urea cycle, a series of complex enzymatic reactions by which ammonia is detoxified in the liver. In the series of steps for disposing of the ammonia molecule, a waste product of protein metabolism, five enzymatic reactions occur as NH2 radicals are combined with carbon and oxygen atoms from carbon dioxide to form urea, which is excreted. The amino acid arginine is synthesized during the same process. Also called Krebs-Henseleit cycle.
