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The present work was aimed to obtain information about age-dependent changes of g-glutamyltransferase (GGT) activity and the levels of non-protein sulfhydryl compounds (NPSH) in rat kidneys. In addition, protein-bound cysteine (PB-Cys), sulfane sulfur compounds and reactive oxygen species (ROS) were estimated.The results indicate that the activity of GGT and NPSH levels in the kidneys are reduced with age. At the same time, a significant increase in the level of protein-bound cysteine was observed. Simultaneously, the content of sulfane sulfur compounds was increased in the group of the oldest animals. These findings indicate that the capacity for extracellular glutathione degradation and, in consequence, the availability of cysteine for intracellular glutathione biosynthesis may be impaired. The increased PB-Cys level indicates potentiation of the thiolation reaction, i.e. development of protein-mixed disulfides. These results reveal age dependent disturbances in the thiol-disulfide equilibrium in the kidneys which leads to an imbalance between proand antioxidatory processes.The intracellular glutathione (GSH) level depends upon the equilibrium between processes during which it is consumed and its biosynthesis limited by cysteine availability. The metabolism of GSH is closely connected to Meister's g-glutamyl cycle (Meister, 1983) in which a pivotal role is played by membrane g-glutamyltransferase (GGT), an enzyme whose active centre is directed extracellularly (Scheme 1). This enzyme participates in the salvage pathway of extracellular GSH by catalyzing its hydrolysis to amino-acid components, of which cysteine is used for intracellular GSH biosynthesis. Normally, due to Vol. 49 No. 2/2002 501-507 QUARTERLY ½

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The CAPD as a replacement therapy helps to preserve thiol levels and anaerobic sulfur metabolism in erythrocytes.