-2S] cluster environments at the Q(o) site. These findings led us to conclude that the mobility of the Fe/S protein is involved in its response to the occupancy of the Q(i) site by different molecules. We propose that the heterogeneity seen might reflect the distinct responses of the two Fe/S proteins at the Q(o) sites of the dimeric enzyme upon the occupancy of the Q(i) sites and discuss it in terms of the function of the dimeric cyt bc(1) during its multiple turnovers.

Chapter 25 Analysis of electron transfer and superoxide generation in the cytochrome bc1 complex

During the electron transfer through the cytochrome bc(1) complex (ubiquinol-cytochrome c oxidoreductase or complex III), protons are translocated across the membrane, and production of superoxide anion radicals (O(2)(*-)) is observed. The bc(1) complex is purified from broken mitochondrial preparation prepared from frozen heart muscles by repeated detergent solubilization and salt fractionation. The electron transfer of the purified complex is determined spectrophotometrically. The activity depends on the choice of detergent, protein concentration, and ubiquinol derivatives used. The proton translocation activity of 2H(+)/e(-) is determined in the reconstituted bc(1)-PL vesicles. The O(2)(*-) production by bc(1) is determined by measuring the chemiluminescence of the 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazol[1,2-1]pyrazin-3-one hydrochloride (MCLA)-O(2)(*-) adduct during a single turnover of bc(1) complex, with the Applied Photophysics stopped-flow reaction analyzer SX.18MV, by leaving the excitation light source off and registering the light emission. Production of O(2)(*-) by bc(1) is in an inverse relationship to its electron transfer activity. Inactivation of the bc(1) complex by incubating at elevated temperature (37 degrees C) or by treatment with proteinase K results in an increase in O(2)(*-)-generating activity to the same level as that of the antimycin A-inhibited complex. These results suggest that the structural integrity of protein subunits is not required for O(2)(*-)-generating activity in the bc(1) complex.

Chapter 27 An improved method for introducing point mutations into the mitochondrial cytochrome B gene to facilitate studying the role of cytochrome B in the formation of reactive oxygen species

Cytochrome b is a pivotal protein subunit of the cytochrome bc(1) complex and forms the ubiquinol oxidation site in the enzyme that is generally thought to be the primary site where electrons are aberrantly diverted from the enzyme, reacting with oxygen to form superoxide anion. In addition, recent studies have shown that mutations in cytochrome b can substantially increase rates of oxygen radical formation by the bc(1) complex. It would, thus, be advantageous to be able to manipulate cytochrome b by mutagenesis of the cytochrome b gene to better understand the role of cytochrome b in oxygen radical formation. Cytochrome b is encoded in the mitochondrial genome in eukaryotic cells, and introduction of point mutations into the gene is generally cumbersome because of the tedious screening process for positive clones. In addition, previously it has been especially difficult to introduce point mutations that lead to loss of respiratory function, as might be expected of mutations that markedly enhance oxygen radical formation. To more efficiently introduce amino acid changes into cytochrome b we have devised a method for mutagenesis of the Saccharomyces cerevisiae mitochondrial cytochrome b gene that uses a recoded ARG8 gene as a "placeholder" for the wild-type b gene. In this method ARG8, a gene that is normally encoded by nuclear DNA, replaces the naturally occurring mitochondrial cytochrome b gene, resulting in ARG8 expressed from the mitochondrial genome (ARG8(m)). Subsequently replacing ARG8(m) with mutated versions of cytochrome b results in arginine auxotrophy. Respiratory-competent cytochrome b mutants can be selected directly by virtue of their ability to restore growth on nonfermentable substrates. If the mutated cytochrome b is nonfunctional, the presence of the COX2 respiratory gene marker on the mitochondrial transforming plasmid enables screening for cytochrome b mutants with a stringent respiratory deficiency (mit(-)).

Synthesis, characterization and potent superoxide dismutase-like activity of novel bis(pyrazole)-2,2'-bipyridyl mixed ligand copper(II) complexes

Eleven new complexes of Cu(II) chloride and nitrate with bis(pyrazol-1-yl)propane and bis[2-(pyrazol-1-yl)ethyl]ether ligands were prepared and characterized by spectral and electrochemical methods. X-Ray crystal structure determination of bis[2-(3,5-dimethylpyrazol-1-yl)ethyl]etherdinitratocopper revealed a hepta-coordinated structure with the bis(pyrazole) ligand coordinated in a tridentate NNO-fashion and both of the nitrate ions in a bidentate fashion. Reaction of Cu(II) nitrate complexes with 2,2'-bipyridyl led to the displacement of one of the nitrate ions into the outer sphere and the formation of mixed-ligand complexes. Mixed-ligand bipyridyl Cu(II) complexes demonstrated the highest superoxide dismutase (SOD)-like activity in a chemical superoxide anion-generating system, with IC(50) values in the low micromolar range. Density functional theory calculations showed that introduction of a bipypidyl ligand into the complexes dramatically lowered the lowest unoccupied molecular orbital (LUMO) energy level, which explains the increased SOD-like activity of these complexes compared to non-bipy species. These bipy complexes were also effective scavengers of reactive oxygen species generated by phagocytes (human neutrophils and murine bone marrow leukocytes) ex vivo. Thus, these bipy mixed-ligand complexes represent a promising class of SOD mimetics for future development.

Membrane potential greatly enhances superoxide generation by the cytochrome bc1 complex reconstituted into phospholipid vesicles

The mitochondrial cytochrome bc(1) complex (ubiquinol/cytochrome c oxidoreductase) is generally thought to generate superoxide anion that participates in cell signaling and contributes to cellular damage in aging and degenerative disease. However, the isolated, detergent-solubilized bc(1) complex does not generate measurable amounts of superoxide except when inhibited by antimycin. In addition, indirect measurements of superoxide production by cells and isolated mitochondria have not clearly resolved the contribution of the bc(1) complex to the generation of superoxide by mitochondria in vivo, nor did they establish the effect, if any, of membrane potential on superoxide formation by this enzyme complex. In this study we show that the yeast cytochrome bc(1) complex does generate significant amounts of superoxide when reconstituted into phospholipid vesicles. The rate of superoxide generation by the reconstituted bc(1) complex increased exponentially with increased magnitude of the membrane potential, a finding that is compatible with the suggestion that membrane potential inhibits electron transfer from the cytochrome b(L) to b(H) hemes, thereby promoting the formation of a ubisemiquinone radical that interacts with oxygen to generate superoxide. When the membrane potential was further increased, by the addition of nigericin or by the imposition of a diffusion potential, the rate of generation of superoxide was further accelerated and approached the rate obtained with antimycin. These findings suggest that the bc(1) complex may contribute significantly to superoxide generation by mitochondria in vivo, and that the rate of superoxide generation can be controlled by modulation of the mitochondrial membrane potential.

New extension of the Mitchell Theory for oxidative phosphorylation in mitochondria of living organisms

The Mitchell Theory implies the proton motive force Deltap across the inner mitochondrial membrane as the energy-rich intermediate of oxidative phosphorylation. Deltap is composed mainly of an electrical (DeltaPsi(m)) and a chemical part (DeltapH) and generated by the respiratory chain complexes I, III and IV. It is consumed mostly by the ATP synthase (complex V) to produce ATP. The free energy of electron transport within the proton pumps is sufficient to generate Deltap of about 240 mV. The proton permeability of biological membranes, however, increases exponentially above 130 mV leading to a waste of energy at high values (DeltaPsi(m)>140 mV). In addition, at DeltaPsi(m)>140 mV, the production of the superoxide radical anion O(2)(-) at complexes I, II and III increases exponentially with increasing DeltaPsi(m). O(2)(-) and its neutral product H(2)O(2) (=ROS, reactive oxygen species) induce oxidative stress which participates in aging and in the generation of degenerative diseases. Here we describe a new mechanism which acts independently of the Mitchell Theory and keeps DeltaPsi(m) at low values through feedback inhibition of complex IV (cytochrome c oxidase) at high ATP/ADP ratios, thus preventing the formation of ROS and maintaining high efficiency of oxidative phosphorylation.

Tissue-, substrate-, and site-specific characteristics of mitochondrial reactive oxygen species generation

Reactive oxygen species are a by-product of mitochondrial oxidative phosphorylation, derived from a small quantity of superoxide radicals generated during electron transport. We conducted a comprehensive and quantitative study of oxygen consumption, inner membrane potentials, and H(2)O(2) release in mitochondria isolated from rat brain, heart, kidney, liver, and skeletal muscle, using various respiratory substrates (alpha-ketoglutarate, glutamate, succinate, glycerol phosphate, and palmitoyl carnitine). The locations and properties of reactive oxygen species formation were determined using oxidative phosphorylation and the respiratory chain modulators oligomycin, rotenone, myxothiazol, and antimycin A and the uncoupler CCCP. We found that in mitochondria isolated from most tissues incubated under physiologically relevant conditions, reactive oxygen release accounts for 0.1-0.2% of O(2) consumed. Our findings support an important participation of flavoenzymes and complex III and a substantial role for reverse electron transport to complex I as reactive oxygen species sources. Our results also indicate that succinate is an important substrate for isolated mitochondrial reactive oxygen production in brain, heart, kidney, and skeletal muscle, whereas fatty acids generate significant quantities of oxidants in kidney and liver. Finally, we found that increasing respiratory rates is an effective way to prevent mitochondrial oxidant release under many, but not all, conditions. Altogether, our data uncover and quantify many tissue-, substrate-, and site-specific characteristics of mitochondrial ROS release.

Effect of subunit IV on superoxide generation by Rhodobacter sphaeroides cytochrome bc(1) complex

Previous studies indicate that the three-subunit cytochrome bc(1) core complex of Rhodobacter sphaeroides contains a fraction of the electron transfer activity of the wild-type enzyme. Addition of subunit IV to the core complex increases electron transfer activity to the same level as that of the wild-type complex. This activity increase may result from subunit IV preventing electron leakage, from the low potential electron transfer chain, and reaction with molecular oxygen, producing superoxide anion. This suggestion is based on the following observations: (1) the extent of cytochrome b reduction in the three-subunit core complex, by ubiquinol, in the presence of antimycin A, never reaches the same level as that in the wild-type complex; (2) the core complex produces 4 times as much superoxide anion as does the wild-type complex; and (3) when the core complex is reconstituted with subunit IVs having varying reconstitutive activities, the activity increase in reconstituted complexes correlates with superoxide production decrease and extent of cytochrome b reduction increase.

The rate-limiting step in the cytochrome bc1 complex (Ubiquinol-Cytochrome c Oxidoreductase) is not changed by inhibition of cytochrome b-dependent deprotonation: implications for the mechanism of ubiquinol oxidation at center P of the bc1 complex

Quinol oxidation at center P of the cytochrome bc(1) complex involves bifurcated electron transfer to the Rieske iron-sulfur protein and cytochrome b. It is unknown whether both electrons are transferred from the same domain close to the Rieske protein, or if an unstable semiquinone anion intermediate diffuses rapidly to the vicinity of the b(L) heme. We have determined the pre-steady state rate and activation energy (E(a)) for quinol oxidation in purified yeast bc(1) complexes harboring either a Y185F mutation in the Rieske protein, which decreases the redox potential of the FeS cluster, or a E272Q cytochrome b mutation, which eliminates the proton acceptor in cytochrome b. The rate of the bifurcated reaction in the E272Q mutant (<10% of the wild type) was even lower than that of the Y185F enzyme ( approximately 20% of the wild type). However, the E272Q enzyme showed the same E(a) (61 kJ mol(-1)) with respect to the wild type (62 kJ mol(-1)), in contrast with the Y185F mutation, which increased E(a) to 73 kJ mol(-1). The rate and E(a) of the slow reaction of quinol with oxygen that are observed after cytochrome b is reduced were unaffected by the E272Q substitution, whereas the Y185F mutation modified only its rate. The Y185F/E272Q double mutation resulted in a synergistic decrease in the rate of quinol oxidation (0.7% of the wild type). These results are inconsistent with a sequential "movable semiquinone" mechanism but are consistent with a model in which both electrons are transferred simultaneously from the same domain in center P.

Degenerative diseases, oxidative stress and cytochrome c oxidase function

Aging and degenerative diseases are associated with increased levels of reactive oxygen species (ROS). ROS are mostly produced in mitochondria, and their levels increase with higher mitochondrial membrane potential. Cellular respiratory control is based on inhibition of respiration by high membrane potentials. However, we have described a second mechanism of respiratory control based on allosteric inhibition of cytochrome c oxidase (CcO), the terminal enzyme of the respiratory chain, at high ATP:ADP ratios. The mechanism is independent of membrane potential. We have proposed that feedback inhibition of CcO by ATP keeps the membrane potential and ROS production at low levels. Various forms of stress switch off allosteric ATP-inhibition via reversible dephosphorylation of CcO, resulting in increased membrane potential and cellular ROS levels. This mechanism is proposed to represent a missing molecular link between stress and degenerative diseases.

A role for mitochondrial oxidative stress in sulfur mustard analog 2-chloroethyl ethyl sulfide-induced lung cell injury and antioxidant protection

Sulfur mustards (SMs) have been used as warfare agents since World War I and still pose a significant threat against civilian and military personnel. SM exposure can cause significant blistering of the skin, respiratory injury, and fibrosis. No antidote currently exists for SM exposure, but recent studies, using the SM analog 2-chloroethyl ethyl sulfide (CEES), have focused on the ability of antioxidants to prevent toxicity. Although antioxidants can prevent CEES-induced toxicity, the mechanisms by which these compounds are effective against SM agents are largely unknown. Using human bronchial epithelial (16HBE) cells and primary small airway epithelial cells, we show that CEES causes a significant increase in mitochondrial dysfunction as early as 4 h, which is followed by increases in mitochondrial reactive oxygen species (ROS), peaking 12 h after exposure. We also have identified a catalytic antioxidant metalloporphyrin that can rescue airway cells from CEES-induced toxicity when added 1 h after CEES exposure. In addition, the cytoprotective effects of the catalytic antioxidant are associated with correcting mitochondrial dysfunction ROS, DNA oxidation, and decreases in intracellular GSH. These findings suggest a role for oxidative stress in CEES toxicity and provide a rationale to investigate antioxidants as rescue agents in SM exposures.