Effect of thiol reagents on the proton conductivity of the H+-ATPase of mitochondria

Mitochondrial Oxidative Stress Is the General Reason for Apoptosis Induced by Different-Valence Heavy Metals in Cells and Mitochondria

This review analyzes the causes and consequences of apoptosis resulting from oxidative stress that occurs in mitochondria and cells exposed to the toxic effects of different-valence heavy metals (Ag+, Tl+, Hg2+, Cd2+, Pb2+, Al3+, Ga3+, In3+, As3+, Sb3+, Cr6+, and U6+). The problems of the relationship between the integration of these toxic metals into molecular mechanisms with the subsequent development of pathophysiological processes and the appearance of diseases caused by the accumulation of these metals in the body are also addressed in this review. Such apoptosis is characterized by a reduction in cell viability, the activation of caspase-3 and caspase-9, the expression of pro-apoptotic genes (Bax and Bcl-2), and the activation of protein kinases (ERK, JNK, p53, and p38) by mitogens. Moreover, the oxidative stress manifests as the mitochondrial permeability transition pore (MPTP) opening, mitochondrial swelling, an increase in the production of reactive oxygen species (ROS) and H2O2, lipid peroxidation, cytochrome c release, a decline in the inner mitochondrial membrane potential (ΔΨmito), a decrease in ATP synthesis, and reduced glutathione and oxygen consumption as well as cytoplasm and matrix calcium overload due to Ca2+ release from the endoplasmic reticulum (ER). The apoptosis and respiratory dysfunction induced by these metals are discussed regarding their interaction with cellular and mitochondrial thiol groups and Fe2+ metabolism disturbance. Similarities and differences in the toxic effects of Tl+ from those of other heavy metals under review are discussed. Similarities may be due to the increase in the cytoplasmic calcium concentration induced by Tl+ and these metals. One difference discussed is the failure to decrease Tl+ toxicity through metallothionein-dependent mechanisms. Another difference could be the decrease in reduced glutathione in the matrix due to the reversible oxidation of Tl+ to Tl3+ near the centers of ROS generation in the respiratory chain. The latter may explain why thallium toxicity to humans turned out to be higher than the toxicity of mercury, lead, cadmium, copper, and zinc.

The vacuolar ATPase: sulfite stabilization and the mechanism of nitrate inactivation

Using vacuolar membranes from Neurospora crassa, we observed that sulfite prevented the loss of vacuolar ATPase activity that otherwise occurred during 36 h at room temperature. Sulfite neither activated nor changed the kinetic behavior of the enzyme. Further, in the presence of sulfite, the vacuolar ATPase was not inhibited by nitrate. We tested the hypothesis that sulfite acts as a reducing agent to stabilize the enzyme, while nitrate acts as an oxidizing agent, inhibiting the enzyme by promoting the formation of disulfide bonds. All reducing agents tested, dithionite, selenite, thiophosphate, dithiothreitol and glutathione, prevented the loss of ATPase activity. On the other hand, all oxidizing agents tested, bromate, iodate, arsenite, perchlorate, and hydrogen peroxide, were potent inhibitors of ATPase activity. The inhibitory effect of the oxidizing agents was specific for the vacuolar ATPase. The mitochondrial ATPase, assayed under identical conditions, was not inhibited by any of the oxidizing agents. Analysis of proteins with two-dimensional gel electrophoresis indicated that nitrate can promote the formation of disufide bonds between proteins in the vacuolar membrane. These data suggest a mechanism to explain why nitrate specifically inhibits vacuolar ATPases, and they support the proposal by Feng and Forgac (Feng, Y., and Forgac, M. (1994) J. Biol. Chem. 269, 13244-13230) that oxidation and reduction of critical cysteine residues may regulate the activity of vacuolar ATPases in vivo.

Role of F0 and F1 subunits in the gating and coupling function of mitochondrial H(+)-ATP synthase. The effect of dithiol reagents

A study is presented on the role of F0 and F1 subunits in oligomycin-sensitive H+ conduction and energy transfer reactions of bovine heart mitochondrial F0F1 H(+)-ATP synthase. Mild treatment with azodicarboxylic acid bis(dimethylamide) (diamide) enhanced oligomycin-sensitive H+ conduction in submitochondrial particles containing F1 attached to F0. This effect was associated with stimulation of the ATPase activity, with no effect on its inhibition by oligomycin, and depression of the 32Pi-ATP exchange. The stimulatory effect of diamide on H+ conduction decreased in particles from which F1 subunits were partially removed by urea. The stimulatory effect exerted by diamide in the submitochondrial particles with F1 attached to F0 was directly correlated with a decrease of the original electrophoretic bands of a subunit of F0 (F0I-PVP protein) and the gamma subunit of F1, with corresponding formation of their cross-linking product. In F0 liposomes, devoid of gamma subunit, diamide failed to stimulate H+ conduction and to cause disappearance of F0I-PVP protein, unless purified gamma subunit was added back. The addition to F0 liposomes of gamma subunit, but not that of alpha and beta subunits, caused per se inhibition of H+ conduction. It is concluded that F0I-PVP and gamma subunits are directly involved in the gate of the F0F1 H(+)-ATP synthase. Data are also presented indicating contribution to the gate of oligomycin-sensitivity conferral protein and of another protein subunit of F0, F6.

Substitution of the cysteinyl residue (Cys21) of subunit b of the ATP synthase from Escherichia coli

The Fo complex of the ATP synthase (F1Fo) of Escherichia coli contains only two cysteinyl residues, Cys21, of the two copies of subunit b. Modification of Cys21 with the hydrophobic maleimide N-(7-dimethylamino-4-methyl-coumarinyl)maleimide resulted in impairment of Fo functions [Schneider, E. & Altendorf, K. (1985) Eur. J. Biochim. 153, 105-109]. We replaced this residue (via cassette mutagenesis) by Ser, Gly, Ala, Thr, Asp and Pro. None of the replacements resulted in detectable alterations of the function of the ATP synthase, making a functional role for these sulfhydryl residues unlikely. Due to its high tolerance towards amino acid substitutions, the region around Cys21 seems not to be a protein-protein contact area.

Further study on the role of Mg2+ in lipid-protein interaction in reconstituted porcine heart mitochondrial H+-ATPase

Porcine heart mitochondrial H+-ATPase was reconstituted by cholate dialysis method in liposomes containing neutral (PC, PE), acidic (PG, PI, PA, PS, DPG) or neutral and acidic phospholipids. The Mg2+ effect on the ATPase activity and its sensitivity to oligomycin, ATP-induced delta psi and delta pH formation was observed for the proteoliposomes containing acidic but not neutral phospholipids. Maleimide spin labels with varying arm lengths or bromoacetamide spin probe were used to monitor the conformational difference of H+-ATPase in the Mg2+-containing and Mg2+-'free' samples. A difference in W/S ratio (weakly immobilized/strongly immobilized component in the ESR spectra) could be detected for the F0.F1-containing and F1-depleted, (F0)-containing proteoliposomes, suggesting conformational difference in the F0-F1 complex and F0 portion induced by the Mg2+ effect. A conformational change of the beta-subunits in the F1 portion was also deduced from the ATP-induced fluorescence quenching of aurovertin-complex for Mg2+-containing samples. The results obtained are in favor of our previous assumption that Mg2+ may play its role by altering the physical state of the lipid bilayer, which would induce a conformational change in F0 (buried in the lipid core), which in turn is transmitted to the catalytic F1, resulting in a higher enzyme activity.

Inactivation of the mitochondrial ATPase inhibitor protein by chemical modification with diethylpyrocarbonate

Modification of histidine residue(s) by diethylpyrocarbonate treatment of submitochondrial particles obtained by sonication results in inhibition of ATPase activity and stimulation of oligomycin-sensitive H+ conduction. The inhibition of the ATPase (EC 3.6.1.3) activity persisted in F1 isolated from diethylpyrocarbonate-treated submitochondrial particles, which exhibited the absorbance spectrum of modified histidine. Thus the inhibition of the ATPase activity results from histidine modification in F1 subunits. Removal of the natural inhibitor protein from submitochondrial particles resulted in stimulation of proton conduction. After removal of F1 inhibitor protein from the particles the stimulatory effect exerted by diethylpyrocarbonate treatment on proton conduction was lost. Reconstitution experiments showed that purified F1 inhibitor protein lost, after histidine modification, its capacity to inhibit the ATPase activity and proton conduction. These observations show that the stimulation of proton conduction by the ATPase complex effected by diethylpyrocarbonate treatment results from histidine modification in F1 inhibitor protein.

Proton translocation by the H+-ATPase of mitochondria. Effect of modification by monofunctional reagents of thiol residues in F0 polypeptides

A study is presented on the effect of chemical modification of thiol groups on proton conduction by the H+-ATPase complex in 'inside out' submitochondrial particles, before and after removal of the F1 moiety, and by F0 liposomes. The results obtained show that modification with monofunctional reagents [N-ethylmaleimide, 2,2'-dithiobispyridine, mersalyl and N-(7-dimethylamino-4-methyl-coumarinyl)-maleimide] of thiol residues in membrane integral proteins of F0 results in inhibition of proton conduction. Comparison of the inhibitory effects with the binding of [14C]N-ethylmaleimide to the various F0 polypeptides indicates that the inhibition of proton conduction by thiol reagents was correlated with modification of the 25-kDa, 11-kDa and 9-kDa (N,N'-dicyclohexylcarbodiimide-binding protein) proteins. Involvement of the last component is supported by the observation that modification by thiol reagents depressed the binding of N,N'-dicyclo[14C]hexylcarbodiimide to the 9-kDa protein.

Regulatory role of the ATPase inhibitor protein on proton conduction by mitochondrial H+-ATPase complex

This study shows that the natural inhibitor protein of mitochondrial H+-ATPase complex (IF1) inhibits, in addition to the catalytic activity, the proton conductivity of the complex. The inhibition of ATPase activity by IF1 is less effective in the purified F1 than in submitochondrial particles where F1 is bound to F0. No inhibition of H+ conductivity by F0 is observed in F1-depleted particles.

Modification of subunit b of the F0 complex from Escherichia coli ATP synthase by a hydrophobic maleimide and its effects on F0 functions

Purified F0 from Escherichia coli ATP synthase was labelled with N-(7-dimethylamino-4-methyl-coumarinyl)-maleimide (DACM), a hydrophobic reagent which forms a stable, strongly fluorescent adduct with SH groups. Sodium dodecyl sulfate gel electrophoresis clearly demonstrated that subunit b was exclusively labelled, most likely at Cys-21, the only cysteine residue in E. coli F0. The amount of two molecules of DACM bound per F0, which was calculated from the absorption spectrum at 380 nm, is in full agreement with the postulated stoichiometry of two copies of subunit b/F0 complex. Thus the label provides a useful tool for simply detecting subunit b in protein chemical studies. DACM-labelled F0 was incorporated into liposomes and assayed for H+ translocating activity and its capacity to bind purified F1. Whereas the initial rate of H+ uptake was inhibited about 40% the reconstitution of a dicyclohexylcarbodiimide-sensitive F1F0 ATPase activity was completely unaffected. In a second set of experiments we reconstituted an F0 complex from DACM-labelled purified subunit b and an ac complex. In contrast to the results obtained with intact, DACM-labelled F0, both H+ translocating activity and F1 binding capacity were greatly reduced. Our data indicate that cysteine-21, probably together with other amino acids, is involved in maintaining a proper interaction of the hydrophobic N-terminal region of subunit b with the ac complex. This interplay seems to be a prerequisite for at least the in vitro assembly of a functional F0 complex.

Effect of diamide on proton translocation by the mitochondrial H+-ATPase

Treatment of sonic submitochondrial particles with the bifunctional thiol reagent, diamide, results in an enhancement of proton conductivity and ATPase activity, which is reversed by the reducing agent dithiothreitol, is suppressed by Fo inhibitors like oligomycin and is absent in particles that are deprived of peripheral Fo polypeptides. The effect of diamide is apparently due to oxidation of dithiols to disulfides in peripheral polypeptide(s) of Fo.

Differential effects of triphenyltin and 8-azido-ATP on the ATP synthesis, ATP-Pi exchange, and ATP hydrolysis in liposomes containing ATP synthase and bacteriorhodopsin

The ATP hydrolysis activity of purified ATP synthase reconstituted in liposomes was inhibited by triphenyltin in a manner different from that of other thiol-specific reagents. In liposomes containing ATP synthase and bacteriorhodopsin, ATP hydrolysis and ATP-Pi exchange were inhibited by triphenyltin to a greater extent than the ATP synthesis, in contrast to what was found with an F1-specific inhibitor, 8-azido-ATP. The possibility is discussed that ATP hydrolysis and ATP synthesis are differently coupled to proton conduction through F0.

On the mechanism of H+ translocation by mitochondrial H+ -ATPase. Studies with chemical modifier of tyrosine residues

In this paper a detailed study of the effect of nitration of tyrosine residues by tetranitromethane on H+ conduction and other reactions catalyzed by the H+ -ATPase complex in phosphorylating submitochondrial particles, uncoupled particles, and the purified complex is presented. Tetranitromethane treatment of submitochondrial particles results in marked inhibition of ATP hydrolysis, ATP-33Pi exchange, and proton conduction by the H+ -ATPase complex. These effects are caused by nitration of tyrosine residues of H+ -ATPase complex as shown by the appearance of the absorption peak at 360 nm (specific for nitrotyrosine formation) and inhibition of ATP hydrolysis and ATP-33Pi exchange in the complex purified from tetranitromethane-treated particles. H+ conduction in phospholipid vesicles inlaid with F0 is also inhibited by tetranitromethane treatment. These observations indicate that tyrosine residue(s) of F0 are critically involved in energy-linked proton translocation in the ATP-ase complex.

Studies on the mechanism of action of local anesthetics on proton translocating ATPase from Mycobacterium phlei

We have measured the inhibitory potencies of local anesthetics (procaine, lidocaine, tetracaine and dibucaine) on ATP-mediated H+-translocation, Ca2+-transport and ATPase activity in membrane vesicles from Mycobacterium phlei. Procaine and lidocaine up to 1 mM concentration did not inhibit ATP-dependent H+-translocation, Ca2+-transport and ATPase activity. However, tetracaine and dibucaine at 0.2 mM concentration caused dissipation of the proton gradient, measured by the reversal of the quenching of fluorescence of quinacrine, and inhibition of active Ca2+-transport. Tetracaine (1 mM) inhibited membrane-bound ATPase activity without affecting solubilized F1-ATPase activity. Studies show that these local anesthetics do not prevent the inactivation of F0-F1 ATPase by dicyclohexylcarbodiimide (DCCD). Binding of [14C]DCCD to F0-proteolipid component remained unchanged in the presence of tetracaine indicating that DCCD and tetracaine do not share common binding sites on the F0-proteolipid sector. The inhibition of H+-translocation and membrane-bound ATPase activity by tetracaine was substantially additive in the presence of vanadate.

Studies on the mechanism of action of triphenyltin on proton conduction by the H+-ATPase of mitochondria

A study is presented of the action of triphenyltin on the kinetics of the anaerobic relaxation of the proton gradient set up by respiration in various type of 'inside-out' inner membrane vesicles obtained by exposure of beef-heart mitochondria to ultrasonic energy. Triphenyltin is shown to act as a powerful inhibitor of the proton conductivity of the H+-ATPase. The inhibition persists after removal of the ATPase protein inhibitor, F1 and the oligomycin-sensitivity conferral protein (OSCP) from the particles. The inhibitory effect of triphenyltin is exerted, as in the case of oligomycin and N,N'-dicyclohexylcarbodiimide, on the F0 moiety of the ATPase complex. Comparison of the characteristics of the effect of triphenyltin on proton translocation in chloride and nitrate media shows that the inhibition of passive proton conductivity studied here is unrelated to the hydroxide/anion exchange induced by the organotin. Lack of additivity of the inhibition of H+ conduction by triphenyltin with that exerted by oligomycin and N,N'-dicyclohexylcarbodiimide and the kinetic pattern of the effect of triphenyltin show that the mechanism of action of the organotin is different from that of the other two inhibitors. The relevance of the results obtained with respect to the subunit location and chemical nature of the reaction site of triphenyltin in the H+-ATPase complex is discussed.