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

Detection and quantification of nitric oxide-derived oxidants in biological systems

The free radical nitric oxide (NO^•) exerts biological effects through the direct and reversible interaction with specific targets (e.g. soluble guanylate cyclase) or through the generation of secondary species, many of which can oxidize, nitrosate or nitrate biomolecules. The NO^•-derived reactive species are typically short-lived, and their preferential fates depend on kinetic and compartmentalization aspects. Their detection and quantification are technically challenging. In general, the strategies employed are based either on the detection of relatively stable end products or on the use of synthetic probes, and they are not always selective for a particular species. In this study, we describe the biologically relevant characteristics of the reactive species formed downstream from NO^•, and we discuss the approaches currently available for the analysis of NO^•, nitrogen dioxide (NO₂ ^•), dinitrogen trioxide (N₂O₃), nitroxyl (HNO), and peroxynitrite (ONOO^-/ONOOH), as well as peroxynitrite-derived hydroxyl (HO^•) and carbonate anion (CO₃ ^•-) radicals. We also discuss the biological origins of and analytical tools for detecting nitrite (NO₂ ^-), nitrate (NO₃ ^-), nitrosyl-metal complexes, S-nitrosothiols, and 3-nitrotyrosine. Moreover, we highlight state-of-the-art methods, alert readers to caveats of widely used techniques, and encourage retirement of approaches that have been supplanted by more reliable and selective tools for detecting and measuring NO^•-derived oxidants. We emphasize that the use of appropriate analytical methods needs to be strongly grounded in a chemical and biochemical understanding of the species and mechanistic pathways involved.

The prognostic value of nitrotyrosine levels in coronary heart disease: long-term evaluation in the Acute Coronary Syndrome Registry Strategy (ERICO study)

INTRODUCTION

We aimed to analyze the association of nitrotyrosine (N-TYR) levels and long-term survival in an ongoing coronary heart disease (CHD) prospective cohort, the Acute Coronary Syndrome Registry Strategy (ERICO study).

METHODS

N-TYR levels collected during acute and subacute phase from onset of acute coronary syndrome (ACS) symptoms (myocardial infarction and unstable angina) were evaluated in 342 patients. We calculated case-fatality rates (180-days, 1 year, 2 years and 4 years) and survival analyses up to 4 years using Kaplan-Meier curves and Cox regression with respective cumulative hazard ratios (95% confidence interval; 95%CI), according to N-TYR tertiles up to 4 years of follow-up. Models are presented as crude, age and sex-adjusted and further adjusted for lipids and other confounders.

RESULTS

Overall, median level of N-TYR was 208.33 nmol/l (range: 3.09 to 1500 nmol/l), regardless ACS subtype. During follow-up of 4 years, we observed 44 (12.9%) deaths. Overall survival rate was 298 (87.1%) (Survival days: 1353, 95%CI: 1320-1387 days). N-TYR levels did not associate with mortality / survival rates up to 4 years.

CONCLUSIONS

No relationship was found between N-TYR levels and mortality rates after ACS during 4-year follow-up in the ERICO study.

Measurement and Clinical Significance of Biomarkers of Oxidative Stress in Humans

Oxidative stress is the result of the imbalance between reactive oxygen species (ROS) formation and enzymatic and nonenzymatic antioxidants. Biomarkers of oxidative stress are relevant in the evaluation of the disease status and of the health-enhancing effects of antioxidants. We aim to discuss the major methodological bias of methods used for the evaluation of oxidative stress in humans. There is a lack of consensus concerning the validation, standardization, and reproducibility of methods for the measurement of the following: (1) ROS in leukocytes and platelets by flow cytometry, (2) markers based on ROS-induced modifications of lipids, DNA, and proteins, (3) enzymatic players of redox status, and (4) total antioxidant capacity of human body fluids. It has been suggested that the bias of each method could be overcome by using indexes of oxidative stress that include more than one marker. However, the choice of the markers considered in the global index should be dictated by the aim of the study and its design, as well as by the clinical relevance in the selected subjects. In conclusion, the clinical significance of biomarkers of oxidative stress in humans must come from a critical analysis of the markers that should give an overall index of redox status in particular conditions.

Bioorthogonal labelling of 3-nitrotyrosine in peptides and proteins through diazotisation mediated azidation

A bioorthogonal method of transforming 3-nitrotyrosine to 3-azidotyrosine is described, providing new opportunities to study 3-nitrotyrosine in biological samples.

Determination of nitration degrees for the birch pollen allergen Bet v 1

Nitration of tyrosine residues in the major birch pollen allergen Bet v 1 may alter the allergenic potential of the protein. The kinetics and mechanism of the nitration reaction, however, have not yet been well characterized. To facilitate further investigations, an efficient method to quantify the nitration degree (ND) of small samples of Bet v 1 is required. Here, we present a suitable method of high-performance liquid chromatography coupled to a diode array detector (HPLC-DAD) that can be photometrically calibrated using the amino acids tyrosine (Tyr) and nitrotyrosine (NTyr) without the need for nitrated protein standards. The new method is efficient and in agreement with alternative methods based on hydrolysis and amino acid analysis of tetranitromethane (TNM)-nitrated Bet v 1 standards as well as samples from nitration experiments with peroxynitrite. The results confirm the applicability of the new method for the investigation of the reaction kinetics and mechanism of protein nitration.

A simple and robust LC-MS/MS method for quantification of free 3-nitrotyrosine in human plasma from patients receiving on-pump CABG surgery

We have developed a simple, sensitive, and robust liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) method to determine free 3-nitrotyrosine concentrations in human plasma of patients receiving on-pump coronary artery bypass grafting surgery. A one-step solid-phase extraction protocol was optimized to enrich the analyte at low nanomolar concentrations. The processed samples were analyzed by LC-MS/MS with a 2.1 × 100 mm Kinetex PFP column and a triple quadrupole mass spectrometer. The method was validated for 3-nitrotyrosine concentrations close to real patient plasma levels. The relative standard deviations or relative errors of the intraday and interday determinations were all within 10%. Limit of detection and limit of quantitation were determined to be 0.034 nM and 0.112 nM, respectively, while lower limit of quantitation was below 0.625 nM. No deterioration of the column performance was noticed after running a large number of patient samples. The results showed that the 3-nitrotyrosine concentrations in coronary sinus plasma samples were elevated after cardiopulmonary bypass (CPB) procedure. The pre-CPB and post-CPB concentrations of 3-nitrotyrosine in patient plasmas were 1.494 ± 0.107 nM and 2.167 ± 0.177 nM (mean ± SEM), respectively. Application of this method to more patients in clinical studies may help validate 3-nitrotyrosine as a meaningful biomarker for nitrosative stress and link patient characteristics, clinical outcomes, and cardioprotective treatments to endogenous nitrosative stress levels.

Good mass spectrometry and its place in good science

The mass spectrometry community has expanded as instruments became more powerful, user-friendly, affordable and readily available. This opens up opportunities for novice users to perform high impact research, using highly advanced instrumentation. This introductory tutorial is targeted at the novice user working in a research setting. It aims to offer the benefit of other people's experiences and to help newcomers avoid known pitfalls and problematic issues. It discusses some of the essential features of sound analytical chemistry and highlights the need to use validated analytical methods that provide high quality results along with a measure of their uncertainty. Examples are used to illustrate potential pitfalls and their consequences.

Effects of taurine on nitric oxide and 3-nitrotyrosine levels in spleen during endotoxemia

Taurine (2-aminoethanesulfonic acid) is a free sulfur-containing β-amino acid which has antioxidant, antiinflammatory and detoxificant properties. In the present study, the role of endotoxemia on peroxynitrite formation via 3-nitrotyrosine (3-NT) detection, and the possible antioxidant effect of taurine in lipopolysaccharide (LPS)-treated guinea pigs were aimed. 40 adult male guinea pigs were divided into four groups; control, endotoxemia, taurine and taurine+endotoxemia. Animals were administered taurine (300 mg/kg), LPS (4 mg/kg) or taurine plus LPS intraperitoneally. After 6 h of incubation, when highest blood levels of taurine and endotoxin were attained, the animals were sacrificed and spleen samples were collected. The amounts of 3-nitrotyrosine and taurine were measured by HPLC, and reactive nitrogen oxide species (NOx) which are stable end products of nitric oxide was measured spectrophotometrically in spleen tissues. LPS administration significantly decreased the concentration of taurine whilst increased levels of 3-NT and NOx compared with control group. It was determined that taurine treatment decreased the levels of 3-nitrotyrosine and NOx in taurine+endotoxemia group. The group in which taurine was administered alone, contradiction to well-known antioxidant effect, taurine caused elevated concentration of 3-NT and NOx. This data suggest that taurine protects spleen against oxidative damage in endotoxemic conditions. However, the effect of taurine is different when it is administered alone. In conclusion, taurine may act as an antioxidant during endotoxemia, and as a prooxidant in healthy subjects at this dose.

Highly sensitive and positively charged precolumn derivatization reagent for amines and amino acids in liquid chromatography/electrospray ionization tandem mass spectrometry

We have developed a highly sensitive and positively charged precolumn derivatization reagent, (5-N-succinimidoxy-5-oxopentyl)triphenylphosphonium bromide (SPTPP), for amines and amino acids in liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS). The handling of the derivatization reaction is quite simple and the reagent reacts with the analytes rapidly and with high efficiency. The derivatized analytes were observed to form regular and intense product ions upon MS/MS analysis; thus, highly sensitive and selective detection was possible in the selected reaction monitoring (SRM) mode. The limits of detection of the SPTPP-derivatized analytes were less than sub-femtomole levels. The sensitivities of the derivatized analytes increased about 500-fold compared to those of underivatized analytes. Since the hydrophobicities of the samples increased after their derivatization, the resolution of the analytes improved dramatically when a reversed-phase system was used. The relative standard deviations of intra-day and inter-day variations were below 10.6% and 13.3%, respectively. The accuracy ranged between 86.6-113% and 83.4-113%, respectively. Furthermore, the developed reagent was used for the analysis of the neurotransmitter 4-aminobutanoic acid (GABA) and oxidative stress markers such as oxidized, nitrated, and halogenated tyrosines in rat serum.

Effects of long-term exposure of extremely low frequency magnetic field on oxidative/nitrosative stress in rat liver

Thirty-two adult Wistar-Albino female and male rats were used to investigate the long-term (45 days) effects of extremely low frequency magnetic field (ELF-MF; 50Hz, 1mT, 4h/day) exposure on oxidative/nitrosative stress in liver tissues of rats. The rats were divided randomly into four groups: female control (FC; n = 8) and MF-exposed female rats (F-MF; n = 8); male control (MC; n = 8) and MF-exposed male rats (M-MF; n = 8). Liver tissue from each animal was harvested and utilized for malondialdehyde (MDA) and 3-nitrotyrosine (3-NT) detection. MDA levels were measured by MDA-TBA method, while the 3-NT levels were determined by the HPLC-UV system. There were no significant differences between the MDA levels of the control (FC; MC) and MF-exposed (F-MF; M-MF) rats (P > 0.05). In the F-MF rats, 3-NT levels were significantly increased when compared to those of the FC rats (P < 0.05). There were no significant differences between the 3-NT levels of the MC and M-MF rats. In conclusion, our study suggests that the long-term ELF-MF exposure may enhance the oxidative/nitrosative stress in liver tissue of the female rats and could have a deteriorative effect on cellular proteins rather than lipids by enhancing 3-NT formation.

Methods to detect nitric oxide and its metabolites in biological samples

Nitric oxide (NO) methodology is a complex and often confusing science and the focus of many debates and discussion concerning NO biochemistry. NO is involved in many physiological processes including regulation of blood pressure, immune response, and neural communication. Therefore its accurate detection and quantification are critical to understanding health and disease. Due to the extremely short physiological half-life of this gaseous free radical, alternative strategies for the detection of reaction products of NO biochemistry have been developed. The quantification of NO metabolites in biological samples provides valuable information with regard to in vivo NO production, bioavailability, and metabolism. Simply sampling a single compartment such as blood or plasma may not always provide an accurate assessment of whole body NO status, particularly in tissues. Therefore, extrapolation of plasma or blood NO status to specific tissues of interest is no longer a valid approach. As a result, methods continue to be developed and validated which allow the detection and quantification of NO and NO-related products/metabolites in multiple compartments of experimental animals in vivo. The methods described in this review is not an exhaustive or comprehensive discussion of all methods available for the detection of NO but rather a description of the most commonly used and practical methods which allow accurate and sensitive quantification of NO products/metabolites in multiple biological matrices under normal physiological conditions.

The relationship between taurine and 3-nitrotyrosine level of hepatocytes in experimental endotoxemia

It has been proposed that taurine may function as an oxidant in a dose-dependent manner in vivo and in vitro. The present study was carried out to investigate the relationship between taurine concentration and 3-nitrotyrosine level, a stable marker of peroxynitrite action, in hepatocytes of guinea pig in endotoxemia before and after taurine administration. The levels of taurine and 3-nitrotyrosine were measured by HPLC method. In the present study, taurine was low concentration in hepatocytes exposed to endotoxemia. In taurine plus endotoxin treated animals, HPLC analysis showed higher taurine level compared with animals only supplemented with taurine. But 3-nitrotyrosine levels were same in both taurine alone and taurine plus endotoxin groups. In conclusion, taurine is able to prevent the damaging effect of peroxynitrite, at concentration measured in hepatocytes, in our experimental conditions.

Determination of plasma free 3-nitrotyrosine and tyrosine by reversed-phase liquid chromatography with 4-fluoro-7-nitrobenzofurazan derivatization

Oxidative stress plays an important role in pathogenesis of many diseases. Measurement of 3-nitrotyrosine (NO(2)Tyr), as a potential biomarker for nitric oxide-mediated damage, has recently been the focus of particular attention. We have developed an HPLC method with NBD-F pre-column derivatization followed by C(18) cartridge cleaning. Using this method we achieved limits of detection of 0.5 and 1.1 nm for NO(2)Tyr and tyrosine (Tyr), respectively, close to that achieved by LS-MS/MS. NO(2)Tyr and tyrosine concentrations were linear over the calibration ranges 0.5-100 nm and 1-320 microm, respectively, with correlation coefficients greater than 0.95. To evaluate the utility of this assay in plasma we analysed samples obtained from smokers and non-smoking subjects. Consistent with the presence of elevated oxidative stress, the plasma NO(2)Tyr concentration and NO(2)Tyr:Tyr ratio of smokers were 17.42 +/- 11.6 nm and 0.263 +/- 0.192 nm/microm with 3.8 and 3.9 times higher (both p < 0.05), respectively, than that of non-smoker controls (4.54 +/- 2.75 nm and 0.067 +/- 0.050 nm/microm, respectively). In conclusion, we have developed a novel HPLC assay for NO(2)Tyr without MS detection that is applicable to clinical studies addressing the pathophysiology and importance of oxidative stress.

Chromatographic and mass spectrometric methods for quantitative determination of 3-nitrotyrosine in biological samples and their application to human samples

The permanent modification of soluble and protein-associated tyrosine by nitration results in the formation of 3-nitrotyrosine, which can be used as a marker of "nitro-oxidative" damage to proteins. Based on the analysis of patient materials, over 40 different diseases and/or conditions have been linked to increased nitration of tyrosine. They include many cardiovascular diseases, conditions associated with immunological reactions and neurological diseases. In this article we review the existing chromatographic and mass spectrometric methods for quantitative measurements of 3-nitrotyrosine in different human biological samples including plasma, either from the free amino acid pool or from hydrolyzed proteins from different matrices.

The Influence of Free 3-Nitrotyrosine and Saliva on the Quantitative Analysis of Protein-Bound 3-Nitrotyrosine in Sputum

Background

We have recently developed a new technique for quantitatively measuring protein-bound 3-nitrotyrosine (3-NT), a footprint of nitrosative stress, utilizing high-performance liquid chromatography with an electrochemical detection (HPLC-ECD) system. Using this system, we showed that 3-NT formation was upregulated in the sputum of both COPD and asthmatic patients. However, in order to improve the accuracy of the measurement system, We have to resolve some problems which were the influence of free amino acid form of 3-NT and of salivary contamination.

Objectives

We initially investigated the amount of the free amino acid form of 3-NT in induced sputum and compared with that of protein-bound 3-NT. Next, we evaluated the concentration of protein-bound 3-NT in saliva and compared with that in induced sputum by means of HPLC-ECD.

Methods

Five male COPD patients were enrolled. Induced sputum and saliva were obtained from the patients. The free amino acid form of 3-NT in sputum and saliva was measured by HPLC-ECD, and the protein-bound 3-NT and tyrosine in sputum and saliva were enzymatically hydrolyzed by Streptomyces griseus Pronase and measured for the protein hydrolysate by HPLC-ECD.

Results

The mean value of the amount of protein-bound 3-NT was 65.0 fmol (31.2 to 106.4 fmol). On the other hand, the amount of the free amino acid form of 3-NT was under the detection limit (<10 fmol). The levels of both 3-NT (sputum: 0.55 ± 0.15 pmol/ml, saliva: 0.02 ± 0.01 pmol/ml, p < 0.01) and tyrosine (sputum: 0.81 ± 0.43 μmol/ml, saliva: 0.07 ± 0.04 μmol/ml, p < 0.01) in saliva were significantly lower than in sputum. The percentage of 3-NT in saliva to that in sputum was about 3.1%, and that of tyrosine was about 9.0%.

Conclusion

The free amino acid form of 3-NT does not affect the measurement of protein-bound 3-NT. Furthermore, the influence of salivary contamination on the measurement of protein-bound 3-NT in induced sputum by means of HPLC-ECD was very small and could be negligible.

Quantification of 3-nitrotyrosine levels using a benchtop ion trap mass spectrometry method

Oxidative damage by reactive nitrogen species is linked to the pathogenesis of numerous inflammatory disorders, including atherosclerosis. 3-Nitrotyrosine (NO2Tyr), a posttranslational modification of proteins generated by reactive nitrogen species, serves as a "molecular fingerprint" for protein modification by nitric oxide (NO)-derived oxidants. Studies demonstrate that systemic levels of protein-bound NO2Tyr serve as an independent predictor of cardiovascular risks and are modulated by statin therapy. Measurement of NO2Tyr in biological matrices may thus serve both as a quantitative index of nitrative stress in vivo and an important new prognostic marker of clinical relevance. Analytical methods for the accurate detection and quantification of trace levels of NO2Tyr in biological tissues and fluids are, thus, of considerable interest. Here, we describe a rapid, sensitive, and specific method for the quantification of NO2Tyr in biological matrices using readily available benchtop ion-trap mass spectrometry instrumentation (e.g., LCQDeca) combined with high-performance liquid chromatography (HPLC) interface. Through judicious use of stable isotopically labeled precursors as synthetic internal standards, the tandem mass spectrometric method described simultaneously adjusts for potential intrapreparative sample losses and monitors potential artifactual generation of NO2Tyr during processing. The described method permits rapid and reproducible quantification of NO2Tyr in biological and clinical specimens at the 100 fmol on column detection limit and should prove useful for studies defining the impact of reactive nitrogen species in cardiovascular disease and other inflammatory disorders.

Mechanisms underlying resistance of pancreatic islets from ALR/Lt mice to cytokine-induced destruction

Nuclear and mitochondrial genomes combine in ALR/Lt mice to produce systemically elevated defenses against free radical damage, rendering these mice resistant to immune-mediated pancreatic islet destruction. We analyzed the mechanism whereby isolated islets from ALR mice resisted proinflammatory stress mediated by combined cytokines (IL-1beta, TNF-alpha, and IFN-gamma) in vitro. Such damage entails both superoxide and NO radical generation, as well as peroxynitrite, resulting from their combination. In contrast to islets from other mouse strains, ALR islets expressed constitutively higher glutathione reductase, glutathione peroxidase, and higher ratios of reduced to oxidized glutathione. Following incubation with combined cytokines, islets from control strains produced significantly higher levels of hydrogen peroxide and NO than islets from ALR mice. Nitrotyrosine was generated in NOD and C3H/HeJ islets but not by ALR islets. Western blot analysis showed that combined cytokines up-regulated the NF-kappaB inducible NO synthase in NOD-Rag and C3H/HeJ islets but not in ALR islets. This inability of cytokine-treated ALR islets to up-regulate inducible NO synthase and produce NO correlated both with reduced kinetics of IkappaB degradation and with markedly suppressed NF-kappaB p65 nuclear translocation. Hence, ALR/Lt islets resist cytokine-induced diabetogenic stress through enhanced dissipation and/or suppressed formation of reactive oxygen and nitrogen species, impaired IkappaB degradation, and blunted NF-kappaB activation. Nitrotyrosylation of beta cell proteins may generate neoantigens; therefore, resistance of ALR islets to nitrotyrosine formation may, in part, explain why ALR mice are resistant to type 1 diabetes when reconstituted with a NOD immune system.

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

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

Comparison of spectrophotometric, HPLC and chemilumines-cence methods for 3-nitrotyrosine and peroxynitrite interaction

We have studied the interaction of 3-nitrotyrosine with peroxynitrite using three different methods; chemiluminescence, spectrophotometry and HPLC. Peroxynitrite-induced luminol or lucigenin chemiluminescence were significantly decreased by 3-nitrotyrosine, in concentration-dependent manners. The intensity of the peroxynitrite spectrum was also markedly reduced in the presence of 3-nitrotyrosine in the spectrophometric assay. However, there was no attenuation of the 3-nitrotyrosine signal in the HPLC assay after mixing with peroxynitrite. The interaction of 3-nitrotyrosine and hypochlorous acid (HOCl) was also studied via the chemiluminescence assay, where the HOCl-induced responses were markedly inhibited by 3-nitrotyrosine. These results suggest that caution should be taken when studying the levels or interactions of 3-nitrotyrosine.

Recent methodological advances in the mass spectrometric analysis of free and protein-associated 3-nitrotyrosine in human plasma

L-Tyrosine and L-tyrosine residues in proteins are attacked by various reactive-nitrogen species (RNS) including peroxynitrite to form 3-nitrotyrosine (NO(2)Tyr) and protein-associated 3-nitrotyrosine (NO(2)TyrProt). Circulating NO(2)Tyr and NO(2)TyrProt have been suggested and are widely used as biomarkers of oxidative stress in humans. In this article the mass spectrometry (MS)-based analytical methods recently reported for the quantification of circulating levels of NO(2)Tyr and NO(2)TyrProt are discussed. These methodologies differ in sensitivity, selectivity, specificity and accessibility to interferences with the latter mainly arising from artifactual formation of NO(2)Tyr and NO(2)TyrProt during sample treatment such as acidification and chemical derivatization. Application of these methodologies to healthy normal humans revealed basal circulating levels for NO(2)Tyr which range between 0.7 and 64 nM, i.e. by two orders of magnitude. Application of gas chromatography-tandem mass spectrometry (GC-tandem MS) methods by two independent research groups by using two different protocols to avoid artifactual nitration of L-tyrosine revealed almost identical mean plasma levels of the order of 1.0 nM in healthy humans. The lower limits of quantitation (LOQ) of these methods were 0.125 and 0.3n M, respectively. This order of magnitude for basal NO(2)Tyr is supported by two liquid chromatography-tandem mass spectrometry (LC-tandem MS) methods with LOQ values of 4.4 and 1.4 nM. On the basis of the data provided by GC-tandem MS and LC-tandem MS the use of a range of 0.5-3 nM for NO(2)Tyr and of 0.6 pmol/mg plasma protein or a molar ratio of 3-nitrotyrosine to tyrosine in plasma proteins of the order of 1:10(6) for NO(2)TyrProt in plasma of healthy humans as reference values appear reasonably justified. Recently reported clinical studies involving 3-nitrotyrosine as a biomarker of oxidative stress are discussed in particular from the analytical point of view.

Large-volume sample stacking-capillary electrophoresis used for the determination of 3-nitrotyrosine in rat urine

Large-volume sample stacking using the electroosmotic flow (EOF) pump technique has been investigated for the quantification of 3-nitrotyrosine in urine of diabetic rats. The best separation conditions for these highly complex samples were obtained using capillary electrophoresis (CE) in the reversed polarity mode (i.e., injecting at the cathode and detecting at the anode) using cetyltrimethylammonium bromide (CTAB) in the running buffer. The optimum CE separation conditions were achieved using a phosphate buffer prepared with 0.15M phosphoric acid and 0.5 mM CTAB adjusted to pH 6.4 with sodium hydroxide. In such CE conditions, the limit of detection (LOD) was 1.77 microM for 3-nitrotyrosine with normal injection mode, meanwhile with the large-volume sample stacking technique a more than 20-fold improvement was observed (i.e., LOD = 0.08 microM was obtained) without noticeable loss of resolution. This value allowed the detection of 3-nitrotyrosine in urine from diabetic rats. To our knowledge, this work is one of the few applications showing the great possibilities of these stacking procedures to analyse biological samples by CE.

Effects of diabetes on plasma nitrotyrosine levels

BACKGROUND

Oxidative stress plays a major role in disease processes such as atherosclerosis and diabetes. Peroxynitrite is a reaction product of nitric oxide (NO) and superoxide and a potent oxidant. The peroxynitrite-mediated tyrosine nitration, which forms nitrotyrosine (NT), is associated with several pathological conditions.

METHODS

We measured plasma NT levels using the HPLC method in 40 Mexican Americans with diabetes, but not taking medications, and 40 age- and sex-matched euglycaemic controls.

RESULTS

Plasma-free NT levels were not different between subjects with diabetes (11.0 +/- 1.7 nmol/l, n = 40) and with non-diabetes (10.4 +/- 1.5 nmol/l, n = 40). There was also no association with levels of fasting glucose (r = -0.049, P = 0.663) or 2-h glucose (r = -0.099, P = 0.390). However, females had significantly lower free NT level (7.6 +/- 1.4 nmol/l, n = 40) than males (13.8 +/- 1.7 nmol/l, n = 40, P = 0.005), which were not affected by age, smoking status, BMI and glucose levels.

CONCLUSIONS

In contrast to some earlier reports, our study shows that diabetes has no effect on plasma NT levels in Mexican Americans. We have also demonstrated lower free NT levels in females than males, which may partly explain the lower risk profile to vascular disease in women.

Inhibition of reactive nitrogen species effects in vitro and in vivo by isoflavones and soy-based food extracts

Recent studies have shown that soy isoflavone inhibits inducible nitric oxide (NO) synthase activities and is reported to have peroxynitrite scavenging ability. Consequently, we investigated whether isoflavones (daidzein and genistein) and extracts from soy-based products (miso, soymilk, tofu, soy sprout, black soybean, soybean, and yuba) would inhibit the reactive nitrogen species (RNS) effect in vitro and in vivo. In the in vitro experiments [including the protection of cellular DNA from peroxynitrite or sodium nitroprusside damage, an inhibitory effect on nitric oxide production from lipopolysaccharide (LPS)-induced RAW 264.7 cells, and nitric oxide scavenging ability], extracts from soy-based foods showed a potent antioxidant activity and an inhibiting effect on RNS activity. These effects were correlated with total isoflavone content. In the in vivo experiments, rats were given isoflavones (4.0 mg/kg bw) or soy-based product extracts (1.0 g/kg bw) orally for 1 week and were injected with vehicle H(2)O (1 mL/kg bw) or LPS (10 mg/kg bw) on the day 7. Twelve hours after treatment, the rats were killed, and blood serum was collected for analysis. The intraperitoneal administration of LPS resulted in an increase in serum nitrite, nitrate, and nitrotyrosine concentrations. These are stable metabolite end products of nitric oxide, to 4-, 16-, and 5-fold levels, (4, 10 microM and 58 +/- 14 pmol/mL), of the placebo control, respectively. Results showed that oral administration of isoflavones and extracts from soy-based products significantly decreased serum nitrite, nitrate, and nitrotyrosine levels in LPS-induced rats. This study demonstrates that soy isoflavone supplementation may inhibit RNS-induced oxidation both in vitro and in vivo.

A Novel Competitive ELISA for Both Free and Protein-Bound Nitrotyrosine

3-Nitro-L-tyrosine (nitrotyrosine) has recently been considered to be useful as a biomarker of endogenous production of several reactive nitrogen species including peroxynitrite. In the present study, nitrotyrosine was coupled to human serum albumin (HSA) using a two-step glutaraldehyde method and immunized mouse with multifocal intradermal injections. Using a conventional immunization protocol, 12 stable monoclonal antibodies (MAbs) producing cell lines recognizing nitrotyrosine were obtained. Six MAbs were selected for further characterization. A study of cross-reactions with nitrotyrosine-like compounds showed that the antibodies had a high specificity for nitrotyrosine, but no detectable reactivity with L-tyrosine, p-nitro-L-phenylalanine, o-phospho-L-tyrosine or 3-amino-L-tyrosine. Using these high titer and affinity antibodies, a competitive inhibition ELISA was developed with a lower detection limit of approximately 20 nmol/L to detect both free and protein-bound nitrotyrosine in biological systems.

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.

Accurate quantification of basal plasma levels of 3-nitrotyrosine and 3-nitrotyrosinoalbumin by gas chromatography-tandem mass spectrometry

Measurement of 3-nitro-L-tyrosine (NO(2)Tyr) and protein-related 3-nitro-L-tyrosine in human plasma is associated with numerous methodological problems which result in highly divergent basal plasma levels often ranging within two orders of magnitude. Recently, we have described an interference-free GC-tandem MS-based method for NO(2)Tyr which yielded the lowest basal plasma NO(2)Tyr levels reported thus far. This method was extended to quantify protein-associated 3-nitrotyrosine and in particular 3-nitrotyrosinated albumin (NO(2)TyrALB) in human plasma. NO(2)TyrALB and albumin (ALB) were extracted from plasma by affinity column extraction and digested enzymatically at neutral pH. 3-Nitro- L-[2H(3)]tyrosine was used as internal standard. In plasma of 18 healthy young volunteers the molar ratio of NO(2)TyrALB to albumin-derived tyrosine (TyrALB), i.e. NO(2)TyrALB/TyrALB, was determined to be 1.55+/-0.54x1:10(6) (mean+/-SD). The plasma concentration of NO(2)TyrALB was estimated as 24+/-4 nM. The NO(2)Tyr plasma levels in these volunteers were determined to be 0.73+/-0.53 nM. In the same volunteers, NO(2)TyrALB/TyrALB, NO(2)TyrALB and NO(2)Tyr were measured 15 days later and the corresponding values were determined to be 1.25+/-0.58x1:10(6), 25+/-6 nM and 0.69+/-0.16 nM. For comparison, NO(2)Tyr and NO(2)TyrALB were measured in six plasma samples from healthy volunteers by GC-MS and GC-tandem MS. Different values were found for NO(2)Tyr, i.e. 5.4+/-2.8 versus 2.7+/-1.5 nM, and comparable values for NO(2)TyrALB/TyrALB, i.e. 0.5+/-0.2x1:10(6) versus 0.4+/-0.1x1:10(6), by these methods. The ratio of the values measured by GC-MS to those measured by GC-tandem MS were 2.9+/-3.1 for NO(2)Tyr and 1.2+/-0.2 for NO(2)TyrALB/TyrALB. The present GC-tandem MS method provides accurate values of NO(2)Tyr and NO(2)TyrALB in human plasma.

Choice of different dyes to label tyrosine and nitrotyrosine

In this work, we will present some attempts to analyze tyrosine and nitrotyrosine using capillary electrophoresis and either UV-Visible detection or laser-induced fluorescence (LIF) detection. An argon ion (488 nm) laser is used for fluorescein isothiocyanate (FITC) and 7-fluoro-4-nitro-2,1,3-benzoxadiazole (NBD-F). A near infrared (780 nm) laser is used for NIR 780 derivatives. The UV-Visible limit of detection is 2.5 microM whereas it is in the range of 30 nM for LIF detection.

Simultaneous determination of 3-nitrotyrosine and tyrosine in plasma proteins of rats and assessment of artifactual tyrosine nitration

A sensitive and specific isotope dilution liquid chromatography-electrospray tandem mass spectrometry method was developed for the determination of the 3-nitrotyrosine residue levels in rat plasma proteins. The assay is based on the cleavage of proteins with concentrated hydrochloric acid to release both 3-nitrotyrosine and tyrosine. To control the potential artifactual nitration of tyrosine residues during the proteolysis, samples are spiked with (13)C(9)-labeled tyrosine and the level of (13)C(9)-labeled 3-nitrotyrosine is measured. The clean-up process entails hydrolysate fortification with 2,5,6-d(3)-3-nitrotyrosine, followed by solid-phase extraction on octadecylsilyl (to isolate tyrosine) and aminopropylsilyl (to isolate 3-nitrotyrosine) cartridges. Tyrosine and 3-nitrotyrosine fractions are mixed in an appropriate ratio prior to the analysis. The method was applied to animals exposed to ferric nitrilotriacetate to induce oxidative stress.

Artifactual nitration controlled measurement of protein-bound 3-nitro-L-tyrosine in biological fluids and tissues by isotope dilution liquid chromatography electrospray ionization tandem mass spectrometry

A sensitive and selective method is presented to accurately determine the level of protein-bound 3-nitro-L-tyrosine (NTyr) in rat plasma and kidney samples. This assay is based on isotope dilution liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). The sample preparation entails protein precipitation, acid hydrolysis with 6 N HCl, and solid-phase extraction (using reverse and aminopropyl phase cartridges) prior to the determinative step. For kidney samples, NTyr is converted into its butyl ester to improve sensitivity. The potential formation of artifactual NTyr during the acid hydrolysis step was carefully followed and determined by supplementation of the samples with (13)C-labeled L-tyrosine (Tyr) prior to protein digestion. Hence, the concomitant measurement of formation of (13)C-enriched NTyr enabled the accurate determination of artifactual NTyr. This approach was employed to measure the basal level of protein-bound NTyr in rat plasma and kidney samples, revealing levels in the range of 4-18 micromol/mol of Tyr and 50-68 micromol/mol of Tyr, respectively. No artifactual nitration of Tyr was observed in kidney proteins, whereas in the case of plasma the contribution of the artifactual response ranged from 16 to 40%. This method allows the analysis of protein-bound NTyr with a full control of the artifactual nitration of tyrosine during the proteolysis and/or sample preparation. Reliable detection of NTyr in proteins may allow insight into the role of nitric oxide-derived oxidants under various pathological conditions.

Determination of nitrotyrosine and related compounds in biological specimens by competitive enzyme immunoassay

A gas mediator, nitric oxide is converted to peroxynitrite in the presence of superoxide anion. Peroxynitrite is a potent oxidant, which injures various tissues and organs by nitration of the tyrosine residues of proteins, and it enhances the late response of inflammation. The determination of nitrated tyrosine, nitrotyrosine, which is a stable final metabolite of peroxynitrite, provides an important indicator of tissue disorders caused by peroxynitrite. This paper reports a competitive solid-phase immunoassay for measuring nitrotyrosine in various biological specimens. In this study, peroxidase-conjugated nitrotyrosine was prepared by reaction of nitrotyrosine with 1,4-benzoquinone treatment, and then it was allowed to compete with nitrotyrosine on an anti-nitrotyrosine antibody-coated 96-well multiplate. No amino acids or related compounds tested in the experiments interfered with the immune reaction of nitrotyrosine, except cysteine, which only slightly inhibited the immune reaction at the concentrations higher than 1000 times the concentration of nitrotyrosine. The limit of detection of free nitrotyrosine was approximately 500 pg/mL (2 nM) at a competition ratio (B/B(o)%) of 80%. The newly developed enzyme immunoassay (EIA) method was used for assay of nitrotyrosine in biological specimens, with the following results: (i) Lipopolysaccharide (LPS) activation of RAW264.7 cells induced a significant increase in nitrotyrosine production compared to that with nonactivated cells. N(omega)-nitro-L-arginine methyl ester decreased nitrotyrosine production with either LPS-activated or nonactivated RAW cells. There is a relationship between nitrotyrosine production and nitrite ion. (ii) The nitrotyrosine level detected in the plasma specimens from healthy volunteers was 35.21 +/- 4.87 ng/mL (135.4 +/- 18.7 nM). (iii) The concentration of nitrotyrosine in the nasal lavage fluid of allergic rhinitis patients was 41.40 +/- 20.96 ng/mL (159.02 +/- 80.6 nM). Thus, the EIA method combines sensitivity and specificity with the ability to process a large number of specimens to quantify nitrotyrosine produced with in vivo and in vitro sources.

Determination of nitrite plus nitrate and malondialdehyde in human plasma: analytical performance and the effect of smoking and exercise

The aim of this study was to evaluate the analytical performance and clinical usefulness of spectrophotometric assays for the measurement of the plasma levels of nitrite plus nitrate (NOx), and malondialdehyde (MDA), as an index of nitric oxide release and lipid peroxidation, respectively. We studied 30 healthy sedentary volunteers, 12 endurance athletes and 12 regular heavy smokers. The lower limit of quantification for plasma NOx concentration was 1 micromol/l, and linearity was observed from 1 to 40 micromol/l of NOx concentration. Variation in replicate samples within or between days was always below 5%. NOx levels were significantly higher in athletes compared to both control subjects and smokers (p<0.05 and p<0.001, respectively), as well as in healthy subjects compared to smokers (p<0.05). The analytical limit of quantification for plasma MDA concentration was 0.03 micromol/l, and linearity was observed from 0.03 to 20 micromol/l of MDA concentration. Variation in replicate samples within or between runs was <5%. Mean MDA concentration was significantly higher in smokers compared to control subjects and athletes (p<0.001). A significant inverse relationship (p<0.001) was observed when comparing NOx with MDA (r=-0.49) or LDL levels (r=-0.30) in the total population. The assays evaluated in this study proved to be sensitive, specific and practicable, and therefore suitable for routine application in clinical chemistry laboratories and/or physiopathological studies involving human blood samples.

3-Nitrotyrosine butyl ester: a novel derivative to assess tyrosine nitration in rat plasma by liquid chromatography-tandem mass spectrometry detection

A novel, sensitive, and specific method is presented for the quantification of endogenous 3-nitrotyrosine in rat plasma based on isotope dilution liquid chromatography-electrospray ionization tandem mass spectrometry, using 3-nitro-2,5,6-d(3)-l-tyrosine as an internal standard. The extraction and cleanup method entails three major steps: protein precipitation, solid-phase extraction with an aminopropyl cartridge, followed by derivatization of 3-nitrotyrosine to the corresponding butyl ester. The analysis of the stable butyl ester derivative circumvented matrix interferences, which were encountered on the analysis of the nonderivatized analyte in plasma, and thus significantly improved sensitivity. The mass spectral acquisition of 3-nitrotyrosine butyl ester was done in the positive ion mode using selected reaction monitoring of two specific transitions. The response was linear over the concentration range 1.4-28.5 nM, and the recoveries of spiked 3-nitrotyrosine in rat plasma exceeded 75%. The detection and quantification limits of 3-nitrotyrosine in rat plasma (165 microL equivalent injected) approached 0.43 and 1.4 nM (0.07 and 0.23 pmol, on column), respectively. This study also addresses the potential artifactual formation of 3-nitrotyrosine, which may lead to an overestimation of the background levels of the biomarker. Solid-phase extraction of 3-nitrotyrosine was required prior to esterification to avoid artifactual nitration of tyrosine. In this context, analysis of eight rat plasma samples showed quantifiable levels in only four of the samples of the order of 1.4-1.5 nM.

Artifact-free quantification of free 3-chlorotyrosine, 3-bromotyrosine, and 3-nitrotyrosine in human plasma by electron capture-negative chemical ionization gas chromatography mass spectrometry and liquid chromatography-electrospray ionization tandem mass spectrometry

Halogenation and nitration of biomolecules have been proposed as key mechanisms of host defense against bacteria, fungi, and viruses. Reactive oxidants also have the potential to damage host tissue, and they have been implicated in disease. In the current studies, we describe specific, sensitive, and quantitative methods for detecting three stable markers of oxidative damage: 3-chlorotyrosine, 3-bromotyrosine, and 3-nitrotyrosine. Our results indicate that electron capture-negative chemical ionization-gas chromatography/mass spectrometry (EC-NCI GC/MS) is 100-fold more sensitive than liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-MS/MS) for analyzing authentic 3-chlorotyrosine, 3-bromotyrosine, and 3-nitrotyrosine. Using an isotopomer of tyrosine to evaluate artifactual production of the analytes during sample preparation and analysis, we found that artifact generation was negligible with either technique. However, LC-MS/MS proved cumbersome for analyzing multiple samples because it required 1.5 h of run and equilibration time per analysis. In contrast, EC-NCI GC/MS required only 5 min of run time per analysis. Using EC-NCI GC/MS, we were able to detect and quantify attomole levels of free 3-chlorotyrosine, 3-bromotyrosine, and 3-nitrotyrosine in human plasma. Our results indicate that EC-NCI GC/MS is a sensitive and specific method for quantifying free 3-chlorotyrosine, 3-bromotyrosine, and 3-nitrotyrosine in biological fluids in a single, rapid analysis and that it avoids generating any of the analytes ex vivo.

The effects of peroxynitrite on erythrocytes

Endotoxin-induced peroxynitrite formation has been demonstrated in plasma. The aim of this study is to evaluate whether this has an effect on erythrocytes. For this purpose erythrocyte 3-nitrotyrosine (3-NT) level, Na+-K+ ATPase and glutathione peroxidase activities were measured both in vivo and in vitro. In vivo peroxynitrite formation was induced in rats by intraperitoneal Escherichia coli (E.coli) injection. Erythrocytes were directly incubated with peroxynitrite in the in vitro experiment. 3-NT levels were measured by reverse-phase HPLC, glutathione peroxidase, and Na+-K+ ATPase activities were measured by spectrophotometric techniques. There was a marked increase in the 3-NT levels in both experiments. However, glutathione peroxidase activity was significantly increased in in vivo experiments, while decreasing in in vitro conditions. Although Na+-K+ ATPase activities were significantly reduced by peroxynitrite in vitro, Na+-K+ ATPase activities were similar in control and E.coli-injected rat erythrocytes. Although nitrating effect of peroxynitrite does not seem to be preventable by endogenous antioxidants, this effect of peroxynitrite may not endanger erythrocytes if the oxidative damage of peroxynitrite is prevented.