Peroxynitrite-mediated nitration of peptides: characterization of the products by electrospray and combined gas chromatography-mass spectrometry

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.

Supraspinal peroxynitrite modulates pain signaling by suppressing the endogenous opioid pathway

Peroxynitrite (PN, ONOO(-)) is a potent oxidant and nitrating agent that contributes to pain through peripheral and spinal mechanisms, but its supraspinal role is unknown. We present evidence here that PN in the rostral ventromedial medulla (RVM) is essential for descending nociceptive modulation in rats during inflammatory and neuropathic pain through PN-mediated suppression of opioid signaling. Carrageenan-induced thermal hyperalgesia was associated with increased 3-nitrotyrosine (NT), a PN biomarker, in the RVM. Furthermore, intra-RVM microinjections of the PN decomposition catalyst Fe(III)-5,10,15,20-tetrakis(N-methyl-pyridinium-4-yl)porphyrin (FeTMPyP(5+)) dose-dependently reversed this thermal hyperalgesia. These effects of FeTMPyP(5+) were abrogated by intra-RVM naloxone, implicating potential interplay between PN and opioids. In support, we identified NT colocalization with the endogenous opioid enkephalin (ENK) in the RVM during thermal hyperalgesia, suggesting potential in situ interactions. To address the functional significance of such interactions, we exposed methionine-enkephalin (MENK) to PN and identified the major metabolite, 3-nitrotyrosine-methionine-sulfoxide (NSO)-MENK, using liquid chromatography-mass spectrometry. Next, we isolated, purified, and tested NSO-MENK for opioid receptor binding affinity and analgesic effects. Compared to MENK, this NSO-MENK metabolite lacked appreciable binding affinity for δ, μ, and κ opioid receptors. Intrathecal injection of NSO-MENK in rats did not evoke antinociception, suggesting that PN-mediated chemical modifications of ENK suppress opioid signaling. When extended to chronic pain, intra-RVM FeTMPyP(5+) produced naloxone-sensitive reversal of mechanical allodynia in rats following chronic constriction injury of the sciatic nerve. Collectively, our data reveal the central role of PN in RVM descending facilitation during inflammatory and neuropathic pain potentially through anti-opioid activity.

Controlling murine and rat chronic pain through A3 adenosine receptor activation

Clinical management of chronic neuropathic pain is limited by marginal effectiveness and unacceptable side effects of current drugs. We demonstrate A(3) adenosine receptor (A(3)AR) agonism as a new target-based therapeutic strategy. The development of mechanoallodynia in a well-characterized mouse model of neuropathic pain following chronic constriction injury of the sciatic nerve was rapidly and dose-dependently reversed by the A(3)AR agonists: IB-MECA, its 2-chlorinated analog (Cl-IB-MECA), and the structurally distinct MRS1898. These effects were naloxone insensitive and thus are not opioid receptor mediated. IB-MECA was ≥1.6-fold more efficacious than morphine and >5-fold more potent. In addition, IB-MECA was equally efficacious as gabapentin (Neurontin) or amitriptyline, but respectively >350- and >75-fold more potent. Besides its potent standalone ability to reverse established mechanoallodynia, IB-MECA significantly increased the antiallodynic effects of all 3 analgesics. Moreover, neuropathic pain development in rats caused by widely used chemotherapeutics in the taxane (paclitaxel), platinum-complex (oxaliplatin), and proteasome-inhibitor (bortezomib) classes was blocked by IB-MECA without antagonizing their antitumor effect. A(3)AR agonist effects were blocked with A(3)AR antagonist MRS1523, but not with A(1)AR (DPCPX) or A(2A)AR (SCH-442416) antagonists. Our findings provide the scientific rationale and pharmacological basis for therapeutic development of A(3)AR agonists for chronic pain.

Peroxynitrite-induced nitrative and oxidative modifications alter tau filament formation

Tau undergoes numerous posttranslational modifications during the progression of Alzheimer's disease (AD). Some of these changes accelerate tau aggregation, while others are inhibitory. AD-associated inflammation is thought to create oxygen and nitrogen radicals such as peroxynitrite (PN). In vitro, PN can nitrate many proteins, including tau. We have previously demonstrated that tau's ability to form filaments is profoundly affected by treatment with PN and have attributed this inhibition to tyrosine nitration. However, PN is highly reactive and unstable leading to oxidative amino acid modifications through its free radical byproducts. To test whether PN can modify other amino acids in tau via oxidative modifications, a mutant form of the tau protein lacking all tyrosines (5XY → F) was constructed. 5XY → F tau readily forms filaments; however, like wild-type tau the extent of polymerization was greatly reduced following PN treatment. Since 5XY → F tau cannot be nitrated, it was clear that nonnitrative modifications are generated by PN treatment and that these modifications change tau filament formation. Mass spectrometry was used to identify these oxidative alterations in wild-type tau and 5XY → F tau. PN-treated wild-type tau and 5XY → F tau consistently displayed lysine formylation throughout tau in a nonsequence-specific distribution. Lysine formylation likely results from reactive free radical exposure caused by PN treatment. Therefore, our results indicate that PN treatment of proteins in vitro cannot be used to study protein nitration as it likely induces numerous other random oxidative modifications clouding the interpretations of any functional consequences of tyrosine nitration.

Proteolytic degradation by cathepsin D of glycated hemoglobin from diabetes patients gives rise to hemorphin-7 peptides

Previous studies showed a significantly reduced level of hemorphins in the serum of diabetes patients. In order to elucidate the biochemical mechanisms responsible for this anomaly, the influence of hemoglobin glycation on hemorphin generation was studied. The glycation of hemoglobin occurs in the blood of diabetes patients and this could modify its enzymatic digestion and the resulting proteolytic products. Several samples of hemoglobin were obtained from the blood of type 1 diabetes patients (n=8) and normal healthy control subjects (n=2). The glycated hemoglobin samples were classified on the basis of their HbA1c values expressed as a percentage of total hemoglobin. Four solutions of glycated hemoglobin characterized by HbA1c values of 6%, 9.1%, 10.7% and 12.1% were treated with cathepsin D and the hemorphins obtained following the proteolysis were compared to controls. It was found that hemorphins were produced whatever the level of glycation of hemoglobin and also that the degree of glycation had no effect on the quantity of hemorphins released. Thus the alteration of hemoglobin does not seem to be the essential reason for the decrease in hemorphin concentrations in the sera of diabetic patients.

Method for quantifying nitromethane in blood as a potential biomarker of halonitromethane exposure

The cytotoxicity and genotoxicity of nitromethane and its halogenated analogues in mammals raise concerns about potential toxicity to humans. This study shows that halonitromethanes are not stable in human blood and undergo dehalogenation to form nitromethane. We quantified nitromethane in human blood using solid-phase microextraction (SPME) headspace sampling coupled with gas chromatography (GC) and high resolution mass spectrometry (HRMS). The limit of detection was 0.01 microg/L with a linear calibration curve spanning 3 orders of magnitude. This method employs isotope dilution to precisely quantify trace amounts of nitromethane (coefficient of variation <6%). At three spiked concentrations of nitromethane, method accuracy ranged from 88 to 99%. We applied this method to blood samples collected from 632 people with no known occupational exposure to nitromethane or halonitromethanes. Nitromethane was detected in all blood samples tested (range: 0.28-3.79 microg/L, median: 0.66 microg/L). Time-course experiments with trichloronitromethane- and tribromonitromethane-spiked blood showed that nitromethane was the major product formed (1 nmole tribromonitromethane formed 0.59 nmole of nitromethane, whereas 1 nmole trichloronitromethane formed 0.77 nmole nitromethane). Nitromethane may form endogenously from peroxynitrite: nitromethane concentrations increased proportionately in blood samples spiked with peroxynitrite. Blood nitromethane can be a biomarker of exposure to both nitromethane and halonitromethanes. This sensitive, accurate, and precise analytical method can be used to determine baseline blood nitromethane level in the general population. It can also be used to study the health impact from exposure to nitromethane and halonitromethanes in occupational environments and to assess trichloronitromethane (chloropicrin) exposure in chemical terrorism investigations.

Investigation of tyrosine nitration and nitrosylation of angiotensin II and bovine serum albumin with electrospray ionization mass spectrometry

Protein tyrosine nitration is one of the important regulatory mechanisms in various cellular phenomena such as cell adhesion, endo/exo-cytosis of cellular materials, and signal transduction. In the present study, electrospray ionization tandem mass spectrometry (ESI-MS/MS) with a linear ion-trap mass spectrometer was applied for identification of nitrated proteins and localization of the modified tyrosine residues. When angiotensin II(DRVYIHPF) was nitrated in vitro with tetranitromethane (TNM), the mass spectrum showed a shift of +45 Da which corresponded to tyrosine nitration. An additional +29 Da mass shift was also detected by ESI-MS. This differed from nitrated peptide analysis with matrix-associated laser desorption/ionization mass spectrometry (MALDI-MS), which showed oxygen neutral loss from the nitrated tyrosine residues upon laser irradiation. Hence the +29 Da mass shift of the nitrated peptide observed by ESI-MS suggested the introduction of an NO group for nitrosylation of tyrosine residues. To confirm this in vitro nitrosylation on the protein level, bovine serum albumin was in vitro nitrated with TNM and analyzed by ESI-MS/MS. As expected, +29 as well as +45 Da mass shifts were detected, and the +29 Da mass shift was found to correspond to the modification on tyrosine residues by NO. Although the chemical mechanism by which this occurs in ESI-MS is not clear, the +29 Da mass shift could be a new potential marker of nitrosylated peptides.

Enzyme immunoassays for the investigation of protein nitration by air pollutants

Two enzyme immunoassays have been developed, characterised, and applied to investigate protein nitration in birch pollen extract (BPE) and bovine serum albumin (BSA) samples exposed to air pollutants. The monoclonal antibody CAY-189542 against nitrotyrosine (raised against peroxynitrite-treated keyhole limpet hemocyanine) was characterised in an indirect competitive assay (affinity and cross-reactivities) and applied in a new one-sided enzyme immunoassay for nitrated proteins. The one-sided assay was calibrated against a nitrated BSA standard with an average of 14 nitrotyrosine residues per molecule (nitro-(14)-BSA; detection limit 8.3 pmol L(-1)), and the sensitivity of the test was found to be significantly enhanced by a multivalent binding mode of the monoclonal antibody (bonus effect of multivalency). The same antibody and a polyclonal antibody against Bet v 1, the most prominent birch pollen allergen, were used in a new sandwich immunoassay for specific determination of nitrated Bet v 1. This assay was calibrated against a nitrated Bet v 1 standard with an average of 3 nitrotyrosine residues per molecule (nitro-(3)-Bet v 1; detection limit 0.2 nmol L(-1)). Bet v 1 and BSA exposed to polluted urban outdoor air and to synthetic gas mixtures containing NO2 and O3 at atmospherically relevant concentration levels were found to be efficiently nitrated within hours to days. Pronounced correlations of nitro-(14)-BSA equivalent concentrations with exposure time and with nitro-(3)-Bet v 1 equivalent concentrations in nitrated BPE samples were observed. Test experiments indicated that the efficiency of protein nitration was strongly enhanced by reactive species formed upon interaction of NO2 with O3 and H2O (e.g. NO3 and HNO3). Potential implications of protein nitration by air pollutants are outlined and discussed.

Lacidipine, a potential peroxynitrite scavenger: investigation of activity by liquid chromatography and mass spectrometry

Inflamed tissues are often characterised by the production of *NO and O(2)(-) radicals, which are known to react at an extremely fast rate to produce peroxynitrite (ONOO(-)). This highly oxidising entity reacts with protein-bound tyrosine to give 3-nitrotyrosine, which is considered a biochemical marker of peroxynitrite-induced damage. Lacidipine is a calcium antagonist indicated for the treatment of mild to moderate hypertension. In the present work, electrospray mass spectrometry with and without liquid chromatography was used to evaluate the capability of lacidipine and two other related molecules as ONOO(-) scavengers. This capability is compared with that associated with a number of commercial polyphenols described in the literature as efficient scavengers of this cytotoxic agent. The use of mass spectrometry provided rapid quantitative assessment of both the nitration and its reduction, and showed that lacidipine possesses a reasonable capability for reducing in vitro nitration of superoxide dismutase.

Selective nitration of histone tyrosine residues in vivo in mutatect tumors

Nitric oxide-derived reactive species have been implicated in many disorders. Protein nitrotyrosine is often used as a stable marker of these reactive species. Using immunohistochemistry, we have previously detected nitrotyrosine in murine Mutatect tumors, where neutrophils are the principal source of nitric oxide. We now report on the identification of several prominent nitrotyrosine-containing proteins. Using Western blot analysis, nitrotyrosine in higher molecular mass proteins (>20 kDa) was detected in tumors containing a high number of neutrophils but not in tumors with fewer neutrophils. Staining for nitrotyrosine was consistently seen in low molecular mass proteins (< or =15 kDa), regardless of the level of neutrophils. Protein nitrotyrosine was not seen in Mutatect cells growing in vitro. Treatment with nitric oxide donors produced nitration of < or =15-kDa proteins, but only after extended periods. These small proteins, both from tumors and cultured cells, were identified by mass spectrometry to be histones. Only a subset of tyrosine residues was nitrated. Selective nitration may reflect differential accessibility of different tyrosine residues and the influence of neighboring residues within the nucleosome. The prominence of histone nitration may reflect its relative stability, making this post-translational modification a potentially useful marker of extended exposure of cells or tissues to nitric oxide-derived reactive species.

Fibrillar beta-amyloid evokes oxidative damage in a transgenic mouse model of Alzheimer's disease

Beta-amyloid is one of the most significant features of Alzheimer's disease, and has been considered to play a pivotal role in neurodegeneration through an unknown mechanism. However, it has been noted that beta-amyloid accumulation is associated with markers of oxidative stress including protein oxidation (Smith et al., 1997), lipid peroxidation (Mark et al., 1997; Sayre et al., 1997), advanced glycation end products (Smith et al., 1994), and oxidation of nucleic acids (Nunomura et al., 1999). Furthermore, studies from cultured cells have shown that beta-amyloid leads to an increase in hydrogen peroxide levels (Behl et al., 1994), and the production of reactive oxygen intermediates (Harris et al., 1995). Taken together, this evidence supports the idea that beta-amyloid plays a key role in oxidative stress-evoked neuropathology. In this study, we examined the induction of oxidative stress in response to amyloid load in a mouse model of Alzheimer's disease. The mice carrying mutant amyloid precursor protein and presenilins-1 (Goate et al., 1991; Hardy, 1997), develops beta-amyloid deposits at 10-12 weeks of age and show several features of the human disease (Holcomb et al., 1998; Matsuoka et al., 2001; McGowan et al., 1999; Takeuchi et al., 2000; Wong et al., 1999). Both 3-nitrotyrosine and 4-hydroxy-2-nonenal (protein and lipid oxidative stress markers, respectively) associate strongly with fibrillar beta-amyloid, but not with diffuse (thioflavine S negative) beta-amyloid, and the levels increase in relation to the age-associated increase in fibrillar amyloid load.From these data we suggest that fibrillar beta-amyloid is associated with oxidative damage which may influence disease progression in the Alzheimer's disease brain.

Investigation of tyrosine nitration in proteins by mass spectrometry

In vivo nitration of tyrosine residues is a post-translational modification mediated by peroxynitrite that may be involved in a number of diseases. The aim of this study was to evaluate possibilities for site-specific detection of tyrosine nitration by mass spectrometry. Angiotensin II and bovine serum albumin (BSA) nitrated with tetranitromethane (TNM) were used as model compounds. Three strategies were investigated: (i) analysis of single peptides and protein digests by matrix-assisted laser desorption/ionization (MALDI) peptide mass mapping, (ii) peptide mass mapping by electrospray ionization (ESI) mass spectrometry and (iii) screening for nitration by selective detection of the immonium ion of nitrotyrosine by precursor ion scanning with subsequent sequencing of the modified peptides. The MALDI time-of-flight mass spectrum of nitrated angiotensin II showed an unexpected prompt fragmentation involving the nitro group, in contrast to ESI-MS, where no fragmentation of nitrated angiotensin II was observed. The ESI mass spectra showed that mono- and dinitrated angiotensin II were obtained after treatment with TNM. ESI-MS/MS revealed that the mononitrated angiotensin II was nitrated on the side-chain of tyrosine. The dinitrated angiotensin II contained two nitro groups on the tyrosine residue. Nitration of BSA was confirmed by Western blotting with an antibody against nitrotyrosine and the sites for nitration were investigated by peptide mass mapping after in-gel digestion. Direct mass mapping by ESI revealed that two peptides were nitrated. Precursor ion scanning for the immonium ion for nitrotyrosine revealed two additional partially nitrated peptides. Based on the studies with the two model compounds, we suggest that the investigation of in vivo nitration of tyrosine and identification of nitrated peptides might be performed by precursor ion scanning for the specific immonium ion at m/z 181.06 combined with ESI-MS/MS for identification of the specific nitration sites.

Analysis of peptides and proteins containing nitrotyrosine by matrix-assisted laser desorption/ionization mass spectrometry

Oxidative damage to proteins can occur under physiological conditions through the action of reactive oxygen species, including those containing nitrogen such as peroxynitrite (ONO2-). Peroxynitrite has been shown in vitro to target tyrosine residues in proteins through free radical addition to produce 3-nitrotyrosine. In this work, we show that mass spectral patterns associated with 3-nitrotyrosine containing peptides allow identification of peptides containing this modification. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry was used to characterize a synthetic peptide AAFGY(m-NO2)AR and several peptides containing 3-nitrotyrosine derived from bovine serum albumin treated with tetranitromethane. A unique series of ions were found for these peptides in addition to the mass shift of +45 Da corresponding to the addition of the nitro group. Specifically, two additional ions were observed at roughly equal abundance that correspond to the loss of one and two oxygens, and at lower abundances, two ions are seen that suggest the formation of hydroxylamine and amine derivatives. These latter four components appear to originate by laser-induced photochemical decomposition. MALDI-MS analysis of the synthetic peptide containing 3-nitrotyrosine revealed this same pattern. Post-source decay (PSD) MALDI-time-of-flight (TOF) and collisional activation using a prototype MALDI quadrupole TOF yielded extensive fragmentation that allowed site-specific identification of 3-nitrotyrosine. Conversion of peptides containing 3-nitrotyrosine to 3-aminotyrosine with Na2S2O4 yielded a single molecular ion by MALDI with an abundant sidechain loss under PSD conditions. These observations suggest that MALDI can provide a selective method for the analysis and characterization of 3-nitrotyrosine-containing peptides.

Unraveling peroxynitrite formation in biological systems

Peroxynitrite promotes oxidative damage and is implicated in the pathophysiology of various diseases that involve accelerated rates of nitric oxide and superoxide formation. The unambiguous detection of peroxynitrite in biological systems is, however, difficult due to the combination of a short biological half-life, limited diffusion, multiple target molecule reactions, and participation of alternative oxidation/nitration pathways. In this review, we provide the conceptual framework and a comprehensive analysis of the current experimental strategies that can serve to unequivocally define the existence and quantitation of peroxynitrite in biological systems of different levels of organization and complexity.

Nitration and hydroxylation of aromatic amino acid and guanine by the air pollutant peroxyacetyl nitrate

Peroxyacetyl nitrate (PAN) is a common gaseous photochemical compound in polluted air and cigarette smog. The toxicity of PAN has been found to depend on three pathways: (1) its oxidizing property that mimics peroxide or peroxynitrite; (2) its nitrating and hydroxylating properties similar to peroxynitrite; and (3) its acetylating property like acetic anhydride. The present investigations were intended to focus on the reactions of PAN with aromatic amino acids and guanine. When PAN interacted with tyrosine and guanine the major products were 3-nitrotyrosine, 3, 5-dinitrotyrosine, 8-hydroxyguanine and 8-nitroguanine. These compounds have been used as indicators for the presence of peroxynitrite in previous studies. When PAN interacted with phenylalanine, the products were 3-nitrotyrosine, 4-nitrophenylalanine, p-tyrosine, o-tyrosine and m-tyrosine. 5-Hydroxytryptophan is produced from the reaction of PAN with tryptophan. Furthermore, the formation of nitrated tyrosines was also found in the PAN-treated HL-60 cells. A high yield of dityrosine was formed when PAN and peroxynitrite were reacted with tyrosine, probably through free radical oxidation. We also found that peroxynitrite and PAN are similar in their oxidizing activity. From these findings, we suggest that peroxynitrite may be considered as the reactive intermediate of PAN.

Quantification of 3-nitrotyrosine in biological tissues and fluids: generating valid results by eliminating artifactual formation

Reactive nitrogen species such as peroxynitrite can nitrate specific amino acids, whether free or protein bound, and 3-nitrotyrosine is believed to be one marker of this reaction. To examine the significance of this pathway in biological systems we have developed an accurate, sensitive, and specific assay for 3-nitrotyrosine based on combined liquid chromatography tandem mass spectrometry. Our approach allowed simultaneous analysis of both tyrosine and 3-nitrotyrosine and employs isotopomer standards (i.e., [15N1, 13C9]-tyrosine and [13C6]-3nitrotyrosine). Calibration curves were linear (r2 = 0.999) across the range 0.5-100 pg/microL (i.e., 2.2-442 fmol/microL), and the detection limit for standard samples was 0.5 pg/microL (2.2 fmol/microL, or 10 fmol on column; S/N = 5) or 1 pg/microL (4.4 fmol/microL) for extracted (biological) samples. As a component of this study we have undertaken an extensive investigation of artifactual formation of 3-nitrotyrosine under conditions that exist during sample extraction and derivatization. Our studies show that under appropriate conditions (low pH, elevated temperatures, and in the presence of a vast excess of the two substrates, tyrosine and the nitrate anion), 3-nitrotyrosine can readily be formed as an artifact.

Stable markers of oxidant damage to proteins and their application in the study of human disease

The mechanisms of formation and the nature of the altered amino acid side chains formed on proteins subjected to oxidant attack are reviewed. The use of stable products of protein side chain oxidation as potential markers for assessing oxidative damage in vivo in humans is discussed. The methods developed in the authors laboratories are outlined, and the advantages and disadvantages of these techniques compared with other methodologies for assessing oxidative damage to proteins and other macromolecules. Evidence is presented to show that protein oxidation products are sensitive markers of oxidative damage, that the pattern of products detected may yield information as to the nature of the original oxidative insult, and that the levels of oxidized side-chains can, in certain circumstances, be much higher than those of other markers of oxidation such as lipid hydroperoxides.

A covalent thymine-tyrosine adduct involved in DNA-protein crosslinks: synthesis, characterization, and quantification

A thymine-tyrosine adduct, (3-[(1,3-dihydro-2,4-dioxopyrimidin-5-yl)methyl]-L-tyrosine), was synthesized using a simple, single-step condensation between 5-(hydroxymethyl)uracil and L-tyrosine. This approach provides access to useful quantities (mg-g) of analytically pure reference material, and with minor modification, to stable isotope-labeled analogues (isotopomers). With reference material and a suitable internal standard available, isotope-dilution liquid chromatography-electrospray ionization-tandem mass spectrometry (LC/MS/MS) was used to assay the adduct in a model system purged of oxygen, i.e., a gamma-irradiated N2O-saturated aqueous solution of thymine and tyrosine. The convenient synthetic route to standards and the method for quantification reported here will prove useful in assessing the significance of the adduct in biological systems. These studies also highlight the potential for artefactual adduct formation if the appropriate substrates are present under acidic conditions.

Peroxyacetyl nitrate-induced apoptosis through generation of reactive oxygen species in HL-60 cells

Peroxyacetyl nitrate (PAN), an ubiquitous air pollutant, induced apoptosis in human leukemia HL-60, human chronic myelogenous leukemia K-562, and mouse monocyte-macrophage RAW 264.7 cell lines. In the HL 60 cells, characteristic apoptosis morphology could be observed 4 h after the cells were treated with 50 microM PAN. Exposure of HL-60 cells to increasing concentrations of PAN (from 1 microM to 100 microM) confirmed the concentration dependence of apoptosis as evidenced by DNA fragmentation in HL-60 cells, chromatin condensation by acridine-orange staining, and the appearance of the DNA apoptotic peak in flow cytometry. During apoptosis in HL-60 cells, 3-nitrotyrosine and 3,5-dinitrotyrosine were detected by high-performance liquid chromatography and liquid chromatography-mass spectrometry-mass spectrometry. We hypothesized that PAN might induce cell death in human leukemia cells by releasing peroxynitrite and other reactive oxygen species (ROS) such as superoxide and hydrogen peroxide. Moreover, exogenous superoxide dismutase promoted PAN-induced apoptosis, and in contrast, a combination of superoxide dismutase and catalase suppressed this apoptosis. We also hypothesize that the generation of ROS during PAN-induced apoptosis in HL-60 cells could activate stress-activated protein kinase/jun N-terminal kinase activity. The formation of H2O2 produced from the dismutation of PAN-elicited superoxide anion contributed to the apoptotic mechanism in HL-60 cells through ROS pathways. These findings suggested that induction of apoptosis by the air pollutant PAN might occur as a result of the release of ROS.

Electrochemical analysis of protein nitrotyrosine and dityrosine in the Alzheimer brain indicates region-specific accumulation

HPLC with electrochemical array detection (HPLC-ECD) was used to quantify 3,3'-dityrosine (diTyr) and 3-nitrotyrosine (3-NO2-Tyr) in four regions of the human brain that are differentially affected in Alzheimer's disease (AD). DiTyr and 3-NO2-Tyr levels were elevated consistently in the hippocampus and neocortical regions of the AD brain and in ventricular cerebrospinal fluid (VF), reaching quantities five- to eightfold greater than mean concentrations in brain and VF of cognitively normal subjects. Uric acid, a proposed peroxynitrite scavenger, was decreased globally in the AD brain and VF. The results suggest that AD pathogenesis may involve the activation of oxidant-producing inflammatory enzyme systems, including nitric oxide synthase.

Nitrite and nitrate analyses: a clinical biochemistry perspective

OBJECTIVE

To review the assays available for measurement of nitrite and nitrate ions in body fluids and their clinical applications.

DESIGN AND METHODS

Literature searches were done of Medline and Current Contents to November 1997.

RESULTS

The influence of dietary nitrite and nitrate on the concentrations of these ions in various body fluids is reviewed. An overview is presented of the metabolism of nitric oxide (which is converted to nitrite and nitrate). Methods for measurement of the ions are reviewed. Reference values are summarized and the changes reported in various clinical conditions. These include: infection, gastroenterological conditions, hypertension, renal and cardiac disease, inflammatory diseases, transplant rejection, diseases of the central nervous system, and others. Possible effects of environmental nitrite and nitrate on disease incidence are reviewed.

CONCLUSIONS

Most studies of changes in human disease have been descriptive. Diagnostic utility is limited because the concentrations in a significant proportion of affected individuals overlap with those in controls. Changes in concentration may also be caused by diet, outside the clinical investigational setting. The role of nitrite and nitrate assays (alongside direct measurements of nitric oxide in breath) may be restricted to the monitoring of disease progression, or response to therapy in individual patients or subgroups. Associations between disease incidence and drinking water nitrate content are controversial (except for methemoglobinemia in infants).