Catalyzes the formation of 5-oxoproline from gamma-glutamyl dipeptides and plays a significant role in glutathione (GSH) homeostasis. Converts both GSH and gamma-glutamyl-L-alanine to 5-oxoproline in vitro. Plays a role in detoxification of heavy metals and metalloids by recycling glutamate and maintaining GSH homeostasis. an alpha-(gamma-L-glutamyl)-L-amino acid = 5-oxo-L-proline + an L-alpha-amino acid Binds 2 Mn(2+) ions per subunit. Expressed in the central vascular bundle of roots, leaf veins, hydathodes, cauline leaves, shoot apex, sepal veins, flower receptacles and developing seeds. By cadmium and lead (PubMed:15708574). Induced by arsenite (PubMed:24214398). No visible phenotype under normal growth conditions, but mutant plants show increased tolerance to arsenite and cadmium. Belongs to the gamma-glutamylcyclotransferase family.