Characterization of sulfur-centered radical intermediates formed during the oxidation of thiols and sulfite by peroxynitrite. ESR-spin trapping and oxygen uptake studies

Alteration of nitric oxide synthase activity upon oxidative stress in cultured retinal cells

In the present study we examined the effect of oxidative stress-mediated hydroperoxide formation on the activity of nitric oxide synthase (NOS) in retinal cells in culture. Oxidative stress was induced in the presence of Fe2+ and ascorbate or Fe2+ alone and compared to H2O2-induced maximal cellular oxidation, and was measured by following the formation of intracellular hydroperoxides with the probe DCFH2 (2',7'-dichlorodihydrofluorescein). After a 15-min exposure to the oxidants, formation of hydroperoxides was significantly increased in the presence of 100 microM Fe2+ (about twofold), as compared to the control. Coadministration of Fe2+ and ascorbate (Fe-Asc) did not affect DCF fluorescence, but highly reduced the intracellular pH (pHi = 6.32 +/- 0.08), in comparison with control conditions (pHi = 7.05 +/- 0.11), as determined with the probe BCECF (2',7'-bis-(carboxyethyl)-5(and-6) carboxyfluorescein). Nevertheless, preincubation of Fe-Asc at acidic pH also increased the formation of hydroperoxides. Oxidative stress induced in the presence of Fe-Asc (at pH 6.5) significantly decreased the activity of NOS by 20% of control activity, as determined by the formation of [14C]citrulline. Fe-Asc (pH 6.5) also reduced the production of cyclic GMP (cGMP) in retinal cells by 1.5-fold, although a decrement in pH from 7.4 to 6.5 was not sufficient to decrease cGMP production. These data suggest that NO. production may be compromised in the presence of Fe-Asc. Moreover, neither 4 mM dithiotreitol (DTT) nor 4 mM glutathione (GSH) altered the production of cGMP in retinal cells submitted to oxidative stress. A reduction in NO. generation upon oxidative stress may reduce major damaging effects induced by ONOO- in cultured retinal cells.

Sulphite enhances peroxynitrite-dependent alpha1-antiproteinase inactivation. A mechanism of lung injury by sulphur dioxide?

Sulphite is toxic to the lung and can cause allergic reactions, the most common of which is bronchoconstriction in asthmatics. We show that sulphite can considerably potentiate the inactivation of alpha1-antiproteinase caused by peroxynitrite. Addition of peroxynitrite to sulphite generated inactivating species that persisted at pH 7.4 and 37 degrees C for at least 30 min. We propose that formation of protein-modifying sulphite radicals from SO3(2-) exposed to ONOO- is a mechanism by which SO2 could cause lung injury, both by enhancing proteolysis and by creating new antigens that could provoke an immune response.

One-electron oxidation pathway of thiols by peroxynitrite in biological fluids: bicarbonate and ascorbate promote the formation of albumin disulphide dimers in human blood plasma

Recent studies have shown that peroxynitrite oxidizes thiol groups through competing one- and two-electron pathways. The two-electron pathway is mediated by the peroxynitrite anion and prevails quantitatively over the one-electron pathway, which is mediated by peroxynitrous acid or a reactive species derived from it. In CO2-containing fluids the oxidation of thiols might follow a different mechanism owing to the rapid formation of a different oxidant, the nitrosoperoxycarbonate anion (ONOOCO2(-)). Here we present evidence that in blood plasma peroxynitrite induces the formation of a disulphide cross-linked protein identified by immunological (anti-albumin antibodies) and biochemical criteria (peptide mapping) as a dimer of serum albumin. The albumin dimer did not form in plasma devoid of CO2 and its formation was enhanced by ascorbate. However, analysis of thiol groups showed that reconstituting dialysed plasma with NaHCO3 protected protein thiols against the oxidation mediated by peroxynitrite and that the simultaneouspresence of ascorbate provided further protection. Ascorbate alone did not protect thiol groups from peroxynitrite-mediated oxidation. ESR spin-trapping studies with N-t-butyl-alpha-phenylnitrone (PBN) revealed that peroxynitrite induced the formation of protein thiyl radicals and their intensity was markedly decreased by plasma dialysis and restored by reconstitution with NaHCO3. PBN completely inhibited the formation of albumin dimer. Moreover, the addition of iron-diethyldithiocarbamate to plasma demonstrated that peroxynitrite induced the formation of protein S-nitrosothiols and/or S-nitrothiols. Our results are consistent with the hypothesis that NaHCO3 favours the one-electron oxidation of thiols by peroxynitrite with formation of thiyl radicals, ;NO2, and RSNOx. Thiyl radicals, in turn, are involved in chain reactions by which thiols are oxidized to disulphides.

Formation of spin trap adducts during the decomposition of peroxynitrite

Peroxynitrite-mediated one-electron oxidations may be an important event in its cytotoxic mechanisms, and yet, free radical formation in the presence of peroxynitrite is difficult to study by EPR-spin trapping because adducts from most spin traps are destroyed by the oxidant. This led to some controversy with regard to the interpretation of experiments in the presence of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), an adequate spin trap to study most of free radicals. In this report we reexamined peroxynitrite-mediate formation of spin-trap adducts. Kinetic studies and EPR experiments with water labeled with 17O are in agreement with the reaction of DMPO with a highly reactive intermediate derived from peroxynitrite to produce the DMPO-hydroxyl radical adduct by a mechanism not involving the oxidation of DMPO to a cation radical followed by water addition. The results cannot discriminate between two mechanisms of DMPO-hydroxyl radical formation, either spontaneous peroxynitrite homolysis to the hydroxyl radical or DMPO-assisted peroxynitrite homolysis. The formation of DMPO adducts during peroxynitrite-mediated oxidation of dimethyl sulfoxide, ethanol, and formate occurs through free radical mechanisms as confirmed by studies of oxygen consumption and product formation. Accordingly, spin-trapping experiments in the presence of 3,5-dibromo-4-nitrosobenzenesulfonic acid, a spin trap that is more resistant to nitrogen dioxide, led to the detection of the methyl and the beta-hydroxyethyl radical during peroxynitrite-mediated oxidation of dimethyl sulfoxide and ethanol, respectively. Oxidation of these hydroxyl radical scavengers to detectable radicals favors the hypothesis that the hydroxyl radical is produced during peroxynitrite homolysis. Bicarbonate was able to modulate peroxynitrite-mediated one-electron oxidations.

Endothelial nitric oxide synthase-dependent superoxide generation from adriamycin

Adriamycin (or doxorubicin) is an active and broad spectrum chemotherapeutic agent. Unfortunately, its clinical use is severely restricted by a dose-limiting cardiotoxicity which has been linked to the formation of superoxide. Enzymatic one-electron reduction of adriamycin forms adriamycin semiquinone radical, which rapidly reacts with oxygen to form superoxide and adriamycin. In this way, adriamycin provides a kinetic mechanism for the one-electron reduction of oxygen by flavoenzymes such as NADPH-cytochrome P450 reductase and mitochondrial NADH dehydrogenase. We demonstrate here that the endothelial isoform of nitric oxide synthase (eNOS) reduces adriamycin to the semiquinone radical. As a consequence, superoxide formation is enhanced and nitric oxide production is decreased. Adriamycin binds to eNOS with a Km of approximately 5 microM, as calculated from both eNOS-dependent NADPH consumption and superoxide generation. Adriamycin stimulated superoxide formation is not affected by calcium/calmodulin and is abolished by the flavoenzyme inhibitor, diphenyleneiodonium. This strongly suggests that adriamycin undergoes reduction at the reductase domain of eNOS. A consequence of eNOS-mediated reductive activation of adriamycin is the disruption of the balance between nitric oxide and superoxide. This may lead eNOS to generate peroxynitrite and hydrogen peroxide, potent oxidants implicated in several vascular pathologies.

Thiols and disulphides can aggravate peroxynitrite-dependent inactivation of alpha1-antiproteinase

Peroxynitrite (ONOO-) is a cytotoxic species formed in vivo. There is considerable interest in the development of ONOO- 'scavengers' as therapeutic agents; several thiols have been suggested to fulfil this role. One protein inactivated by ONOO- is alpha1-antiproteinase (alpha1AP), the major inhibitor of serine proteinases in human body fluids. At low thiol:ONOO- concentration ratios, several thiols (captopril, penicillamine, cysteine, cystine and penicillamine disulphide) aggravated inactivation of alpha1AP by ONOO- , whereas GSH, GSSG, homocysteine, ergothioneine, N-acetylcysteine, lipoate and dihydrolipoate did not. We suggest that sulphur-containing radicals are produced by reaction of certain thiols/disulphides with ONOO- or ONOO- -derived products and could mediate biological damage, including inactivation of alpha1AP. This must be considered in attempts to use thiols as 'peroxynitrite scavengers'.

Carbon dioxide modulation of hydroxylation and nitration of phenol by peroxynitrite

We have examined the formation of hydroxyphenols, nitrophenols, and the minor products 4-nitrosophenol, benzoquinone, 2,2'-biphenol, and 4,4'-biphenol from the reaction of peroxynitrite with phenol in the presence and absence of added carbonate. In the absence of added carbonate, the product yields of nitrophenols and hydroxyphenols have different pH profiles. The rates of nitration and hydroxylation also have different pH profiles and match the trends observed for the product yields. At a given pH, the sum of the rate constants for nitration and hydroxylation is nearly identical to the rate constant for the spontaneous decomposition of peroxynitrite. The reaction of peroxynitrite with phenol is zero-order in phenol, both in the presence and absence of added carbonate. In the presence of added carbonate, hydroxylation is inhibited, whereas the rate of formation and yield of nitrophenols increase. The combined maximum yield of o- and p-nitrophenols is 20 mol% (based on the initial concentration of peroxynitrite) and is about fourfold higher than the maximal yield obtained in the absence of added carbonate. The o/p ratio of nitrophenols is the same in the presence and absence of added carbonate. These results demonstrate that hydroxylation and nitration occur via two different intermediates. We suggest that the activated intermediate formed in the isomerization of peroxynitrous acid to nitrate, ONOOH*, is the hydroxylating species. We propose that intermediate 1, O=N-OO-CO2-, or secondary products derived from it, is (are) responsible for the nitration of phenol. The possible mechanisms responsible for nitration are discussed.

Spin-trapping of free radicals formed during the oxidation of glutathione by tetramethylammonium peroxynitrite

Glutathionyl radical (GS.) formed during the oxidation of glutathione by tetramethylammonium peroxynitrite ([NMe4][ONOO]) was spin-trapped with 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) and 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO). This radical reacted with ammonium formate to form the carbon dioxide anion radical (CO2-.). The superoxide anion formed during oxidation of GSH by peroxynitrite salt was trapped with DMPO and detected as the DMPO-hydroxyl adduct. Addition of SOD mimic completely abolished the spectrum of the hydroxyl adduct but not the spectrum of the DMPO-glutathionyl radical adduct. Addition of seleno-DL-cystine or its reduced form caused a dramatic inhibition in the formation of spin adducts, suggesting that seleno-DL-cysteine is a more effective scavenger of peroxynitrite. The oxygen uptake observed during oxidation of GSH by peroxynitrite salt was inhibited by spin traps. In the presence of catalase, approximately 50% of the oxygen consumed was restored, indicating stoichiometric conversion of O2 to H2O2 during oxidation of GSH by peroxynitrite salt. Results indicate that nitrite and glutathione disulfide are formed as the major products during oxidation of GSH by peroxynitrite.

Peroxynitrite-mediated decarboxylation of pyruvate to both carbon dioxide and carbon dioxide radical anion

There has been a recent renewal of interest in the antioxidant properties of pyruvate which are usually attributed to its capacity to undergo oxidative decarboxylation in the presence of hydrogen peroxide. The interaction of pyruvate with other oxidizing biological intermediates, however, has been scarcely considered in the literature. Here we report that peroxynitrite, the oxidant produced by the reaction between superoxide anion and nitric oxide, reacts with pyruvate with an apparent second-order rate constant of 88 +/- 7 M-1 s-1 at pH 7.4 and 37 degrees C. Kinetic studies indicated that pyruvate reacts with peroxynitrite anion (k = 100 +/- 7 M-1 s-1, peroxynitrous acid (k = 49 +/- 7 M-1 s-1, and a highly oxidizing species derived from peroxynitrous acid. Pyruvate decarboxylation was proved by anion exchange chromatography detection of acetate in incubations of peroxynitrite and pyruvate at pH 7.4 and 5.5. Formation of carbon dioxide radical anion was ascertained by EPR spin-trapping studies in the presence of GSH and the spin-trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The use of pyruvate labeled with 13C at the 1-position led to the detection of the labeled DMPO carbon dioxide radical anion adduct. In the absence of GSH, oxygen consumption studies confirmed that peroxynitrite mediates the decarboxylation of pyruvate to free radical intermediates. Comparing the yields of acetate and free radicals estimated from the oxygen uptake studies, it is concluded that pyruvate is oxidized by both one- and two-electron oxidation pathways, the latter being preponderant. Hydrogen peroxide-mediated pyruvate oxidation does not produce detectable levels of carbon dioxide radical anion except in the presence of iron(II)-ethylenediamine-N,N,N',N'-tetraacetate (EDTA). The apparent second-order rate constant of the reaction between pyruvate and hydrogen peroxide was determined to be 1 order of magnitude lower than that of the reaction between pyruvate and peroxynitrite. The latter process may contribute to the antioxidant properties of pyruvate.

Inhibition of peroxynitrite-mediated oxidation of glutathione by carbon dioxide

Peroxynitrite reacts with CO2 to from an adduct containing a weak O--O bond that can undergo homolytic and/or heterolytic cleavage to give other reactive intermediates. Because the peroxynitrite/CO2 reaction is fast and physiological concentrations of CO2 are relatively high, peroxynitrite-mediated oxidations of biological species probably involve the peroxynitrite-CO2 adduct and its subsequent reactive intermediates. We have examined the reaction of glutathione with peroxynitrite in the presence and absence of added bicarbonate. In the presence of added bicarbonate, CO2 competes with glutathione for peroxynitrite, resulting in a markedly decreased consumption of glutathione compared with that observed in the absence of added bicarbonate. However, the consumption of glutathione still is much higher than predicted from the assumption that the glutathione-peroxynitrite reaction is the only reaction that can consume glutathione in this system. These results suggest that glutathione partially, but not completely, traps intermediate(s) derived from the peroxynitrite and CO2 reaction. Some rate constants for the trapping of the intermediates are estimated by simulating the reactions, and possible mechanisms for the reaction of peroxynitrite with glutathione in the presence of added bicarbonate are discussed.

Pathways of peroxynitrite oxidation of thiol groups

Peroxynitrite mediates the oxidation of the thiol group of both cysteine and glutathione. This process is associated with oxygen consumption. At acidic pH and a cysteine/peroxynitrite molar ratio of < or = 1.2, there was a single fast phase of oxygen consumption, which increased with increasing concentrations of both cysteine and oxygen. At higher molar ratios the profile of oxygen consumption became biphasic, with a fast phase (phase I) that decreased with increasing cysteine concentration, followed by a slow phase (phase II) whose rate of oxygen consumption increased with increasing cysteine concentration. Oxygen consumption in phase I was inhibited by desferrioxamine and 5,5-dimethyl-1-pyrroline N-oxide, but not by mannitol; superoxide dismutase also inhibited oxygen consumption in phase I, while catalase added during phase II decreased the rate of oxygen consumption. For both cysteine and glutathione, oxygen consumption in phase I was maximal at neutral to acidic pH: in contrast, total thiol oxidation was maximal at alkaline pH. EPR spin-trapping studies using N-tert-butyl-alpha-phenylnitrone indicated that the yield of thiyl radical adducts had a pH profile comparable with that found for oxygen consumption. The apparent second-order rate constants for the reactions of peroxynitrite with cysteine and glutathione were 1290 +/- 30 M-1.S-1 and 281 +/- 6 M-1.S-1 respectively at pH 5.75 and 37 degrees C. These results are consistent with two different pathways participating in the reaction of peroxynitrite with low-molecular-mass thiols: (a) the reaction of the peroxynitrite anion with the protonated thiol group, in a second-order process likely to involve a two-electron oxidation, and (b) the reaction of peroxynitrous acid, or a secondary species derived from it, with the thiolate in a one-electron transfer process that yields thiyl radicals capable of initiating an oxygen-dependent radical chain reaction.

Reactive oxygen species participate in peroxynitrite-induced apoptosis in HL-60 cells

Peroxynitrite (ONOO-) is a physiological product generated by the interaction of superoxide (O2.-) and nitric oxide (.NO). We have previously shown that peroxynitrite induces apoptosis in HL-60 cells. In the present study, we demonstrated that peroxynitrite generates reactive oxygen species (ROS) in HL-60 cells. Brief exposure of HL-60 cells to ONOO- induced elevation of lucigenin chemiluminescence, indicating generation of superoxide anion. Exogenous superoxide dismutase (SOD), a scavenger of O2.-, fully abolished the chemiluminescence response, further supporting this notion. Following O2.- generation, the accumulation of hydrogen peroxide (H2O2) was observed. The addition of SOD exacerbated but that of catalase attenuated peroxynitrite-induced DNA fragmentation, suggesting that this H2O2 production contributes to the apoptotic process. In addition, pre-treatment of HL-60 cells with N-acetyl-L-cysteine (15 mM), a ROS scavenger, fully scavenged peroxynitrite-elicited ROS generation and effectively inhibited (ONOO-)-induced apoptosis, further enforcing this hypothesis. In summary, our results suggest that (ONOO-)-stimulated ROS formation may serve as a mechanism for the propagation of peroxynitrite-mediated apoptotic cell death in an intact cell system.

The chemistry of the S-nitrosoglutathione/glutathione system

S-Nitrosothiols have generated considerable interest due to their ability to act as nitric oxide (NO) donors and due to their possible involvement in bioregulatory systems-e.g., NO transfer reactions. Elucidation of the reaction pathways involved in the modification of the thiol group by S-nitrosothiols is important for understanding the role of S-nitroso compounds in vivo. The modification of glutathione (GSH) in the presence of S-nitrosoglutathione (GSNO) was examined as a model reaction. Incubation of GSNO (1 mM) with GSH at various concentrations (1-10 mM) in phosphate buffer (pH 7.4) yielded oxidized glutathione, nitrite, nitrous oxide, and ammonia as end products. The product yields were dependent on the concentrations of GSH and oxygen. Transient signals corresponding to GSH conjugates, which increased by one mass unit when the reaction was carried out with 15N-labeled GSNO, were identified by electrospray ionization mass spectrometry. When morpholine was present in the reaction system, N-nitrosomorpholine was formed. Increasing concentrations of either phosphate or GSH led to lower yields of N-nitrosomorpholine. The inhibitory effect of phosphate may be due to reaction with the nitrosating agent, nitrous anhydride (N2O3), formed by oxidation of NO. This supports the release of NO during the reaction of GSNO with GSH. The products noted above account quantitatively for virtually all of the GSNO nitrogen consumed during the reaction, and it is now possible to construct a complete set of pathways for the complex transformations arising from GSNO + GSH.