Reactions of b-cytochromes with ATP and antimycin A in pigeon heart mitochondria

Evidence of ROS generation by mitochondria in cells with impaired electron transport chain and mitochondrial DNA damage

Mitochondrial damage is a well known cause of mitochondria-related diseases. A major mechanism underlying the development of mitochondria-related diseases is thought to be an increase in intracellular oxidative stress produced by impairment of the mitochondrial electron transport chain (ETC). However, clear evidence of intracellular free radical generation has not been clearly provided for mitochondrial DNA (mtDNA)-damaged cells. In this study, using the novel fluorescence dye, 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (HPF), which was designed to detect hydroxyl radicals (*OH), intracellular free radical formation was examined in 143B cells (parental cells), 143B-rho(0) cells (mtDNA-lacking cells), 87 wt (cybrid), and cybrids of 4977-bp mtDNA deletion (common deletion) cells containing the deletion with 0%, 5%, 50% and >99% frequency (HeLacot, BH5, BH50 and BH3.12, respectively), using a laser confocal microscope detection method. ETC inhibitors (rotenone, 3-nitropropionic acid, thenoyltrifluoroacetone, antimycin A and sodium cyanide) were also tested to determine whether inhibitor treatment increased intracellular reactive oxygen species (ROS) generation. A significant increase in ROS for 143B-rho(0) cells was observed compared with 143B cells. However, for the 87 wt cybrid, no increase was observed. An increase was also observed in the mtDNA-deleted cells BH50 and BH3.12. The ETC inhibitors increased intracellular ROS in both 143B and 143B-rho(0) cells. Furthermore, in every fluorescence image, the fluorescence dye appeared localized around the nuclei. To clarify the localization, we double-stained cells with the dye and MitoTracker Red. The resulting fluorescence was consistently located in mitochondria. Furthermore, manganese superoxide dismutase (MnSOD) cDNA-transfected cells had decreased ROS. These results suggest that more ROS are generated from mitochondria in ETC-inhibited and mtDNA-damaged cells, which have impaired ETC.

Tumor necrosis factor-alpha increases the steady-state reduction of cytochrome b of the mitochondrial respiratory chain in metabolically inhibited L929 cells

The mechanism of tumor necrosis factor alpha (TNFalpha)-induced cytotoxicity in metabolically inhibited cells is unclear, although some studies have suggested that mitochondrial dysfunction and generation of reactive oxygen species may be involved. Here we studied the effect of TNFalpha on the redox state of mitochondrial cytochromes and its involvement in the generation of reactive oxygen species in metabolically inhibited L929 cells. Treatment with TNFalpha and cycloheximide (TNFalpha/CHX) induced mitochondrial cytochrome c release, increased the steady-state reduction of cytochrome b, and decreased the steady-state reduction of cytochromes cc(1) and aa(3). TNFalpha/CHX treatment also induced lipid peroxidation, intracellular generation of reactive oxygen species, and cell death. Furthermore, as the cells died mitochondrial morphology changed from an orthodox to a hyperdense and condensed and finally to a swollen conformation. Antimycin A, a mitochondrial respiratory chain complex III inhibitor that binds to cytochrome b, blocked the formation of reactive oxygen species, suggesting that the free radicals are generated at the level of cytochrome b. Moreover, antimycin A, when added after 3 h of TNFalpha/CHX treatment, arrested the further release of cytochrome c and the cytotoxic response. We propose that the reduced cytochrome b promotes the formation of reactive oxygen species, lipid peroxidation of the cell membrane, and cell death.

Inhibitor effects on redox-linked protonations of the b haems of the mitochondrial bc1 complex

The effects of pH and inhibitors on the spectra and redox properties of the haems b of the bc1 complex of beef heart submitochondrial particles were investigated. The major findings were: (1) both haems have a weakly redox-linked protonatable group with pKox and pKred of around 6 and 8; (2) at pH values above 7, haem bH becomes heterogeneous in its redox behaviour. This heterogeneity is removed by the Qi site inhibitors antimycin A, funiculosin and HQNO, but not by the Qo site inhibitors myxothiazol or stigmatellin; (3) of all inhibitors tested only funiculosin had a large effect on the Em/pH profile of either haem b. In all cases where definite effects were found, the haem most affected was that thought to be closest to the site of inhibitor binding; (4) spectral shifts of haem groups caused by inhibitor binding were usually, but not always, of the haem group closest to the binding site; (5) titrations with succinate/fumarate were in reasonable agreement with redox-mediated data provided that strict anaerobiosis was maintained. Apparent large shifts of haem midpoint potentials with antimycin A and myxothiazol could be produced in aerobic succinate/fumarate titrations in the presence of cyanide, as already reported in the literature, but these were artefactual; (6) the heterogeneous haem bH titration behaviour can be simulated with a model similar to that proposed by Salerno et al. (J. Biol. Chem. (1989) 264, 15398-15403) in which there is redox interaction between haem bH and ubiquinone species bound at the Qi site. Simulations closely fit both the haem bH data and known semiquinone data only if it is assumed that semiquinone bound to oxidised haem bH is EPR-silent.

Nuclearly inherited diuron-resistant mutations conferring a deficiency in the NADH--or succinate--ubiquinone oxidoreductase activity in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, diuron, antimycin and myxothiazol block the respiratory pathway at the bc1 complex level. Nuclearly inherited mutations located at the DIU3 and DIU4 loci confer in vitro resistance to diuron and cross-resistance to antimycin and myxothiazol at the NADH oxidase level. The mutant strains do not exhibit diuron resistance at the quinol-cytochrome-c oxidoreductase level. Thus, the apparent resistance does not seem to be the result of a modification of the inhibitory sites. Instead, the quinone reduction rate was found to be altered in the mutant. The diu3 mutations lead to a deficiency of the NADH--ubiquinone oxidoreductase activity, and the diu4 mutations to a deficiency of the succinate--ubiquinone oxidoreductase activity. On the basis of the model of Kröger and Klingenberg, a decrease of quinone reduction could explain the resistance to the bc1 complex inhibitors. Thus, the apparent resistance to the bc1 complex inhibitors was found to be due to a modification of the electron transfer kinetics.

Effect of b-c1-site inhibitors on the midpoint potentials of mitochondrial cytochromes b

Anaerobic potentiometric titrations of b cytochromes have been carried out in beef heart submitochondrial particles in the presence of several specific inhibitors of electron transfer through the b-c1-site of the respiratory chain. Whereas antimycin shows no significant effect on the titration curve of cytochrome b-562, NoHOQnO is found to shift the Em of b-562 by 20-30 mV to the positive. Funiculosin raises the Em of b-562 by greater than 100 mV and also appears to bring about a minor shift of b-566 midpoint potential. In the presence of myxothiazol, both b cytochromes titrate with Em values 15-30 mV more positive than in the control.

Control of electron transfer in the cytochrome system of mitochondria by pH, transmembrane pH gradient and electrical potential. The cytochromes b-c segment

1. A study is presented of the effects of pH, transmembrane pH gradient and electrical potential on oxidoreductions of b and c cytochromes in ox heart mitochondria and 'inside-out' submitochondrial particles. 2. Kinetic analysis shows that, in mitochondria at neutral pH, there is a restraint on the aerobic oxidation of cytochrome b566 with respect to cytochrome b562. Valinomycin plus K+ accelerates cytochrome b566 oxidation and retards net oxidation of cytochrome b562. At alkaline pH the rate of cytochrome b566 oxidation approaches that of cytochrome b562 and the effects of valinomycin on b cytochromes are impaired. 3. At slightly acidic pH, oxygenation of antimycin-supplemented mitochondria causes rapid reduction of cytochrome b566 and small delayed reduction of cytochrome b562. Valinomycin or a pH increase in the medium promote reduction of cytochrome b562 and decrease net reduction of cytochrome b566. 4. Addition of valinomycin to mitochondria and submitochondrial particles in the respiring steady state causes, at pH values around neutrality, preferential oxidation of cytochrome b566 with respect to cytochrome b562. The differential effect of valinomycin on oxidation of cytochromes b566 and b562 is enhanced by substitution of 1H2O of the medium with 2H2O and tends to disappear as the pH of the medium is raised to alkaline values. 5. Nigericin addition in the aerobic steady state causes, both in mitochondria and submitochondrial particles, preferential oxidation of cytochrome b562 with respect to cytochrome b566. This is accompanied by c cytochrome oxidation in mitochondria but c cytochrome reduction in submitochondrial particles. 6. In mitochondria as well as in submitochondrial particles, the aerobic transmembrane potential (delta psi) does not change by raising the pH of the external medium from neutrality to alkalinity. The transmembrane pH gradient (delta pH) on the other hand, decrease slightly. 7. The results presented provide evidence that the delta psi component of the aerobic delta microH+ (the sum of the proton chemical and electrical activities) exerts a pH-dependent constraint on forward electron flow from cytochrome b566 to cytochrome b562. This effect is explained as a consequence of anisotropic location of cytochromes b566 and b562 in the membrane and the pH-dependence of the redox function of these cytochromes. Transmembrane delta pH, on the other hand, exerts control on electron flow from cytochrome b562 to c cytochromes.

bc1-Complex from beef heart. One-step purification by hydroxyapatite chromatography in Triton X-100, polypeptide pattern and respiratory chain characteristics

A new simple method for the purification of the bc1-complex has been developed. The polypeptide composition of the complex was analysed by dodecyl sulfate-polyacrylamide gel electrophoresis. The content of chain components and phospholipids was determined. The b-type cytochromes were further characterized by their absorbance spectra and midpoint potentials. (1) Starting from a Triton X-100 extract of submitochondrial particles supplemented with antimycin, the bc1-complex is purified by adsorption chromatography on hydroxyapatite with citrate as specific eluant. (2) The complex splits in dodecyl sulfate into five main polypeptides with apparent molecular weight of 47, 44, 31, 11 and less than 10 kdalton. (3) The purified complex has a heme-b content of 8.0 mumol/g protein and a cytochrome c1 content of 3.8 mumol/g protein. (4) The cytochromes show the typical absorbance spectra of cytochromes b-562 and b-565 and are present in approximately equal amounts with midpoint potentials of Em7 = + 100 mV and Em7 = + mV respectively. Carbon monoxide does not bind to the cytochromes. (5) The nonheme iron protein content of the complex is diminished to 0.6 mumol/g protein. (6) The use of the nonionic surfactant Triton X-100 leads to a complete loss of lipids and ubiquinone of the bc1-complex. (7) The complex contains no succinate dehydrogenase as indicated by the absence of the 69 kdalton subunit in the dodecyl sulfate gel electrophoresis. In addition, it lacks an ubiquinone cytochrome c reductase activity and other electron transferring activities. This may be inferred from an inhibition by antimycin and depletion of ubiquinone and phospholipids. The highly purified and relative stable complex can be prepared giving 50% yield and may be suitable for protein chemistry studies.

Stepwise reduction of cytochromes b-562, b-566 and b-558 in rat liver mitochondria

1. Addition of KCN to aerobic, rotenone-inhibited rat liver mitochondria with out addition of substrate caused reduction of cytochromes b-562 (having an alpha-band at 562 nm at room temperature), c + c1, and a + a3. The effect of KCN on cytochrome b-562 was reversed by pentachlorophenol, though the effect of KCN on cytochromes c+c1 and a+a3 was not reversed by this uncoupler.2. Addition of ATP to aerobic, rat liver mitochondria inhibited with 500 muM KCN under conditions were cytochromes b-562, c+c1 and a+a3 were reduced, caused reduction of cytochrome b-566. The absorbance spectrum of cytochrome b-566 had an alpha-band at 565.5 nm, a beta-band at 538 nm and a gamma-band at 431 nm, but no shoulder around 558 nm at room temperature. 3. Addition of succinate to rotenone-KCN-inhibited and ATP-treated rat liver mitochondria under conditions where cytochromes b-566, b-562, c+c1 and a+a3 were already fully reduced, caused reduction of cytochrome b-558 (having an alpha-band at 558 nm, a beta-band at 527 nm and a gamma-band at 426 nm at room temperature) after exhaustion of molecular oxygen in the reaction medium, without any contribution from a long-wavelength species (cytochrome b-566). 4. It was concluded that the 558-nm band is not a short-wavelength shoulder of cytochrome b-566, but is due to a different species from cytochrome b-566.

Thermodynamic and EPR characterization of iron-sulfur centers in the NADH-ubiquinone segment of the mitochondrial respiratory chain in pigeon heart

Several iron-sulfur centers in the NADH-ubiquinone segment of the respiratory chain in pigeon heart mitochondria and in submitochondrial particles were analyzed by the combined application of cryogenic EPR (between 30 and 4.2 degrees K) and potentiometric titration. Center N-1 (iron-sulfur centers associated with NADH dehydrogenase are designated with the prefix "N") resolves into two single electron titratins with EM7.2 values of minus 380 plus or minus 20 mV and minus 240 plus or minus 20 mV (Centers N-1a and N-1b, respectively). Center N-1a exhibits an EPR spectrum of nearly axial symmetry with g parellel = 2.03, g = 1.94, while that of Center N-1b shows more apparent rhombic symmetry with gz = 2.03, gy = 1.94 and gx = 1.91. Center N-2 also reveals EPR signals of axial symmetry at g parallel = 2.05 and g = 1.93 and its principal signal overlaps with those of Centers N-1a and N-1b. Center N-2 can be easily resolved from N-1a and N-1b because of its high EM7.2 value (minus 20 plus or minus 20 mV). Resolution of Centers N-3 and N-4 was achieved potentiometrically in submitochondrial particles. The component with EM7.2 = minus 240 plus or minus 20 mV is defined as Center N-3 (gz = 2.10, (gz = 2.10, (gy = 1.93?), GX = 1.87); the minus 405 plus or minus 20 mV component as Center N-4 (gz = 2.11, (gy = 1.93?), gx = 1.88). At temperatures close to 4.2 degrees K, EPR signals at g = 2.11, 2.06, 2.03, 1.93, 1.90 and 1.88 titrate with EM7.2 = minus 260 plus or minus 20 mV. The multiplicity of peaks suggests the presence of at least two different iron-sulfur centers having similar EM7.2 values (minus 260 plus or minus 20 mV); HENCE, tentatively assigned as N-5 and N-6. Consistent with the individual EM7.2 values obtained, addition of succinate results in the partial reduction of Center N-2, but does not reduce any other centers in the NADH-ubiquinone segment of the respiratory chain. Centers N-2, N-1b, N-3, N-5 and N-6 become almost completely reduced in the presence of NADH, while Centers N-1a and N-4 are only slightly reduced in pigeon heart submitochondrial particles. In pigeon heart mitochondria, the EM7.2 of Center N-4 lies much closer to that of Center N-3, so that resolution of the Center N-3 and N-4 spectra is not feasible in mitochondrial preparations. EM7.2 values and EPR lineshapes for the other iron-sulfur centers of the NADH-ubiquinone segment in the respiratory chain of intact mitochondria are similar to those obtained in submitochondrial particle preparations. Thus, it can be concluded that, in intact pigeon heart mitochondria, at least five iron-sulfur centers show EM7.2 values around minus 250 mV; Center N-2 exhibits a high EM7.2 (minus 20 plus or minus 20 mV), while Center N-1a shows a very low EM7.2 (minus 380 plus or minus 20 mV).

Inhibition of electron transfer from ferrocytochrome b to ubiquinone, cytochrome c1 and duroquinone by antimycin

The effect of antimycin on (i) the respiratory activity of the KCN-insensitive pathway of mitochondria of Neurospora grown on chloramphenicol (chloramphenicol-grown) with durohydroquinone and succinate or NADH as substrate, (ii) the electron transfer from the b-type cytochromes to ubiquinone with durohydroquinone as electron donor as well as (iii) the electron transfer from the b-type cytochromes to duroquinone with succinate as electron donor in chloramphenicol-grown Neurospora and beef heart submitochondrial particles was studied. All experiments were performed in the uncoupled state. 1. The respiratory chain of chloramphenicol-grown Neurospora mitochondria branches at ubiquinone into two pathways. Besides the cytochrome oxidase-dependent pathway, a KCN-insensitive branch equiped with a salicylhydroxamate-sensitive oxidase exists. Durohydroquinone, succinate or NADH are oxidized via both pathways. The durohydroquinone oxidation via the KCN-insensitive pathway is inhibited by antimycin, wheras the succinate or NADH oxidation is not. The titer for ful inhibition is one mol antimycin per mol cytochrome b-563 or cytochrome b-557. 2. The electron transfer from durohydroquinone to ubiquinone, which takes place in the KCN-inhibited state, does not occur in the antimycin-inhibited state. 3. The reduction of duroquinone by succinate in the presence of KCN is inhibited by antimycin. The titer for full inhibition is one mol antimycin per mol cytochrome b-566 or cytochrome b-562 for beef heart (or cytochrome b-563 or cytochrome b-557 for Neurospora). 4. When electron transfer from the b-type cytochromes to cytochrome C1, ubiquinone and duroquinone is inhibited by antimycin, the hemes of cytochrome b-566 and cytochrome b-562 (or cytochrome b-563 and cytochrome b-557) are in the reduced state. 5. The experimental results suggest that the two b-type cytochromes form a binary complex the electron transferring activity of which is inhibited by antimycin, the titer for full inhibition being one mol of antimycin per mol of complex. The electron transfer from the b-type cytochromes to ubiquinone is inhibited in a non-linear fashion.

On the lack of ATP-induced midpoint potential shift for cytochrome b-566 in plant mitochondria

Recently proposed mechanisms of site II energy transduction that assign a key role to cytochrome b-566 are based on the finding that the apparent midpoint potential of b-566 in animal mitochondria increases by more than 250 mV upon addition of ATP [Chance et al. (1970) Proc. Nat. Acad. Sci. USA 66, 1175-1182]. However, since it has never been shown that the redox mediators used in the midpoint potential measurements equilibrate directly with b-566, the observed midpoint potential shift could merely reflect reversed electron transport. In mung bean mitochondria, the apparent midpoint potential of b-566 is known to be unaffected by addition of ATP [Dutton and Storey (1971) Plant Physiol. 47, 282-288]. In the present work, mung bean b-566 is shown to undergo an ATP-induced reduction similar to that observed for b-566 in animal mitochondria. However, in mung bean mitochondria the reduction is found to be rapidly relaxed by addition of redox mediator (phenazine methosulfate, PMS) and concomitantly PMS causes a marked, antimycinsensitive stimulation of ATPase activity. These results suggest that the ATP-induced reduction in mung bean mitochondria is due to reversed electron transport and that PMS can effectively short-circuit reversed electron transport in this system, bringing it close to equilibrium. Moreover, since mung bean and animal b-566 are identical in all other respects tested, the results support the idea that the apparent midpoint potential shift in animal mitochondria is also merely due to reversed electron transport, and that the mediators are now not effective enough to bring the system to equilibrium.