Urinary thymidine glycol as a biomarker for oxidative stress after kidney transplantation

Abstract

Reactive oxygen species are generated during ischemia-reperfusion tissue injury. Oxidation of thymidine by hydroxyl radicals (HO·) causes formation of 5,6-dihydroxy-5,6-dihydrothymidine (thymidine glycol). Thymidine glycol excreted in urine can be used as a biomarker of oxidative DNA damage. The aim of this study was to investigate the oxidative DNA damage in patients showing immediate allograft function after kidney transplantation, and to find out whether this damage correlates with glomerular and tubular lesions. Time dependent changes in urinary excretion rates of thymidine glycol, but also of total protein, albumin, low molecular weight (α1-microglobulin, β2- microglobulin) and high molecular weight proteins (transferrin, IgG, α2- macroglobulin) were analyzed quantitatively and by polyacrylamide-gel electrophoresis in six patients. Urinary thymidine glycol was determined by a fluorimetric assay in combination with affinity chromatography and HPLC. After kidney transplantation the urinary excretion rate of thymidine glycol increased gradually reaching a maximum within the first 48 hours (16.56 ± 11.3 nmol/mmol creatinine, ref. 4.3 ± 0.97). Severe proteinuria with an excretion rate of up to 7.2 g/mmol creatinine was observed and declined within the first 24 hours of allograft function (0.35 ± 0.26 g/mmol creatinine). The gel-electrophoretic pattern showed a nonselective glomerular and tubular proteinuria. The initial nonselective glomerular proteinuria disappeared within 48 hours, changing to a mild selective glomerular proteinuria. In this period (12-48 hours) higher levels of thymidine glycol excretion were observed, when compared to the initial posttransplant phase (13.66 ± 9.76 vs. 4.31 ± 3.61 nmol/mmol creatinine; p < 0.05). An increased excretion of thymidine glycol is seen after kidney transplantation and is explained by the ischemia-reperfusion induced oxidative DNA damage in the kidney. In the second phase higher levels of excretion were observed parallel to the change from a nonselective to a selective glomerular and tubular proteinuria. An explanation may be sought in the repair process of DNA in the glomerular and tubular epithelial cells, appearing simultaneously with functional recovery.