Impairment of glutathione biosynthetic pathway in uraemia and dialysis

Abstract

Glutathione (GSH), the predominant intracellular antioxidant, reportedly has been shown to be decreased in chronic renal failure patients, which renders these patients more susceptible to oxidative damage by free radicals. To our knowledge, the ability of erythrocytes to normalize the GSH level by de novo synthesis in uraemic and dialysis patients has not been studied previously. The main goal of the present study was to measure the activities of the enzymes that are responsible for de novo GSH generation, namely gamma-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase (GSH-S), in erythrocytes from uraemic and dialysis patients. Erythrocyte total GSH level and gamma-GCS and GSH-S activities as well as plasma malondialdehyde (MDA) levels were measured in 19 non-dialysis patients (ND), 34 haemodialysis patients (HD), 22 continuous ambulatory peritoneal dialysis patients (CAPD) and 21 normal healthy controls. The effect of a single haemodialysis session was determined in 16 HD patients. Significant decreases in GSH levels and gamma-GCS activity but not GSH-S were observed in ND, HD and CAPD patients compared with controls. However, GSH levels as well as gamma-GCS and GSH-S activities were not different among the ND, HD and CAPD patients. The decrease in GSH was strongly and positively correlated with the decrease in gamma-GCS in ND, HD and CAPD patients (r = 0.717, P<0.001; r = 0.854, P<0.001; and r = 0.603, P<0.01, respectively). In addition, plasma MDA was negatively correlated with gamma-GCS in ND, HD and CAPD patients (r = 0.721, P<0.001; r = 0.560, P<0.01; and r = 0.585, P<0.01, respectively). A single dialysis session had no effect on GSH level or on gamma-GCS and GSH-S activities. Only a significant reduction in MDA was observed at the end of dialysis. The activity of the rate-limiting enzyme in GSH biosynthesis, gamma-GCS, was significantly decreased in uraemic and dialysis patients, which explains, at least in part, frequent reports of reduced GSH levels in these patients. The decrease in gamma-GCS activity may have been secondary to inhibitory effects from uraemic factors that are not removed by standard dialysis. However, this assumption does not exclude the possibility of down-regulation of gamma-GCS protein expression and further studies in this context are recommended.