An amino acid formed as a metabolic intermediate, but not involved in proteins, and of no nutritional importance.
An amino acid produced by the liver as a by-product during the conversion of ammonia to urea.
An amino acid, C6H13N303, formed from ornithine. It is sometimes used to treat patients with urea cycle defects because citrulline is not taken up by the liver but is converted to arginine in the kidney.