Protein Tyrosine Amination: Detection, Imaging, and Chemoproteomic Profiling with Synthetic Probes

Protein tyrosine nitration (PTN) by oxidative and nitrative stress is a well-known post-translational modification that plays a role in the initiation and progression of various diseases. Despite being recognized as a stable modification for decades, recent studies have suggested the existence of a reduction in PTN, leading to the formation of 3-aminotyrosine (3AT) and potential denitration processes. However, the vital functions of 3AT-containing proteins are still unclear due to the lack of selective probes that directly target the protein tyrosine amination. Here, we report a novel approach to label and enrich 3AT-containing proteins with synthetic salicylaldehyde (SAL)-based probes: SALc-FL with a fluorophore and SALc-Yn with an alkyne tag. These probes exhibit high selectivity and efficiency in labeling and can be used in cell lysates and live cells. More importantly, SALc-Yn offers versatility when integrated into multiple platforms by enabling proteome-wide quantitative profiling of cell nitration dynamics. Using SALc-Yn, 355 proteins were labeled, enriched, and identified to carry the 3AT modification in oxidatively stressed RAW264.7 cells. These findings provide compelling evidence supporting the involvement of 3AT as a critical intermediate in nitrated protein turnover. Moreover, our probes serve as powerful tools to investigate protein nitration and denitration processes, and the identification of 3AT-containing proteins contributes to our understanding of PTN dynamics and its implications in cellular redox biology.

Ribonucleotide reductase: Implications of thiol S-nitrosylation and tyrosine nitration for different subunits

Ribonucleotide reductase (RNR) is a multi-subunit enzyme responsible for catalyzing the rate-limiting step in the production of deoxyribonucleotides essential for DNA synthesis and repair. The active RNR complex is composed of multimeric R1 and R2 subunits. The RNR catalysis involves the formation of tyrosyl radicals in R2 subunits and thiyl radicals in R1 subunits. Despite the quaternary structure and cofactor diversity, all the three classes of RNR have a conserved cysteine residue at the active site which is converted into a thiyl radical that initiates the substrate turnover, suggesting that the catalytic mechanism is somewhat similar for all three classes of the RNR enzyme. Increased RNR activity has been associated with malignant transformation, cancer cell growth, and tumorigenesis. Efforts concerning the understanding of RNR inhibition in designing potent RNR inhibitors/drugs as well as developing novel approaches for antibacterial, antiviral treatments, and cancer therapeutics with improved radiosensitization have been made in clinical research. This review highlights the precise and potent roles of NO in RNR inhibition by targeting both the subunits. Under nitrosative stress, the thiols of the R1 subunits have been found to be modified by S-nitrosylation and the tyrosyl radicals of the R2 subunits have been modified by nitration. In view of the recent advances and progresses in the field of nitrosative modifications and its fundamental role in signaling with implications in health and diseases, the present article focuses on the regulations of RNR activity by S-nitrosylation of thiols (R1 subunits) and nitration of tyrosyl residues (R2 subunits) which will further help in designing new drugs and therapies.

Quantitation of Nitration, Chlorination, and Oxidation in Hemoglobin of Breast Cancer Patients by Nanoflow Liquid Chromatography Tandem Mass Spectrometry

Cells are continually exposed to endogenous reactive oxygen, nitrogen, and halogen species, causing damage to biomolecules. Among them, peroxynitrite and hypochlorous acid are not only oxidants but also biological nitrating and chlorinating agents, leading to the formation of 3-nitrotyrosine and 3-chlorotyrosine, respectively, in proteins. 3-Nitrotyrosine has been detected in vivo under several pathophysiological conditions, including breast cancer. Studies show that the concentrations of 3-nitrotyrosine in plasma proteins and platelets were significantly elevated in breast cancer patients. Compared to blood serum albumin, hemoglobin adducts represent biomonitoring of exposure with a longer lifetime. In this study, human hemoglobin was freshly isolated from blood and digested into peptides with trypsin, and the levels of protein adducts, including nitration, nitrosylation, and chlorination of tyrosine as well as oxidation of methionine residues, were simultaneously quantified by nanoflow liquid chromatography nanoelectrospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS) with selected reaction monitoring. The results demonstrated that the relative extents of nitration at α-Tyr-42 and β-Tyr-130, nitrosylation at α-Tyr-24, and chlorination at α-Tyr-24 and β-Tyr-130 are significantly higher in globin of 25 breast cancer patients compared to those in 25 healthy subjects (p < 0.05). In particular, nitration at α-Tyr-42 and chlorination at α-Tyr-24 showed the area under the receiver operating characteristic curve of >0.8. While the age of the subjects is correlated with the extents of some of these adducts, the body mass index does not have an effect on any of them. Starting with 1 drop of blood, our results indicated that this highly sensitive and specific nanoLC-NSI/MS/MS is useful in investigating the role of reactive nitrogen oxide species and reactive chlorine species in the etiology of breast cancer.

Electrochemistry at Bimetallic Pd/Au Thin Film Surfaces for Selective Detection of Reactive Oxygen Species and Reactive Nitrogen Species

In this work, we designed and fabricated Pd/Au bimetallic thin film electrodes with isolated Pd nanoparticles via underpotential deposition of copper on a gold substrate followed by in situ redox replace reaction in a Pd salt solution. The Pd/Au electrode was characterized by AFM and XPS as well as multiple electrochemical techniques including CV and electrochemical quartz crystal microbalance (EQCM) in sulfuric acid and phosphate buffer electrolytes. Results show that the reduction reactions of the analytes (i.e., H₂O₂ and 3-nitrotyrosine (3-NT)) at the Pd/Au thin film surfaces affect the nature and reactivity of Pd/Au surface electrochemistry including the adsorbed/absorbed hydrogen and/or the premonolayer palladium oxide redox processes at Pd. The EQCM experiment supports the arrangement of small size Pd nanoparticles in the Pd thin film in the presence of gold exhibits unusual properties, acting as a new physicochemical dimension between the electrode and target H₂O₂ and 3-NT molecules. The Pd/Au thin film was demonstrated as an extremely sensitive and selective probe for detection of common ROS and RNS (i.e., H₂O₂ and 3-NT). The integration of two different metallic species, Pd and Au, into a surface structure on nanoscale by exploiting their unique surface electrochemistry establishes an innovative analytical method for highly sensitive and selective detection of H₂O₂ and 3-NT simultaneously. This method has a general scope for detecting a broad range of redox active and nonredox active species simultaneously, which opens up new opportunities to develop new electrocatalytic materials and innovative sensing approaches.

Age-Associated Methylation in Human Hemoglobin and Its Stability on Dried Blood Spots As Analyzed by Nanoflow Liquid Chromatography Tandem Mass Spectrometry

Methylation of biomolecules is involved in many important biological processes. The contributing methylating agents arise from endogenous and exogenous sources (such as cigarette smoking). Human hemoglobin is easily accessible from blood and has been used as a molecular dosimeter for monitoring chemical exposure. We recently developed a method for characterization and quantification of the extents of methylation and ethylation in hemoglobin by nanoflow liquid chromatography tandem mass spectrometry under the selected reaction monitoring mode. Using this method, the relative extents of methylated and ethylated peptides in hemoglobin were quantified in nonsmoking subjects at various ages in this study. Among the nine methylation sites, we found that the extents of methylation were significantly higher in elderly subjects at the N-terminal and His-20 of α-globin, and at the N-terminal and Glu-26 of β-globin. Moreover, the extents of methylation at these sites were significantly correlated with the age of the subjects. On the other hand, no statistically significant difference was found in the ethylated peptides. We also examined the stability of methylated and ethylated hemoglobin when stored on dried blood spot cards. The extents of these modifications on hemoglobin are stable for at least 4 weeks stored at room temperature. Our results suggest that age should be considered as a factor when measuring hemoglobin methylation and that dried blood spot is a valuable biomonitoring technique for hemoglobin modifications in epidemiological studies.

Nitration of tyrosine and its effect on DNA hybridization

One major marker of nitrosative stress is the formation of 3-Nitrotyrosine (3-NT) from Tyrosine (Tyr) by adding a nitro group (-NO₂) with nitrating agents. Nitration of Tyr often causes loss of protein activity and is linked with many diseases. In this article, we detect 3-NT and discriminate it from Tyr with Differential Pulse Voltammetry (DPV) as it is a very important biomarker. We first examined redox (oxidation/reduction) properties and stability of 3-NT in detail. Second, we provided the Tyr and 3-NT discrimination with DPV and compared with the chromatography. We then explored the interaction of 3-NT and DNA oligonucleotides. Our findings demonstrate that 3-NT can be used as a new electrochemical indicator, which is able to detect hybridization of probe (single stranded DNA-ssDNA) and hybrid (double stranded DNA-dsDNA) both via 3-NT reduction and guanine oxidation signal changes at the same time. The signal differences enabled us to distinguish ssDNA and dsDNA without using a label or a tag. Moreover, we achieved to detect hybridization of DNA by using the reduction signal of 3-NT obtained at -0.4V vs. Ag/AgCl. More importantly, we observed the changes of the reduction signals of 3-NT after the interaction of probe and hybrid sequences. We showed that 3-NT signal decreases more with hybrid than the probe. Our platform, for the first time, demonstrates the detection of hybridization both guanine oxidation and indicator reduction signal changes at the same time. Moreover, we, for the first time, demonstrated the interaction between 3-NT and DNA.

Analysis of Chlorination, Nitration, and Nitrosylation of Tyrosine and Oxidation of Methionine and Cysteine in Hemoglobin from Type 2 Diabetes Mellitus Patients by Nanoflow Liquid Chromatography Tandem Mass Spectrometry

The post-translational modification (PTM) of proteins by endogenous reactive chlorine, nitrogen, and oxygen species is implicated in certain pathological conditions, including diabetes mellitus. Evidence showed that the extents of modifications on a number of proteins are elevated in diabetic patients. Measuring modification on hemoglobin has been used to monitor the extent of exposure. This study develops an assay for simultaneous quantification of the extent of chlorination, nitration, and oxidation in human hemoglobin and to examine whether the level of any of these modifications is higher in poorly controlled type 2 diabetic mellitus patients. This mass spectrometry-based assay used the bottom-up proteomic strategy. Due to the low amount of endogenous modification, we first characterized the sites of chlorination at tyrosine in hypochlorous acid-treated hemoglobin by an accurate mass spectrometer. The extents of chlorination, nitration, and oxidation of a total of 12 sites and types of modifications in hemoglobin were measured by nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry under the selected reaction monitoring mode. Relative quantification of these PTMs in hemoglobin extracted from blood samples shows that the extents of chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in diabetic patients (n = 19) than in nondiabetic individuals (n = 18). After excluding the factor of smoking, chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in the nonsmoking diabetic patients (n = 12) than in normal nonsmoking subjects (n = 11). Multiple regression analysis performed on the combined effect of age, body-mass index (BMI), and HbA1c showed that the diabetes factor HbA1c contributes significantly to the extent of chlorination at α-Tyr-24 in nonsmokers. In addition, age contributes to oxidation at α-Met-32 significantly in all subjects and in nonsmokers. These results suggest the potential of using chlorination at α-Tyr-24-containing peptide to evaluate protein damage in nonsmoking type 2 diabetes mellitus.

Mass spectrometry and 3-nitrotyrosine: strategies, controversies, and our current perspective

Reactive-nitrogen species (RNS) such as peroxynitrite (ONOO(-)), that is, the reaction product of nitric oxide ((•)NO) and superoxide (O2(-•)), nitryl chloride (NO2Cl) and (•)NO2 react with the activated aromatic ring of tyrosine to form 3-nitrotyrosine. This modification, which has been known for more than a century, occurs to both the free form of the amino acid (i.e., soluble/free tyrosine) and to tyrosine residues covalently bound within the backbone of peptides and proteins. Nitration of tyrosine is thought to be of biological significance and has been linked to health and disease, but determining its role has proved challenging. Several key questions have been the focus of much of the research activity: (a) to what extent is free/soluble tyrosine nitrated in biological tissues and fluids, and (b) are there specific site(s) of nitration within peptides/proteins and to what extent (i.e., stoichiometry) does this modification occur? These issues have been addressed in a wide range of sample types (e.g., blood, urine, CSF, exhaled breath condensate and various tissues) and a diverse array of physiological/pathophysiological scenarios. The accurate determination of nitrated tyrosine is, however, a stumbling block. Despite extensive study, the extent to which nitration occurs in vivo, the specificity of the nitration reaction, and its importance in health and disease, remain unclear. In this review, we highlight the analytical challenges and discuss the approaches adopted to address them. Mass spectrometry, in combination with either gas chromatography (GC-MS, GC-MS/MS) or liquid chromatography (LC-MS/MS), has played the central role in the analysis of 3-nitrotyrosine and tyrosine-nitrated biological macromolecules. We discuss its unique attributes and highlight the role of stable-isotope labeled 3-nitrotyrosine analogs in both accurate quantification, and in helping to define the biological relevance of tyrosine nitration. We show that the application of sophisticated mass spectrometric techniques is advantageous if not essential, but that this alone is by no means a guarantee of accurate findings. We discuss the important analytical challenges in quantifying 3-nitrotyrosine, possible workarounds, and we attempt to make sense of the disparate findings that have been reported so far.

Functional roles of protein nitration in acute and chronic liver diseases

Nitric oxide, when combined with superoxide, produces peroxynitrite, which is known to be an important mediator for a number of diseases including various liver diseases. Peroxynitrite can modify tyrosine residue(s) of many proteins resulting in protein nitration, which may alter structure and function of each target protein. Various proteomics and immunological methods including mass spectrometry combined with both high pressure liquid chromatography and 2D PAGE have been employed to identify and characterize nitrated proteins from pathological tissue samples to determine their roles. However, these methods contain a few technical problems such as low efficiencies with the detection of a limited number of nitrated proteins and labor intensiveness. Therefore, a systematic approach to efficiently identify nitrated proteins and characterize their functional roles is likely to shed new insights into understanding of the mechanisms of hepatic disease pathophysiology and subsequent development of new therapeutics. The aims of this review are to briefly describe the mechanisms of hepatic diseases. In addition, we specifically describe a systematic approach to efficiently identify nitrated proteins to study their causal roles or functional consequences in promoting acute and chronic liver diseases including alcoholic and nonalcoholic fatty liver diseases. We finally discuss translational research applications by analyzing nitrated proteins in evaluating the efficacies of potentially beneficial agents to prevent or treat various diseases in the liver and other tissues.

Oxidative stress-induced posttranslational modifications of human hemoglobin in erythrocytes

Posttranslational modifications (PTMs) have been reported in hemoglobin (Hb) treated with ROS/RNS in cell-free experiments. However, little is known about oxidative PTMs of Hb occurring within the erythrocytes. The aim of this study is to characterize the patterns of Hb PTMs in erythrocytes under oxidative stress. Using mass spectrometry, we investigated specifically methionine/tryptophan oxidation, tyrosine nitration, and the modification via 4-hydroxynonenal (HNE), a product of lipid-peroxidation, on Hb. We demonstrated that the treatment with H(2)O(2)/nitrite induced higher levels of Hb oxidation/nitration in purified Hb preparations than in unpurified hemolysates and erythrocytes, indicating that ROS/RNS are primarily removed by antioxidative mechanisms. We further studied Hb from erythrocytes exposed to γ-irradiation. An irradiation of 30-100 Gy triggered a remarkable increase of intracellular ROS. However, 30 Gy did not induce apparent changes in Hb oxidation/nitration and hemolysis, while Hb oxidation/nitration and hemolysis were significantly enhanced by 100 Gy, suggesting that Hb oxidation/nitration are the consequence of overwhelmed antioxidative mechanisms after oxidative attack and reflect the severity of the oxidative damage of erythrocytes. Although irradiation was known to induce lipid-peroxidation, we could not detect HNE-Hb adducts in irradiated erythrocytes. Analyzing PTM patterns suggests Hb nitration as a more suitable indicator of the oxidative damage of erythrocytes.

Reactive nitrogen oxide species-induced post-translational modifications in human hemoglobin and the association with cigarette smoking

Nitric oxide (NO) is essential for normal physiology, but excessive production of NO during inflammatory processes can damage the neighboring tissues. Reactive nitrogen oxide species (RNOx), including peroxynitrite (ONOO(-)), are powerful nitrating agents. Biological protein nitration is involved in several disease states, including inflammatory diseases, and it is evident by detection of 3-nitrotyrosine (3NT) in inflamed tissues. In this study, we identified peroxynitrite-induced post-translational modifications (PTMs) in human hemoglobin by accurate mass measurement as well as by the MS(2) and MS(3) spectra. Nitration on Tyr-24, Tyr-42 (α-globin), and Tyr-130 (β-globin) as well as nitrosation on Tyr-24 (α-globin) were identified. Also characterized were oxidation of all three methionine residues, α-Met-32, α-Met-76, and β-Met-55 to the sulfoxide, as well as cysteine oxidation determined as sulfinic acid on α-Cys-104 and sulfonic acid on α-Cys-104, β-Cys-93, and β-Cys-112. These modifications are detected in hemoglobin freshly isolated from human blood and the extents of modifications were semiquantified relative to the reference peptides by nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS) under the selected reaction monitoring (SRM) mode. The results showed a statistically significant positive correlation between cigarette smoking and the extents of tyrosine nitration at α-Tyr-24 and at α-Tyr-42. To our knowledge, this is the first report on identification and quantification of multiple PTMs in hemoglobin from human blood and association of a specific 3NT-containing peptide with cigarette smoking. This highly sensitive and specific assay only requires hemoglobin isolated from one drop (∼10 μL) of blood. Thus, measurement of these PTMs in hemoglobin might be feasible for assessing nitrative stress in vivo.

Iron overload-induced rat liver injury: Involvement of protein tyrosine nitration and the effect of baicalin

Baicalin has been reported to protect against liver injury in iron-overload mice, however, the mechanisms underlying the hepatoprotective properties of baicalin are poorly understood. In this study, we systematically studied the protective effect of baicalin on iron overload induced liver injury, as well as the underlying mechanism based on nitrative stress in rat model. We found that when iron overload rats (500mgiron/kg) were fed baicalin-containing diet (0.3% and 1% w/w) for 45days, baicalin dose dependently protected against iron overload induced liver injury, including alleviation of hepatic pathological damage, decrease of SOD activity, iron content, carbonyl content, and the thiobarbituric acid-reactive substances level in hepatic tissues. It also increased serum iron content, SH content and GPx activity, decreased serum ALT and AST activities. Immunohistochemistry and immunoprecipitation analysis revealed that baicalin could also inhibit iron overload induced protein tyrosine nitration in liver. Moreover, in iron overload rat liver, we found that baicalin decreased the iron overload increased level of glutathione-S-transferases (GSTs) expression, oxidation and nitration. These results suggest that not only oxidative stress, but also nitrative stress, is involved in iron overload induced liver injury, and the underlying mechanism might partially relate to the involvement of GSTs expression and post-translational modification. Baicalin can effectively prevent iron overload caused abnormality and can be a candidate medicine for iron overload diseases.

Anti-stress effects of Glycine tomentella Hayata in tilapia: inhibiting COX-2 expression and enhancing EPA synthesis in erythrocyte membrane and fish growth

The objective of this study was to elucidate the in vivo effects of the ethanol extract of wooly Glycine tomentella Hayata (GTE) root on tilapia to elucidate whether GTE has antistress activity. Tilapia as an animal model were fed with or without GTE, then injected with lipopolysaccharide (LPS) or ammonium chloride (NH(4)Cl). The tilapia were exposed to 100 mg/L of aqueous NH(4)Cl, and/or acute cold stress. Growth parameters of the tilapia were measured during the feeding trials. Tilapia injected with GTE (20 μg/g of fish), NH(4)Cl (100 μg/g of fish) and/or LPS (1 μg/g of fish) were then sampled 2 h poststimulation. GTE significantly inhibited cyclooxygenase-2 expression and hemoglobin (Hb) dimer formation (36 kDa). GTE also improved growth and blood viscosity and upregulated eicosapentaenoic acid content of erythrocytes. The in vivo results indicated that GTE (20 μg/g of fish) can be applied as a stress-tolerance enhancing agent for the aquaculture industry.

In vivo protein tyrosine nitration in Arabidopsis thaliana

Nitration of tyrosine (Y) residues of proteins is a low abundant post-translational modification that modulates protein function or fate in animal systems. However, very little is known about the in vivo prevalence of this modification and its corresponding targets in plants. Immunoprecipitation, based on an anti-3-nitroY antibody, was performed to pull-down potential in vivo targets of Y nitration in the Arabidopsis thaliana proteome. Further shotgun liquid chromatography-mass spectrometry (LC-MS/MS) proteomic analysis of the immunoprecipitated proteins allowed the identification of 127 proteins. Around 35% of them corresponded to homologues of proteins that have been previously reported to be Y nitrated in other non-plant organisms. Some of the putative in vivo Y-nitrated proteins were further confirmed by western blot with specific antibodies. Furthermore, MALDI-TOF (matrix-assisted laser desorption ionization-time of flight) analysis of protein spots, separated by two-dimensional electrophoresis from immunoprecipitated proteins, led to the identification of seven nitrated peptides corresponding to six different proteins. However, in vivo nitration sites among putative targets could not be identified by MS/MS. Nevertheless, an MS/MS spectrum with 3-aminoY318 instead of the expected 3-nitroY was found for cytosolic glyceraldehyde-3-phosphate dehydrogenase. Reduction of nitroY to aminoY during MS-based proteomic analysis together with the in vivo low abundance of these modifications made the identification of nitration sites difficult. In turn, in vitro nitration of methionine synthase, which was also found in the shotgun proteomic screening, allowed unequivocal identification of a nitration site at Y287.

Mass spectra and ion collision cross sections of hemoglobin

Mass spectra of commercially obtained hemoglobin (Hb) show higher levels of monomer and dimer ions, heme-deficient dimer ions, and apo-monomer ions than hemoglobin freshly prepared from blood. This has previously been attributed to oxidation of commercial Hb. Further, it has been reported that that dimer ions from commercial bovine Hb have lower collision cross sections than low charge state monomer ions. To investigate these effects further, we have recorded mass spectra of fresh human Hb, commercial human and bovine Hb, fresh human Hb oxidized with H(2)O(2), lyophilized fresh human Hb, fresh human Hb both lyophilized and chemically oxidized, and commercial human Hb oxidized with H(2)O(2). Masses of α-monomer ions of all hemoglobins agree with the masses expected from the sequences within 3 Da or better. Mass spectra of the β chains of commercial Hb and oxidized fresh human Hb show a peak or shoulder on the high mass side, consistent with oxidation of the protein. Both commercial proteins and oxidized fresh human Hb produce heme-deficient dimers with masses 32 Da greater than expected and higher levels of monomer and dimer ions than fresh Hb. Lyophilization or oxidation of Hb both produce higher levels of monomer and dimer ions in mass spectra. Fresh human Hb, commercial human Hb, commercial bovine Hb, and oxidized commercial human Hb all give dimer ions with cross sections greater than monomer ions. Thus, neither oxidation of Hb or the difference in sequence between human and bovine Hb make substantial differences to cross sections of ions.

Redox signaling (cross-talk) from and to mitochondria involves mitochondrial pores and reactive oxygen species

This review highlights the important role of redox signaling between mitochondria and NADPH oxidases. Besides the definition and general importance of redox signaling, the cross-talk between mitochondrial and Nox-derived reactive oxygen species (ROS) is discussed on the basis of 4 different examples. In the first model, angiotensin-II is discussed as a trigger for NADPH oxidase activation with subsequent ROS-dependent opening of mitochondrial ATP-sensitive potassium channels leading to depolarization of mitochondrial membrane potential followed by mitochondrial ROS formation and respiratory dysfunction. This concept was supported by observations that ethidium bromide-induced mitochondrial damage suppressed angiotensin-II-dependent increase in Nox1 and oxidative stress. In another example hypoxia was used as a stimulator of mitochondrial ROS formation and by using pharmacological and genetic inhibitors, a role of mitochondrial ROS for the induction of NADPH oxidase via PKCvarepsilon was demonstrated. The third model was based on cell death by serum withdrawal that promotes the production of ROS in human 293T cells by stimulating both the mitochondria and Nox1. By superior molecular biological methods the authors showed that mitochondria were responsible for the fast onset of ROS formation followed by a slower but long-lasting oxidative stress condition based on the activation of an NADPH oxidase (Nox1) in response to the fast mitochondrial ROS formation. Finally, a cross-talk between mitochondria and NADPH oxidases (Nox2) was shown in nitroglycerin-induced tolerance involving the mitochondrial permeability transition pore and ATP-sensitive potassium channels. The use of these redox signaling pathways as pharmacological targets is briefly discussed.

Protein tyrosine nitration: selectivity, physicochemical and biological consequences, denitration, and proteomics methods for the identification of tyrosine-nitrated proteins

Protein tyrosine nitration (PTN) is a post-translational modification occurring under the action of a nitrating agent. Tyrosine is modified in the 3-position of the phenolic ring through the addition of a nitro group (NO2). In the present article, we review the main nitration reactions and elucidate why nitration is not a random chemical process. The particular physical and chemical properties of 3-nitrotyrosine (e.g., pKa, spectrophotometric properties, reduction to aminotyrosine) will be discussed, and the biological consequences of PTN (e.g., modification of enzymatic activity, sensitivity to proteolytic degradation, impact on protein phosphorylation, immunogenicity and implication in disease) will be reviewed. Recent data indicate the possibility of an in vivo denitration process, which will be discussed with respect to the different reaction mechanisms that have been proposed. The second part of this review article focuses on analytical methods to determine this post-translational modification in complex proteomes, which remains a major challenge.