Involved in the uric acid degradation pathway. Catalyzes the conversion of oxamate to oxalate. H2O + oxamate = NH4(+) + oxalate kcat is 5.5 sec(-1) with oxamate as substrate. Heterodimer that consists of a 35.5 kDa large (alpha) subunit and a 20 kDa small (beta) subunit, which are synthesized from a single polypeptide. Cleaved by autocatalysis into a large (alpha) and a small (beta) subunit. Belongs to the gamma-glutamyltransferase family.