# Urea Cycle

The **Urea Cycle** is a biochemical pathway that occurs in the **liver** to eliminate excess nitrogen as urea. Nitrogen is delivered in high amounts to the liver (refer to [Cahill Cycle](/content/view/4613/index.html)), and begins in this pathway as **ammonia**.

Ammonia combines with **carbon dioxide** to form **carbamoyl phosphate**, in a reaction catalyzed by the **mitochondrial** enzyme, **CPS1**.

Carbamoyl phosphate is then combined with **ornithine** to produce **citrulline**, in a reaction catalyzed by another **mitochondrial** enzyme, **ornithine transcarbamylase.** Notably, [Ornithine Transcarbamylase Deficiency](/content/view/4273/index.html) can block the urea cycle, leading to hyperammonemia.

Citrulline and **aspartate** are converted into **argininosuccinate**, and this reaction occurs via the **argininosuccinate synthetase** enzyme. Argininosuccinate is then broken down by **argininosuccinase**, to form **arginine** and **fumarate**.

**Arginase** splits arginine into **urea** and ornithine. The urea is excreted in the urine, thereby eliminating nitrogen, whereas the ornithine is regenerated, completing the urea cycle.