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      Comparison of Cytotoxicity of Cysteine and Homocysteine for Renal Epithelial Cells

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          Background: Although the cytotoxic effects of cysteine (Cys) on renal cells have been established, the effects of homocysteine (Hcy), which causes endothelial cell dysfunction, have not been well tested. We compared the direct toxicity of Hcy on renal tubular cells to that of Cys and examined the mechanism of cell toxicity. Methods: LLC-PK1 cells were incubated with test media containing 500 µ M Cys or Hcy in the presence or absence of 100 µ M copper. Lactate dehydrogenase release and thiobarbituric acid reactive substance were measured for estimating cytolysis and lipid peroxidation, respectively. The generation of hydrogen peroxide and hydroxyl radical, and the cell redox state were analyzed using the scopoletin method, salicylate-trap method, and glutathione (GSH) content, respectively. Superoxide dismutase, catalase, and vitamin E also were used for clarifying the mechanism of toxicity. Results: In the presence of copper (+ Cu), cytolysis at 16 h was more prominent in cells exposed to Cys than Hcy. In accordance with cytotoxicity, lipid peroxidation at 4 h of incubation, as well as hydrogen peroxide and hydroxyl radical formation in a shorter incubation, were remarkably greater in Cys + Cu than Hcy + Cu. The addition of Hcy, but not Cys, decreased GSH content significantly. Conclusion: In the presence of copper, Cys was extraordinarily more cytotoxic to renal cells than Hcy. Cytotoxicity from Hcy may be dependent upon depletion of cellular GSH, while Cys cytotoxicity is primarily dependent upon the generation of reactive oxygen species and lipid peroxidation.

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          Most cited references 20

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          Oxygen free radicals in ischemic acute renal failure in the rat.

          During renal ischemia, ATP is degraded to hypoxanthine. When xanthine oxidase converts hypoxanthine to xanthine in the presence of molecular oxygen, superoxide radical (O-2) is generated. We studied the role of O-2 and its reduction product OH X in mediating renal injury after ischemia. Male Sprague-Dawley rats underwent right nephrectomy followed by 60 min of occlusion of the left renal artery. The O-2 scavenger superoxide dismutase (SOD) was given 8 min before clamping and before release of the renal artery clamp. Control rats received 5% dextrose instead. Plasma creatinine was lower in SOD treated rats: 1.5, 1.0, and 0.8 mg/dl vs. 2.5, 2.5, and 2.1 mg/dl at 24, 48, and 72 h postischemia. 24 h after ischemia inulin clearance was higher in SOD treated rats than in controls (399 vs. 185 microliter/min). Renal blood flow, measured after ischemia plus 15 min of reflow, was also greater in SOD treated than in control rats. Furthermore, tubular injury, judged histologically in perfusion fixed specimens, was less in SOD treated rats. Rats given SOD inactivated by prior incubation with diethyldithiocarbamate had plasma creatinine values no different from those of control rats. The OH X scavenger dimethylthiourea (DMTU) was given before renal artery occlusion. DMTU treated rats had lower plasma creatinine than did controls: 1.7, 1.7, and 1.3 mg/dl vs. 3.2, 2.2, and 2.4 mg/dl at 24, 48, and 72 h postischemia. Neither SOD nor DMTU caused an increase in renal blood flow, urine flow rate, or solute excretion in normal rats. The xanthine oxidase inhibitor allopurinol was given before ischemia to prevent the generation of oxygen free radicals. Plasma creatinine was lower in allopurinol treated rats: 2.7, 2.2, and 1.4 mg/dl vs. 3.6, 3.5, and 2.3 mg/dl at 24, 48, and 72 h postischemia. Catalase treatment did not protect against renal ischemia, perhaps because its large size limits glomerular filtration and access to the tubular lumen. Superoxide-mediated lipid peroxidation was studied after renal ischemia. 60 min of ischemia did not increase the renal content of the lipid peroxide malondialdehyde, whereas ischemia plus 15 min reflow resulted in a large increase in kidney lipid peroxides. Treatment with SOD before renal ischemia prevented the reflow-induced increase in lipid peroxidation in renal cortical mitochondria but not in crude cortical homogenates. In summary, the oxygen free radical scavengers SOD and DMTU, and allopurinol, which inhibits free radical generation, protected renal function after ischemia. Reperfusion after ischemia resulted in lipid peroxidation; SOD decreased lipid peroxidation in cortical mitochondria after renal ischemia and reflow. We concluded that restoration of oxygen supply to ischemic kidney results in the production of oxygen free radicals, which causes renal injury by lipid peroxidation.
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            Endothelial cell injury due to copper-catalyzed hydrogen peroxide generation from homocysteine.

            We have examined whether the toxic effects of homocysteine on cultured endothelial cells could result from the formation and action of hydrogen peroxide. In initial experiments with a cell-free system, micromolar amounts of copper were found to catalyze an oxygen-dependent oxidation of homocysteine. The molar ratio of homocysteine oxidized to oxygen consumed was approximately 4.0, which suggests that oxygen was reduced to water. The addition of catalase, however, decreased oxygen consumption by nearly one-half, which suggests that H2O2 was formed during the reaction. Confirming this hypothesis, H2O2 formation was detected using the horseradish peroxidase-dependent oxidation of fluorescent scopoletin. Ceruloplasmin was also found to catalyze oxidation of homocysteine and generation of H2O2 in molar amounts equivalent to copper sulfate. Finally, homocysteine oxidation was catalyzed by normal human serum in a concentration-dependent manner. Using cultured human and bovine endothelial cells, we found that homocysteine plus copper could lyse the cells in a dose-dependent manner, an effect that was completely prevented by catalase. Homocystine plus copper was not toxic to the cells. Specific injury to endothelial cells was seen only after 4 h of incubation with homocysteine plus copper. Confirming the biochemical studies, ceruloplasmin was also found to be equivalent to Cu++ in its ability to cause injury to endothelial cells in the presence of homocysteine. Since elevated levels of homocysteine have been implicated in premature development of atherosclerosis, these findings may be relevant to the mechanism of some types of chronic vascular injury.
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              The oxidant stress of hyperhomocyst(e)inemia.

               J Loscalzo (1996)

                Author and article information

                Nephron Exp Nephrol
                Cardiorenal Medicine
                S. Karger AG
                May 2005
                03 March 2005
                : 100
                : 1
                : e11-e20
                Internal Medicine, Division of Kidney and Dialysis, Hyogo College of Medicine, Nishinomiya, Japan
                84108 Nephron Exp Nephrol 2005;100:e11–e20
                © 2005 S. Karger AG, Basel

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                Page count
                Figures: 6, References: 31, Pages: 1
                Self URI (application/pdf):
                Original Paper

                Cardiovascular Medicine, Nephrology

                Glutathione, Lipid peroxidation, Cysteine, LLC-PK1 cells, Homocysteine


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