Tyrosine nitration in plasma proteins from patients undergoing hemodialysis

The Mutual Contribution of 3-NT, IL-18, Albumin, and Phosphate Foreshadows Death of Hemodialyzed Patients in a 2-Year Follow-Up

Patients with chronic kidney disease (CKD), especially those who are hemodialyzed (HD), are at significantly high risk of contracting cardiovascular disease and having increased mortality. This study aimed to find potential death predictors, the measurement of which may reflect increased mortality in HD patients, and then combine the most promising ones in frames of a simple death risk assessment model. For this purpose, HD patients (n=71) with acute myocardial infarction in the last year (HD group) and healthy people (control group) as a comparative group (n=32) were included in the study. Various laboratory determinations and non-invasive cardiovascular tests were performed. Next, patients were followed for two years, and data on cardiovascular (CV) deaths were collected. On this basis, two HD groups were formed: patients who survived (HD-A, n=51) and patients who died (HD-D, n=20). To model HD mortality, 21 out of 90 potential variables collected or calculated from the raw data were selected. The best explanatory power (95.5%) was reached by a general linear model with four variables: interleukin 18, 3-nitrotyrosine, albumin, and phosphate. The interplay between immuno-inflammatory processes, nitrosative and oxidative stress, malnutrition, and calcium-phosphate disorders has been indicated to be essential in predicting CV-related mortality in studied HD patients. ClinicalTrials.gov Identifier: NCT05214872.

SIRT2 tyrosine nitration by peroxynitrite in response to renal ischemia/reperfusion injury

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are the production of renal ischemia/reperfusion (I/R). The current study is to elucidate a mechanism of SIRT2 tyrosine nitration to accelerate the cell apoptosis induced by peroxynitrite (ONOO‾), the most reactive and deleterious RNS type in renal ischemia/reperfusion (I/R) injury. Our results demonstrate that there is a significant enhancement of the 3-nitrotyrosine levels in renal tissues of Acute Kidney Injury (AKI) patients and rats that underwent renal I/R, and a positive correlation between the 3-nitrotyrosine level and renal function impairment, indicative of an accumulation of peroxynitrite. Notably, peroxynitrite-evoked nitration of SIRT2 destroyed its enzymatic activity and the capability to deacetylate FOXO3a, and enhanced expression of Bim and caspase3, facilitating renal cell apoptosis in renal ischemia/reperfusion and SIN-1(peroxynitrite donor) treatment in vitro, and these effects were reversed by FeTMPyP, a peroxynitrite decomposition scavenger. Importantly, we identified that the tyrosine 86 is responsible for SIRT2 nitration and inactivation using site-mutation assay and Mass Spectrography analysis. Altogether, these findings point to a novel protective mechanism that an inhibition of SIRT2 tyrosine nitration can be a promising strategy to prevent ischemic renal diseases involving AKI.

Common and Novel Markers for Measuring Inflammation and Oxidative Stress Ex Vivo in Research and Clinical Practice-Which to Use Regarding Disease Outcomes?

Many chronic conditions such as cancer, chronic obstructive pulmonary disease, type-2 diabetes, obesity, peripheral/coronary artery disease and auto-immune diseases are associated with low-grade inflammation. Closely related to inflammation is oxidative stress (OS), which can be either causal or secondary to inflammation. While a low level of OS is physiological, chronically increased OS is deleterious. Therefore, valid biomarkers of these signalling pathways may enable detection and following progression of OS/inflammation as well as to evaluate treatment efficacy. Such biomarkers should be stable and obtainable through non-invasive methods and their determination should be affordable and easy. The most frequently used inflammatory markers include acute-phase proteins, essentially CRP, serum amyloid A, fibrinogen and procalcitonin, and cytokines, predominantly TNFα, interleukins 1β, 6, 8, 10 and 12 and their receptors and IFNγ. Some cytokines appear to be disease-specific. Conversely, OS-being ubiquitous-and its biomarkers appear less disease or tissue-specific. These include lipid peroxidation products, e.g., F2-isoprostanes and malondialdehyde, DNA breakdown products (e.g., 8-OH-dG), protein adducts (e.g., carbonylated proteins), or antioxidant status. More novel markers include also -omics related ones, as well as non-invasive, questionnaire-based measures, such as the dietary inflammatory-index (DII), but their link to biological responses may be variable. Nevertheless, many of these markers have been clearly related to a number of diseases. However, their use in clinical practice is often limited, due to lacking analytical or clinical validation, or technical challenges. In this review, we strive to highlight frequently employed and useful markers of inflammation-related OS, including novel promising markers.

Redox Proteomes in Human Physiology and Disease Mechanisms

Redox proteomics is a field of proteomics that is concerned with the characterization of the oxidation state of proteins to gain information about their modulated structure, function, activity, and involvement in different physiological pathways. Oxidative modifications of proteins have been shown to be implicated in normal physiological processes of cells as well as in pathomechanisms leading to the development of cancer, diabetes, neurodegenerative diseases, and some rare hereditary metabolic diseases, like classic galactosemia. Reactive oxygen species generate a variety of reversible and irreversible modifications in amino acid residue side chains and within the protein backbone. These oxidative post-translational modifications (Ox-PTMs) can participate in the activation of signal transduction pathways and mediate the toxicity of harmful oxidants. Thus the application of advanced redox proteomics technologies is important for gaining insights into molecular mechanisms of diseases. Mass-spectrometry-based proteomics is one of the most powerful methods that can be used to give detailed qualitative and quantitative information on protein modifications and allows us to characterize redox proteomes associated with diseases. This Review illustrates the role and biological consequences of Ox-PTMs under basal and oxidative stress conditions by focusing on protein carbonylation and S-glutathionylation, two abundant modifications with an impact on cellular pathways that have been intensively studied during the past decade.

Hemodiafiltration is associated with reduced inflammation, oxidative stress and improved endothelial risk profile compared to high-flux hemodialysis in children

Randomized trials in adults have shown reduced all-cause and cardiovascular mortality on hemodiafiltration (HDF) compared to high-flux hemodialysis (HD), but the mechanisms leading to improved outcomes are not clear. We studied biomarkers of inflammation, oxidative stress, anti-oxidant capacity and endothelial dysfunction in 22 children (13 female, age 8–15 years). All children received HD for at least 3 months, and were then switched to HDF, keeping all dialysis related parameters and dialysis time constant. All the biomarkers of inflammation (ß2-microglobulin, IL-6, IL-10, high sensitive C-reactive protein [hsCRP]), oxidative stress (nitrotyrosine, advanced glycation end-products [AGEs], oxidized low density lipoprotein [ox-LDL] and anti-oxidant capacity) and endothelial dysfunction (asymmetric dimethyl arginine [ADMA], symmetric dimethyl arginine [SDMA]), were comparable between incident and prevalent patients on HD, suggesting that even a short dialysis vintage of 3 months on HD increases inflammation and endothelial stress. After 3 months of HDF therapy there was a significant reduction in ß2-microglobulin (p<0.001), hCRP, ADMA, SDMA, AGEs, ox-LDL (p<0.01 for all) and an increase in total antioxidant capacity (p<0.001) compared to HD. All children were maintained on the same dialyser, dialysis water quality, dialysis time and blood flow speeds suggesting that improved clearances on HDF led to an improved biomarker profile. Even in children with residual renal function there was a significant reduction in ß2 microglobulin, hsCRP, SDMA, ox-LDL and AGEs on HDF compared to HD. Children with a lower blood flow had higher inflammatory status (higher IL-6/IL-10 ratio; p = 0.04, r = -0.43). Children who achieved a higher convective volume (≥median 12.8L/m²) had lower ox-LDL (p = 0.02). In conclusion, we have shown that a significant improvement in inflammation, antioxidant capacity and endothelial risk profile is achieved even within a short time (3 months) on HDF compared to HD treatment.

Trial Registration: ClinicalTrials.gov: NCT02063776.

Alterations in conformational state of albumin in plasma in chronic hemodialyzed patients

OBJECTIVE

In chronic hemodialyzed (CH) patients the balance between production of reactive oxygen species and antioxidant defense system is disturbed and shifted towards oxidative conditions. The properties of albumin in CH patients were studied before hemodialysis (HD) and post-HD.

METHODS

Two oxidants were applied, organic t-butyl hydroperoxide (t-BOOH) and inorganic hydroperoxide (H2O2), for oxidation of albumin molecules. By comparison, albumin from healthy donors was also modified by both oxidants. The thiol content in albumin was determined by the Ellman method. Albumin properties were evaluated with the spin labelling technique using two covalently bound spin labels, maleimide (MSL) and iodoacetamide (ISL), and fatty acid spin probe, 16-doxylstearic acid (16-DS).

RESULTS

A decrease in thiols level in HD albumin was greater than in control albumin. The t-BOOH modified the microenvironment at the binding site of MSL and ISL in control albumin molecules to a greater extent than hydrogen peroxide. Control albumin treated with t-BOOH and H2O2 showed an increase in the mobility of 16-DS. However, no changes were observed in albumin from CH patients treated with either of the oxidizing agents.

CONCLUSION

Both oxidants induced strong conformational changes in albumin from healthy volunteers, but were less effective or ineffective in modification of albumin derived from CH patients. These results show that albumin from CH patients is highly modified in vivo and is not vulnerable to oxidation in the same way as normal albumin.

Carbamylation and oxidation of proteins lead to apoptotic death of lymphocytes

The apoptotic/necrotic changes in isolated human peripheral blood mononuclear cells (MNCs) subjected to hydrogen peroxide (H₂O₂), cyanate (NaOCN) and their combination were examined. The mitochondrial potential (ΔΨm), the activities of caspases (-2, -3, -6, -8 and -9) and the level of carbonyls and amino groups in proteins were determined and DNA fragmentation. Apoptotic or necrotic cells were identified by fluorescence microscopy using double staining with Hoechst 33258/propidium iodide. Treatment of MNCs with NaOCN (1 mmol/L and 2 mmol/L), alone and in combination with H₂O₂ (100 μmol/L), led to a significant decrease in the content of amine groups and a significant increase in the carbonyl level of MNCs in comparison with the control. Measurements taken at three time points (30, 60 and 150 min) showed a significant decrease in ΔΨm in MNCs incubated with H₂O₂, cyanate and their combination. The highest decrease in ΔΨm was observed after 150 min, when a combination of NaOCN and H₂O₂ was applied. We observed significant increases in the activities of caspases-2 and -3 in cells exposed to H₂O₂ and the combination of NaOCN and H₂O₂. An increase in caspase-2 but not in caspase-3 activity was noted in cells incubated with cyanate. A significant increase in caspase-9 activity in MNCs was observed in all arrangements of tested compounds in comparison with the control. In H₂O₂-treated cells, a higher level of necrotic cells was noted in comparison to apoptotic cells, whereas carbamylation led mainly to apoptotic cell death. The combination of cyanate and H₂O₂ increased the population of necrotic cells.

Changes in the conformational state of hemoglobin in hemodialysed patients with chronic renal failure

The aim of this study was to evaluate the properties of internal components of erythrocytes in chronic renal failure (CRF) patients undergoing hemodialysis (HD) in comparison to control subjects. For investigation of conformational state of hemoglobin and nonheme proteins (NHP) the maleimide spin label (MSL) in electron paramagnetic resonance (EPR) was applied. The studies were performed using MSL in whole cells and hemolysate as well as proteins separated by ion exchange chromatography and checked by electrophoresis. Additionally the level of –SH groups in hemolysate and isolated internal proteins of CRF erythrocytes was determined using 4,4′-dithiodipyridine. All measurements were performed before and after hemodialysis. Oxidative stress accompanying CRF/hemodialysed patients caused a significant decrease in the mobility of internal components inside erythrocytes indicated by MSL (P < 0.02). The significant decrease in mobility of spin labeled HbA_1c and HbA both before and after HD (P < 0.0002) as well as in nonheme proteins before hemodialysis (P < 0.05) versus control was indicated. Decrease in mobility of internal components of erythrocytes was accompanied by loss of thiols before and after hemodialysis versus control in NHP (P < 0.05), HbA_1c (P < 0.0002), and HbA (P < 0.0005). These findings showed oxidative influence of hemodialysis on hemoglobins and internal nonheme proteins in erythrocytes of CRF patients.

Mass spectrometry and redox proteomics: applications in disease

Proteomics techniques are continuously being developed to further understanding of biology and disease. Many of the pathways that are relevant to disease mechanisms rely on the identification of post-translational modifications (PTMs) such as phosphorylation, acetylation, and glycosylation. Much attention has also been focused on oxidative PTMs which include protein carbonyls, protein nitration, and the incorporation of fatty acids and advanced glycation products to amino acid side chains, amongst others. The introduction of these PTMs in the cell can occur due to the attack of reactive oxygen and nitrogen species (ROS and RNS, respectively) on proteins. ROS and RNS can be present as a result of normal metabolic processes as well as external factors such as UV radiation, disease, and environmental toxins. The imbalance of ROS and RNS with antioxidant cellular defenses leads to a state of oxidative stress, which has been implicated in many diseases. Redox proteomics techniques have been used to characterize oxidative PTMs that result as a part of normal cell signaling processes as well as oxidative stress conditions. This review highlights many of the redox proteomics techniques which are currently available for several oxidative PTMs and brings to the reader's attention the application of redox proteomics for understanding disease pathogenesis in neurodegenerative disorders and others such as cancer, kidney, and heart diseases.

Low serum ferroxidase I activity is associated with mortality in heart failure and related to both peroxynitrite-induced cysteine oxidation and tyrosine nitration of ceruloplasmin

RATIONALE

Ceruloplasmin antioxidant function is mainly related to its ferroxidase I (FeOxI) activity, which influences iron-dependent oxidative and nitrosative radical species generation. Peroxynitrite, whose production is increased in heart failure (HF), can affect ceruloplasmin antioxidant function through amino acid modification.

OBJECTIVE

We investigated the relationship between FeOxI and ceruloplasmin tyrosine and cysteine modification and explored in a cohort of patients with HF the potential clinical relevance of serum FeOxI.

METHODS AND RESULTS

In patients with chronic HF (n=96, 76 ± 9 years; New York Heart Association class, 2.9 ± 0.8) and age-matched controls (n=35), serum FeOxI, FeOxII, ceruloplasmin, nitrotyrosine-bound ceruloplasmin, B-type natriuretic peptide, norepinephrine, and high-sensitivity C-reactive protein were measured, and the patients were followed up for 24 months. Ceruloplasmin, B-type natriuretic peptide, norepinephrine, and high-sensitivity C-reactive protein were increased in HF versus controls. FeOxI was decreased in HF (-20%) and inversely related to nitrotyrosine-bound ceruloplasmin (r, -0.305; P=0.003). In HF, FeOxI lower tertile had a mortality rate doubled compared with middle-higher tertiles. FeOxI emerged as a mortality predictor (hazard ratio, 2.95; 95% confidence intervals [1.29-6.75]; P=0.011) after adjustment for age, sex, hypertension, smoking, sodium level, estimated glomerular filtration rate, and high-sensitivity C-reactive protein. In experimental settings, peroxynitrite incubation of serum samples and isolated purified ceruloplasmin reduced FeOxI activity while increasing ceruloplasmin tyrosine nitration and cysteine thiol oxidation. Reduced glutathione prevented peroxynitrite-induced FeOxI drop, tyrosine nitration, and cysteine oxidation; flavonoid(-)-epicatechin, which prevented ceruloplasmin tyrosine nitration but not cysteine oxidation, partially impeded peroxynitrite-induced FeOxI drop.

CONCLUSIONS

Reduced activity of serum FeOxI is associated with ceruloplasmin nitration and reduced survival in patients with HF. Both ceruloplasmin tyrosine nitration and cysteine thiol oxidation may be operant in vivo in peroxynitrite-induced FeOxI activity inhibition.

Redox proteomics in study of kidney-associated hypertension: new insights to old diseases

SIGNIFICANCE

The kidney helps to maintain low blood pressure in the human body, and impaired kidney function is a common attribute of aging that is often associated with high blood pressure (hypertension). Kidney-related pathologies are important contributors (either directly or indirectly) to overall human mortality. In comparison with other organs, kidney has an unusually wide range of oxidative status, ranging from the well-perfused cortex to near-anoxic medulla.

RECENT ADVANCES

Oxidative stress has been implicated in many kidney pathologies, especially chronic kidney disease, and there is considerable research interest in oxidative stress biomarkers for earlier prediction of disease onset. Proteomics approaches have been taken to study of human kidney tissue, serum/plasma, urine, and animal models of hypertension.

CRITICAL ISSUES

Redox proteomics, in which oxidative post-translational modifications can be identified in protein targets of oxidative or nitrosative stress, has not been very extensively pursued in this set of pathologies.

FUTURE DIRECTIONS

Proteomics studies of kidney and related tissues have relevance to chronic kidney disease, and redox proteomics, in particular, represents an under-exploited toolkit for identification of novel biomarkers in this commonly occurring pathology.

Selective chemoprecipitation and subsequent release of tagged species for the analysis of nitropeptides by liquid chromatography-tandem mass spectrometry

Tyrosine nitration is a low-abundance post-translational protein modification that requires appropriate enrichment techniques to enable proteomic analyses. We report a simple yet highly specific method to enrich nitropeptides by chemoprecipitation involving only two straightforward chemical modifications of the nitropeptides before capturing the obtained derivatives with a strategically designed solid-phase active ester reagent. Specifically, capping of the aliphatic amines in the peptides is done first by reductive methylation to preserve the charge state of peptides for electrospray ionization mass spectrometric analysis, followed by reduction of nitrotyrosines to the corresponding aminotyrosines. These peptides are then immobilized on the solid-phase active ester reagent, whereas other peptides carrying no free amino groups are separated from the immobilized species by thoroughly washing the beads from which the tagged peptide derivatives can easily be released by acid-catalyzed hydrolysis at room temperature. The benefits of selective enrichment from a matrix of unmodified peptides for liquid chromatography-tandem mass spectrometry are demonstrated on three synthetic nitropeptides that are nitrated fragments of biologically relevant proteins. Identification of several in vitro nitrated human plasma proteins, also implicated under various pathological processes, by database searches from the enriched and tagged tryptic nitropeptides is presented as a practical application. We also show that converting the nitro-group to the small 4-formylbenzoylamido tag does not significantly alter fragmentation properties upon collision-induced dissociation compared with those of the native nitropeptides, and at the same time this derivatization actually improves electron capture dissociation due to conversion of the electron-predator nitro-group to this novel tag.

Assay of 3-nitrotyrosine in tissues and body fluids by liquid chromatography with tandem mass spectrometric detection

3-Nitrotyrosine (3-NT) is a marker of protein nitration in physiological systems. It is present as 3-nitrotyrosine residues in proteins of tissue, extracellular matrix, plasma, and other body fluids and food. It is also present in body fluids and some beverages as free nitrotyrosine and is excreted in urine with the major urinary metabolite 3-nitro-4-hydroxyphenylacetic acid. Quantitation of 3-nitrotyrosine requires tandem mass spectrometry for specific detection. The method developed to determine 3-nitrotyrosine (along with protein glycation and oxidation adducts in a quantitative screening assay) by liquid chromatography with tandem mass spectrometric detection is described. The 3-NT residue contents of plasma protein, hemoglobin, lipoproteins, and cerebrospinal fluid protein and the concentrations of free 3-nitrotyrosine in plasma, urine, and cerebrospinal fluid are given. Changes of 3-nitrotyrosine residue and free 3-nitrotyrosine in diabetes, cirrhosis, acute and chronic renal failure, and neurological disorders, including Alzheimer's disease, are presented and compared with independent estimates.

Differential effects of dialysis and ultrafiltrate from individuals with CKD, with or without diabetes, on platelet phosphatidylserine externalization

Individuals with chronic kidney disease (CKD) and/or diabetes mellitus (DM) are at increased risk of cardiovascular events and have elevated externalization of phosphatidylserine (PS; which propagates thrombus formation) in a small subpopulation of platelets. The purpose of this study was to examine the effect of 1) removing uremic toxins by hemodialysis on PS externalization in patients with either CKD or CKD and DM and 2) ultrafiltrate (UF) from these individuals on PS externalization in healthy platelets. PS externalization was quantified by a fluorescence-activated cell sorter using annexin V in platelet-rich plasma. PS externalization was elevated threefold in CKD patients and returned to basal values during 3-h hemodialysis. In contrast, it was elevated fivefold in individuals with CKD and DM and was still threefold above control after 3-h treatment. UF significantly increased PS externalization in a small subpopulation of platelets from healthy controls. The effect of UF from individuals with CKD and DM was significantly greater than that from patients with CKD alone, and the responses were partially inhibited by the protein kinase Cδ (PKCδ) inhibitor rottlerin and the 5-hydroxytryptamine (5-HT)2A/2Creceptor antagonist ritanserin. The data suggest that uremic toxins present in UF mediate PS externalization in a small subpopulation of platelets, at least in part, via the 5-HT2A/2Creceptor and PKCδ and demonstrate that DM further enhances platelet PS externalization in CKD patients undergoing hemodialysis. This may explain, at least in part, the additional increase in vascular damage observed in CKD patients when DM is present.

Hydrogen peroxide inhibits caspase-dependent apoptosis by inactivating procaspase-9 in an iron-dependent manner

Apoptosis represents a physiological form of cell death, the perturbation of which may contribute to the development of several diseases connected with accumulation of unwanted cells or excessive cell loss. We have previously shown that the continuous presence of low concentrations of H2O2 (generated by the action of glucose oxidase) was able to inhibit caspase-mediated apoptosis in Jurkat cells. The main purpose of the present study was to elucidate the exact molecular mechanism(s) underlying this inhibitory action of H2O2. The results presented show that events like outer mitochondrial membrane permeabilization, release of cytochrome c from mitochondria, oligomerization of Apaf-1, and recruitment of procaspase-9 to apoptosomes were taking place normally, but further advancement toward activation of the execution caspases was interrupted when H2O2 was present during the apoptotic process. From the results presented in this work, it emerges that the inhibition of procaspase-9 autoactivation was probably due to the reversible oxidation of sensitive cysteine residues in this molecule. Remarkably, caspase-9 activation and the ensuing caspase cascade proceeded normally in the presence of H2O2 under conditions of iron deprivation, indicating that the inhibition of procaspase-9 activation was an iron-dependent process. Collectively, these results highlighted the potential role of available intracellular iron ions in signaling mechanisms related to apoptotic cell death.

Phenylalanine and tyrosine metabolism in chronic kidney failure

In chronic kidney failure, there is impairment in the conversion of phenylalanine to tyrosine. As a result, tyrosine and the tyrosine/phenylalanine ratio are reduced in plasma and many tissues, and phenylalanine concentrations tend to be normal or slightly increased. Although animal studies indicate that the kidney is not a major contributor to the conversion of phenylalanine to tyrosine, human studies conducted in the postabsorptive state suggest that the kidney plays a major role in the uptake of phenylalanine and its hydroxylation and release as tyrosine. The human splanchnic bed in the postabsorptive state also displays net uptake of both phenylalanine and tyrosine and hydroxylation of substantial amounts of phenylalanine to form tyrosine. In chronic renal failure (CRF) patients, splanchnic uptake of tyrosine appears to be reduced in the postabsorptive state. After an amino acid meal, there is net release of phenylalanine from the splanchnic bed in normal subjects and to an even greater degree in CRF patients; tyrosine is released postprandially in both normal subjects and CRF patients. In the postabsorptive state, tyrosine release from the kidney is largely derived from the hydroxylation of phenylalanine. In CRF, the release of tyrosine from the kidney is reduced and this reduction may be marked with advanced CRF. These observations, as well as isotope studies indicating normal phenylalanine flux, reduced tyrosine flux and impaired conversion of phenylalanine to tyrosine in CRF patients, raise the possibility that tyrosine may be an essential amino acid in this condition. Further research will be necessary to answer this question. Oxidative stress, which often increases in CRF patients, may lead to increased formation of chlorotyrosine and nitrotyrosine in plasma proteins and of nitrotyrosine in the brain. Increased nitrotyrosine is also found in kidneys of patients with diabetic nephropathy or allograft nephropathy. Increased serum concentrations of oxidation products of phenylalanine have also been observed in patients with CRF. Impaired urinary excretion also may lead to accumulation of metabolic products of both phenylalanine and tyrosine in CRF. It is not known whether the elevated protein chlorotyrosine or nitrotyrosine or increased oxidative products of phenylalanine cause adverse metabolic or toxic effects in patients with CRF.

Oxidatively-modified and glycated proteins as candidate pro-inflammatory toxins in uremia and dialysis patients

End stage renal disease (ESRD) patients accumulate blood hallmarks of protein glycation and oxidation. It is now well established that these protein damage products may represent a heterogeneous class of uremic toxins with pro-inflammatory and pro-oxidant properties. These toxins could be directly involved in the pathogenesis of the inflammatory syndrome and vascular complications, which are mainly sustained by the uremic state and bioincompatibility of dialysis therapy. A key underlying event in the toxicity of these proteinaceous solutes has been identified in scavenger receptor-dependent recognition and elimination by inflammatory and endothelial cells, which once activated generate further and even more pronounced protein injuries by a self-feeding mechanism based on inflammation and oxidative stress-derived events. This review examines the literature and provides original information on the techniques for investigating proteinaceous pro-inflammatory toxins. We have also evaluated therapeutic - either pharmacological or dialytic - strategies proposed to alleviate the accumulation of these toxins and to constrain the inflammatory and oxidative burden of ESRD.

Oxidative stress is progressively enhanced with advancing stages of CKD

BACKGROUND

Oxidative stress appears to have a central role in the pathophysiological process of uremia and its complications, including cardiovascular disease. However, there is little evidence to suggest how early oxidative stress starts developing during the progression of chronic kidney disease (CKD). The aim of this study is to assess oxidative stress activity in a cross-sectional study of patients with CKD stages 1 to 4.

METHODS

Eighty-seven steady patients (47 men, 40 women) with a median age of 62 years (range, 28 to 84 years) and mean estimated glomerular filtration rate (eGFR) of 57 mL/min (0.95 mL/s) were studied. Levels of plasma 8-isoprostanes (8-epiPGF2a) and serum total antioxidant status (TAS) were used as markers of oxidative stress. 8-epiPGF2a levels were determined by using an enzyme-linked immunosorbent assay method, whereas a chromatometric method was used to determine TAS.

RESULTS

Plasma 8-epiPGF2a levels increased significantly as CKD stages advanced (P < 0.001). There was a highly significant inverse correlation between 8-epiPGF2a level and GFR (P < 0.01). Serum TAS levels also increased in a similar fashion (P < 0.009) and showed a significant inverse correlation with GFR (P < 0.01). 8-epiPGF2a and TAS levels showed a positive correlation (P < 0.05). Multiple regression analysis showed that the most significant predictor variable for 8-epiPGF2a level was eGFR, whereas the association between eGFR and TAS was affected strongly by confounding variables, mainly uric acid level.

CONCLUSION

Oxidative stress appears to increase as CKD progresses and correlates significantly with level of renal function. Increased TAS seems to be dependent on several confounding variables, including increased uric acid levels, and therefore does not seem to be a reliable method for assessing the antioxidant capacity of patients with CKD. These results suggest that larger studies using the correct markers to assess the timing and complex interplay of oxidative stress and other risk factors during the progression of CKD should be carried out.

Nitrotyrosine and Chlorotyrosine: Clinical Significance and Biological Functions in the Vascular System

The heme-containing enzyme myeloperoxidase (MPO) is both present and active in inflammatory conditions. This enzyme is potentiated by its formation of multiple inflammatory mediators. The two most common mediators are the modified tyrosines: nitrotyrosine and 3-chlorotyrosine. Along with other modified tyrosines, these molecules have been found to be elevated in atherosclerosis, lung disease, sepsis, vasculitis, and other inflammatory diseases. By treating some of these diseases, the levels of modified tyrosines have been shown to decrease. There have been a wide range of animal models designed to study the in vivo effects of these tyrosine molecules. In addition, there are also several reports in the literature of the in vitro actions of modified tyrosine molecules demonstrated by various cell-culture models. The purpose of this review is to evaluate the clinical significance and biological functions of these modified tyrosine molecules in atherosclerosis as well as a variety of other inflammatory conditions. It is timely information because of their association with diseases as well as lack of overview of their molecular actions. This review will focus on the formation, clinical significance, and animal and cell-culture models of these important molecules.

Abnormal nitric oxide metabolism in systemic sclerosis: increased levels of nitrated proteins and asymmetric dimethylarginine

OBJECTIVES

Endothelial dysfunction is a primary event in systemic sclerosis; however, the aetiology of events and the role of nitric oxide (NO) is still unclear. The aim of the present study is to investigate whether there are abnormalities in NO metabolism in plasma from patients with primary Raynaud's phenomenon (RP) and in the pathogenesis of systemic sclerosis (SSc): limited SSc (lSSc) and diffuse (dSSc). We also wanted to investigate the effect of factors within patients' SSc serum on NO metabolism in human microvascular endothelial cells (HMECs).

METHODS

Plasma (n=89) or serum (n=80) was assayed for total nitrate and nitrite (NOx), nitration of proteins and the NO inhibitor asymmetric dimethylarginine (ADMA). HMECs were treated with patients' SSc serum and assayed for indicators of NO metabolism.

RESULTS

Plasma NOx was elevated in patients with RP or lSSc (P<0.002), but not in patients with dSSc, compared with controls. Nitrated proteins in plasma, however, were found to be very high in dSSc patients (P<0.03), compared with RP, lSSc or controls. Patients with dSSc also showed increased levels of serum ADMA (P<0.05). The high level of nitrated proteins in dSSc was strongly associated with the severity and duration of dSSc disease. Skin biopsy sections from dSSc patients also showed enhanced nitrotyrosine staining compared with controls. In HMECs, pre-incubation with SSc serum impaired the activity of nitric oxide synthase (NOS) but not the expression of inducible or endothelial NOS. SSc serum also induced a reduction in intracellular cGMP synthesis, and NOx production in the cell culture medium, but was not associated with increased cell cytotoxicity.

CONCLUSIONS

NO formation is increased in patients with primary RP or lSSc, but nitration of proteins and elevated ADMA is a particular feature of dSSc and may reflect abnormal NO regulation and/or contribute to endothelial dysfunction in SSc.

Cellular oxidative processes in relation to renal disease

This article summarizes the biochemical processes that produce reactive oxygen species (ROS) and other mediators that account for 'oxidative stress'. Formation of ROS in signal transduction cascades is illustrated from studies of kidney cell systems. The pathophysiological implications for the nephrologist are then reviewed.