Gamma-glutamyl compounds in human tissues

The unusual structure of y-glutamyl amino acids makes them resistant to hydrolysis by peptidases. Instead they are degraded by y-glutamyltransferase (y-GT) and y-glutamyl amino acid cyclotransferase. In this study the compounds excreted by patients with the rare disorder, y-GT deficiency, demonstrated the probable physiological substrates and role of y-GT in the human kidney. Abnormal metabolites excreted were reduced glutathione, y-glutamylcysteine, y-glutamylglutamine, y-glutamyl-a-aminobutyrylglycine and cysteine. The last three have not been previously described in this disorder. Cystine, a proven in vitro substrate of y-GT, was found in normal amounts. Disulphides and mixed disulphides of the above thiols were present even in fresh urine but may have been products of thiol-thiol oxidation and thiol-disulphide exchange in the bladder. Plasma levels of the y-glutamyl compounds and cysteine seemed not to be increased. pgamma-Giutamylglutamine was positively identified in plasma and cerebrospinal fluid from hyperammonaemic patients with urea cycle defects and was reabsorbed by the kidney even when plasma levels of y-glutamylglutamine were >214 f.Lmoi/L. However in patients with phenylalanine hydroxylase deficiency, y-glutamylphenylalanine was detected in their urine but not their plasma. This contrast in the renal handling of y-glutamyl amino acids may occur because y-glutamylphenylalanine, unlike y-glutamylglutamine, is a poor substrate for intracellular y-glutamyl amino acid cyclotransferase. Enzyme studies on serum, urine and fibroblasts from the two Australian patients confirmed that y-GT activity was deficient and that the y-glutamyl amino acid cyclotransferase activity in their red cells was normal. Human lens, a tissue model for the degradation of y-glutamyl compounds, was analysed for y-glutamyl amino acid cyclotransferase activity. The investigation found that this activity was located on the periphery of the lens, was higher in a child compared with adults and was lower in a cataractous (and probably GSH depleted) lens compared with lenses from normal controls of similar age. This study has shown for the first time that y-glutamylglutamine, ophthalmic acid and cysteine are physiological substrates of human renal y-GT. Hence both the hydrolysis of y-glutamyl compounds and the transpeptidation of cysteine appear to be important physiological functions of human y-GT in the renal tubule, allowing for direct synthesis of y-glutamylcysteine and some role in renal ammonium metabolism.