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

pH-Dependent Nitrotyrosine Formation in Ribonuclease A is Enhanced in the Presence of Polyethylene Glycol (PEG)

Protein nitration can occur as a result of peroxynitrite-mediated oxidative stress. Excess production of peroxynitrite (PN) within the cellular medium can cause oxidative damage to biomolecules. The in vitro nitration of Ribonuclease A (RNase A) results in nitrotyrosine (NT) formation with a strong dependence on the pH of the medium. In order to mimic the cellular environment in this study, PN-mediated RNase A nitration has been carried out in a crowded medium. The degree of nitration is higher at pH 7.4 (physiological pH) compared to pH 6.0 (tumor cell pH). The extent of nitration increases significantly when PN is added to RNase A in the presence of crowding agents PEG 400 and PEG 6000. PEG has been found to stabilize PN over a prolonged period, thereby increasing the degree of nitration. NT formation in RNase A also results in a significant loss in enzymatic activity.

Mechanisms of peroxynitrite-mediated nitration of tyrosine

The mechanisms of tyrosine nitration by peroxynitrous acid or nitrosoperoxycarbonate were investigated with the CBS-QB3 method. Either the protonation of peroxynitrite or a reaction with carbon dioxide gives a reactive peroxide intermediate. Peroxynitrous acid-mediated nitration of phenol occurs via unimolecular decomposition to give nitrogen dioxide and hydroxyl radicals. Nitrosoperoxycarbonate also undergoes unimolecular decomposition to give carbonate and nitrogen dioxide radicals. The reactions of tyrosine with the hydroxyl or carbonate radicals give a phenoxy radical intermediate. The reaction of the nitrogen dioxide with this radical intermediate followed by tautomerization gives nitrated tyrosine in both cases. According to CBS-QB3 calculations, the rate-limiting step for the nitration of phenol is the decomposition of peroxynitrous acid or nitrosoperoxycarbonate.

Phosphatidylinositol 3-kinase is a target for protein tyrosine nitration

A major mechanism of injury associated with the production of nitric oxide (NO*) in vivo is due to its diffusion-limited reaction with superoxide to form peroxynitrite, which in turn may cause nitration of protein tyrosine residues. To assess the physiological role of tyrosine nitration, it is crucial to identify the proteins that become nitrated. Therefore, we treated lysates from RAW 264.7 cells with 1 mM peroxynitrite and immunoprecipitated tyrosine nitrated proteins. This treatment resulted in the nitration of several proteins, with molecular weights ranging from 60-250 kD. One of these proteins was immunologically identified as the p85 regulatory subunit of the phosphatidylinositol 3-kinase, a key enzyme involved in the signal transduction cascade initiated by many agonists including growth factors. Treatment of RAW 264.7 macrophages with the NO* donor spermine NONOate also induced a nitration of the p85 subunit, demonstrating that this covalent modification also occurs in intact cells. Immunoprecipitation of the p110 catalytic subunit of the phosphatidylinositol 3-kinase co-immunoprecipitated p85 in control lysates. However, p85 could not be detected in the same immunoprecipitates when the lysates had been preincubated with 1 mM peroxynitrite, indicating that the nitration of the p85 subunit may abrogate its interaction with the p110 subunit.

Peroxynitrite-mediated inactivation of manganese superoxide dismutase involves nitration and oxidation of critical tyrosine residues

Previous studies from our laboratory have demonstrated that the mitochondrial protein manganese superoxide dismutase is inactivated, tyrosine nitrated, and present as higher molecular mass species during human renal allograft rejection. To elucidate mechanisms whereby tyrosine modifications might result in loss of enzymatic activity and altered structure, the effects of specific biological oxidants on recombinant human manganese superoxide dismutase in vitro have been evaluated. Hydrogen peroxide or nitric oxide had no effect on enzymatic activity, tyrosine modification, or electrophoretic mobility. Exposure to either hypochlorous acid or tetranitromethane (pH 6) inhibited (approximately 50%) enzymatic activity and induced the formation of dityrosine and higher mass species. Treatment with tetranitromethane (pH 8) inhibited enzymatic activity 67% and induced the formation of nitrotyrosine. In contrast, peroxynitrite completely inhibited enzymatic activity and induced formation of both nitrotyrosine and dityrosine along with higher molecular mass species. Combination of real-time spectral analysis and electrospray mass spectroscopy revealed that only three (Y34, Y45, and Y193) of the nine total tyrosine residues in manganese superoxide dismutase were nitrated by peroxynitrite. Inspection of X-ray crystallographic data suggested that neighboring glutamate residues associated with two of these tyrosines may promote targeted nitration by peroxynitrite. Tyr34, which is present in the active site, appeared to be the most susceptible residue to peroxynitrite-mediated nitration. Collectively, these observations are consistent with previous results using chronically rejecting human renal allografts and provide a compelling argument supporting the involvement of peroxynitrite during this pathophysiologic condition.

Kinetics of superoxide dismutase- and iron-catalyzed nitration of phenolics by peroxynitrite

Superoxide dismutase and Fe3+EDTA catalyzed the nitration by peroxynitrite (ONOO-) of a wide range of phenolics including tyrosine in proteins. Nitration was not mediated by a free radical mechanism because hydroxyl radical scavengers did not reduce either superoxide dismutase or Fe3+EDTA-catalyzed nitration and nitrogen dioxide was not a significant product from either catalyst. Rather, metal ions appear to catalyze the heterolytic cleavage of peroxynitrite to form a nitronium-like species (NO2+). The calculated energy for separating peroxynitrous acid into hydroxide ion and nitronium ion is 13 kcal.mol-1 at pH 7.0. Fe3+EDTA catalyzed nitration with an activation energy of 12 kcal.mol-1 at a rate of 5700 M-1.s-1 at 37 degrees C and pH 7.5. The reaction rate of peroxynitrite with bovine Cu,Zn superoxide dismutase was 10(5) M-1.s-1 at low superoxide dismutase concentrations, but the rate of nitration became independent of superoxide dismutase concentration above 10 microM with only 9% of added peroxynitrite yielding nitrophenol. We propose that peroxynitrite anion is more stable in the cis conformation, whereas only a higher energy species in the trans conformation can fit in the active site of Cu,Zn superoxide dismutase. At high superoxide dismutase concentrations, phenolic nitration may be limited by the rate of isomerization from the cis to trans conformations of peroxynitrite as well as by competing pathways for peroxynitrite decomposition. In contrast, Fe3+EDTA appears to react directly with the cis anion, resulting in greater nitration yields.

Structural and functional modifications induced by diamide on the F0 sector of the mammalian ATP synthase

In this report data are presented which firmly establish that by treating isolated F0 with the thiol reagent diamide, two 25 kDa F0 subunits react to form a dimer of 45 kDa apparent molecular mass. This dimerising effect is correlated to the impairment of the binding of F1 to F0, both at microM and mM diamide concentrations. Under the latter condition, modification of other F0 subunits also occurs. Passive proton conductance through F0, as well as its sensitivity to N,N'-dicyclohexylcarbodiimide, are affected at low diamide concentration. Thus perturbation of the cysteine residue of the 25 kDa F0 subunit is sufficient for altering the ATP synthase proton channel.

Mitochondrial F0F1 H+-ATP synthase. Characterization of F0 components involved in H+ translocation

The membrane F0 sector of mitochondrial ATP synthase complex was rapidly isolated by direct extraction with CHAPS from F1-depleted submitochondrial particles. The preparation thus obtained is stable and can be reconstituted in artificial phospholipid membranes to result in oligomycin-sensitive proton conduction, or recombined with purified F1 to give the oligomycin-sensitive F0F1-ATPase complex. The F0 preparation and constituent polypeptides were characterized by SDS-polyacrylamide gel electrophoresis and immunoblot analysis. The functional role of F0 polypeptides was examined by means of trypsin digestion and reconstitution studies. It is shown that, in addition to the 8 kDa DCCD-binding protein, the nuclear encoded protein [(1987) J. Mol. Biol. 197, 89-100], characterized as an intrinsic component of F0 (F0I, PVP protein [(1988) FEBS Lett. 237,9-14]) [corrected] is involved in H+ translocation and the sensitivity of this process to the F0 inhibitors, DCCD and oligomycin.

Effect of cetyltrimethylammonium on ATP hydrolysis and proton translocation in the F0-F1 H+-ATP synthase of mitochondria

The amphiphylic alkyl cation cetyltrimethylammonium inhibits the catalytic activity of soluble and membrane-bound F1 in a noncompetitive fashion. In sonic submitochondrial particles the Dixon plot showed a peculiar pattern with upward deviation at cetyltrimethylammonium concentration higher than 80 microM. In membrane-bound F1 the inhibition by cetyltrimethylammonium was potentiated by the F0 inhibitor ologomycin. Cetyltrimethylammonium also inhibited the oligomycin-sensitive proton conductivity in F1-containing particles but was without any effect in F1-depleted particles. Also this inhibitory effect was potentiated by oligomycin. These results indicate functional cooperative interactions between F0 and F1.

Bacterial adenosine 5'-triphosphate synthase (F1F0): purification and reconstitution of F0 complexes and biochemical and functional characterization of their subunits

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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.

Thiols in oxidative phosphorylation: thiols in the F0 of ATP synthase essential for ATPase activity

It was shown previously that the ATP synthase complex of bovine heart mitochondria contains an essential set of thiols or dithiols in its membrane sector (F0), whose modification by various reagents results in uncoupling [Yagi, T., and Hatefi, Y. (1984) Biochemistry 23, 2449-2455]. The sensitivity to modifiers was increased by membrane energization, and the uncoupling was reversed by membrane-permeable thiol compounds when modifiers other than alkylating agents were used to uncouple. The present paper demonstrates that there exists in the F0 of bovine ATP synthase another set of essential thiols, whose modification results in reversible inhibition of ATPase activity. These thiols are most susceptible to modification by mercurials (p-chloromercuribenzoate greater than p-chloromercuribenzene sulfonate) and do not appear to be modified by N-ethylmaleimide. The reversible modification of these thiols by mercurials protects the ATP synthase against irreversible inhibition in F0 by N,N-dicyclohexylcarbodiimide. The possible location of these two sets of thiols in the F0 of bovine ATP synthase is discussed.

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.