Cleaves the gamma-glutamyl bond of extracellular glutathione (gamma-Glu-Cys-Gly), glutathione conjugates (such as maresin conjugate (13R)-S-glutathionyl-(14S)-hydroxy-(4Z,7Z,9E,11E,16Z,19Z)-docosahexaenoate, MCTR1) and other gamma-glutamyl compounds (such as leukotriene C4, LTC4). The metabolism of glutathione by GGT1 releases free glutamate and the dipeptide cysteinyl-glycine, which is hydrolyzed to cysteine and glycine by dipeptidases. In the presence of high concentrations of dipeptides and some amino acids, can also catalyze a transpeptidation reaction, transferring the gamma-glutamyl moiety to an acceptor amino acid to form a new gamma-glutamyl compound. Contributes to cysteine homeostasis, glutathione homeostasis and in the conversion of the leukotriene LTC4 to LTD4. an alpha-amino acid + an N-terminal (5-L-glutamyl)-[peptide] = 5-L-glutamyl amino acid + N-terminal L-alpha-aminoacyl-[peptide] glutathione + H2O = L-cysteinylglycine + L-glutamate an S-substituted glutathione + H2O = an S-substituted L-cysteinylglycine + L-glutamate H2O + leukotriene C4 = L-glutamate + leukotriene D4 (13R)-S-glutathionyl-(14S)-hydroxy-(4Z,7Z,9E,11E,16Z,19Z)-docosahexaenoate + H2O = (13R)-S-cysteinylglycyl-(14S)-hydroxy-(4Z,7Z,9E,11E,16Z,19Z)-docosahexaenoate + L-glutamate Activated by autocatalytic cleavage. Sulfur metabolism; glutathione metabolism. Lipid metabolism; leukotriene D4 biosynthesis. Heterodimer composed of the light and heavy chains. The active site is located in the light chain. Highly expressed in kidney. Detected at lower levels in liver, lung, plexus choroideus and brain capillary endothelial cells. N-glycosylated on both chains. Cleaved by autocatalysis into a large and a small subunit and the autocatalytic cleavage is essential to the functional activation of the enzyme. Belongs to the gamma-glutamyltransferase family.