The
urea cycle (also known as the
ornithine cycle) is a cycle of
biochemical reactions occurring in many animals that produces
urea ((
NH2)
2CO) from
ammonia (NH
3). This cycle was the first metabolic cycle discovered (
Hans Krebs and Kurt Henseleit, 1932), five years before the discovery of the
TCA cycle. In
mammals, the urea cycle takes place primarily in the
liver, and to a lesser extent in the
kidney.