Quantitation of nitrotyrosine levels in lung sections of patients and animals with acute lung injury

Flavone acetic acid stimulates nitric oxide and peroxynitrite production in subcutaneous mouse tumors

Flavone acetic acid (FAA) has powerful anti-tumor activity against many types of solid murine tumors, but its biochemical mechanism of action is not understood. The present study examined the role of tumor vasculature and nitric oxide in mediating the anti-tumor effects of FAA. Athymic nude mice bearing subcutaneous RJ2-14 tumors were treated with a single dose of FAA, 200 mg/kg i.p., and euthanized at various times. Apoptosis within tumors was apparent during the first six hours of FAA treatment. We found that Type III, endothelial nitric oxide synthase (NOS) activity was significantly increased in tumors, but not in other tissues, as early as two hours after FAA dosing. FAA also stimulated the formation of the toxic peroxynitrite radical in tumors within two hours of treatment as assessed by immunostaining for nitrotyrosine. Staining was observed in dilated tumor vessels and surrounding tumor cells and correlated with the presence of apoptosis. Tumor endothelium may therefore be a critical target for FAA activity via stimulation of the nitric oxide pathway.

Free and protein-associated nitrotyrosine formation following rat liver preservation and transplantation

Nitrotyrosine in human and animal tissues has been associated with pathologic conditions such as atherosclerosis, renal failure, and acute lung disease. In this study, free and protein-associated nitrotyrosine were determined in plasma and tissue samples using a dual-channel electrochemical detection method. Free nitrotyrosine was quantified in acetonitrile-extracted samples while protein-associated nitrotyrosine was determined in proteinase K-digested samples. In human plasma, total nitrotyrosine increased from 2.3 to 4.3 and 13.2 mumol/mol Tyr following addition of 0, 0.5, and 1 mM ONOO-. To determine if nitrotyrosine was produced during ex vivo hypothermic preservation, rat livers were stored in University of Wisconsin solution (UW) for 0, 6, or 8 h and reperfused for 3 h. Total nitro-tyrosine increased 359 and 908% after 6 and 8 h preservation compared to 0 h. To determine if nitrotyrosine was produced in vivo following hepatic ischemia, a rat preservation-transplantation model was utilized in which livers were flushed with cold UW (0-h group) or transplanted following 6 h hypothermic preservation in UW. Free nitrotyrosine increased from 15.7 +/- 0.3 in the 0-h group to 23.6 +/- 2.5 mumol/mol Tyr, 24 h posttransplant of 6-h preserved livers. Protein-associated nitrotyrosine increased from 9.5 +/- 1.1 in the 0-h group to 27.5 +/- 0.7 mumol/mol Tyr in the 6-h preservation-transplantation group. Protein-associated nitrotyrosine provides an integrative determination of nitration. Detection of free and protein-associated nitrotyrosine in biologic samples may allow insight into the role of .NO-derived oxidants in tissue injury associated with various pathologic conditions.

Oxidative modification of tryptophan residues exposed to peroxynitrite

The aim of this study was to clarify the mechanism of loss of Trp residues in proteins exposed to peroxynitrite. The Trp residues in bovine serum albumin and collagen IV were decreased by peroxynitrite treatment. To identify the degradation products of the Trp residue by peroxynitrite, tert-butoxycarbonyl-L-tryptophan (Boc-Trp) was used as a model of the Trp residue in proteins, and the products formed from peroxynitrite-treated Boc-Trp were then isolated. Boc-Trp decreased with an increase in peroxynitrite concentration. N-Formylkynurenine, oxindole, and hydropyrroloindole were identified as major products. The formation of these products may account for the losses of Trp residues in proteins by peroxynitrite.

Reactions of peroxynitrite with gamma-tocopherol

The reaction of peroxynitrite with gamma-tocopherol (gamma-TH) in a methanol/potassium phosphate buffer solution results in the formation of four major products, which were identified as 2,7,8-trimethyl-2-(4,8,12-trimethyldecyl)-5-nitro-6-chromanol++ + (NGT), 2,7,8-trimethyl-2-(4,8,12-trimethyldecyl)-5,6-chromaquinone (tocored), and two diastereomers of 8a-(hydroxy)-gamma-tocopherone. NGT was the major product formed in these reactions, and its formation was modestly increased by increasing amounts of Fe(3+)-EDTA. Tocored and NGT also were formed when gamma-TH was exposed to 3-morpholinosydnonimine (SIN-1), a compound that decomposes to form peroxynitrite. When gamma-TH reacted with the nitrating agent NO2+BF4- in acetonitrile or methanol/potassium phosphate buffer, NGT and tocored also were formed, but the major product detected was gamma-tocopherol quinone (gamma-TQ). This product was not detected in reactions involving peroxynitrite. Oxidation of gamma-TH by peroxynitrite involves nitration and electron transfer reactions. Since the product distribution in oxidations with NO2+BF4- differed substantially from that in oxidations with peroxynitrite and SIN-1, NO2+ appeared not to be the principal species involved in NGT formation. Nitration of gamma-TH may involve either peroxynitrite or some peroxynitrite-derived oxidant other than NO2+. Because of its stability and formation as a novel product of the reaction between gamma-TH with peroxynitrite, NGT may be a useful in vivo marker for peroxynitrite interactions with lipid structures that contain gamma-TH.

Formation of reactive nitrogen species during peroxidase-catalyzed oxidation of nitrite. A potential additional mechanism of nitric oxide-dependent toxicity

Involvement of peroxynitrite (ONOO-) in inflammatory diseases has been implicated by detection of 3-nitrotyrosine, an allegedly characteristic protein oxidation product, in various inflamed tissues. We show here that nitrite (NO2-), the primary metabolic end product of nitric oxide (NO.), can be oxidized by the heme peroxidases horseradish peroxidase, myeloperoxidase (MPO), and lactoperoxidase (LPO), in the presence of hydrogen peroxide (H2O2), to most likely form NO.2, which can also contribute to tyrosine nitration during inflammatory processes. Phenolic nitration by MPO-catalyzed NO2- oxidation is only partially inhibited by chloride (Cl-), the presumed major physiological substrate for MPO. In fact, low concentrations of NO2- (2-10 microM) catalyze MPO-mediated oxidation of Cl-, indicated by increased chlorination of monochlorodimedon or 4-hydroxyphenylacetic acid, most likely via reduction of MPO compound II. Peroxidase-catalyzed oxidation of NO2-, as indicated by phenolic nitration, was also observed in the presence of thiocyanate (SCN-), an alternative physiological substrate for mammalian peroxidases. Collectively, our results suggest that NO2-, at physiological or pathological levels, is a substrate for the mammalian peroxidases MPO and lactoperoxidase and that formation of NO2. via peroxidase-catalyzed oxidation of NO2- may provide an additional pathway contributing to cytotoxicity or host defense associated with increased NO. production.

Nitrotyrosine bound to beta-VLDL-apoproteins: a biomarker of peroxynitrite formation in experimental atherosclerosis

Lipoprotein oxidation plays a key role in the initiation and progression of atherosclerosis. Peroxynitrite is a powerful oxidant and nitrating species formed by the reaction of nitric oxide with superoxide radical. Peroxynitrite can oxidize lipoproteins and generate nitrotyrosine either from free or protein-bound tyrosine. Nitrotyrosine has been used as a fingerprint for peroxynitrite reaction in vivo. In this study, the content of nitrotyrosine bound to beta-VLDL apoproteins was determined in New Zealand rabbits before and at 15, 30, 45 and 60 days of cholesterol feeding. A significant increase of nitrotyrosine bound to beta-VLDL apoproteins was observed in parallel with the hypercholesterolemia induced by 1% cholesterol enriched diet. These data indicate that apolipoprotein-bound nitrotyrosine may be used as a biomarker of peroxynitrite production during the development of atherosclerosis in this experimental model.

Inhibition of peroxynitrite-mediated tyrosine nitration by catechin polyphenols

Peroxynitrite is a cytotoxic species generated by the reaction between superoxide and nitric oxide. The ability of catechins and their gallate esters to decrease peroxynitrite-induced nitration of tyrosine and to limit surface charge alteration of low density lipoprotein (LDL) was investigated. All compounds tested were found to be potent peroxynitrite scavengers preventing the nitration of tyrosine. The ability of the catechin polyphenols at 10 microM to minimise tyrosine nitration induced by peroxynitrite (500 microM) was ECG (38.1 +/- 3.6%) approximately EGCG (32.1 +/- 7.5%) approximately gallic acid (32.1 +/- 1.9%) > catechin (23.9 +/- 5.4%) approximately epicatechin (22.9 +/- 3.3%) approximately EGC (19.9 +/- 2.0%). Trolox (10 microM) was used as the standard for comparative purposes and was found to be less effective than the polyphenols in inhibiting tyrosine nitration (13.6 +/- 2.9%). The catechin polyphenols were also found to offer protection from peroxynitrite-induced modification of critical amino acids of apolipoprotein B-100 of LDL which contribute towards its surface charge.

Expired nitric oxide as a marker for childhood asthma

Expression of the inflammatory isoform of the enzyme nitric oxide synthase (NOS) is increased in airway-lining cells of patients with asthma. The NOS product nitric oxide (NO.) was measured in the expired gas of children with asthma. Vital capacity expirates from 21 control subjects and 13 subjects with asthma were assayed by chemiluminescence. Measurements were highly reproducible (coefficient of variation, 2.6% +/- 1.1%) and did not vary with age, sex, height, or weight. Patients with asthma had mean NO. levels (16.3 parts per billion) that were more than threefold higher than those of control subjects (5.05 ppb; p < 0.001). Expired NO. decreased as airflow obstruction improved during corticosteroid treatment (r2 = 0.77; n = 7; p < 0.001) but remained higher than normal (13.5 ppb; n = 5; p < 0.01) even after airflow obstruction resolved. We demonstrate the use of a reproducible test for asthma in children that is independent of measures of airflow obstruction. We speculate that expired NO assays may prove to be a more sensitive measure of childhood asthma than spirometry.

Superoxide: a two-edged sword

Superoxide (O2-) is the compound obtained when oxygen is reduced by one electron. For a molecule with an unpaired electron, O2- is surprisingly inert, its chief reaction being a dismutation in which it reacts with itself to form H2O2 and oxygen. The involvement of O2- in biological systems was first revealed by the discovery in 1969 of superoxide dismutase, an enzyme that catalyzes the dismutation of O2-. Since then it has been found that biological systems produce a bewildering variety of reactive oxidants, all but a few arising ultimately from O2-. These oxidants include O2- itself, H2O2 and alkyl peroxides, hydroxyl radical and other reactive oxidizing radicals, oxidized halogens and halamines, singlet oxygen, and peroxynitrite. These various oxidants are able to damage molecules in their environment, and are therefore very dangerous. They are thought to participate in the pathogenesis of a number of common diseases, including among others malignancy, by their ability to mutate the genome, and atherosclerosis, by their capacity for oxidizing lipoproteins. Their properties are put to good use, however, in host defense, where they serve as microbicidal and parasiticidal agents, and in biological signalling, where their liberation in small quantities results in redox-mediated changes in the functions of enzymes and other proteins.

Peroxynitrite induces the conversion of xanthine dehydrogenase to oxidase in rabbit liver

Effect of peroxynitrite (ONOO-) on the conversion of xanthine dehydrogenase to oxidase in rabbit liver was examined. ONOO- (25-200 microM) induced the conversion of xanthine dehydrogenase to oxidase in a dose-dependent manner. The addition of hydroxyl radical scavengers (mannitol and dimethyl sulfoxide) gave no alteration in the ONOO(-)-induced conversion of xanthine dehydrogenase to oxidase, implying that the action of ONOO- is not due to hydroxyl radicals which may be formed from ONOO-. The experiment utilizing dithiothreitol also revealed that the action of ONOO- might be due to oxidation of sulfhydryl group of xanthine dehydrogenase. These results suggest that ONOO- has the potential to convert xanthine dehydrogenase to oxidase, and that this effect may be correlated with cytotoxic actions of ONOO-.

Prevention of peroxynitrite-dependent tyrosine nitration and inactivation of alpha1-antiproteinase by antibiotics

Peroxynitrite, formed by reaction of superoxide and nitric oxide, appears to be an important tissue damaging species generated at sites of inflammation. In this paper, we compare the abilities of several antibiotics to protect against peroxynitrite-dependent inactivation of alpha1-antiproteinase, and to inhibit tyrosine nitration by peroxynitrite, in vitro. Tetracycline, minocycline, doxycycline, rifamycin and rifampicin were highly protective in both assay systems, whereas several other antibiotics tested were not. The possibility that antibiotics could affect tissue injury at sites of inflammation by scavenging peroxynitrite is discussed.

Thiourea and dimethylthiourea inhibit peroxynitrite-dependent damage: nonspecificity as hydroxyl radical scavengers

Thiourea and, more recently, dimethylthiourea, have been used as hydroxyl radical (OH.) scavengers in experiments both in vitro and in vivo. We show that both compounds can inhibit nitration of the amino acid tyrosine on addition of peroxynitrite, and also the inactivation of alpha1-antiproteinase by peroxynitrite. Hence, protective effects of (dimethyl) thiourea could be due to inhibition of peroxynitrite-dependent damage as well as to OH. scavenging, and these compounds must not be regarded as specific OH. scavengers.

Peroxynitrite: a two-faced metabolite of nitric oxide

The discovery that nitric oxide (NO) reacts with superoxide (O2.-) forming peroxynitrite (ONOO-) (1) and the proof that this reaction occurs in vivo (2,3) holds enormous implications for the understanding of free radicals in biological systems. Not only in mammalian defense mechanisms against microorganisms, but also in pathophysiology during overexposure of tissues to radicals or other highly reactive species. Peroxynitrite is a highly reactive compound with harmful effects on cells and could therefore be an important microbicidal compound. Furthermore, the reaction of superoxide with NO interferes with NO signalling mechanisms. NO is not only released in response to inflammatory agents by inflammatory cells, but is also an important messenger molecule in paracrine mechanisms and neurotransmission. Whether peroxynitrite formation is a negative side effect of NO and superoxide release, or a functional characteristic is yet to be determined, and will be discussed in this review.

Peroxynitrite-dependent aromatic hydroxylation and nitration of salicylate and phenylalanine. Is hydroxyl radical involved?

There is considerable dispute about whether the hydroxylating ability of peroxynitrite (ONOO-)- derived species involves hydroxyl radicals (OH.). This was investigated by using salicylate and phenylalanine, attack of OH. upon which leads to the formation of 2,3- and 2,5-dihydroxybenzoates, and o- m- and p- tyrosines respectively. On addition of ONOO- to salicylate, characteristic products of hydroxylation (and nitration) were observed in decreasing amounts with rise in pH, although added products of hydroxylation of salicylate were not recovered quantitatively at pH 8.5, suggesting further oxidation of these products and underestimating of hydroxylation at alkaline pH. Hydroxylation products decreased in the presence of several OH. scavengers, especially formate, to extents similar to those obtained when hydroxylation was achieved by a mixture of iron salts, H2O2 and ascorbate. However, OH. scavengers also inhibited formation of salicylate nitration products. Ortho, p- and m-tyrosines as well as nitration products were also observed when ONOO- was added to phenylalanine. The amount of these products again decreased at high pH and were decreased by addition of OH. scavengers. We conclude that although comparison with Fenton systems suggests OH. formation, simple homolytic fission of peroxynitrous acid (ONOOH) to OH. and NO2. would not explain why OH. scavengers inhibit formation of nitration products.

Effects of carbon dioxide/bicarbonate on induction of DNA single-strand breaks and formation of 8-nitroguanine, 8-oxoguanine and base-propenal mediated by peroxynitrite

Carbon dioxide has been reported to react with peroxynitrite (ONOO-), a strong oxidant and nitrating agent, to form an ONO2CO2- adduct, altering the reactivity characteristic of peroxynitrite. We found that bicarbonate (0-10 mM) caused a dose-dependent increase of up to 6-fold in the formation of 8-nitroguanine in calf-thymus DNA incubated with 0.1 mM peroxynitrite, whereas it produced no apparent effect on 8-oxoguanine formation. In contrast, bicarbonate inhibited peroxynitrite-induced strand breakage in plasmid pBR322 DNA and thymine-propenal formation from thymidine. We conclude that C02/HCO3- reacts with peroxynitrite to form a potent nitrating agent, but also to inactivate hydroxyl-radical-like activity of peroxynitrous acid.

Reactive oxygen species produced by macrophage-derived foam cells regulate the activity of vascular matrix metalloproteinases in vitro. Implications for atherosclerotic plaque stability

Vulnerable areas of atherosclerotic plaques often contain lipid-laden macrophages and display matrix metalloproteinase activity. We hypothesized that reactive oxygen species released by macrophage-derived foam cells could trigger activation of latent proforms of metalloproteinases in the vascular interstitium. We showed that in vivo generated macrophage foam cells produce superoxide, nitric oxide, and hydrogen peroxide after isolation from hypercholesterolemic rabbits. Effects of these reactive oxygens and that of peroxynitrite, likely to result from simultaneous production of nitric oxide and superoxide, were tested in vitro using metalloproteinases secreted by cultured human vascular smooth muscle cells. Enzymes in culture media or affinity-purified (pro-MMP-2 and MMP-9) were examined by SDS-PAGE zymography, Western blotting, and enzymatic assays. Under the conditions used, incubation with xanthine/xanthine oxidase increased the amount of active gelatinases, while nitric oxide donors had no noticeable effect. Incubation with peroxynitrite resulted in nitration of MMP-2 and endowed it with collagenolytic activity. Hydrogen peroxide treatment showed a catalase-reversible biphasic effect (gelatinase activation at concentrations of 4 microM, inhibition at > or = 10-50 microM). Thus, reactive oxygen species can modulate matrix degradation in areas of high oxidant stress and could therefore contribute to instability of atherosclerotic plaques.

2-Amino-4-methylpyridine as a potent inhibitor of inducible NO synthase activity in vitro and in vivo

1. The ability of 2-amino-4-methylpyridine to inhibit the catalytic activity of the inducible NO synthase (NOS II) enzyme was characterized in vitro and in vivo. 2. In vitro, 2-amino-4-methylpyridine inhibited NOS II activity derived from mouse RAW 264.7 cells with an IC50 of 6 nM. Enzyme kinetic studies indicated that inhibition is competitive with respect to arginine. 2-Amino-4-methylpyridine was less potent on human recombinant NOS II (IC50 = 40 nM) and was still less potent on human recombinant NOS I and NOS III (IC50 = 100 nM). NG-monomethyl-L-arginine (L-NMMA), N6-iminoethyl-L-lysine (L-NIL) and aminoguanidine were much weaker inhibitors of murine NOS II than 2-amino-4-methylpyridine but, unlike 2-amino-4-methylpyridine, retained similar activity on human recombinant NOS II. L-NMMA inhibited all three NOS isoforms with similar potency (IC50S 3-7 microM). In contrast, compared to activity on human recombinant NOS III, L-NIL displayed 10 x selectivity for murine NOS II and 11 x selectivity for human recombinant NOS II while aminoguanidine displayed 7.3 x selectivity for murine NOS II and 3.7 x selectivity for human recombinant NOS II. 3. Mouse RAW 264.7 macrophages produced high levels of nitrite when cultured overnight in the presence of lipopolysaccharide (LPS) and interferon-gamma. Addition of 2-amino-4-methylpyridine at the same time as the LPS and IFN-gamma, dose-dependently reduced the levels of nitrite (IC50 = 1.5 microM) without affecting the induction of NOS II protein. Increasing the extracellular concentration of arginine decreased the potency of 2-amino-4-methylpyridine but at concentrations up to 10 microM, 2-amino-4-methylpyridine did not inhibit the uptake of [3H]-arginine into the cell. Addition of 2-amino-4-methylpyridine after the enzyme was induced also dose-dependently inhibited nitrite production. Together, these data suggest that 2-amino-4-methylpyridine reduces cellular production of NO by competitive inhibition of the catalytic activity of NOS II, in agreement with results obtained from in vitro enzyme kinetic studies. 4. When infused i.v. in conscious unrestrained rats, 2-amino-4-methylpyridine inhibited the rise in plasma nitrate produced in response to intraperitoneal injection of LPS (ID50 = 0.009 mg kg-1 min-1). Larger doses of 2-amino-4-methylpyridine were required to raise mean arterial pressure in untreated conscious rats (ED50 = 0.060 mg kg-1 min-1) indicating 6.9 x selectivity for NOS II over NOS III in vivo. Under the same conditions, L-NMMA was nonselective while L-NIL and aminoguanidine displayed 5.2 x and 8.6 x selectivity respectively. All of these compounds caused significant increases in mean arterial pressure at doses above the ID50 for inhibition of NOS II activity in vivo. 5. 2-Amino-4-methylpyridine also inhibited LPS-induced elevation in plasma nitrate after either subcutaneous (ID50 = 0.3 mg kg-1) or oral (ID50 = 20.8 mg kg-1) administration. 6. These data indicate that 2-amino-4-methylpyridine is a potent inhibitor of NOS II activity in vitro and in vivo with a similar degree of isozyme selectivity to that of L-NIL and aminoguanidine in rodents.

Effect of 2,3-dimercaptosuccinic acid on nephrosclerosis in the Dahl rat. I. Role of reactive oxygen species

2,3-Dimercaptosuccinic acid (DMSA), a sulfhydryl-containing chelator, has previously been shown to reduce mean blood pressure in lead-treated rats. In the present study we have demonstrated that DMSA (0.5% for 5 days every 2 weeks) also reduces mean blood pressure in the Dahl salt-sensitive (SS) rat. Six-week-old Dahl SS and salt resistant (SR) rats were placed on a 0.3% NaCl diet for two weeks, followed by an 8% NaCl diet for four weeks. Eight SS and 8 SR rats remained untreated while 8 SS and 8 SR rats were treated with DMSA. DMSA treatment ameliorated the mean blood pressure rise in the Dahl SS rats (141 +/- 5 vs. 120 +/- 4 mm Hg at 6 weeks, P < 0.001). Nephrosclerosis was severe in untreated SS rats but absent in treated SS rats as well as in both treated and untreated SR rats. Reactive oxygen species formation, as assessed by kidney cortex content of malondialdehyde (MDA) and immunohistochemical demonstration of nitrotyrosine (a byproduct of peroxynitrite) in interlobular arteries, was increased in Dahl SS rats, but abolished by DMSA (MDA 9.65 +/- 0.33 nmol/g wet wt, untreated SS, vs. 6.46 +/- 0.51, treated SS, P < 0.001). The anti-nephrosclerotic action of DMSA was clearly disproportionate to the reduction in blood pressure. We conclude that the effect of DMSA was related instead to the reactive oxygen species scavenging properties of the thiol groups.

Sensitive determination of nitrotyrosine in human plasma by isocratic high-performance liquid chromatography

A highly sensitive and simple isocratic high-performance liquid chromatographic method was developed for determination of 3-nitrotyrosine in human plasma with precolumn derivatization with 4-fluoro-7-nitrobenzo-2-oxa-1,3-diazole. The precision of the method was satisfactory (coefficient of variation 4.8%), and the detection limit was established at 0.1 pmol of 3-nitrotyrosine allowing the determination at the level of 6 pmol/ml in human plasma. The recoveries of 3-nitrotyrosine and alpha-methyltyrosine, an internal standard, were 89.3 +/- 7.1 and 85.7 +/- 7.6%, respectively. The 3-nitrotyrosine level was 31 +/- 6 pmol/ml (mean +/- S.D., n = 9) in plasma from healthy volunteers. Since 3-nitrotyrosine is a stable product of peroxynitrite, an oxidant formed by a reaction of nitric oxide and superoxide radicals, the measurement of its plasma concentration may be useful as a marker of nitric oxide-dependent oxidative damage.

Sensitivity of human pancreatic islets to peroxynitrite-induced cell dysfunction and death

Nitric oxide and peroxynitrite (generated by the reaction of nitric oxide with the superoxide anion) may both be mediators of beta-cell damage in early insulin-dependent diabetes mellitus. We observed that acute exposure of primary cultured human pancreatic islets to peroxynitrite results in a significant decrease in glucose oxidation and islet retrieval. DNA strand breaks in single human and rat islet cells are detectable after acute peroxynitrite exposure, followed by a decrease in islet cell survival after 1 h and 24 h. Cell death appeared to occur via a toxic cell death mechanism (necrosis) rather than apoptosis, as suggested by vital staining and ultrastructural evidence of early membrane and organelle degradation, mitochondrial swelling and loss of matrix. This study demonstrates for the first time that cultured human pancreatic islets are susceptible to the noxious effects of peroxynitrite.

Base modification and strand breakage in isolated calf thymus DNA and in DNA from human skin epidermal keratinocytes exposed to peroxynitrite or 3-morpholinosydnonimine

Exposure of isolated calf thymus DNA and human skin epidermal keratinocytes to peroxynitrite or the peroxynitrite generator, 3-morpholinosydnonimine (SIN-1), led to extensive DNA base modification. Large increases in xanthine and hypoxanthine, possible deamination products of guanine and adenine, respectively, and in 8-nitroguanine were observed, but only small changes in some oxidized base products were seen. This pattern of damage suggests that hydroxyl radicals were not major contributors to base modification caused by peroxynitrite, as OH is known to cause multiple oxidative modifications to all four DNA bases. Instead, it seems that reactive nitrogen species play a much greater role in the mechanism of base damage, producing both nitration and deamination of purine bases when DNA or whole cells are exposed to peroxynitrite. If this pattern of damage is unique to peroxynitrite, it might act as a marker of cellular damage by this species in vivo.

Cytokine-treated human neutrophils contain inducible nitric oxide synthase that produces nitration of ingested bacteria

Although the production of NO within rodent phagocytes is well-characterized, its production and function within human phagocytes are less clear. We show here that neutrophils within human buffy coat preparations stimulated with a mixture of interleukin 1, tumor necrosis factor alpha, and interferon gamma contain inducible NO synthase mRNA and protein, one of the enzymes responsible for NO production. The protein colocalizes with myeloperoxidase within neutrophil primary granules. Using an inhibitor of NO synthase, L-N-monomethyl arginine, we show that activity of this enzyme is required for the formation of nitrotyrosine around phagocytosed bacteria, most likely through the intermediate production of peroxynitrite, a reaction product of NO and superoxide anions.

Nitrotyrosine residues in placenta. Evidence of peroxynitrite formation and action

The interaction of nitric oxide and superoxide produces peroxynitrite anion, a strong, long-lived oxidant with pronounced deleterious effects that may cause vascular damage. The formation and action of peroxynitrite can be detected by immunohistochemical localization of nitrotyrosine residues. We compared the presence and localization of nitrotyrosine and of the endothelial isoform of nitric oxide synthase in placental villous tissue from normotensive pregnancies (n = 5) with pregnancies complicated by preeclampsia (n = 5), intrauterine growth restriction (n = 5), and preeclampsia plus intrauterine growth restriction (n = 4), conditions characterized by increases in fetoplacental vascular resistance, fetal platelet consumption, and fetal morbidity and mortality. In all tissues, absent or faint nitrotyrosine immunostaining but prominent nitric oxide synthase immunostaining were found in syncytiotrophoblast. In tissues from normotensive pregnancies, faint nitrotyrosine immunostaining was found in vascular endothelium, and nitric oxide synthase was present in stem villous endothelium but not in the terminal villous capillary endothelium. In contrast, in preeclampsia and/or intrauterine growth restriction, moderate to intense nitrotyrosine immunostaining was seen in villous vascular endothelium, and immunostaining was also seen in surrounding vascular smooth muscle and villous stroma. The intensity of nitrotyrosine immunostaining in preeclampsia (with or without intrauterine growth restriction) was significantly greater than that of controls. Intense nitric oxide synthase staining was seen in endothelium of stem villous vessels and the small muscular arteries of the terminal villous region in these tissues and may be an adaptive response to the increased resistance. The presence of nitrotyrosine residues, particularly in the endothelium, may indicate the formation and action of peroxynitrite, resulting in vascular damage that contributes to the increased placental vascular resistance.

Protection against peroxynitrite-dependent tyrosine nitration and alpha 1-antiproteinase inactivation by ascorbic acid. A comparison with other biological antioxidants

Peroxynitrite, formed by reaction of superoxide and nitric oxide, appears to be an important tissue-damaging species generated at sites of inflammation. In this paper, we compare the abilities of several biological antioxidants to protect against peroxynitrite-dependent inactivation of alpha 1-antiproteinase, and to inhibit tyrosine nitration upon addition of peroxynitrite. GSH and ascorbate protected efficiently in both systems. Uric acid inhibited tyrosine nitration but not alpha 1-antiproteinase inactivation. The possibility that ascorbic acid is an important scavenger of reactive nitrogen species in vivo is discussed.

Formation of nitrating and chlorinating species by reaction of nitrite with hypochlorous acid. A novel mechanism for nitric oxide-mediated protein modification

Detection of 3-nitrotyrosine has served as an in vivo marker for the production of the cytotoxic species peroxynitrite (ONOO-). We show here that reaction of nitrite (NO2-), the autoxidation product of nitric oxide (.NO), with hypochlorous acid (HOCl) forms reactive intermediate species that are also capable of nitrating phenolic substrates such as tyrosine and 4-hydroxyphenylacetic acid, with maximum yields obtained at physiological pH. Monitoring the reaction of NO2- with HOCl by continuous flow photodiode array spectrophotometry indicates the formation of a transient species with spectral characteristics similar to those of nitryl chloride (Cl-NO2). Reaction of synthetic Cl-NO2 with N-acetyl-L-tyrosine results in the formation of 3-chlorotyrosine and 3-nitrotyrosine in ratios that are similar to those obtained by the NO2-/HOCl reaction (4:1). Tyrosine residues in bovine serum albumin are also nitrated and chlorinated by NO2-/HOCl and synthetic Cl-NO2. The reaction of N-acetyl-L-tyrosine with NO2-/HOCl or authentic Cl-NO2 also produces dityrosine, suggesting that free radical intermediates are involved in the reaction mechanism. Our data indicate that while chlorination reactions of Cl-NO2 are mediated by direct electrophilic addition to the aromatic ring, a free radical mechanism appears to be operative in nitrations mediated by NO2-/HOCl or Cl-NO2, probably involving the combination of nitrogen dioxide (.NO2) and tyrosyl radical. We propose that NO2- reacts with HOCl by Cl+ transfer to form both cis- and trans-chlorine nitrite (Cl-ONO) and Cl-NO2 as intermediates that modify tyrosine by either direct reaction or after decomposition to reactive free and solvent-caged Cl. and .NO2 as reactive species. Formation of Cl-NO2 and/or Cl-ONO in vivo may represent previously unrecognized mediators of inflammation-mediated protein modification and tissue injury, and offers an additional mechanism of tyrosine nitration independent of ONOO-.

Counterprotective effect of erythrocytes in experimental bacterial peritonitis is due to scavenging of nitric oxide and reactive oxygen intermediates

Erythrocytes (RBC) in the peritoneal cavity significantly increase the lethality of bacterial peritonitis. The lethality is known to be associated with, and perhaps due to, increased bacterial counts in the peritoneal cavity. The mechanism is unknown. In this study, we investigated the hypothesis that RBC scavenge reactive oxygen intermediates (ROI) and nitric oxide (NO), so that the counterprotective effect is due to a loss of the microbiostatic activity of both ROI and NO. To study this effect, rats were subjected to a peritoneal inoculation of live Escherichia coli without RBC (nonlethal dose) or with RBC (lethal dose). The adjuvant effect of RBC was not modified by NG-monomethyl-L-arginine (NMA, an NO synthase inhibitor), superoxide dismutase, catalase, mannitol, or a combination of these agents. Furthermore, the increased number of bacteria in the peritoneal cavity in the presence of RBC was unaffected by these treatments. The administration of NMA with bacteria alone (no RBC) converted a nonlethal model into a lethal one associated with higher intraperitoneal bacterial counts. A similar effect was seen with superoxide dismutase and catalase but not with mannitol. During bacterial peritonitis in the absence of RBC, superoxide and NO formation (determined by the total nitrite plus nitrate formed) was detected in the ascites and inducible NO synthase mRNA expression was present in the peritoneal cells. In the absence of RBC, superoxide was detected and oxidation of dihydrorhodamine to rhodamine was observed, indicating that peroxynitrite was produced. Both were blocked by the inclusion of RBC. Preinjection with a low inoculum of killed bacteria protected the rats from a subsequent lethal peritoneal bacterial challenge; this effect was reversed by scavenging ROI and NO. The protective effect of killed bacterial pretreatment was lost when RBC were placed in the peritoneal cavity. In vitro bactericidal activity of NO- and ROI-generating macrophages was also inhibited by RBC or by inhibiting ROI and NO formation. Taken together, these data are consistent with the hypothesis that RBC can impair bacterial clearance by removing both NO and ROI, suggesting that NO in combination with superoxide may be important to the antimicrobial defenses of the peritoneal cavity.

Nitric oxide synthase generates superoxide and nitric oxide in arginine-depleted cells leading to peroxynitrite-mediated cellular injury

Besides synthesizing nitric oxide (NO), purified neuronal NO synthase (nNOS) can produce superoxide (.O2-) at lower L-Arg concentrations. By using electron paramagnetic resonance spin-trapping techniques, we monitored NO and .O2- formation in nNOS-transfected human kidney 293 cells. In control transfected cells, the Ca2+ ionophore A23187 triggered NO generation but no .O2- was seen. With cells in L-Arg-free medium, we observed .O2- formation that increased as the cytosolic L-Arg levels decreased, while NO generation declined. .O2- formation was virtually abolished by the specific NOS blocker, N-nitro-L-arginine methyl ester (L-NAME). Nitrotyrosine, a specific nitration product of peroxynitrite, accumulated in L-Arg-depleted cells but not in control cells. Activation by A23187 was cytotoxic to L-Arg-depleted, but not to control cells, with marked lactate dehydrogenase release. The cytotoxicity was largely prevented by either superoxide dismutase or L-NAME. Thus, with reduced L-Arg availability NOS elicits cytotoxicity by generating .O2- and NO that interact to form the potent oxidant peroxynitrite. Regulating arginine levels may provide a therapeutic approach to disorders involving .O2-/NO-mediated cellular injury.

Increased nitric oxide formation in recurrent thrombotic microangiopathies: a possible mediator of microvascular injury

The term thrombotic microangiopathy (TMA) has been used extensively to encompass hemolytic uremic syndrome and thrombotic thrombocytopenic purpura, two syndromes of hemolytic anemia, and thrombocytopenia associated with renal or brain involvement or both. There is evidence that endothelial damage is a crucial feature in the sequence of events that precedes the development of microvascular lesions. More recent studies would suggest that endothelial dysfunction could be a consequence of neutrophil activation. Activated neutrophils generate superoxide anions (O2-) that, combining with endothelial-derived nitric oxide (NO), form the highly cytotoxic hydroxyl radical. Seven patients with recurrent forms of TMA and seven healthy volunteers were studied. Plasma concentrations of the NO metabolites, nitrites/nitrates, were elevated in the acute phase of TMA, indicating an increased NO synthesis in vivo. In addition, elevated serum concentrations of tumor necrosis factor, a potent inducer of endothelial NO synthase, were found in acute TMA. Serum from patients with acute TMA induced NO synthesis in cultured endothelial cells more than normal serum. Enhanced stimulatory activity was no longer found in the recovery phase. Release of O2- by neutrophils ex vivo was higher than normal in patients with acute TMA, but decreased in the recovery phase. Exactly the same trend was observed for plasma malondialdehyde and conjugated dienes, indicating that excessive oxygen radical formation in acute TMA is associated with increased lipid peroxidation. Thus, in recurrent forms of TMA, NO formation was increased as compared with controls. This was associated with signs of lipid peroxidation, likely the consequence of the interaction of NO with neutrophil-derived oxygen products.

Protection against peroxynitrite dependent tyrosine nitration and alpha 1-antiproteinase inactivation by some anti-inflammatory drugs and by the antibiotic tetracycline

OBJECTIVE

To examine in vitro the ability of several drugs to protect against deleterious effects of peroxynitrite, a cytotoxic agent formed by reaction of nitric oxide with superoxide radical, that may be generated in the rheumatoid joint and could cause joint damage.

METHODS

The ability of several drugs to protect against such possible toxic actions of peroxynitrite as inactivation of alpha 1-antiproteinase and nitration of tyrosine was evaluated.

RESULTS

Most non-steroidal anti-inflammatory drugs were moderately (indomethacin, diclofenac, naproxen, tolmetin) or only weakly (sulindac, ibuprofen, aurothioglucose, flurbiprofen, sulphasalazine, salicylate, penicillamine disulphide) effective in preventing tyrosine nitration and alpha 1-antiproteinase inactivation by peroxynitrite, but 5-aminosalicylate and penicillamine were much more effective, as was the antibiotic tetracycline (but not ampicillin). Phenylbutazone and flufenamic acid protected effectively against tyrosine nitration, but could not be tested in the alpha 1-antiproteinase system. The analgesic paracetamol was highly protective in both assay systems.

CONCLUSION

Many drugs used in the treatment of rheumatoid arthritis are unlikely to act by scavenging peroxynitrite. The feasibility of peroxynitrite scavenging as a mechanism of penicillamine, 5-aminosalicylate, and paracetamol action in vivo is discussed.

Nitric oxide production and perivascular nitration in brain after carbon monoxide poisoning in the rat

Nitric oxide is a short-lived free radical and physiological mediator which has the potential to cause cytotoxicity. Studies were conducted to investigate whether nitric oxide, and the potent oxidant peroxynitrite, were generated in brain during experimental carbon monoxide (CO) poisoning in the rat. Nitric oxide production was documented by electron paramagnetic resonance spectroscopy, and found to be increased by ninefold immediately after CO poisoning. Evidence that peroxynitrite was generated was sought by looking for nitrotyrosine in the brains of CO-poisoned rats. Nitrotyrosine was found deposited in vascular walls, and also diffusely throughout the parenchyma in inummocytochemical studies. The affinity and specificity of an anti-nitrotyrosine antibody was investigated and a solid phase immunoradiochemical assay was developed to quantity nitrotyrosine in brain homogenates. A 10-fold increase in nitrotyrosine was found in the brains of CO-poisoned rats. Platelets were involved with production of nitrotyrosine in the early phase of exposure to CO. However, nitrotyrosine formation and leukocyte sequestration were not decreased in thrombocytopenic rats poisoned with CO according to the standard model. When rats were pre-treated with the nitric oxide synthase inhibitor, L-nitroarginine methyl ester, formation of both nitric oxide and nitrotyrosine in response to CO poisoning were abolished, as well as leukocyte sequestration in the microvasculature, endothelial xanthine dehydrogenase conversion to xanthine oxidase, and brain lipid peroxidation. We conclude that perivascular reactions mediated by peroxynitrite are important in the cascade of events which lead to brain oxidative stress in CO poisoning.

Effects of peroxynitrite-induced protein modifications on tyrosine phosphorylation and degradation

The ability of protein tyrosine kinases to phosphorylate a synthetic peptide was inhibited 51% by peroxynitrite-mediated nitration of tyrosine. Exposure of endothelial cells to peroxynitrite decreased the intensity of tyrosine phosphorylated proteins and increased the intensity of nitrotyrosine-containing proteins. Peroxynitrite-modified BSA was degraded by human red blood cell lysates. However, human plasma in a concentration-, time-, and temperature-dependent manner, removed the protein nitrotyrosine epitope. These results suggest that tyrosine nitration interferes with phosphorylation and targets proteins for degradation. Specific enzymatic process(es) for removing nitrotyrosine may be present in vivo.

Pulmonary edema in a woman following fetal surgery

Most cases of acute lung injury in pregnancy are attributed to hydrostatic pulmonary edema. In this report, however, we describe a 20-year-old pregnant woman who developed a unique case of increased permeability pulmonary edema following surgery for the repair of a fetal congenital diaphragmatic hernia. Two days after surgery, the patient developed acute respiratory failure and diffuse alveolar edema, requiring intubation and positive pressure ventilation for 5 days. The diagnosis of increased permeability pulmonary edema was confirmed by the ratio of pulmonary edema fluid to plasma protein (ratio=0.99). The patient received IV nitroglycerine for tocolysis. As a nitric oxide donor, the nitroglycerine may have combined with exogenous oxygen to form peroxynitrite, a known impediment to alveolar epithelial cell function. Many cases of pulmonary edema in pregnancy are diagnosed as hydrostatic based on clinical parameters, such as positive maternal fluid balance. In this case, these parameters would have been misleading. Measurement of the protein concentration in the pulmonary edema fluid allowed us to accurately determine that the patient had increased permeability pulmonary edema as the cause of her acute respiratory failure. Sampling of pulmonary fluid can differentiate the type of edema formation and in some cases help to identify mechanisms of acute lung injury.

Inactivation of tissue inhibitor of metalloproteinase-1 by peroxynitrite

Peroxynitrite (ONOO-) has recently been implicated in connective tissue destruction in vivo. We have studied the effect of ONOO- on the activity of tissue inhibitor of metalloproteinase-1 (TIMP-1) in vitro. The inactivation of TIMP-1 by ONOO- was dose dependent with 50 microM ONOO- reducing the inhibitory activity of TIMP-1 towards gelatinase-A by 50%. High concentrations of ONOO- (500 microM-5 mM) caused protein fragmentation whilst lower concentrations (<250 microM) inactivated TIMP-1 without altering the molecular weight. Inactivation could be blocked by ONOO- scavengers but not by hydroxyl radical scavengers. Our results show that ONOO- is capable of inactivating TIMP-1, a process which could potentiate metalloproteinase-mediated tissue breakdown.

Protection against peroxynitrite-dependent tyrosine nitration and alpha 1-antiproteinase inactivation by oxidized and reduced lipoic acid

Peroxynitrite, formed by combination of superoxide radical with nitric oxide, is a reactive tissue-damaging species apparently involved in the pathology of several human diseases. Peroxynitrite nitrates tyrosine residues and inactivates alpha 1-antiproteinase. We show that both lipoic acid and dihydrolipoic acid efficiently protect against damage by peroxynitrite. By contrast, other disulphides tested did not. The biological antioxidant effects of lipoate/dihydrolipoate may involve scavenging of reactive nitrogen species as well as reactive oxygen species.

Reactions of nitric oxide and peroxynitrite with organic molecules and ferrihorseradish peroxidase: interference with the determination of hydrogen peroxide

Endothelial, inflammatory, and other cell types, in addition to forming reactive oxygen species, under proper stimulation release free radicals such as nitric oxide (.NO) and strong oxidants such as peroxynitrite (ONOO-), which is the product of the reaction of nitric oxide with superoxide. Several methods for the detection of H2O2 are based on the ferrihorseradish peroxidase catalyzed oxidation of organic molecules. We investigated the reactions of nitric oxide and peroxynitrite with organic molecules as well as with ferrihorseradish peroxidase and examined the potential interference with the detection of H2O2. Peroxynitrite at low concentrations (0-10 microM) induced a concentration-dependent oxidation of 1,2 phenylenediamine, 3-3' dimethoxybenzidine (o-dianisidine) and para-hydroxyphenylacetic acid (pHPA). With the exception of pHPA, the oxidation of the above compounds by peroxynitrite was not affected by the presence of ferrihorseradish peroxidase. The yield of HPA-dimmer, the oxidation product of pHPA by peroxynitrite, was decreased because ferrihorseradish peroxidase catalyzed the formation of a different product, 3-nitro-HPA. Nitrogen oxides, formed by the reaction of nitric oxide with oxygen, oxidized the aromatic amines o-phenylenediamine and o-dianisidine. A 10-fold excess of nitric oxide over H2O2 decreased the yield of pHPA and dihydrorhodamine 123 (DHR123) by 58 and 72%, respectively, as compared to H2O2 plus ferrihorseradish peroxidase. The inhibition of pHPA oxidation by nitric oxide was in part to the reaction of nitric oxide with compound I and compound II and in part due to the reaction with the phenoxyl radical. These data suggest that the simultaneous generation of nitric oxide and peroxynitrite can interfere with the detection of H2O2. The overestimation or underestimation of the H2O2 detected is dependent upon the organic molecule utilized for detection and by the relative rate of nitric oxide, superoxide, and peroxynitrite generation.

Role of oxidants/antioxidants in smoking-induced lung diseases

An imbalance between oxidants and antioxidants has been considered in the pathogenesis of smoking-induced lung diseases, such as chronic obstructive pulmonary disease (COPD), particularly emphysema. Recent evidence indicates that increased neutrophil sequestration and activation occurs in the pulmonary microvasculature in smokers and in patients with COPD, with the potential to release reactive oxygen species (ROS). ROS generated by airspace phagocytes or inhaled directly from the environment also increase the oxidant burden and may contribute to the epithelial damage. Although much research has focused on the protease/antiprotease theory of the pathogenesis of emphysema, less attention has been paid to the role of ROS in this condition. The injurious effects of the increased oxidant burden in smokers and in patients with COPD are opposed by the lung antioxidant defences. Hence, determining the mechanisms regulating the antioxidant responses is critical to our understanding of the role of oxidants in the pathogenesis of smoking-induced lung disease and to devising future strategies for antioxidant therapy. In this article we have reviewed the evidence for the presence of an oxidant/antioxidant imbalance in smoking-induced lung disease and its relevance to therapy in these conditions.

Nitric oxide regulation of superoxide-dependent lung injury: oxidant-protective actions of endogenously produced and exogenously administered nitric oxide

The influence of endogenous cell .NO production and .NO derived from exogenous sources on oxidant injury to cultured fetal rat lung alveolar epithelium and an animal model of pulmonary oxidant injury was examined. Confluent fetal rat alveolar epithelial cell monolayers were stimulated to produce .NO after treatment with a combination of cytokines (IL-1 beta, TNF-alpha, IFN-gamma), LPS and zymosan-activated serum (CZ). Cell injury, assessed by 14C-adenine release, was significantly increased compared to basal and CZ-induced cells after inhibition of .NO synthesis by L-NMMA. Cell monolayer macromolecule barrier function was determined by the rate of diffusion of 125I-albumin from the apical to basolateral side of monolayers. Following exposure to CZ and/or O2.- generated by xanthine oxidase + lumazine (XO), endogenous cell .NO production and exogenously administered .NO (from .NO donors S-nitrosyl-glutathione and S-nitroso-N-acetylpenicillamine) significantly inhibited the increased monolayer permeability induced by exposure to reactive oxygen species. Furthermore, inhalation of 5-10 ppm of .NO significantly reduced the toxicity of > 95% oxygen to adult rats. We conclude that when cultured pulmonary epithelial cells and lung tissue in vivo are subjected to inflammatory mediators or acute oxidative stress, .NO can play a protective role by inhibiting O2.(-)-dependent toxicity.

Activation of the inducible form of nitric oxide synthase in the brains of patients with multiple sclerosis

Nitric oxide (NO) has been implicated as a pathogenic mediator in a variety of central nervous system (CNS) disease states, including the animal model of multiple sclerosis (MS) and experimental allergic encephalomyelitis. We have examined post-mortem brain tissues collected from patients previously diagnosed with MS, as well as tissues collected from the brains of patients dying without neuropathies. Both Northern blot analysis and reverse transcriptase (RT)-driven in situ PCR (RT-in situ PCR) studies demonstrated that inducible NO synthase (iNOS) mRNA was present in the brain tissues from MS patients but was absent in equivalent tissues from normal controls. We have also performed experiments identifying the cell type responsible for iNOS expression by RT-in situ PCR in combination with immunohistochemistry. Concomitantly, we analyzed the tissues for the presence of the NO reaction product nitrotyrosine to demonstrate the presence of a protein nitrosylation adduct. We report here that iNOS mRNA was detectable in the brains of 100% of the CNS tissues from seven MS patients examined but in none of the three normal brains. RT-in situ PCR experiments also demonstrated the presence of iNOS mRNA in the cytoplasm of cells that also expressed the ligand recognized by the Ricinus communis agglutinin 1 (RCA-1), a monocyte/macrophage lineage marker. Additionally, specific labeling of cells was observed when brain tissues from MS patients were exposed to antisera reactive with nitrotyrosine residues but was significantly less plentiful in brain tissue from patients without CNS disease. These results demonstrate that iNOS, one of the enzymes responsible for the production of NO, is expressed at significant levels in the brains of patients with MS and may contribute to the pathology associated with the disease.

Formation of 8-nitroguanine in DNA treated with peroxynitrite in vitro and its rapid removal from DNA by depurination

Peroxynitrite is a strong oxidant formed by reaction of nitric oxide with superoxide in inflamed tissues. We have demonstrated that 8-nitroguanine is formed dose-dependently in calf thymus DNA incubated with low concentrations of peroxynitrite in vitro. 8-Nitroguanine in acid-hydrolyzed DNA was chemically reduced into 8-aminoguanine, which was analyzed using high performance liquid chromatography with electrochemical detection. Only peroxynitrite, but not nitrite, tetranitromethane nor NO-releasing compounds, formed 8-nitroguanine. Antioxidants and desferrioxamine inhibited the reaction. 8-Nitroguanine was depurinated from DNA incubated at pH 7.4, 37 degrees C (t1/2 = approximately 4 h). Peroxynitrite did not increase 8-oxoguanine levels in DNA.