Requirements for heme and thiols for the nonenzymatic modification of nitrotyrosine

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.

iNOS is necessary for GBP-mediated T. gondii clearance in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular parasite of rodents and humans. Interferon-inducible guanylate binding proteins (GBPs) are mediators of T. gondii clearance, however, this mechanism is incomplete. Here, using automated spatially targeted optical micro proteomics we demonstrate that inducible nitric oxide synthetase (iNOS) is highly enriched at GBP2+ parasitophorous vacuoles (PV) in murine macrophages. iNOS expression in macrophages is necessary to limit T. gondii load in vivo and in vitro. Although iNOS activity is dispensable for GBP2 recruitment and PV membrane ruffling; parasites can replicate, egress and shed GBP2 when iNOS is inhibited. T. gondii clearance by iNOS requires nitric oxide, leading to nitration of the PV and collapse of the intravacuolar network of membranes in a chromosome 3 GBP-dependent manner. We conclude that reactive nitrogen species generated by iNOS cooperate with GBPs to target distinct structures in the PV that are necessary for optimal parasite clearance in macrophages.

Unveiling the human nitroproteome: Protein tyrosine nitration in cell signaling and cancer

Covalent amino acid modification significantly expands protein functional capability in regulating biological processes. Tyrosine residues can undergo phosphorylation, sulfation, adenylation, halogenation, and nitration. These posttranslational modifications (PTMs) result from the actions of specific enzymes: tyrosine kinases, tyrosyl-protein sulfotransferase(s), adenylate transferase(s), oxidoreductases, peroxidases, and metal-heme containing proteins. Whereas phosphorylation, sulfation, and adenylation modify the hydroxyl group of tyrosine, tyrosine halogenation and nitration target the adjacent carbon residues. Because aberrant tyrosine nitration has been associated with human disorders and with animal models of disease, we have created an updated and curated database of 908 human nitrated proteins. We have also analyzed this new resource to provide insight into the role of tyrosine nitration in cancer biology, an area that has not previously been considered in detail. Unexpectedly, we have found that 879 of the 1971 known sites of tyrosine nitration are also sites of phosphorylation suggesting an extensive role for nitration in cell signaling. Overall, the review offers several forward-looking opportunities for future research and new perspectives for understanding the role of tyrosine nitration in cancer biology.

Inducible nitric oxide synthase (iNOS) is necessary for GBP-mediated T. gondii restriction in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular, protozoan pathogen of rodents and humans. T. gondii's ability to grow within cells and evade cell-autonomous immunity depends on the integrity of the parasitophorous vacuole (PV). Interferon-inducible guanylate binding proteins (GBPs) are central mediators of T. gondii clearance, however, the precise mechanism linking GBP recruitment to the PV and T. gondii restriction is not clear. This knowledge gap is linked to heterogenous GBP-targeting across a population of vacuoles and the lack of tools to selectively purify the intact PV. To identify mediators of parasite clearance associated with GBP2-positive vacuoles, we employed a novel protein discovery tool automated spatially targeted optical micro proteomics (autoSTOMP). This approach identified inducible nitric oxide synthetase (iNOS) enriched at levels similar to the GBPs in infected bone marrow-derived myeloid cells. iNOS expression on myeloid cells was necessary for mice to control T. gondii growth in vivo and survive acute infection. T. gondii infection of IFNγ-primed macrophage was sufficient to robustly induce iNOS expression. iNOS restricted T. gondii infection through nitric oxide synthesis rather than arginine depletion, leading to robust and selective nitration of the PV. Optimal parasite restriction by iNOS and vacuole nitration depended on the chromosome 3 GBPs. Notably, GBP2 recruitment and ruffling of the PV membrane occurred in iNOS knockouts, however, these vacuoles contained dividing parasites. iNOS activity was necessary for the collapse of the intravacuolar network of nanotubular membranes which connects parasites to each other and the host cytosol. Based on these data we conclude reactive nitrogen species generated by iNOS cooperate with the chromosome 3 GBPs to target distinct biology of the PV that are necessary for optimal parasite clearance in murine myeloid cells.

Modeling oxidative injury response in human kidney organoids

Background

Hemolysis occurs in many injury settings and can trigger disease processes. In the kidney, extracellular hemoglobin can induce damage via several mechanisms. These include oxidative stress, mitochondrial dysfunction, and inflammation, which promote fibrosis and chronic kidney disease. Understanding the pathophysiology of these injury pathways offers opportunities to develop new therapeutic strategies.

Methods

To model hemolysis-induced kidney injury, human kidney organoids were treated with hemin, an iron-containing porphyrin, that generates reactive oxygen species. In addition, we developed an induced pluripotent stem cell line expressing the biosensor, CytochromeC-GFP (CytoC-GFP), which provides a real-time readout of mitochondrial morphology, health, and early apoptotic events.

Results

We found that hemin-treated kidney organoids show oxidative damage, increased expression of injury markers, impaired functionality of organic anion and cation transport and undergo fibrosis. Injury could be detected in live CytoC-GFP organoids by cytoplasmic localization of fluorescence. Finally, we show that 4-(phenylthio)butanoic acid, an HDAC inhibitor with anti-fibrotic effects in vivo, reduces hemin-induced human kidney organoid fibrosis.

Conclusion

This work establishes a hemin-induced model of kidney organoid injury. This platform provides a new tool to study the injury and repair response pathways in human kidney tissue and will assist in the development of new therapeutics.

Reductive modification of genetically encoded 3-nitrotyrosine sites in alpha synuclein expressed in E.coli

Tyrosine nitration is a post-translational protein modification relevant to various pathophysiological processes. Chemical nitration procedures have been used to generate and study nitrated proteins, but these methods regularly lead to modifications at other amino acid residues. A novel strategy employs a genetic code modification that allows incorporation of 3-nitrotyrosine (3-NT) during ribosomal protein synthesis to generate a recombinant protein with defined 3-NT-sites, in the absence of other post-translational modifications. This approach was applied to study the generation and stability of the 3-NT moiety in recombinant proteins produced in E.coli. Nitrated alpha-synuclein (ASYN) was selected as exemplary protein, relevant in Parkinson's disease (PD). A procedure was established to obtain pure tyrosine-modified ASYN in mg amounts. However, a rapid (t_1/2 = 0.4 h) reduction of 3-NT to 3-aminotyrosine (3-AT) was observed. When screening for potential mechanisms, we found that 3-NT can be reduced enzymatically to 3-AT, whilst biologically relevant low molecular weight reductants, such as NADPH or GSH, did not affect 3-NT. A genetic screen for E.coli proteins, involved in the observed 3-NT reduction, revealed the contribution of several, possibly redundant pathways. Green fluorescent protein was studied as an alternative model protein. These data confirm 3-NT reduction as a broadly-relevant pathway in E.coli. In conclusion, incorporation of 3-NT as a genetically-encoded non-natural amino acid allows for generation of recombinant proteins with specific nitration sites. The potential reduction of the 3-NT moiety by E.coli, however, requires attention to the design of the purification strategy for obtaining pure nitrated protein.

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3-nitrotyrosine in proteins is reduced to 3-aminotyrosine in E.coli.

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3-aminotyrosine is stable in E.coli.

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3-nitrotyrosine reduction is independent of the harboring protein but affected by protein structure.

ANSID: A Solid-Phase Proteomic Approach for Identification and Relative Quantification of Aromatic Nitration Sites

Nitration of tyrosine and other aromatic amino acid residues in proteins occurs in the setting of inflammatory, neurodegenerative, and cardiovascular diseases—importantly, this modification has been implicated in the pathogenesis of diverse diseases and the physiological process of aging. To understand the biological consequences of aromatic nitration in both health and disease, it is critical to molecularly identify the proteins that undergo nitration, specify their cognate modification sites and quantify their extent of nitration. To date, unbiased identification of nitrated proteins has often involved painstaking 2D-gel electrophoresis followed by Western Blotting with an anti-nitrotyrosine antibody for detection. Apart from being relatively slow and laborious, this method suffers from limited coverage, the potential for false-positive identifications, and failure to reveal specific amino acid modification sites. To overcome these shortcomings, we have developed a solid-phase, chemical-capture approach for unbiased and high-throughput discovery of nitrotyrosine and nitrotryptophan sites in proteins. Utilizing this method, we have successfully identified several endogenously nitrated proteins in rat brain and a total of 244 nitrated peptides from 145 proteins following in vitro exposure of rat brain homogenates to the nitrating agent peroxynitrite (1 mM). As expected, Tyr residues constituted the great majority of peroxynitrite-mediated protein nitration sites; however, we were surprised to discover several brain proteins that contain nitrated Trp residues. By incorporating a stable-isotope labeling step, this new Aromatic Nitration Site IDentification (ANSID) method was also adapted for relative quantification of nitration site abundances in proteins. Application of the ANSID method offers great potential to advance our understanding of the role of protein nitration in disease pathogenesis and normal physiology.

Nitric oxide mediates glial-induced neurodegeneration in Alexander disease

Glia play critical roles in maintaining the structure and function of the nervous system; however, the specific contribution that astroglia make to neurodegeneration in human disease states remains largely undefined. Here we use Alexander disease, a serious degenerative neurological disorder caused by astrocyte dysfunction, to identify glial-derived NO as a signalling molecule triggering astrocyte-mediated neuronal degeneration. We further find that NO acts through cGMP signalling in neurons to promote cell death. Glial cells themselves also degenerate, via the DNA damage response and p53. Our findings thus define a specific mechanism for glial-induced non-cell autonomous neuronal cell death, and identify a potential therapeutic target for reducing cellular toxicity in Alexander disease, and possibly other neurodegenerative disorders with glial dysfunction.

Alexander disease is a rare neurological disorder caused by mutations in GFAP, yet it is unclear how glial disruptions lead to neural death. Here, Wang et al. identify a mechanism by which glial-derived nitric oxide leads to neuronal degeneration in fly and mouse models of the disease.

Leghemoglobin is nitrated in functional legume nodules in a tyrosine residue within the heme cavity by a nitrite/peroxide-dependent mechanism

Protein tyrosine (Tyr) nitration is a post-translational modification yielding 3-nitrotyrosine (NO2 -Tyr). Formation of NO2 -Tyr is generally considered as a marker of nitro-oxidative stress and is involved in some human pathophysiological disorders, but has been poorly studied in plants. Leghemoglobin (Lb) is an abundant hemeprotein of legume nodules that plays an essential role as an O2 transporter. Liquid chromatography coupled to tandem mass spectrometry was used for a targeted search and quantification of NO2 -Tyr in Lb. For all Lbs examined, Tyr30, located in the distal heme pocket, is the major target of nitration. Lower amounts were found for NO2 -Tyr25 and NO2 -Tyr133. Nitrated Lb and other as yet unidentified nitrated proteins were also detected in nodules of plants not receiving NO3- and were found to decrease during senescence. This demonstrates formation of nitric oxide (˙NO) and NO2- by alternative means to nitrate reductase, probably via a ˙NO synthase-like enzyme, and strongly suggests that nitrated proteins perform biological functions and are not merely metabolic byproducts. In vitro assays with purified Lb revealed that Tyr nitration requires NO2- + H2 O2 and that peroxynitrite is not an efficient inducer of nitration, probably because Lb isomerizes it to NO3-. Nitrated Lb is formed via oxoferryl Lb, which generates nitrogen dioxide and tyrosyl radicals. This mechanism is distinctly different from that involved in heme nitration. Formation of NO2 -Tyr in Lb is a consequence of active metabolism in functional nodules, where Lb may act as a sink of toxic peroxynitrite and may play a protective role in the symbiosis.

Proteomic approaches to analyze protein tyrosine nitration

SIGNIFICANCE

The conversion of protein-bound Tyr residues to 3-nitrotyrosine (3NY) can occur during nitrative stress and has been correlated to aging and many disease states. Proteomic analysis of this post-translational modification, using mass spectrometry-based techniques, is crucial for understanding its potential role in pathological and physiological processes.

RECENT ADVANCES

To overcome some of the disadvantages inherent to well-established nitroproteomic methods using anti-3NY antibodies and gel-based separations, methods involving multidimensional chromatography, precursor ion scanning, and/or chemical derivatization have emerged for both identification and quantitation of protein nitration sites. A few of these methods have successfully detected endogenous 3NY modifications from biological samples.

CRITICAL ISSUES

While model systems often show promising results, identification of endogenous 3NY modifications remains largely elusive. The frequently low abundance of nitrated proteins in vivo, even under inflammatory conditions, is especially challenging, and sample loss due to derivatization and cleaning may become significant.

FUTURE DIRECTIONS

Continued efforts to avoid interference from non-nitrated peptides without sacrificing recovery of nitrated peptides are needed. Quantitative methods are emerging and are crucial for identifying endogenous modifications that may have significant biological impacts.

Characterization of a cellular denitrase activity that reverses nitration of cyclooxygenase

Protein 3-nitrotyrosine (3-NT) formation is frequently regarded as a simple biomarker of disease, an irreversible posttranslational modification that can disrupt protein structure and function. Nevertheless, evidence that protein 3-NT modifications may be site selective and reversible, thus allowing for physiological regulation of protein activity, has begun to emerge. We have previously reported that cyclooxygenase (COX)-1 undergoes heme-dependent nitration of Tyr(385), an internal and catalytically essential residue. In the present study, we demonstrate that nitrated COX-1 undergoes a rapid reversal of nitration by substrate-selective and biologically regulated denitrase activity. Using nitrated COX-1 as a substrate, denitrase activity was validated and quantified by analytic HPLC with electrochemical detection and determined to be constitutively active in murine and human endothelial cells, macrophages, and a variety of tissue samples. Smooth muscle cells, however, contained little denitrase activity. Further characterizing this denitrase activity, we found that it was inhibited by free 3-NT and may be enhanced by endogenous nitric oxide and exogenously administered carbon monoxide. Finally, we describe a purification protocol that results in significant enrichment of a discrete denitrase-containing fraction, which maintains activity throughout the purification process. These findings reveal that nitrated COX-1 is a substrate for a denitrase in cells and tissues, implying that the reciprocal processes of nitration and denitration may modulate bioactive lipid synthesis in the setting of inflammation. In addition, our data reveal that denitration is a controlled process that may have broad importance for regulating cell signaling events in nitric oxide-generating systems during oxidative/nitrosative stress.

Proteomic analysis of endogenous nitrotryptophan-containing proteins in rat hippocampus and cerebellum

Nitration of tryptophan residues is a novel post-translational modification. In the present study, we examined whether NO2Trp (nitrotryptophan)-containing proteins are produced in the hippocampus and cerebellum of the adult rat under physiological conditions in vivo. Using Western blot analysis with anti-6-NO2Trp-specific antibody, we found many similar immunoreactive spots in the protein extracts from both regions. These spots were subsequently subjected to trypsin digestion and LC-ESI-MS/MS (LC-electrospray ionization-tandem MS) analysis. We identified several cytoskeletal proteins and glycolytic enzymes as NO2Trp-containing proteins and determined the position of nitrated tryptophan residues with significant ion score levels (P<0.05) in several proteins in both regions. We also observed that the total amount of NO2Trp-containing proteins in the cerebellum was significantly greater than that in the hippocampus (P<0.05). Moreover, IP (immunoprecipitation) assays using anti-aldolase C antibody showed that the relative intensity of immunostaining for NO2Trp over aldolase C was much higher in cerebellum than in hippocampus. The amounts of nNOS (neuronal nitric oxide synthase) and eNOS (endothelial nitric oxide synthase) were much greater in cerebellum than in hippocampus. This is the first evidence of several specific sites of nitrated tryptophan in proteins under physiological conditions in vivo.

Conversion of 3-nitrotyrosine to 3-aminotyrosine residues facilitates mapping of tyrosine nitration in proteins by electrospray ionization-tandem mass spectrometry using electron capture dissociation

Protein tyrosine nitration is associated with oxidative stress and various human diseases. Tandem mass spectrometry has been the method of choice for the identification and localization of this posttranslational modification to understand the underlying mechanisms and functional consequences. Due to the electron predator effect of the nitro group limiting fragmentation of the peptide backbone, electron-based dissociation has not been applicable, however, to nitrotyrosine-containing peptides. A straightforward conversion of the nitrotyrosine to the aminotyrosine residues is introduced to address this limitation. When tested with nitrated ubiquitin and human serum albumin as model proteins in top-down and bottom-up approaches, respectively, this chemical derivatization enhanced backbone fragmentation of the corresponding nitroproteins and nitropeptides by electron capture dissociation (ECD). Increased sequence coverage has been obtained by combining in the bottom-up strategy the conversion of nitrotyrosine to aminotyrosine and introducing, in addition to trypsin, a further digesting enzyme of complementary specificity, when protein nitration was mapped by liquid chromatography-electrospray ionization tandem mass spectrometry using both collision-induced dissociation (CID) and ECD.

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.

Proteins from Avastin® (bevacizumab) show tyrosine nitrations for which the consequences are completely unclear

Avastin® (bevacizumab) is a protein drug widely used for cancer treatment although its further use is questionable due to serious side effects reported. As no systematic proteomic study on posttranslational modifications (PTMs) was reported so far, it was the aim of the current study to use a gel-based proteomics method for determination of Avastin®-protein(s).

Avastin® was run on two-dimensional gel electrophoresis (2-DE), spots were picked, followed by multi-enzyme in-gel digestion. Subsequently, the resulting peptides and posttranslational modifications were identified by mass spectrometry (nano-LC-ESI-MS/MS; HCT and LTQ Orbitrap MS). Heavy and light chains were observed and the 9 spots that were picked from 2DE-gels were identified as bevacizumab with high sequence coverage. MS/MS results showed multiple tyrosine nitrations on the Avastin® light and heavy chains that were either represented as nitrotyrosine or as aminotyrosine, which was shown to be generated from nitrotyrosine under reducing conditions. Protein nitration is known to significantly change protein functions and interactions and it may well be that some of the adverse effects of the protein drug Avastin® may be due to this PTM, which may have been generated during production- thus, nitration of Avastin® is a challenge for the pharmaceutical industry.

Effects of nitric oxide synthase-3 overexpression on post-translational modifications and cell survival in HepG2 cells

Hepatocarcinoma is the fifth most common neoplasm and the third cause of cancer-related death. The development of genetic- and/or molecular-based therapies is urgently required. The administration of high doses of nitric oxide (NO) promotes cell death in hepatocytes. NO contributes to cell signaling by inducing oxidative/nitrosative-dependent post-translational modifications. The aim of the present study was to investigate protein modifications and its relation with alteration of cell proliferation and death in hepatoma cells. Increased intracellular NO production was achieved by stable nitric oxide synthase-3 (NOS-3) overexpression in HepG2 cells. We assessed the pattern of nitration, nitrosylation and carbonylation of proteins by proteomic analysis. The results showed that NOS-3 cell overexpression increased oxidative stress, which affected proteins mainly involved in cell protein folding. Carbonylation also altered metabolism, as well as immune and antioxidant responses. The interaction of nitrosative and oxidative stress generated tyrosine nitration, which affected the tumor marker Serpin B3, ATP synthesis and cytoskeleton. All these effects were associated with a decrease in chaperone activity, a reduction in cell proliferation and an increased cell death. Our study showed that alteration of nitration, nitrosylation and carbonylation pattern of proteins by NO-dependent oxidative/nitrosative stress was related to a reduction of cell survival in a hepatoma cell line.

Site-specific incorporation of 3-nitrotyrosine as a probe of pKa perturbation of redox-active tyrosines in ribonucleotide reductase

E. coli ribonucleotide reductase catalyzes the reduction of nucleoside 5'-diphosphates into 2'-deoxynucleotides and is composed of two subunits: alpha2 and beta2. During turnover, a stable tyrosyl radical (Y*) at Y(122)-beta2 reversibly oxidizes C(439) in the active site of alpha2. This radical propagation step is proposed to occur over 35 A, to use specific redox-active tyrosines (Y(122) and Y(356) in beta2, Y(731) and Y(730) in alpha2), and to involve proton-coupled electron transfer (PCET). 3-Nitrotyrosine (NO(2)Y, pK(a) 7.1) has been incorporated in place of Y(122), Y(731), and Y(730) to probe how the protein environment perturbs each pK(a) in the presence of the second subunit, substrate (S), and allosteric effector (E). The activity of each mutant is <4 x 10(-3) that of the wild-type (wt) subunit. The [NO(2)Y(730)]-alpha2 and [NO(2)Y(731)]-alpha2 each exhibit a pK(a) of 7.8-8.0 with E and E/beta2. The pK(a) of [NO(2)Y(730)]-alpha2 is elevated to 8.2-8.3 in the S/E/beta2 complex, whereas no further perturbation is observed for [NO(2)Y(731)]-alpha2. Mutations in pathway residues adjacent to the NO(2)Y that disrupt H-bonding minimally perturb its pK(a). The pK(a) of NO(2)Y(122)-beta2 alone or with alpha2/S/E is >9.6. X-ray crystal structures have been obtained for all [NO(2)Y]-alpha2 mutants (2.1-3.1 A resolution), which show minimal structural perturbation compared to wt-alpha2. Together with the pK(a) of the previously reported NO(2)Y(356)-beta2 (7.5 in the alpha2/S/E complex; Yee, C. et al. Biochemistry 2003, 42, 14541-14552), these studies provide a picture of the protein environment of the ground state at each Y in the PCET pathway, and are the starting point for understanding differences in PCET mechanisms at each residue in the pathway.

Redox-control of matrix metalloproteinase-1: a critical link between free radicals, matrix remodeling and degenerative disease

Many degenerative disease processes associated with aging result from enhanced extracellular matrix (ECM) breakdown. Concomitant with aberrant matrix destruction are alterations in levels of reactive oxygen species (ROS) generating and detoxification systems. ROS function as second messengers due to their ability to react with wide range of biomolecules resulting in modification of an array of signaling networks. ROS can activate upstream kinases (MKK) responsible for MAPK activation and restrict the activity of their inhibitory phosphatases. Here we focus on the redox-sensitive signaling components that control the expression of MMP-1, which is largely responsible for maintaining ECM homeostasis. Numerous disease processes are associated with shifts in steady state ROS levels that influence overall ECM degradation. This review highlights the redox-sensitive regulatory signals that control the expression of the primary initiating protease MMP-1 and provides strong rational for the use of antioxidant based therapies for treatment of degenerative disorders associated with aberrant matrix destruction.

Oxidative protein labeling in mass-spectrometry-based proteomics

Oxidation of proteins and peptides is a common phenomenon, and can be employed as a labeling technique for mass-spectrometry-based proteomics. Nonspecific oxidative labeling methods can modify almost any amino acid residue in a protein or only surface-exposed regions. Specific agents may label reactive functional groups in amino acids, primarily cysteine, methionine, tyrosine, and tryptophan. Nonspecific radical intermediates (reactive oxygen, nitrogen, or halogen species) can be produced by chemical, photochemical, electrochemical, or enzymatic methods. More targeted oxidation can be achieved by chemical reagents but also by direct electrochemical oxidation, which opens the way to instrumental labeling methods. Oxidative labeling of amino acids in the context of liquid chromatography(LC)-mass spectrometry (MS) based proteomics allows for differential LC separation, improved MS ionization, and label-specific fragmentation and detection. Oxidation of proteins can create new reactive groups which are useful for secondary, more conventional derivatization reactions with, e.g., fluorescent labels. This review summarizes reactions of oxidizing agents with peptides and proteins, the corresponding methodologies and instrumentation, and the major, innovative applications of oxidative protein labeling described in selected literature from the last decade.

Protein nitration is associated with increased proteolysis in skeletal muscle of bile duct ligation-induced cirrhotic rats

Cirrhosis is characterized by skeletal muscle wasting. In this study, the effects of nitric oxide production on skeletal muscle protein nitration and degradation in cirrhosis were investigated. Cirrhosis was induced by bile duct ligation (BDL) in Sprague-Dawley rats for 4 weeks. The BDL-induced cirrhotic rats and sham-operated rats were then injected daily with either saline or N(G)-l-nitro-arginine methyl ester (l-NAME) for 7 days from week 4 to week 5, after which nitrite/nitrate, glutathione reduction, as well as protein nitration, ubiquitination, and degradation were assessed in skeletal muscle. Elevated muscular nitrite/nitrate concentrations, protein nitration, total ubiquitin conjugates, and degradation fragments of myosin heavy chain as well as diminished glutathione reduction levels were observed in BDL-induced cirrhotic rats as compared with controls. Administration of l-NAME for 1 week led to reduction of nitrite/nitrate levels; protein nitration was also decreased in the skeletal muscle. In addition, ubiquitination of muscular proteins and degradation of myosin heavy chain were significantly diminished after treatment of l-NAME. In conclusion, nitrosative stress occurred in the skeletal muscle of BDL-induced cirrhotic rats and may lead to increased proteolysis of muscle-specific structural proteins.

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.

In vitro and in vivo protein-bound tyrosine nitration characterized by diagonal chromatography

A new proteomics technique for analyzing 3-nitrotyrosine-containing peptides is presented here. This technique is based on the combined fractional diagonal chromatography peptide isolation procedures by which specific classes of peptides are isolated following a series of identical reverse-phase HPLC separation steps. Here dithionite is used to reduce 3-nitrotyrosine to 3-aminotyrosine peptides, which thereby become more hydrophilic. Our combined fractional diagonal chromatography technique was first applied to characterize tyrosine nitration in tetranitromethane-modified BSA and further led to a high quality list of 335 tyrosine nitration sites in 267 proteins in a peroxynitrite-treated lysate of human Jurkat cells. We then analyzed a serum sample of a C57BL6/J mouse in which septic shock was induced by intravenous Salmonella infection and identified six in vivo nitration events in four serum proteins, thereby illustrating that our technique is sufficiently sensitive to identify rare in vivo tyrosine nitration sites in a very complex background.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

The nitration of platelet vasodilator stimulated phosphoprotein following exposure to low concentrations of hydrogen peroxide

Hydrogen peroxide (H2O2) at biologically relevant concentrations acts as a signaling molecule. We have shown previously that H2O2 acts synergistically with nitric oxide (NO) to inhibit platelet aggregation. We found that this synergism may be associated with the increased serine phosphorylation of vasodilator-sensitive phosphoprotein (VASP) by H2O2. In this study we demonstrate that H2O2 in the absence of NO or exogenous haem- containing proteins induces nitration of plateletVASP and other unidentified proteins by a mechanism that may involve the formation of peroxynitrite. The nitration was NO-dependent, but independent of oxidative stress and guanylyl cyclcase. The flavanoid epigallocatechin gallate (ECGC) completely suppressed nitration and was also shown to inhibit partially platelet activation by other agonists. Importantly, protein nitration was reversible, or at least the nitrated tyrosine residues are converted to a form not recognized by anti-nitrotyrosine antibodies. The loss of nitrated VASP was still evident in the presence of membrane permeable protease inhibitors. In conclusion, as H2O2 can inhibit platelet function, the nitration of VASP, a protein critical for actin cytoskeletal rearrangement, may represent a novel mechanism important in the regulation of platelets shape change leading to inhibition of platelets aggregation and the formation of blood clot.

Protein tyrosine phosphorylation and protein tyrosine nitration in redox signaling

Reversible phosphorylation of protein tyrosine residues by polypeptide growth factor-receptor protein tyrosine kinases is implicated in the control of fundamental cellular processes including the cell cycle, cell adhesion, and cell survival, as well as cell proliferation and differentiation. During the last decade, it has become apparent that receptor protein tyrosine kinases and the signaling pathways they activate belong to a large signaling network. Such a network can be regulated by various extracellular cues, which include cell adhesion, agonists of G protein-coupled receptors, and oxidants. It is well documented that signaling initiated by receptor protein tyrosine kinases is directly dependent on the intracellular production of oxidants, including reactive oxygen and nitrogen species. Accumulated evidence indicates that the intracellular redox environment plays a major role in the mechanisms underlying the actions of growth factors. Oxidation of cysteine thiols and nitration of tyrosine residues on signaling proteins are described as posttranslational modifications that regulate, positively or negatively, protein tyrosine phosphorylation (PTP). Early observations described the inhibition of PTP activities by oxidants, resulting in increased levels of proteins phosphorylated on tyrosine. Therefore, a redox circuitry involving the increasing production of intracellular oxidants associated with growth-factor stimulation/cell adhesion, oxidative reversible inhibition of protein tyrosine phosphatases, and the activation of protein tyrosine kinases can be delineated.

Low frequency and low intensity pulsed electromagnetic field exerts its antiinflammatory effect through restoration of plasma membrane calcium ATPase activity

Rheumatoid arthritis (RA) is a chronic inflammatory disorder affecting 1% of the population worldwide. Pulsed electromagnetic field (PEMF) has a number of well-documented physiological effects on cells and tissues including antiinflammatory effect. This study aims to explore the antiinflammatory effect of PEMF and its possible mechanism of action in amelioration of adjuvant induced arthritis (AIA). Arthritis was induced by a single intradermal injection of heat killed Mycobacterium tuberculosis at a concentration of 500 microg in 0.1 ml of paraffin oil into the right hind paw of rats. The arthritic animals showed a biphasic response regarding changes in the paw edema volume. During the chronic phase of the disease, arthritic animals showed an elevated level of lipid peroxides and depletion of antioxidant enzymes with significant radiological and histological changes. Besides, plasma membrane Ca(2+) ATPase (PMCA) activity was inhibited while intracellular Ca(2+) level as well as prostaglandin E(2) levels was noticed to be elevated in blood lymphocytes of arthritic rats. Exposure of arthritic rats to PEMF at 5 Hzx4 microT x 90 min, produced significant antiexudative effect resulting in the restoration of the altered parameters. The antiinflammatory effect could be partially mediated through the stabilizing action of PEMF on membranes as reflected by the restoration of PMCA and intracellular Ca(2+) levels in blood lymphocytes subsequently inhibiting PGE(2) biosynthesis. The results of this study indicated that PEMF could be developed as a potential therapy for RA in human beings.

Reduction of the nitro group during sample preparation may cause underestimation of the nitration level in 3-nitrotyrosine immunoblotting

We noted differences in the antibody response to 3-nitrotyrosine (NO(2)Tyr) in fixed and non-fixed tissues, and studied therefore potential problems associated with non-fixed tissues in Western blot analyses. Three different monoclonal anti-nitrotyrosine antibodies in Western blot analysis of inflammatory stimulated rat abdominal, liver and lung tissue homogenates caused no immunoreactivity, in contrast to a polyclonal nitrotyrosine antibody applied in fixed and non-fixed tissues. Western blot studies using both mono- and polyclonal antibodies showed a temperature- and heme group-dependent reduction of NO(2)Tyr in nitrated rat and bovine serum albumin incubated with dithiothreitol. Mass spectrometric analyses of a nitrated peptide angiotensin II revealed under similar conditions a positive temperature effect between 56 and 70 degrees C on reduction of NO(2)Tyr to 3-aminotyrosine which is not detected by anti-NO(2)Tyr antibodies. Western blot analysis may therefore underestimate the level of tissue nitration, and factors causing a reduction of NO(2)Tyr during sample preparation might conceal the actual nitration of proteins.

Nitric oxide and oxygen radical attack on GDP-dissociation inhibitor 2 (GDI-2) in spinal cord injury of the rat

Protein oxidation and nitration have been described during spinal cord injury (SCI) in animal models. Herein, mass spectrometry unambiguously identified GDP-dissociation inhibitor-2 (GDI-2) in SCI with post-translational modifications of 3-aminotyrosine (8 h post-injury) and an acrolein adduct of GDI-2 (72 h post-injury). On the basis of mass spectrometry evidence, we conclude that lipid-peroxidation and protein nitration do take place on an important signalling protein that may be prevented by specific experimental therapeutic interventions.

Nitrotyrosine formation and heme oxygenase-1 expression in endotoxemic cirrhotic rats

BACKGROUND

Endotoxemia increases hepatic toxicity and mortality in cirrhosis. Because the mechanism of augmented hepatotoxicity in endotoxemic cirrhotic rats is still unclear, we wanted to investigate whether oxidative and nitrosative stress play a causative role in lipopolysaccharide (LPS)-treated cirrhotic rats.

METHODS

Liver cirrhosis was produced by the administration of thioacetamide (0.3 g/L of tap water) for a period of 3 months in rats. At the end of this period, cirrhotic rats were sacrificed 6 h after LPS injection (5 mg/kg, intraperitoneally). Serum transaminase activities, plasma total nitrite and nitrotyrosine (NT) levels as well as hepatic lipid peroxides, NT formation and heme oxygenase 1 (HO-1) expression were determined.

RESULTS

LPS administration to cirrhotic rats caused further increases in serum transaminase activities, and plasma total nitrite and NT levels as well as hepatic lipid peroxide levels as compared to cirrhotic rats. Hepatic NT formation and HO-1 expression were also found to be increased in LPS-injected cirrhotic rats.

CONCLUSIONS

Our results indicate that increased oxidative and nitrosative stress may have a synergistic effect in LPS-augmented hepatotoxicity in cirrhotic rats.

Nitration of p38 MAPK in the placenta: association of nitration with reduced catalytic activity of p38 MAPK in pre-eclampsia

Peroxynitrite, a potent pro-oxidant formed from the interaction of superoxide and nitric oxide, has been widely reported to be nitrating tyrosine residues in proteins resulting in the formation of nitrotyrosine. Biological nitration of tyrosine, a footprint of oxidative injury, has been found to occur in various pathological states including pre-eclampsia, a leading cause of maternal mortality and increased perinatal mortality. Oxidative stress is a major contributor to endothelial dysfunction in pre-eclampsia. Previously, we have demonstrated increased nitrotyrosine immunostaining in placental villous vascular endothelium, surrounding vascular smooth muscle and villous stroma from pre-eclamptic or diabetic pregnancies. Immunoprecipitation (IP) with antinitrotyrosine antibodies followed by immunoblot analysis identified increased nitration of phospho-p38 mitogen-activated protein kinase (MAPK) in the pre-eclamptic placenta. The catalytic activity of p38 MAPK and concentration of phospho-p38 MAPK was also found to be reduced in placentae from pre-eclamptic pregnancies. Comparison of peptide masses of a 42-kDa protein obtained by mass spectrometry with masses of a theoretical tryptic digest of p38 MAPK that was modified by phosphorylation and nitration identified the protein to be p38 MAPK.

Ouabain decreases sarco(endo)plasmic reticulum calcium ATPase activity in rat hearts by a process involving protein oxidation

The effect of cardiac glycosides to increase cardiac inotropy by altering Ca(2+) cycling is well known but still poorly understood. The studies described in this report focus on defining the effects of ouabain signaling on sarcoplasmic reticulum Ca(2+)-ATPase function. Rat cardiac myocytes treated with 50 microM ouabain demonstrated substantial increases in systolic and diastolic Ca(2+) concentrations. The recovery time constant for the Ca(2+) transient, tau(Ca(2+)), was significantly prolonged by ouabain. Exposure to 10 microM H(2)O(2), which causes an increase in intracellular reactive oxygen species similar to that of 50 microM ouabain, caused a similar increase in tau(Ca(2+)). Concurrent exposure to 10 mM N-acetylcysteine or an aqueous extract from green tea (50 mg/ml) both prevented the increases in tau(Ca(2+)) as well as the changes in systolic or diastolic Ca(2+) concentrations. We also observed that 50 microM ouabain induced increases in developed pressure in addition to diastolic dysfunction in the isolated perfused rat heart. Coadministration of ouabain with N-acetylcysteine prevented these increases. Analysis of sarcoplasmic reticulum Ca(2+)-ATPase protein revealed increases in both the oxidation and nitrotyrosine content in the ouabain-treated hearts. Liquid chromatography-mass spectrometric analysis confirmed that the sarcoplasmic reticulum Ca(2+)-ATPase protein from ouabain-treated hearts had modifications consistent with oxidative and nitrosative stress. These data suggest that ouabain induces oxidative changes of the sarcoplasmic reticulum Ca(2+)-ATPase structure and function that may, in turn, produce some of the associated changes in Ca(2+) cycling and physiological function.

Identification of Rotenone-Induced Modifications in α-Synuclein Using Affinity Pull-Down and Tandem Mass Spectrometry

Parkinson's disease is a movement disorder that results from a loss of dopaminergic neurons in the substantia nigra. The disease is characterized by mitochondrial dysfunction, oxidative stress, and the presence of "Lewy body" inclusions enriched with aggregated forms of alpha-synuclein, a presynaptic protein. Although alpha-synuclein is modified at various sites in Lewy bodies, it is unclear how sequence-specific posttranslational modifications modulate the aggregation of the protein in oxidatively stressed neurons. To begin to address this problem, we developed an affinity pull-down/mass spectrometry method to characterize the primary structure of histidine-tagged alpha-synuclein isolated from catecholaminergic neurons. Using this method, we mapped posttranslational modifications of alpha-synuclein from untreated neurons and neurons exposed to rotenone, an inhibitor of mitochondrial complex I. Various posttranslational modifications suggestive of oxidative damage or repair were identified in a region comprising a 20-residue stretch in the C-terminal part of the protein. The results indicate that alpha-synuclein is subject to discrete posttranslational modifications in neurons with impaired mitochondrial function. Our affinity pull-down/mass spectrometry method is a useful tool to examine how specific modifications of alpha-synuclein contribute to neurologic disorders such as Parkinson's disease.

Improved tandem mass spectrometric characterization of 3-nitrotyrosine sites in peptides

Nitrated tyrosines are easily converted into their aminotyrosine equivalents by a reduction step. We here show that this conversion can be exploited to readily discern 3-aminotyrosine peptides in a background of non-nitrated peptides. Furthermore, aminotyrosine peptides are more stable in single mass spectrometry (MS) mode rendering peptide mass maps easier to interpret. One significant caveat of both 3-nitrotyrosine and 3-aminotyrosine peptides is their lack of efficient fragmentation upon collision-induced dissociation (CID) which, in the case of the latter peptides, also produces unexpected, deviating isotopic patterns of fragment ions containing the aminotyrosine residue. The net result is that sequence database searching becomes daunting as the correct peptide is frequently missed since insufficient and/or inaccurate peptide fragments are used. We show that a simple acetylation step, blocking all amines (including aminotyrosine), produces peptides that undergo extensive backbone fragmentation by CID and are thus easily identifiable in databases. Our procedure is additionally illustrated by doubling the number of nitration events mapped in tetranitromethane-nitrated bovine serum albumin (BSA) as compared to a direct analysis of the nitrated peptides using the same amount of material. In conclusion, we here illustrate that this two-step process, heme-mediated reduction and acetylation, can be used for more efficient characterization of protein-bound nitrated tyrosines.

Inorganic nitric oxide metabolites participating in no-dependent modifications of biopolymers

Biogenous nitric(II) oxide (NO), the higher nitrogen oxides (NO2, isomeric N2O3 and N2O4, ONOO-, etc.) that are NO-derived in vivo, and the products of their transformations are active compounds capable of reactions with biopolymers and low-molecular metabolites. The products of these reactions are often considered to be various NO-dependent modifications (NODMs). The nitrated, nitrosylated, nitrosated, and other NODMs play key roles in the regulation of the most important biochemical processes. In this review, we briefly discuss the metabolic reactions of nitrogen oxides that supply active intermediates for NODMs, the NODM reaction products, and some mechanisms of NODM reparation that allow the recovery of chemically intact biopolymer molecule from a modified (chemically damaged) NODM. For example, residues of 3-nitrotyrosine arising due to the NODM reactions of proteins can be reduced to unsubstituted Tyr residues as a result of alternative NODM reactions through intermediate diazotyrosine derivatives. The heterogeneity of a medium in vivo is an important factor controlling the proceeding of NODM reactions. We showed that many processes determining NODM efficiency proceed differently in the heterogeneous media of organisms and in homogeneous aqueous solutions.

Protein Nitration in a Mouse Model of Familial Amyotrophic Lateral Sclerosis

Multiple mechanisms have been proposed to contribute to amyotrophic lateral sclerosis (ALS) pathogenesis, including oxidative stress. Early evidence of a role for oxidative damage was based on the finding, in patients and murine models, of high levels of markers, such as free nitrotyrosine (NT). However, no comprehensive study on the protein targets of nitration in ALS has been reported. We found an increased level of NT immunoreactivity in spinal cord protein extracts of a transgenic mouse model of familial ALS (FALS) at a presymptomatic stage of the disease compared with age-matched controls. NT immunoreactivity is increased in the soluble fraction of spinal cord homogenates and is found as a punctate staining in motor neuron perikarya of presymptomatic FALS mice. Using a proteome-based strategy, we identified proteins nitrated in vivo, under physiological or pathological conditions, and compared their level of specific nitration. alpha- and gamma-enolase, ATP synthase beta chain, and heat shock cognate 71-kDa protein and actin were overnitrated in presymptomatic FALS mice. We identified by matrix-assisted laser desorption/ionization mass spectrometry 16 sites of nitration in proteins oxidized in vivo. In particular, alpha-enolase nitration at Tyr(43), target also of phosphorylation, brings additional evidence on the possible interference of nitration with phosphorylation. In conclusion, we propose that protein nitration may have a role in ALS pathogenesis, acting directly by inhibiting the function of specific proteins and indirectly interfering with protein degradation pathways and phosphorylation cascades.

Proteins as biomarkers of oxidative/nitrosative stress in diseases: the contribution of redox proteomics

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) contribute to the pathogenesis and/or progression of several human diseases. Proteins are important molecular signposts of oxidative/nitrosative damage. However, it is generally unresolved whether the presence of oxidatively/nitrosatively modified proteins has a causal role or simply reflects secondary epiphenomena. Only direct identification and characterization of the modified protein(s) in a given pathophysiological condition can decipher the potential roles played by ROS/RNS-induced protein modifications. During the last few years, mass spectrometry (MS)-based technologies have contributed in a significant way to foster a better understanding of disease processes. The study of oxidative/nitrosative modifications, investigated by redox proteomics, is contributing to establish a relationship between pathological hallmarks of disease and protein structural and functional abnormalities. MS-based technologies promise a contribution in a new era of molecular medicine, especially in the discovery of diagnostic biomarkers of oxidative/nitrosative stress, enabling early detection of diseases. Indeed, identification and characterization of oxidatively/nitrosatively modified proteins in human diseases has just begun.

Cytoprotective Effect of 3-[2-[4-(3-Chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole Dihydrochloride 3.5 Hydrate (DY-9760e) Against Ischemia/Reperfusion-Induced Injury in Rat Heart Involves Inhibition of Fodrin Breakdown and Protein Tyrosine Nitration

We here assessed the effects of 3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate (DY-9760e), a novel calmodulin antagonist, on infarct size in the rat heart subjected to ischemia/reperfusion. Rats were subjected to a 30-min coronary occlusion followed by a 24-h reperfusion. DY-9760e was intravenously infused for 20 min, starting at 20 min after coronary occlusion. Treatment with DY-9760e (10 mg/kg) significantly reduced the infarct size in the risk area assessed by Evans Blue/TTC (triphenyltetrazolium chloride) staining. DY-9760e treatment also ameliorated contractile dysfunction of the left ventricle 72 h after reperfusion. DY-9760e significantly inhibited fodrin breakdown and caspase-3 activation. The inhibitory effect of DY-9760e on the fodrin breakdown was prominent in the rim rather than in the center of the risk area. DY-9760e also blocked protein tyrosine nitration associated with infarction. These results suggest that the cardioprotective effect of DY-9760e involved inhibition of calpain/caspase activation and protein tyrosine nitration.

Tyrosine nitration in plasma proteins from patients undergoing hemodialysis

BACKGROUND

A growing body of evidence suggesting that oxidative stress might be one of the most important complications occurring during hemodialysis (HD) has accumulated. However, although the role of reactive oxygen species has been investigated extensively, little is known about the involvement of reactive nitrogen species. In the present investigation, levels of protein modifications in the form of tyrosine nitration in patients undergoing long-term HD therapy were evaluated.

METHODS

Tyrosine nitration of plasma proteins was detected by means of Western blotting using a specific nitrotyrosine-recognizing monoclonal antibody, and band intensity was evaluated by using image analysis software. Immunoprecipitation of plasma proteins by antinitrotyrosine-agarose--conjugated antibodies, followed by Western blotting, was used in an attempt to identify the nitrated proteins.

RESULTS

Although several proteins with nitrated tyrosine residues were observed in plasma of healthy individuals, increased nitration levels were observed in some specific proteins in all patients tested (n = 25) compared with controls (n = 6). At least 6 apparent bands appeared to be more nitrated than their counterparts in plasma from controls. Ceruloplasmin was identified as 1 of the proteins with significantly increased nitration in patients.

CONCLUSION

Results of the present investigation show that specific plasma proteins of HD patients are post-translationally modified by nitration of their tyrosine residues. The nature of these proteins, as well as the exact molecular mechanisms and consequences of these modifications, warrant additional investigation.

Induction of motor neuron apoptosis by free 3-nitro-L-tyrosine

Peroxynitrite-dependent tyrosine nitration has been postulated to be involved in motor neuron degeneration in amyotrophic lateral sclerosis (ALS). Evidence supporting this supposition includes the appearance of both free and protein-linked 3-nitro-l-tyrosine (nitrotyrosine) in both sporadic and familial ALS, as well as of increased free nitrotyrosine levels in the spinal cord of transgenic mice expressing ALS-linked superoxide dismutase mutants at symptom onset. Here we demonstrate that incubation with clinically relevant concentrations of nitrotyrosine induced apoptosis in motor neurons cultured with trophic factors. Nitrotyrosine was bound to proteins, but it was not incorporated into alpha-tubulin, as previously demonstrated for other cell types. Neither inhibition of nitric oxide production nor scavenging of superoxide and peroxynitrite prevented increases in cell nitrotyrosine immunoreactivity or motor neuron death, suggesting that these effects are not due to the endogenous formation of reactive nitrogen species. In contrast, some populations of astrocytes incorporated nitrotyrosine into alpha-tubulin, but free nitrotyrosine had no effect on the viability and phenotype of astrocytes in culture, as evaluated by glial fibrillary acidic protein immunoreactivity, cell growth and morphology. Co-culture of motor neurons on astrocyte monolayers delayed, but did not prevent, nitrotyrosine-induced motor neuron death. These results suggest that free nitrotyrosine may play a role in the induction of motor neuron apoptosis in ALS.

Regulation of nitric oxide and soluble guanylyl cyclase

Since the discoveries that have verified nitric oxide (NO) as an endogenously produced cell signaling molecule, research surrounding its production and mechanisms of action have been studied at an exponentially increasing rate. NO is produced by a family of enzymes termed the NO synthases (NOS), which are regulated independently by various stimuli. Once produced, NO can solicit numerous biological events by reacting with various metals, thiols, and oxygen species to modify proteins, DNA and lipids. One of the most biologically relevant actions of NO is its binding to the heme moiety in the heterodimeric enzyme, soluble guanylyl cyclase (sGC). Activation of sGC by NO results in the production of the second messenger molecule, 3',5'-cyclic guanosine monophosphate (cGMP), which can regulate numerous physiological events such as vasodilatation and neurotransmission. Here we will review the synthesis and fate of NO, and discuss the activation and regulation of the NO receptor, sGC.

Rapid and selective oxygen-regulated protein tyrosine denitration and nitration in mitochondria

Growing evidence connects a cumulative formation of 3-nitrotyrosyl adducts in proteins as a marker for oxidative damage with the pathogenesis of various diseases and pathological conditions associated with oxidative stress. A physiological signaling role for protein nitration has also been suggested. Controlled "denitration" would be essential for such a contribution of protein nitration to cellular regulatory processes. Thus, we further characterized such a potentially controlled, reversible tyrosine nitration that occurs in respiring mitochondria during oxygen deprivation followed by reoxygenation, which we recently discovered. Mitochondria constitute cellular centers of protein nitration and are leading candidates for a "nitrative" regulation. Mitochondria are capable of completely eliminating 3-nitrotyrosyl adducts during 20 min of hypoxia-anoxia and undergoing a selective partial reduction after only 5 min. This denitration is independent of protein degradation but depends on the oxygen tension. Reoxygenation re-establishes protein tyrosine nitration patterns that are almost identical to the pattern that occurs before hypoxia-anoxia, with nitration levels that depend on the duration of hypoxia-anoxia. The identified mitochondrial targets of this process are critical for energy and antioxidant homeostasis and, therefore, cell and tissue viability. This cycle of protein nitration and denitration shows analogies to protein phosphorylation, and we demonstrate that the cycle meets most of the criteria for a cellular signaling mechanism. Taken together, our data reveal that protein tyrosine nitration in mitochondria can be controlled, is target-selective and rapid, and is dynamic enough to serve as a nitrative regulatory signaling process that likely affects cellular energy, redox homeostasis, and pathological conditions when these features become disturbed.