Catalyzes the formation of 5-oxoproline from gamma-glutamyl dipeptides and may play a significant role in glutathione homeostasis (PubMed:18515354). Induces release of cytochrome c from mitochondria with resultant induction of apoptosis (PubMed:16765912). . This is the function of ENSG00000006625 (GGCT, gamma-glutamylcyclotransferase).