nitrogen metabolism urea cycle pathway ornithine cycle UPa:UPA00158 The urea cycle, also known as the ornithine cycle, is a cycle of biochemical reactions occurring in many animal organisms that produces urea from ammonia (NH4+). This cycle was the first metabolic cycle discovered (Krebs and Hensenleit, 1932) [Wikipedia]. Arginine from the diet or from protein breakdown is cleaved by the cytosolic enzyme arginase, generating urea and ornithine. In subsequent reactions of the urea cycle a new urea residue is built on the ornithine, regenerating arginine and perpetuating the cycle. KEGG:map00250 KEGG:map01100 GO:0000050 KEGG:map00330 KEGG:map01110