N-acetyl-beta-glucosaminidase urine activity as a marker of early proximal tubule damage and a predictor of the long-term function of the transplanted kidneys

Abstract

Introduction: Ischaemia-reperfusion injury (IRI) is a factor leading to the damages of the transplanted kidney, what affects mainly the proximal tubules. Early monitoring of tubule damage can be an efficient tool to predict the allograft dysfunction. Present in proximal tubules, N-acetyl-beta-glucosaminidase (NAG) is a lysosomal enzyme whose excretion rises as a result of IRI or acute rejection. The aim of this study was to monitor the NAG urine activity to evaluate the early proximal tubule damage, and to try to predict the long-term function of the transplanted kidney. Material and methods: The study enrolled 87 Caucasian renal transplant recipients (61.7% males, 38.3% females, mean age 45.56±14.34 years). Urine samples were collected for NAG and creatinine analysis on the 1st day after transplantation, and then in the 3rd and 12th month. Protocol biopsies were performed in the 3rd and 12th month. Results: A significant positive correlation between NAG urine activity in the 3rd month after transplantation and creatinine concentration on the 14th (p=0.004) and 30th day (p=0.05), in the 3rd month (p=0.009) and after the 1st (p=0.005) and 2nd year (p=0.003) was observed. A statistically significantly higher urinary NAG activity in samples collected in the first 3 days and in the 3rd month after transplantation among patients with DGF (p=0.006 and p=0.03 respectively) was found. There was a significant positive correlation between NAG urine activity in the 3rd month and the grade of tubular atrophy in specimens collected in the 3rd (p=0.03) and 12th (p=0.04) month. Conclusions: Monitoring of NAG urine activity is useful in the evaluation of early proximal tubule damage and predicting the long-term function of the transplanted kidneys. Key words: allograft, CAD, DGF, kidney function, NAG. INTRODUCTION According to the United Network for Organ Sharing (UNOS) records, 40% of renal allografts are lost during the first decade after transplantation N-acetyl-beta-glucosaminidase (NAG) is a lysosomal enzyme present in the proximal tubules of the kidney, normally secreted in small concentrations as a consequence of a natural exocytosis process (Price 1992). It was shown that after administration of cadmium, contrast agents, aminoglycosides and other nephrotoxic drugs, excretion of NAG increases * e-mail: [email protected] Abbreviations: CAN, chronic allograft nephropathy; CIT, cold ischaemia; GFR, glomerular filtration rate; HLA, human leukocyte antigen; IRI, Ischaemia-reperfusion injury; NAG, N-acetyl-beta-glucosaminidase; PRA, panel-reactive antibody; TA/IF, tubular atrophy and interstitial fibrosis Urine samples were collected for NAG and creatinine analysis on the 1st day after transplantation, and then in the 3rd and 12th month. Among subjects that did not produce urine because of delayed graft function (DGF), the first sample was collected when the amount of urine reached 500 ml per day, which was not later than on the 3rd day after the transplantation. These samples corresponded with the urine samples collected on the 1st day after transplantation from patients without DGF, and they were analysed together. The collected samples were centrifuged at 4000 rpm for 10 min, and urine without the sediment was stored at -80ºC until the time of analysis. The activity of NAG was measured by the Maruhn colorimetric method using p-nitrophenyl-N-acetyl-β-Dglucosamine (Sigma-Aldrich) as a substrate RESULTS The GFR and NAG activity during the study are presented in In regard to histopathological changes in renal allograft biopsies, there was a significant positive correlation between NAG urine activity in the 3rd month after transplantation and the grade of tubular atrophy in specimens collected in the 3rd (p=0.03) and 12th (p=0.04) month There was a significant positive correlation between NAG urine activity in the 3rd month after transplantation and the grade of proteinuria in single urine samples in the 3rd month after transplantation (p=0.05), and a significant positive correlation between NAG urine activity in the 12th month, and the grade of proteinuria in single urine samples in the 12th month after transplantation (p=0.01). Moreover, NAG urine activity in the 3rd month after transplantation correlated positively with the time of dialysis before transplantation (p=0.02), and negatively with the volume of residual diuresis (p=0.05). There was no correlation between urinary NAG activity and immunosuppressive agent concentrations. A higher NAG urine activity in the 3rd month after transplantation was observed in recipients of kidneys from donors with ischaemic or haemorrhagic stroke as a reason of death (p=0.03). DISCUSSIO