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

Oxidative and Nitrosative Stress in Oral Squamous Cell Carcinoma

INTRODUCTION

The incidence of oral squamous cell carcinoma (OSCC) shows a constant increase, while the long-term outcome remains poor over the last decades. Radical oxygen and nitrogen species (RONS) - initially released by carcinogens, such as alcohol and tobacco, and later maintained by the tumor microenvironment - appear to be strongly associated to chronic inflammation, tumor induction, progression, and metastatic spread. The aim of this study was to evaluate the role of oxidative and nitrosative stress in primary OSCC compared to healthy tissue specimens and to identify their impact on tumor carcinogenesis.

MATERIALS AND METHODS

In this basic research study, tissue samples of 30 patients with primary OSCC were evaluated for the expression of pAKT, pERK, 3-NT, NOS1, NOS3, MAPK1, and IP-8 by immunohistochemistry and RT-PCR and compared to those of a healthy control group (n = 30).

RESULTS

The results showed a significantly increased expression of pAKT (p < 0.001), pERK (p = 0.01), 3-NT (p = 0.039), NOS1 (p = 0.025), NOS3 (p = 0.046), and MAPK1 (p = 0.032) in OSCC tissue samples compared to healthy controls.

CONCLUSION

The results of this study prove the tested stable degradation products to be suitable for the detection of RONS in OSCC. Moreover, the significantly increased expression underlines the role of RONS in carcinogenesis of OSCC, suggests specific mechanisms of detection, and anticipates supplementary research.

Development of an Analytical Assay for Electrochemical Detection and Quantification of Protein-Bound 3-Nitrotyrosine in Biological Samples and Comparison with Classical, Antibody-Based Methods

Reactive oxygen and nitrogen species (RONS) cause oxidative damage, which is associated with endothelial dysfunction and cardiovascular disease, but may also contribute to redox signaling. Therefore, their precise detection is important for the evaluation of disease mechanisms. Here, we compared three different methods for the detection of 3-nitrotyrosine (3-NT), a marker of nitro-oxidative stress, in biological samples. Nitrated proteins were generated by incubation with peroxynitrite or 3-morpholino sydnonimine (Sin-1) and subjected to total hydrolysis using pronase, a mixture of different proteases. The 3-NT was then separated by high performance liquid chromatography (HPLC) and quantified by electrochemical detection (ECD, CoulArray) and compared to classical methods, namely enzyme-linked immunosorbent assay (ELISA) and dot blot analysis using specific 3-NT antibodies. Calibration curves for authentic 3-NT (detection limit 10 nM) and a concentration-response pattern for 3-NT obtained from digested nitrated bovine serum albumin (BSA) were highly linear over a wide 3-NT concentration range. Also, ex vivo nitration of protein from heart, isolated mitochondria, and serum/plasma could be quantified using the HPLC/ECD method and was confirmed by LC-MS/MS. Of note, nitro-oxidative damage of mitochondria results in increased superoxide (O2•-) formation rates (measured by dihydroethidium-based HPLC assay), pointing to a self-amplification mechanism of oxidative stress. Based on our ex vivo data, the CoulArray quantification method for 3-NT seems to have some advantages regarding sensitivity and selectivity. Establishing a reliable automated HPLC assay for the routine quantification of 3-NT in biological samples of cell culture, of animal and human origin seems to be more sophisticated than expected.

Role of oxidative and nitrosative stress in autogenous bone grafts to the mandible using guided bone regeneration and a deproteinized bovine bone material

The aim of this study was to evaluate the role of oxidative and nitrosative stress in autogenous bone grafts to the mandible based on immunohistochemical analysis.

MATERIAL AND METHODS

Using a well-established sheep model autogenous bone grafts were harvested form the iliac bone. A combination of a Collagen Membrane (CM) and Deproteinized Bovine Bone Material (DBBM) was used to cover the bone graft (Experiment 2). This modification was compared with simple onlay bone grafts (Experiment 1). Immunohistochemically, the expression of specific stable degradation products of oxidative and nitrosative stress was compared between the two experimental groups.

RESULTS

Specific markers for oxidative and nitrosative stress showed statistically significant differences in expression in the different experimental groups. The influence of oxidative and nitrosative stress on osteoblasts (OB), osteoclasts (OC), and osteocytes (OCy) was analysed. Experiment 2 showed increased expression of markers in OB and decreased expression in OC.

CONCLUSIONS

Taking the result of this study and reports from the literature into consideration grafts in Experiment 2 showed less resorption and atrophy, higher activity of OB and inhibition of OC, and less expression of Reactive Oxygen and Nitrogen Species (RONS) as markers of oxidative stress within the graft. These data illustrate the improved remodelling processes in grafts using CM and DBBM.

Rapid and sensitive determination of protein-nitrotyrosine by ELISA: Application to human plasma

3-Nitrotyrosine (3NT) is known as an important indicator of nitrosative stress and has been linked to various diseases. Our aim was to develop an indirect ELISA (enzyme-linked immunosorbent assay) method suitable for the detection of protein-bound 3NT in clinical plasma and serum samples. Nitrated protein standards and reduced protein standards were prepared. Limit of detection was determined for standards; recovery and reproducibility were determined for human plasma samples. The limit of detection for this method is 1.82±0.56 pmol/mg protein. Mean recovery of standards was 95%. 3NT concentration in plasma samples of obese and normal weight subjects was determined to be between 2 pmol/mg and 19 pmol/mg. No time-consuming sample preparation or expensive laboratory equipment is required, and applied antibodies are commercially available. Sensitivity, rapid analysis time, possibilities of high throughput applications and small sample volumes make this ELISA attractive for use in clinical laboratories.

Nitrotyrosine promotes human aortic smooth muscle cell migration through oxidative stress and ERK1/2 activation

Nitrotyrosine is a new biomarker of atherosclerosis and inflammation. The objective of this study was to determine the direct effects of free nitrotyrosine on human aortic smooth muscle cell (AoSMC) migration and molecular mechanisms. By a modified Boyden chamber assay, nitrotyrosine significantly increased AoSMC migration in a concentration-dependent manner. For example, nitrotyrosine at 300 nM increased AoSMC migration up to 152% compared with l-tyrosine-treated control cells (P<0.01). Cell wound healing assay confirmed this effect. Nitrotyrosine significantly increased the expression of some key cell migration-related molecules including PDGF receptor-B, matrix metalloproteinase 2 (MMP2) and integrins alphaV and beta3 at both mRNA and protein levels in AoSMC (P<0.01). In addition, nitrotyrosine increased reactive oxygen species (ROS) production in AoSMC by staining with fluorescent dye DCFHDA. Furthermore, nitrotyrosine induced transient phosphorylation of ERK2 by Bio-Plex luminex immunoassay and western blot analysis. AoSMC were able to uptake nitrotyrosine. Antioxidants including seleno-l-methionine and superoxide dismutase mimetic (MnTBAP) as well as ERK1/2 inhibitor PD98059 effectively blocked the promoting effect of nitrotyrosine on AoSMC migration and the mRNA expression of above cell migration-related molecules. Thus, nitrotyrosine directly increases AoSMC migration in vitro and the expression of migration-related molecules through overproduction of ROS and activation of ERK1/2 pathway. Nitrotyrosine may contribute to cardiovascular pathogenesis.

Protein 3-nitrotyrosine in complex biological samples: quantification by high-pressure liquid chromatography/electrochemical detection and emergence of proteomic approaches for unbiased identification of modification sites

Nitration of tyrosine residues by nitric oxide (NO)-derived species results in the accumulation of 3-nitrotyrosine in proteins, a hallmark of nitrosative stress in cells and tissues. Tyrosine nitration is recognized as one of the multiple signaling modalities used by NO-derived species for the regulation of protein structure and function in health and disease. Various methods have been described for the quantification of protein 3-nitrotyrosine residues, and several strategies have been presented toward the goal of proteome-wide identification of protein tyrosine modification sites. This chapter details a useful protocol for the quantification of 3-nitrotyrosine in cells and tissues using high-pressure liquid chromatography with electrochemical detection. Additionally, this chapter describes a novel biotin-tagging strategy for specific enrichment of 3-nitrotyrosine-containing peptides. Application of this strategy, in conjunction with high-throughput MS/MS-based peptide sequencing, is anticipated to fuel efforts in developing comprehensive inventories of nitrosative stress-induced protein-tyrosine modification sites in cells and tissues.

A comparative study of proteolysis methods for the measurement of 3-nitrotyrosine residues: enzymatic digestion versus hydrochloric acid-mediated hydrolysis

A common approach for the quantification of 3-nitrotyrosine (NY) in routine analyses relies on the cleavage of peptide bonds in order to release the free amino acids from proteins in tissues or fluids. NY is usually monitored by either GC-MS(/MS) or LC-MS/MS techniques. Various proteolysis methods have been employed to combine digestion efficiency with prevention of artifactual nitration of tyrosine. However, so far, no study was designed to compare the HCl-based hydrolysis method with enzymatic digestion in terms of reliability for the measurement of NY. The present work addresses the digestion efficiency of BSA using either 6M HCl, pronase E or a cocktail of enzymes (pepsin, pronase E, aminopeptidase, prolidase) developed in our laboratory. The HCl-based hydrolysis leads to a digestion yield of 95%, while 25 and 75% are achieved with pronase E and the cocktail of enzymes, respectively. These methods were compared in terms of NY measurement and the results indicate that a prior reduction of the disulfide bonds ensures a reliable quantification of NY. We additionally show that the enzyme efficacy is not altered when the digestion is carried out in the presence of BSA with a high content of NY.

Carbon monoxide, oxidative stress, and mitochondrial permeability pore transition

The cellular effects of carbon monoxide (CO) are produced primarily by CO binding to iron or other transition metals, which may also promote prooxidant activities of the more reactive gases, oxygen and nitric oxide. We tested the hypothesis that prooxidant effects of CO deregulate the calcium-dependent mitochondrial pore transition (MPT), which disrupts membrane potential and releases apoptogenic proteins. Rats were exposed to either CO (50 ppm) or hypobaric hypoxia (HH) for 1, 3, or 7 days, and liver mitochondria harvested to study protein expression and sensitivity to MPT by calcium and oxidants. Both exposures induced hypoxia-sensitive protein expression: hypoxia-inducible factor 1alpha (HIF-1alpha), heme oxygenase-1 (HO-1), and manganese SOD (SOD2), but SOD2 induction was greater by CO than by HH, especially at 7 days. Relative to HH, CO also caused significant early mitochondrial oxidative and nitrosative stress shown by decreases in GSH/GSSG and increases in protein 3-nitrotyrosine (3-NT) and protein mixed disulfide formation. This altered MPT sensitivity to calcium through an effect on the "S-site," causing loss of pore protection by adenine nucleotides. By 7 days, despite continued CO, nitrosative stress decreased and adenine nucleotide protection was restored to preexposure levels. This is the first evidence of functional mitochondrial pore stress caused by CO independently of its hypoxic effect, as well as a compensatory response exemplifying a mitochondrial phenotype shift. The implications are that cellular CO can activate or deactivate mitochondria for initiation of apoptosis in vivo.

Electrochemical detection of free 3-nitrotyrosine: Application to microdialysis studies

3-Nitrotyrosine (3-NT) is formed by the reaction of peroxynitrite with either free or protein-bound tyrosine residues and has been proposed as a biomarker of oxidative stress caused by reactive nitrogen species. This study describes the development of an HPLC electrochemical detection assay for free 3-NT capable of measuring this metabolite at the very low (nanomolar) levels encountered physiologically. We employed a dual-cell coulometric approach in which 3-NT is first reduced at an upstream cell to 3-aminotyrosine, which itself is then oxidized at the downstream cell. The method was shown to be linear over the range of 1-500 nM (r = 0.999), with a detection limit (signal/noise ratio of 3) of 0.5 nM (25 fmol on column). Ten consecutive injections of 2 and 20 nM 3-NT standards produced coefficients of variation of 5.88 and 1.87%, respectively. Validation of the identity of the 3-NT peak was confirmed by coelution with authentic standards and by the in vitro production of 3-NT by incubation of 3-morpholinylsydnoneimine (SIN-1, 100 microM), a molecule releasing nitric oxide and superoxide in solution at a pH of 7.0 or higher with tyrosine (10 microM). Using this method, 3-NT was detected in human liver microdialysate (levels up to 2.6 nM), although levels in rat spinal cord dialysate were below the limit of detection.

Effects of melatonin on 3-nitrotyrosine formation and energy charge ratio in guinea pig kidney in LPS-induced stress

The aim of this study was to evaluate the effects of Escherichia coli-derived lipopolysaccharide on guinea pig kidney by measuring the energy charge ratio and 3-nitrotyrosine levels. In addition the possible protective role of melatonin against lipopolysaccharide-mediated peroxynitrite formation and energy depletion of kidney was determined. Guinea pigs were either pretreated with melatonin or saline (for the control) followed by intraperitoneal administration of E. coli. Six hours after the administration of E. coli, guinea pig kidney ATP, ADP, AMP and 3-nitrotyrosine levels were measured by reverse-phase high performance liquid chromatography. There was a significant increase in the formation of 3-nitrotyrosine and decrease in energy charge in the endotoxin-induced group. However melatonin administration prevented 3-nitrotyrosine formation while failing to prevent or restore changes in the energy charge ratio of the kidney.

Nitric oxide release from the unimolecular decomposition of the superoxide radical anion adduct of cyclic nitrones in aqueous medium

Nitrones such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) and 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO) have become the spin-traps of choice for the detection of transient radical species in chemical and biological systems using electron paramagnetic resonance (EPR) spectroscopy. The mechanism of decomposition of the superoxide radical anion (O2(.-)) adducts of DMPO, DEPMPO and EMPO in aqueous solutions was investigated. Our findings suggest that nitric oxide (NO) was formed during the decomposition of the O2(.-) adduct as detected by EPR spin trapping using Fe(II)N-methyl-d-glucamine dithiocarbamate (MGD). Nitric oxide release was observed from the O2(.-) adduct formed from hypoxanthine-xanthine oxidase, PMA-activated human neutrophils, and DMSO solution of KO2. Nitric oxide formation was not observed from the independently generated hydroxyl radical adduct. Formation of nitric oxide was also indirectly detected as nitrite (NO2(.-)) utilizing the Griess assay. Nitrite concentration increases with increasing O2(.-) concentration at constant DMPO concentration, while NO2(.-) formation is suppressed at anaerobic conditions. Moreover, large excess of DMPO also inhibits NO2(.-) formation which can be attributed to the oxidation of DMPO to hydroxamic acid nitroxide (DMPO-X) by nitrogen dioxide (NO2), a precursor to NO2(.-). Product analysis was also conducted to further elucidate the mechanism of adduct decay using gas chromatography-mass spectrometry (GC-MS) technique.

A Study of the Metabolites of Ischemia-Reperfusion Injury and Selected Amino Acids in the Liver Using Microdialysis during Transplantation

BACKGROUND

Preservation and ischemia-reperfusion injury still impact the outcome of orthotopic liver transplantation. The authors used microdialysis with a view to monitoring its effect on graft function.

METHODS

A microdialysis catheter was inserted into the graft immediately after reperfusion and perfused with an isotonic solution for 48 hr. Metabolites of the ischemia-reperfusion injury and selected amino acids were studied. There were 18 patients, with a median age of 52 years (range, 38-62 years), 8 of whom were men. Lactate, pyruvate, glycerol, and glucose levels were measured. In addition, alanine, arginine, citrulline, gamma-aminobutyric acid (GABA), glutamate, glutamine, glycine, and taurine were determined.

RESULTS

All grafts functioned well. High lactate, pyruvate, and glycerol levels were observed in the immediate postoperative period. These showed a significant rapid decrease and stabilized to baseline levels. Alanine, glutamate, GABA, and taurine levels declined significantly to baseline values. Arginine levels were low immediately postreperfusion and then increased, reaching significantly higher values beyond 19 hr.

CONCLUSIONS

These data may represent "normal" changes seen in the immediate posttransplant period because all grafts functioned well. Two important metabolic fates of arginine in the liver are in the detoxification of ammonia by means of the urea cycle, and in the synthesis of nitric oxide (NO). Low extracellular arginine may reflect influx of the amino acid into hepatocytes, resulting in formation of NO in the presence of inducible NO synthase or conversion to ornithine in the presence of arginase in the urea cycle. As the organ stabilizes, restriction of arginine uptake may give rise to the observed increase in extracellular arginine.

Site-specific delivery of acidic fibroblast growth factor stimulates angiogenic and osteogenic responsesin vivo

A major clinical problem in orthopedics is the healing of nonunion fractures. Limitations of this bone repair process include insufficient angiogenesis and mineralization. Integrating appropriate biomaterials with site-specific neovascularization and osteogenesis at the wound site has been the focus of several clinically relevant therapeutic strategies. As an extracellular protein, acidic fibroblast growth factor (FGF-1) induces, coordinates, and sustains site-specific molecular responses associated with angiogenesis and osteogenesis. To establish the ability of this growth factor to coordinate bone regenerative process in vivo, site-specific delivery of FGF-1, entrapped in a fibrin/hydroxyapatite composite, was evaluated. Kinetic analysis in vivo revealed the biocomposite was capable of delivering biologically active FGF-1. Release kinetics revealed an initial delivery of 87.5 ng/h of active FGF-1 in the first 20 h, followed by a reduced delivery of 28 ng/h during the next 20 h. In situ immunohistological analyses demonstrated that FGF-1-containing implants induced increased angiogenesis and infiltration of cells expressing osteogenic related markers (i.e., osteopontin, osteocalcin). Collectively, these efforts support that site-specific delivery of active FGF-1 in a fibrin/hydroxyapatite composite is competent to induce not only angiogenesis but also osteogenic cellular responses.

Contributions of endothelial and neuronal nitric oxide synthases to cerebrovascular responses to hyperoxia

Hyperoxia causes a transient decrease in CBF, followed by a later rise. The mediators of these effects are not known. We used mice lacking endothelial or neuronal nitric oxide synthase (NOS) isoforms (eNOS-/- and nNOS-/- mice) to study the roles of the NOS isoforms in mediating changes in cerebral vascular tone in response to hyperoxia. Resting regional cerebral blood flow (rCBF) did not differ between wild type (WT), eNOS-/- mice, and nNOS-/- mice. eNOS-/- mice showed decreased cerebrovascular reactivities to NG-nitro-L-arginine methyl ester (L-NAME), PAPA NONOate, acetylcholine (Ach), and SOD1. In response to hyperbaric oxygen (HBO2) at 5 ATA, WT and nNOS-/- mice showed decreases in rCBF over 30 minutes, but eNOS-/- mice did not. After 60 minutes HBO2, rCBF increased more in WT mice than in eNOS-/- or nNOS-/- mice. Brain NO-metabolites (NOx) decreased in WT and eNOS-/- mice within 30 minutes of HBO2, but after 45 minutes, NOx rose above control levels, whereas they did not change in nNOS-/- mice. Brain 3NT increased during HBO2 in WT and eNOS-/- but did not change in nNOS-/- mice. These results suggest that modulation of eNOS-derived NO by HBO2 is responsible for the early vasoconstriction responses, whereas late HBO2-induced vasodilation depends upon both eNOS and nNOS.

Oxygen seizure latency and peroxynitrite formation in mice lacking neuronal or endothelial nitric oxide synthases

Nitric oxide (NO) from endothelial or neuronal NO synthases (eNOS or nNOS) may contribute both to the cerebrovascular responses to oxygen and potentially to the peroxynitrite-mediated toxic effects of hyperbaric oxygen (HBO(2)) on the central nervous system (CNS O(2) toxicity). In mice lacking eNOS or nNOS (-/-), regional cerebral blood flow (rCBF) and 3-nitrotyrosine (3-NT), a biochemical marker for peroxynitrite (ONOO(-)) formation, were measured in the brain during HBO(2) exposure. These variables were then correlated with EEG spiking activity related to CNS O(2) toxicity. In wild-type (WT) mice, HBO(2) exposure transiently reduced rCBF, but by 60 min rCBF was restored to baseline levels and above, followed by EEG spikes. Mice lacking nNOS also showed initial depression of rCBF followed by hyperemia but the delay in the onset of EEG discharges was greater. In contrast, in eNOS-deficient mice rCBF did not decrease and hyperemia was less pronounced during HBO(2). EEG spike latency was longer in eNOS(-/-) compared to WT or nNOS(-/-) mice. 3-NT gradually increased in all strains during HBO(2) but accumulation was slower in nNOS(-/-) mice, consistent with less ONOO(-) production. These results indicate that NOS-deficient mice have different cerebrovascular responses and tolerance to HBO(2) depending on which enzyme isoform is affected. The data suggest a key role for eNOS-dependent NO production in cerebral vasoconstriction and in the development of hyperoxic hyperemia preceding O(2) seizures, whereas neuronal NO may mediate toxic effects of HBO(2) mainly by its reaction with superoxide to generate the stronger oxidant, peroxynitrite.

Incorporation of 3-nitrotyrosine into the C-terminus of alpha-tubulin is reversible and not detrimental to dividing cells

The C-terminus of the alpha-chain of tubulin is subject to reversible incorporation of tyrosine by tubulin tyrosine ligase and removal by tubulin carboxypeptidase. Thus, microtubules rich in either tyrosinated or detyrosinated tubulin can coexist in the cell. Substitution of the terminal tyrosine by 3-nitrotyrosine has been claimed to cause microtubule dysfunction and consequent injury of epithelial lung carcinoma A549 cells. Nitrotyrosine is formed in cells by nitration of tyrosine by nitric oxide-derived species. We studied properties of tubulin modified by in vitro nitrotyrosination at the C-terminus of the alpha-subunit, and the consequences for cell functioning. Nitrotyrosinated tubulin was a good substrate of tubulin carboxypeptidase, and showed a similar capability to assemble into microtubules in vitro to that of tyrosinated tubulin. Tubulin of C6 cells cultured in F12K medium in the presence of 500 micro m nitrotyrosine became fully nitrotyrosinated. This nitrotyrosination was shown to be reversible. No changes in morphology, proliferation, or viability were observed during cycles of nitrotyrosination, denitrotyrosination, and re-nitrotyrosination. Similar results were obtained with CHO, COS-7, HeLa, NIH-3T3, NIH-3T3(TTL-), and A549 cells. C6 and A549 cells were subjected to several passages during 45 days or more in the continuous presence of 500 micro m nitrotyrosine without noticeable alteration of morphology, viability, or proliferation. The microtubular networks visualized by immunofluorescence with antibodies to nitrotyrosinated and total tubulin were identical. Furthermore, nitrotyrosination of tubulin in COS cells did not alter the association of tubulin carboxypeptidase with microtubules. Our results demonstrate that substitution of C-terminal tyrosine by 3-nitrotyrosine has no detrimental effect on dividing cells.

The catabolic fate of nitric oxide: the nitric oxide oxidase and peroxynitrite reductase activities of cytochrome oxidase

Stimulation of cardiomyocytes to endogenously evolve nitric oxide is shown by microsensor measurements on single cells to lead to transient nitric oxide concentrations of a few hundred nanomolar. At these submicromolar concentrations, no evidence could be found for the expected reaction between nitric oxide generated and the oxymyoglobin present in the cells: nitric oxide + oxymyoglobin --> nitrate + metmyoglobin. No metmyoglobin formation was detected by electron paramagnetic resonance spectroscopy, and microsensor measurements revealed near quantitative conversion of the nitric oxide to nitrite rather than nitrate ion. Moreover, the rate of nitrite formation is shown to be too rapid to be accounted for by non-enzymatic means. The essentially quantitative and rapid catabolism of nitric oxide to nitrite ion can plausibly be explained on the basis of a cycle of reactions catalyzed by cytochrome c oxidase. It is demonstrated with the purified hemoproteins in vitro that the terminal oxidase can outcompete oxymyoglobin for available nitric oxide. It is proposed that under normal physiological and most pathological (non-inflammatory) conditions, reaction with cytochrome c oxidase is the major route by which NO is removed from mitochondria-rich cells.

Acidic fibroblast growth factor signaling inhibits peroxynitrite-induced death of osteoblasts and osteoblast precursors

After trauma injury to the musculoskeletal system, conditions such as ischemia and inflammation involve excess production of superoxide (O2*), nitric oxide (*NO), and their reaction product, peroxynitrite (ONOO-). Exposure of murine osteoblasts and rat-derived primary osteoblast precursors to ONOO- resulted in a dose- and time-dependent delayed cell death that was more characteristic of apoptosis than necrosis. Exposure of both cell populations to ONOO- immediately enhanced phosphorylation and nitration of tyrosine residues within several polypeptides. Treatment of osteoblasts and osteoblast precursors with exogenous acidic fibroblast growth factor (FGF-1) enhanced cellular growth, increased endogenous levels of tyrosine phosphorylation, and significantly induced expression of both osteopontin and osteocalcin messenger RNA (mRNA) as well as osteopontin protein. Pretreatment of both cell populations with exogenous FGF-1 prevented ONOO(-)-mediated death. Cell signaling induced by FGF-1 pretreatment had no major effect of total levels of tyrosine nitration after ONOO- treatment. Collectively, these in vitro efforts show that FGF-1 signaling renders osteoblasts and osteoblast precursors resistant to the cytotoxic effects of ONOO-. Consequently, results presented here predict the therapeutic use of this growth factor for promoting the progression of bone repair mechanisms after fracture trauma.

Tyrosine nitration: localisation, quantification, consequences for protein function and signal transduction

The nitration of free tyrosine or protein tyrosine residues generates 3-nitrotyrosine the detection of which has been utilised as a footprint for the in vivo formation of peroxynitrite and other reactive nitrogen species. The detection of 3-nitrotyrosine by analytical and immunological techniques has established that tyrosine nitration occurs under physiological conditions and levels increase in most disease states. This review provides an updated, comprehensive and detailed summary of the tissue, cellular and specific protein localisation of 3-nitrotyrosine and its quantification. The potential consequences of nitration to protein function and the pathogenesis of disease are also examined together with the possible effects of protein nitration on signal transduction pathways and on the metabolism of proteins.

LC-MS/MS detection of peroxynitrite-derived 3-nitrotyrosine in rat microvessels

3-Nitrotyrosine (3NT) is used as a biomarker of nitrative pathology caused by peroxynitrite (PN), myeloperoxidase (MPO)-, and/or eosinophil peroxidase (EPO)-dependent nitrite oxidation. 3NT measurements in biological materials are usually based on either antibody staining, HPLC detection, or GC detection methodologies. In this report, a procedure is described for the measurement of 3NT and tyrosine (TYR) by LC-MS/MS that is simple, direct, and sensitive. Though highly specialized in its use as an assay, LC-MS/MS technology is available in many research centers in academia and industry. The critical assay for 3NT was linear below 100 ng/ml and the limit of detection was below 100 pg/ml. Regarding protein digested samples, we found that MRM was most selective with 133.1 m/z as the daughter ion. In comparison, LC-ECD was 100 times less sensitive. Basal levels of 3NT in extracted digests of rat brain homogenate were easily detected by LC-MS/MS, but were below detection by LC-ECD. The LC-MS/MS assay was used to detect 3NT in rat brain homogenate that was filtered through a 180 micron nylon mesh. Three fractions were collected and examined by phase contrast microscopy. The mass ratio (3NT/TYR) of 3NT in fractions of large vessel enrichment, microvessel enrichment, and vessel depletion was 0.6 ng/mg, 1.2 ng/mg, and 0.2 ng/mg, respectively. Ultimately, we found that the basal 3NT/TYR mass ratio as determined by LC-MS/MS was six times greater in microvessel-enriched brain tissue vs. tissue devoid of microvessels.

Superoxide reacts with nitric oxide to nitrate tyrosine at physiological pH via peroxynitrite

Tyrosine nitration is a widely used marker of peroxynitrite (ONOO(-)) produced from the reaction of nitric oxide with superoxide. Pfeiffer and Mayer (Pfeiffer, S., and Mayer, B. (1998) J. Biol. Chem. 273, 27280-27285) reported that superoxide produced from hypoxanthine plus xanthine oxidase in combination with nitric oxide produced from spermine NONOate did not nitrate tyrosine at neutral pH. They suggested that nitric oxide and superoxide at neutral pH form a less reactive intermediate distinct from preformed alkaline peroxynitrite that does not nitrate tyrosine. Using a stopped-flow spectrophotometer to rapidly mix potassium superoxide with nitric oxide at pH 7.4, we report that an intermediate spectrally and kinetically identical to preformed alkaline cis-peroxynitrite was formed in 100% yield. Furthermore, this intermediate nitrated tyrosine in the same yield and at the same rate as preformed peroxynitrite. Equivalent concentrations of nitric oxide under aerobic conditions in the absence of superoxide did not produce detectable concentrations of nitrotyrosine. Carbon dioxide increased the efficiency of nitration by nitric oxide plus superoxide to the same extent as peroxynitrite. In experiments using xanthine oxidase as a source of superoxide, tyrosine nitration was substantially inhibited by urate formed from hypoxanthine oxidation, which was sufficient to account for the lack of tyrosine nitration previously reported. We conclude that peroxynitrite formed from the reaction of nitric oxide with superoxide at physiological pH remains an important species responsible for tyrosine nitration in vivo.

Determination of 3-nitrotyrosine by high-pressure liquid chromatography with a dual-mode electrochemical detector

3-Nitrotyrosine, a product of tyrosine nitration, is useful as a marker for the generation of reactive nitrogen oxide species with short half-lives such as peroxynitrite. A reverse-phase high-pressure liquid chromatographic method using a dual-mode electrochemical detector in series with a photodiode array detector has been developed to determine the levels of 3-nitrotyrosine in biological samples. The principle of this method involves reduction of 3-nitrotyrosine at an upstream gold amalgam electrode and oxidation of the resulting product(s) at a downstream glassy carbon electrode. 3-Nitrotyrosine is quantified by the amount of the current generated at the downstream electrode, and a femtomole detection level can be achieved. The disappearance of the corresponding peak when the electrochemical detector is used only in the single oxidative mode provides additional evidence for the identity of 3-nitrotyrosine in the sample. Tyrosine from the same sample is determined by its UV absorption at 280 nm, thus eliminating the need for an internal standard. With this method a dose-dependent increase of 3- to 10-fold in the levels of protein 3-nitrotyrosine was observed in the blood plasma, and a 2- to 4-fold increase in the lung cytosols, of rats treated with the lung carcinogen and nitrating agent tetranitromethane.

Dityrosine formation outcompetes tyrosine nitration at low steady-state concentrations of peroxynitrite. Implications for tyrosine modification by nitric oxide/superoxide in vivo

Formation of peroxynitrite from NO and O-(*2) is considered an important trigger for cellular tyrosine nitration under pathophysiological conditions. However, this view has been questioned by a recent report indicating that NO and O-(*2) generated simultaneously from (Z)-1-(N-[3-aminopropyl]-N-[4-(3-aminopropylammonio)butyl]-amino) diazen-1-ium-1,2-diolate] (SPER/NO) and hypoxanthine/xanthine oxidase, respectively, exhibit much lower nitrating efficiency than authentic peroxynitrite (Pfeiffer, S. and Mayer, B. (1998) J. Biol. Chem. 273, 27280-27285). The present study extends those earlier findings to several alternative NO/O-(*2)-generating systems and provides evidence that the apparent lack of tyrosine nitration by NO/O-(*2) is due to a pronounced decrease of nitration efficiency at low steady-state concentrations of authentic peroxynitrite. The decrease in the yields of 3-nitrotyrosine was accompanied by an increase in the recovery of dityrosine, showing that dimerization of tyrosine radicals outcompetes the nitration reaction at low peroxynitrite concentrations. The observed inverse dependence on peroxynitrite concentration of dityrosine formation and tyrosine nitration is predicted by a kinetic model assuming that radical formation by peroxynitrous acid homolysis results in the generation of tyrosyl radicals that either dimerize to yield dityrosine or combine with (*)NO(2) radical to form 3-nitrotyrosine. The present results demonstrate that very high fluxes (>2 microM/s) of NO/O-(*2) are required to render peroxynitrite an efficient trigger of tyrosine nitration and that dityrosine is a major product of tyrosine modification caused by low steady-state concentrations of peroxynitrite.

Free radical mediated photoreceptor damage in uveitis

Uveitis is a major cause of blindness, with the visual loss that occurs being due primarily to retinal tissue damage. The tissue damage is mediated mainly by phagocytic inflammatory cells, such as macrophages, by the release of various proteolytic enzymes, arachidonic acid metabolites, cytokines and free radicals. The latter are found to be potent cytotoxic agents that readily cause tissue damage by peroxidation of lipid cell membranes. Recent studies of experimental uveitis indicate that other potent oxidants are generated in uveitis by macrophages. One of these is ONOO-, which is formed from *NO and O(-)2. The macrophages generate *NO preferentially in the outer retina following iNOS expression. In these phagocytes, outer retinal proteins, especially arrestin, are found to be potent iNOS inducers. Current studies of RPE show that these cells protect the retina from ONOO- mediated damage in uveitis by releasing a novel protein called retinal pigment epithelial protective protein. This protein is found to suppress O(-)2 and *NO generation by the phagocytes, in both in vitro and in vivo uveitis models. The protective protein expression is restricted to RPE, its suppressive effect is a result of the inhibition of the phosphorylation of cytosolic proteins, p47-phox, required for the assembly of NADPH and activation of NFkappaB, which are required for generation of 0(-)2 and expression of iNOS respectively. Either pharmacologically or chemically, up-regulation of RPP generation could help in preventing retinal degeneration in uveitis or other degenerative dis

Gas chromatographic-tandem mass spectrometric quantification of free 3-nitrotyrosine in human plasma at the basal state

A fully validated gas chromatographic-tandem mass spectrometric (GC-tandem MS) method for the accurate and precise quantification of free 3-nitrotyrosine in human plasma at the basal state is described. In the plasma of 11 healthy humans a mean concentration of 2.8 nM (range 1.4-4.2 nM) for free 3-nitrotyrosine was determined by this method. This is the lowest concentration reported for free 3-nitrotyrosine in plasma of healthy humans. The presence of endogenous free 3-nitrotyrosine in human plasma was unequivocally shown by generating a daughter mass spectrum. Various precautions had to be taken to avoid artifactual formation of 3-nitrotyrosine from nitrate during sample treatment. Endogenous plasma 3-nitrotyrosine and 3-nitro-l-[(2)H(3)]tyrosine added for use as internal standard were isolated by high-performance liquid chromatographic (HPLC) analysis of 200-microl aliquots of plasma ultrafiltrate samples (20 kDa cut-off), extracted from a single HPLC fraction by solid-phase extraction, derivatized to their n-propyl ester-pentafluoropropionyl amide-trimethylsilyl ether derivatives, and quantified by GC-tandem MS. Overall recovery was determined as 50 +/- 5% using 3-nitro-l-[(14)C(9)]tyrosine. The limit of detection of the method was 4 amol of 3-nitrotyrosine, while the limit of quantitation was 125 pM using 3-nitro-l-[(14)C(9)]tyrosine. 3-Nitrotyrosine added to human plasma at 1 nM was quantitated with an accuracy of > or = 80% and a precision of > or = 94%. The method should be useful to investigate the utility of plasma free 3-nitrotyrosine as an indicator of nitric oxide ((.)NO)-associated oxidative stress in vivo in humans.

Requirements for heme and thiols for the nonenzymatic modification of nitrotyrosine

Peroxynitrite-dependent formation of nitrotyrosine has been associated with inactivation of various enzymes and proteins possessing functionally important tyrosines. We have previously reported an enzymatic activity modifying the nitrotyrosine residues in nitrated proteins. Here we are describing a nonenzymatic reduction of nitrotyrosine to aminotyrosine, which depends on heme and thiols. Various heme-containing proteins can mediate the reaction, although the reaction also is catalyzed by heme. The reaction is most effective when vicinal thiols are used as reducing agents, although ascorbic acid also can replace thiols with lesser efficiency. The reaction could be inhibited by (z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1- ium-1, but not other tested NO donors. HPLC with electrochemical detection analysis of the reaction identified aminotyrosine as the only reaction product. The reduction of nitrotyrosine was most effective at a pH close to physiological and was markedly decreased in acidic conditions. Various nitrophenol compounds also were modified in this reaction. Understanding the mechanism of this reaction could help define the enzymatic modification of nitrotyrosine-containing proteins. Furthermore, this also could assist in understanding the role of nitrotyrosine formation and reversal in the regulation of various proteins containing nitrotyrosine. It also could help define the role of nitric oxide and other reactive species in various disease states.

Role of free radicals in the limb teratogenicity of L-NAME (N(G)-nitro-(L)-arginine methyl ester): a new mechanistic model of vascular disruption

In continuing studies of limb effects resulting from fetal exposure to N(G)-nitro-(L)-arginine methyl ester (L-NAME), we examined the early time course of vascular changes and the effectiveness of fetal intraamniotic injection. Vascular engorgement and hemorrhage occurred within 4 hr of L-NAME treatment on gestational day (gd) 17, and direct injection appeared to be as effective as maternal intraperitoneal injection in inducing limb hemorrhage. Further studies examined protein nitration and electron transport inhibition in tissues of exposed fetuses. L-NAME caused significant increases in nitrotyrosine (NT) formation in limb but not in heart or brain, and reduced electron transport rates in limb. Three agents, alpha-phenyl-N-t-butylnitrone (PBN), a radical trap and inhibitor of inducible nitric oxide synthase (iNOS), allopurinol, an inhibitor of xanthine oxidase, and aminoguanidine, a relatively specific inhibitor of iNOS, significantly moderated limb hemorrhage and protein nitration in distal limb. These results suggest that L-NAME works directly on the fetal limb vasculature and indicate a cytotoxic role for peroxynitrite, a potent oxidant and nitrating agent that is the reaction product of nitric oxide and superoxide anion radical. We propose that L-NAME and other vasoactive toxicants disrupt the fetal limb in a sequential process. Initially, nitric oxide (NO) is depleted, causing hemorrhage and edema in the limb. Within hours, iNOS is induced, resulting in cytotoxic tissue concentrations of NO and reactive nitrogen species that induce apoptosis and/or necrosis in the limb. We suggest that L-NAME exposure may serve as a model of vascular disruptive limb malformations.

Analysis of 3-nitrotyrosine in biological fluids and protein hydrolyzates by high-performance liquid chromatography using a postseparation, on-line reduction column and electrochemical detection: results with various nitrating agents

Nitric oxide reacts rapidly with superoxide to form the strong nitrating agent peroxynitrite, which is responsible for much of the tissue damage associated with diverse pathophysiological conditions such as inflammation. The occurrence of free or protein-bound nitrotyrosine (NTYR) has been considered as evidence for in vivo formation of peroxynitrite. However, various agents can nitrate tyrosine, and their relative significance in vivo has not been determined due to lack of a sensitive method to analyze NTYR in tissue proteins and biological fluids. We have developed a new HPLC-electrochemical detection method to analyze NTYR in protein hydrolyzates or biological fluids. The sample is injected directly into a reversed-phase HPLC column and NTYR is subsequently reduced by a platinum column to 3-aminotyrosine, which is quantified with an electrochemical detector. The method is simple, selective, and sensitive (detection limit, 0.1 pmol per 20-microl injection). We have applied this method to compare in vitro the ability of various nitrating agents to form NTYR in bovine serum albumin and human plasma. Yields of NTYR formed in human plasma proteins incubated with 1 or 10 mM nitrating agent decreased in the following order: synthetic peroxynitrite > 3-morpholinosydonimine, a generator of both NO and superoxide > Angeli's salt, which forms nitroxyl anion (NO-) > spermine-NONOate, which releases NO > sodium nitrite plus hypochlorite, which forms the nitrating agent nitryl chloride (NO2Cl). A simple purification method using a C18 Sep-Pak cartridge is also described for analysis of free NTYR in human plasma.

Tyrosine modifications and inactivation of active site manganese superoxide dismutase mutant (Y34F) by peroxynitrite

Recent studies from this laboratory have demonstrated that human manganese superoxide dismutase (MnSOD) is a target for tyrosine nitration in several chronic inflammatory diseases including chronic organ rejection, arthritis, and tumorigenesis. Furthermore, we demonstrated that peroxynitrite (ONOO-) is the only known biological oxidant competent to inactivate enzymatic activity, nitrate critical tyrosine residues, and induce dityrosine formation in MnSOD. To elucidate the differential contributions of tyrosine nitration and oxidation during enzymatic inactivation, we now compare ONOO- treatment of native recombinant human MnSOD (WT-MnSOD) and a mutant, Y34F-MnSOD, in which tyrosine 34 (the residue most susceptible to ONOO--mediated nitration) was mutated to phenylalanine. Both WT-MnSOD (IC50 = 65 microM, 15 microM MnSOD) and Y34F-MnSOD (IC50 = 55 microM, 15 microM Y34F) displayed similar dose-dependent sensitivity to ONOO--mediated inactivation. Compared to WT-MnSOD, the Y34F-MnSOD mutant demonstrated significantly less efficient tyrosine nitration and enhanced formation of dityrosine following treatment with ONOO-. Collectively, these results suggest that complete inactivation of MnSOD by ONOO- can occur independent of the active site tyrosine residue and includes not only nitration of critical tyrosine residues but also tyrosine oxidation and subsequent formation of dityrosine.

Microtubule dysfunction by posttranslational nitrotyrosination of alpha-tubulin: a nitric oxide-dependent mechanism of cellular injury

NO2Tyr (3-Nitrotyrosine) is a modified amino acid that is formed by nitric oxide-derived species and has been implicated in the pathology of diverse human diseases. Nitration of active-site tyrosine residues is known to compromise protein structure and function. Although free NO2Tyr is produced in abundant concentrations under pathological conditions, its capacity to alter protein structure and function at the translational or posttranslational level is unknown. Here, we report that free NO2Tyr is transported into mammalian cells and selectively incorporated into the extreme carboxyl terminus of alpha-tubulin via a posttranslational mechanism catalyzed by the enzyme tubulin-tyrosine ligase. In contrast to the enzymatically regulated carboxyl-terminal tyrosination/detyrosination cycle of alpha-tubulin, incorporation of NO2Tyr shows apparent irreversibility. Nitrotyrosination of alpha-tubulin induces alterations in cell morphology, changes in microtubule organization, loss of epithelial-barrier function, and intracellular redistribution of the motor protein cytoplasmic dynein. These observations imply that posttranslational nitrotyrosination of alpha-tubulin invokes conformational changes, either directly or via allosteric interactions, in the surface-exposed carboxyl terminus of alpha-tubulin that compromises the function of this critical domain in regulating microtubule organization and binding of motor- and microtubule-associated proteins. Collectively, these observations illustrate a mechanism whereby free NO2Tyr can impact deleteriously on cell function under pathological conditions encompassing reactive nitrogen species production. The data also yield further insight into the role that the alpha-tubulin tyrosination/detyrosination cycle plays in microtubule function.

Detection of 3-nitrotyrosine in human platelets exposed to peroxynitrite by a new gas chromatography/mass spectrometry assay

A new sensitive and specific assay was developed and applied for the quantitative determination of 3-nitrotyrosine in proteins of human platelets. 3-Nitrotyrosine was quantitatively converted into a new pentafluorobenzyl derivative in a single step and detected as an abundant carboxylate anion at m/z 595 using negative ion chemical ionization gas chromatography/mass spectrometry. The internal standard, [13C6]-3-nitrotyrosine, was prepared via a new and efficient method using nitronium borofluorate dissolved in hydrochloric acid. The assay showed excellent linearity and sensitivity. Intact human platelets contained 1.4+/-0.6 ng of 3-nitrotyrosine per milligram of protein. Peroxynitrite increased 3-nitrotyrosine levels 4- to 535-fold at the concentration range of 10 to 300 microM. Decomposed peroxynitrite was without the effect. Nitrogen dioxide (43 microM) was also a potent tyrosine nitrating molecule, increasing the levels of 3-nitrotyrosine 153-fold. HOCl (50 microM) in the presence of nitrite (50 microM) increased the 3-nitrotyrosine levels 3-fold. Exposure of platelets to nitric oxide, nitrite, thrombin, adenosine diphosphate, platelet activating factor, and arachidonic acid had no effect on platelet 3-nitrotyrosine levels.

Analytical methods for 3-nitrotyrosine as a marker of exposure to reactive nitrogen species: a review

Nitric oxide (NO*) is a diatomic free radical which has recently been found to have a key role in both normal physiological processes and disease states. The presence of NO in biological systems leads to the formation of reactive nitrogen species (RNS) such as peroxynitrite which reacts avidly with tyrosine residues in proteins to form nitrotyrosine (NTYR). Since peroxynitrite has a very short half-life at neutral pH, the presence of NTYR has been used as a marker of RNS production in various tissues. A number of methods for separation, detection, and quantitation of NTYR in biological samples have been developed. These methods include immunochemical techniques such as immunhistochemistry, ELISA, and Western blotting, high-performance liquid chromatography (HPLC) in combination with various detection systems including UV and electrochemical detection (ECD), gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), and electrospray mass spectrometry. In terms of sensitivity and specificity, it would appear that methods based on combinations of HPLC and various types of ECD are very versatile giving a limit of detection of 20 fmol per injection of protein hydrolysate. They are only limited by the sample quantity and the preparation that is required to achieve acceptable chromatograms. In addition to the detection of NTYR as a marker of RNS, its role in biological systems may be more subtle with nitration of key tyrosine residues likely to profoundly affect cellular function such as signaling cascades. Further advances are likely to be made in the localization of NTYR residues in peptide fragments using mass spectrometry.

Nitrotyrosine in plasma of celiac disease patients as detected by a new sandwich ELISA

Inflammation is characterized by increased nitric oxide production. Nitrotyrosine has recently been suggested to be useful as a marker for NO-mediated tissue damage. In context of the development of an ELISA for detection of nitrotyrosine in plasma, monoclonal anti-nitrotyrosine antibodies were developed by injecting mice with nitrated keyhole limpet hemocyanin. The specificity of the antibodies was determined by binding to nitrated BSA, lack of binding to unmodified BSA, tyrosine, 3-chlorotyrosine or phenylalanine and inhibition of binding by nitrotyrosine. The antibodies developed are useful for Western blot analysis and immunohistochemical staining. Using these antibodies a nitrotyrosine sandwich ELISA was developed with a lower detection limit of approximately 0.2 nM. The intra- and interassay variance were 2.4% and 11.9%, respectively. Using this newly developed ELISA, 1.27 +/- 1.03 microM nitrotyrosine was detected in plasma samples of celiac disease patients whereas nitrotyrosine was undetectable in control samples. Elevated nitrotyrosine levels were paralleled by an increase in plasma concentrations of NO-oxidation products (NOx), nitrite and nitrate from 15.1 +/- 6.1 microM in controls to 61.0 +/- 28.2 microM in celiac disease patients. Both nitrotyrosine and NOx levels declined when the patients were on a gluten-free diet, suggesting a relation between intestinal inflammation and plasma nitrotyrosine and NOx levels.

An activity in rat tissues that modifies nitrotyrosine-containing proteins

Homogenates from rat spleen and lung could modify nitrotyrosine-containing BSA. With incubation, nitrotyrosine-containing BSA lost its epitope to a monoclonal antibody that selectively recognized nitrotyrosine-containing proteins. In the presence of protease inhibitors, the loss of the nitrotyrosine epitope occurred without protein degradation and hydrolysis. This activity was found in supernatant but not particulate fractions of spleen homogenates. The factor was heat labile, was sensitive to trypsin treatment, and was retained after passage through a membrane with a 10-kDa retention. The activity was time- and protein-concentration dependent. The activity increased about 2-fold in spleen extracts with endotoxin (bacterial lipopolysaccharide) treatment of animals, suggesting that the activity is inducible or regulatable. Other nitrotyrosine-containing proteins also served as substrates, while free nitrotyrosine and some endogenous nitrotyrosine-containing proteins in tissue extracts were poor substrates. Although the product and possible cofactors for this reaction have not yet been identified, this activity may be a "nitrotyrosine denitrase" that reverses protein nitration and, thus, decreases peroxynitrite toxicity. This activity was not observed in homogenates from rat liver or kidney, suggesting that there may also be some tissue specificity for the apparent denitrase activity.

Nitrosation of uric acid by peroxynitrite. Formation of a vasoactive nitric oxide donor

Peroxynitrite (ONOO-), formed by the reaction between nitric oxide (. NO) and superoxide, has been implicated in the etiology of numerous disease processes. Low molecular weight antioxidants, including uric acid, may minimize ONOO---mediated damage to tissues. The tissue-sparing effects of uric acid are typically attributed to oxidant scavenging; however, little attention has been paid to the biology of the reaction products. In this study, a previously unidentified uric acid derivative was detected in ONOO--treated human plasma. The product of the uric acid/ONOO- reaction resulted in endothelium-independent vasorelaxation of rat thoracic aorta, with an EC50 value in the range of 0.03-0.3 microM. Oxyhemoglobin, a .NO scavenger, completely attenuated detectable .NO release and vascular relaxation. Uric acid plus decomposed ONOO- neither released .NO nor altered vascular reactivity. Electrochemical quantification of .NO confirmed that the uric acid/ONOO- reaction resulted in spontaneous (thiol-independent) and protracted (t1/2 approximately 125 min) release of .NO. Mass spectroscopic analysis indicated that the product was a nitrated uric acid derivative. The uric acid nitration/nitrosation product may play a pivotal role in human pathophysiology by releasing .NO, which could decrease vascular tone, increase tissue blood flow, and thereby constitute a role for uric acid not previously described.

Pathophysiology of nitric oxide and related species: free radical reactions and modification of biomolecules

Since its initial discovery as an endogenously produced bioactive mediator, nitric oxide (.NO) has been found to play a critical role in the cellular function of nearly all organ systems. Furthermore, aberrant production of .NO or reactive nitrogen species (RNS) derived from .NO, has been implicated in a number of pathological conditions, such as acute lung disease, atherosclerosis and septic shock. While .NO itself is fairly non-toxic, secondary RNS are oxidants and nitrating agents that can modify both the structure and function of numerous biomolecules both in vitro, and in vivo. The mechanisms by which RNS mediate toxicity are largely dictated by its unique reactivity. The study of how reactive nitrogen species (RNS) derived from .NO interact with biomolecules such as proteins, carbohydrates and lipids, to modify both their structure and function is an area of active research, which is lending major new insights into the mechanisms underlying their pathophysiological role in human disease. In the context of .NO-dependent pathophysiology, these biochemical reactions will play a major role since they: (i) lead to removal of .NO and decreased efficiency of .NO as an endothelial-derived relaxation factor (e.g. in hypertension, atherosclerosis) and (ii) lead to production of other intermediate species and covalently modified biomolecules that cause injury and cellular dysfunction during inflammation. Although the physical and chemical properties of .NO and .NO-derived RNS are well characterised, extrapolating this fundamental knowledge to a complicated biological environment is a current challenge for researchers in the field of .NO and free radical research. In this review, we describe the impact of .NO and .NO-derived RNS on biological processes primarily from a biochemical standpoint. In this way, it is our intention to outline the most pertinent and relevant reactions of RNS, as they apply to a diverse array of pathophysiological states. Since reactions of RNS in vivo are likely to be vast and complex, our aim in this review is threefold: (i) address the major sources and reactions of .NO-derived RNS in biological systems, (ii) describe current knowledge regarding the functional consequences underlying .NO-dependent covalent modification of specific biomolecules, and (iii) to summarise and critically evaluate the available evidence implicating these reactions in human pathology. To this end, three areas of special interest have been chosen for detailed description, namely, formation and role of S-nitrosothiols, modulation of lipid oxidation/nitration by RNS, and tyrosine nitration mechanisms and consequences.

Biological tyrosine nitration: a pathophysiological function of nitric oxide and reactive oxygen species

Analytical and immunological methodologies and occasionally both methodologies have been applied to detect and quantify 3-nitrotyrosine in almost every major organ system. In certain diseases increased levels of 3-nitrotyrosine have been correlated with elevated levels of other indices of oxidative stress. Numerous reports have established that nitration is a biological process derived from the biochemical interaction of nitric oxide or nitric oxide-derived secondary products with reactive oxygen species. This article addresses critical issues regarding this biological process, namely the biochemical pathways for nitration of tyrosine residues in vivo, potential protein targets, and pathophysiological consequences of protein tyrosine nitration.