Glutamate dehydrogenase breaks down glutamate to form ammonia and ct-ketoglutarate. Aspaate aminotransferase transfers the amino group from glutamate to oxaloacetate to form aspaate. Ammonia and aspaate both feed into the urea cycle to supply the nitrogen used to synthesize urea. Argininosuceinate synthase, argininosuccinate lyase, and arginase are enzymes that catalyze other steps of the urea cycle.