The urea cycle yields urea, the major form in which excess nitrogen is excreted from the human body, and the amino acid arginine (Brusilow and Horwich 2001). It consists of four reactions: that of ornithine and carbamoyl phosphate to form citrulline, of citrulline and aspartate to form argininosuccinate, the cleavage of argininosuccinate to yield fumarate and arginine, and the cleavage of arginine to yield urea and re-form ornithine. The carbamoyl phosphate consumed in this cycle is synthesized in the mitochondria from bicarbonate and ammonia, and this synthesis in turn is dependent on the presence of N-acetylglutamate, which allosterically activates carbamoyl synthetase I enzyme. The synthesis of N-acetylglutamate is stimulated by high levels of arginine. Increased levels of free amino acids, indicated by elevated arginine levels, thus stimulate urea synthesis.
Two enzymes catalyze the hydrolysis of arginine to yield ornithine and urea. Cytosolic ARG1 is the canonical urea cycle enzyme. Mitochondrial ARG2 likewise catalyzes urea production from arginine and may have a substantial sparing effect in patients lacking ARG1 enzyme, so its reaction is annotated here although the role of ARG2 under normal physiological conditions remains unclear.