Figure 6: The urea cycle. The urea cycle is a pathway of cytosolic and mitochondrial proteins involved in the conversion of ammonia (NH4) to urea for excretion via the kidneys. The enzymes in the pathway are as follows: carbamoylphosphate synthetase (CPS1), ornithine transcarbamylase (OTC); argininosuccinic acid synthase (ASS); argininosuccinic acid lyase (ASL); arginase I (ARG); nitric oxide synthase (NOS). Pearl [2].