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      Plasma Cysteine/Cystine Reduction Potential Correlates with Plasma Creatinine Levels in Chronic Kidney Disease

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          Abstract

          Background/Aims: Oxidative stress has been considered a nontraditional risk factor for cardiovascular disease in the chronic kidney disease (CKD) population, possibly triggered by uremic toxicity. Methods: A chromatographic method with coulometric detection was adapted to directly and simultaneously determine cysteine (Cys) and cystine (Cyss) in plasma samples. Healthy subjects and CKD subjects in different stages were analyzed. The free Cys and free Cyss levels in their plasma were determined, and the reduction potential [E<sub>h(Cyss/2Cys)</sub>] was calculated with the Nernst equation. Results: Healthy plasma presented E<sub>h(Cyss/2Cys)</sub> of –123 ± 7 mV. Plasma E<sub>h(Cyss/2Cys)</sub> correlated significantly with creatinine levels (p < 0.0001, r = 0.62). Conclusion: Plasma E<sub>h(Cyss/2Cys)</sub> correlated with increased levels of plasma creatinine, supporting the view that uremia triggers oxidative stress. In addition, it may be used as a quantitative oxidative stress biomarker in uremic conditions.

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          Most cited references36

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          The elephant in uremia: oxidant stress as a unifying concept of cardiovascular disease in uremia.

          Cardiovascular disease is the leading cause of mortality in uremic patients. In large cross-sectional studies of dialysis patients, traditional cardiovascular risk factors such as hypertension and hypercholesterolemia have been found to have low predictive power, while markers of inflammation and malnutrition are highly correlated with cardiovascular mortality. However, the pathophysiology of the disease process that links uremia, inflammation, and malnutrition with increased cardiovascular complications is not well understood. We hereby propose the hypothesis that increased oxidative stress and its sequalae is a major contributor to increased atherosclerosis and cardiovascular morbidity and mortality found in uremia. This hypothesis is based on studies that conclusively demonstrate an increased oxidative burden in uremic patients, before and particularly after renal replacement therapies, as evidenced by higher concentrations of multiple biomarkers of oxidative stress. This hypothesis also provides a framework to explain the link that activated phagocytes provide between oxidative stress and inflammation (from infectious and non-infections causes) and the synergistic role that malnutrition (as reflected by low concentrations of albumin and/or antioxidants) contributes to the increased burden of cardiovascular disease in uremia. We further propose that retained uremic solutes such as beta-2 microglobulin, advanced glycosylated end products (AGE), cysteine, and homocysteine, which are substrates for oxidative injury, further contribute to the pro-atherogenic milieu of uremia. Dialytic therapy, which acts to reduce the concentration of oxidized substrates, improves the redox balance. However, processes related to dialytic therapy, such as the prolonged use of catheters for vascular access and the use of bioincompatible dialysis membranes, can contribute to a pro-inflammatory and pro-oxidative state and thus to a pro-atherogenic state. Anti-oxidative therapeutic strategies for patients with uremia are in their very early stages; nonetheless, early studies demonstrate the potential for significant efficacy in reducing cardiovascular complications.
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            The uremic solute indoxyl sulfate induces oxidative stress in endothelial cells.

            Endothelial dysfunction and oxidative stress are matters of concern in patients with chronic renal failure (CRF). Uremic solutes retained in these patients could be involved in these processes. Notably, the protein-bound uremic solute indoxyl sulfate induces endothelial dysfunction in vitro, and has shown pro-oxidant effects. To demonstrate that indoxyl sulfate is a potential mediator of oxidative stress in endothelial cells in vitro. Indoxyl sulfate-induced oxidative stress in human umbilical vein endothelial cells (HUVEC) was studied by measuring reactive oxygen specie (ROS) production by cytofluorimetry, by analyzing the involvement of the pro-oxidative enzymes NAD(P)H oxidase, xanthine oxidase, and NO synthase, and by measuring the levels of the non-enzymatic antioxidant glutathione. We showed that indoxyl sulfate induced a significant production of ROS in HUVEC, with or without human serum albumin. We then investigated the role of pro-oxidative enzymes and measured the levels of the antioxidant glutathione. The NAD(P)H oxidase inhibitors, DPI, and apocynin, inhibited ROS production, whereas inhibitors of xanthine oxidase, NO synthase, and mitochondrial ROS had no effect. Interestingly, indoxyl sulfate strongly decreased the levels of glutathione, one of the most active antioxidant systems of the cell. In addition, the ROS production mediated by indoxyl sulfate was inhibited by the antioxidants vitamin C, vitamin E, and NAC. The uremic solute indoxyl sulfate enhances ROS production, increases NAD(P)H oxidase activity, and decreases glutathione levels in endothelial cells. Thus, indoxyl sulfate induces oxidative stress by modifying the balance between pro- and antioxidant mechanisms in endothelial cells.
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              Increased inorganic phosphate induces human endothelial cell apoptosis in vitro.

              Chronic kidney disease with hyperphosphatemia is associated with accelerated atherosclerosis and endothelial dysfunction. However, the contribution of high serum phosphate levels to endothelial injury is incompletely understood. The aim of this work was to evaluate the responses of endothelial cells to elevated levels of extracellular phosphate in vitro. High phosphate in concentrations similar to those observed in uremia-associated hyperphosphatemia (>2.5 mM) induced apoptosis in two endothelial cell lines (EAhy926 cells and GM-7373 cells). This effect was enhanced when cells were incubated for 24 h in the presence of 2.8 mM calcium instead of 1.8 mM. By treating cells with 0.5 or 1.0 mM phosphonoformic acid, an inhibitor of the phosphate transporter, death was completely prevented. The process of phosphate-induced apoptosis was further characterized by increased oxidative stress, as detected by increased ROS generation and disruption of the mitochondrial membrane potential at approximately 2 h after treatment, followed by caspase activation. These findings show that hyperphosphatemia causes endothelial cell apoptosis, a process that impairs endothelial integrity. Endothelial cell injury induced by high phosphate concentrations may be an initial event leading to vascular complications in patients with chronic kidney disease.
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                Author and article information

                Journal
                BPU
                Blood Purif
                10.1159/issn.0253-5068
                Blood Purification
                S. Karger AG
                0253-5068
                1421-9735
                2012
                March 2013
                08 November 2012
                : 34
                : 3-4
                : 231-237
                Affiliations
                aDepartamento de Patologia Básica, Universidade Federal do Paraná, Centro Politécnico, and bCenter for Health and Biological Sciences, Pontifícia Universidade Católica do Paraná, Curitiba, Brazil
                Author notes
                *Lia S. Nakao, Departamento de Patologia Básica, Room 153, Setor de Ciências Biológicas, Centro Politécnico, Universidade Federal do Paraná, Curitiba 81531-980 (Brazil), E-Mail lia.nakao@ufpr.br
                Article
                342627 Blood Purif 2012;34:231–237
                10.1159/000342627
                23147870
                86366cfb-e2d3-4590-a973-093680593f46
                © 2012 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                History
                : 17 December 2010
                : 09 August 2012
                Page count
                Figures: 3, Tables: 2, Pages: 7
                Categories
                Original Paper

                Cardiovascular Medicine,Nephrology
                Oxidative stress,Uremia,Reduction potential,Cysteine,Chronic kidney disease

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