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 (By similarity). Indirectly regulates multiple aspects of skeletal biology (PubMed:12810527). 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. 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. Defects in Ggt1 are a cause of glutathionuria, severe growth failure, reduced life spans and infertility. Ggt1-deficient mice have multiple metabolic abnormalities and are dwarf. Some abnormalities can be ameliorated by N-acetylcysteine treatment. Deficient mice have growth retardation, skeletal abnormalities. They are sexually immature, develop cataracts, and have coats with a gray cast. Most die between 10 and 18 weeks. They cannot break down GSH into its constituent amino acids as it passes through the proximal tubules of the kidney. They excrete large amounts of GSH in their urine, leading to a fatal cysteine deficiency. Belongs to the gamma-glutamyltransferase family.