Cleaves the gamma-glutamyl peptide bond of glutathione and glutathione-S-conjugate such as leukotriene C4 (PubMed:21447318). Does not cleaves gamma-glutamyl compounds such as gamma-glutamyl leucine (PubMed:21447318). May also catalyze a transpeptidation reaction in addition to the hydrolysis reaction, transferring the gamma-glutamyl moiety to an acceptor amino acid to form a new gamma-glutamyl compound (PubMed:21447318). Acts as a negative regulator of geranylgeranyl glutathione bioactivity by cleaving off its gamma-glutamyl group, playing a role in adaptive immune responses (PubMed:30842656). 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 H2O + leukotriene C4 = L-glutamate + leukotriene D4 an S-substituted glutathione + H2O = an S-substituted L-cysteinylglycine + L-glutamate H2O + S-[(2E,6E,10E)-geranylgeranyl]-L-glutathione = L-glutamate + S-[(2E,6E,10E)-geranylgeranyl]-L-cysteinylglycine Inhibited by serine-borate. 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. Cleaved by autocatalysis into a large and a small subunit. Glycosylated. A previous study reported that GSH and oxidized glutathione (GSSG) are not substrates for murine GGT5 (By similarity). However, this result contrasts with two studies reported that GSH is indeed a substrate for GGT5 (PubMed:21447318, PubMed:1676842). Belongs to the gamma-glutamyltransferase family.