The enzyme, found in bacteria, fungi and animals, is specific for glutathione (cf. EC?4.3.2.9, ¦Ã-glutamylcyclotransferase). The enzyme acts as a cyclotransferase, cleaving the amide bond via transamidation using the ¦Á-amine of the?L-glutamyl residue, releasing it as the cyclic 5-oxo-L-proline.