The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea ((NH2)2CO) from ammonia (NH3). This cycle occurs in ureotelic organisms. The urea cycle converts highly toxic ammonia to urea for excretion. This cycle was the first metabolic cycle to be discovered (Hans Krebs and Kurt Henseleit, 1932), five years before the discovery of the TCA cycle. The urea cycle takes place primarily in the liver and, to a lesser extent, in the kidneys. Amino acid catabolism results in waste ammonia. All animals need a way to excrete this product. Most aquatic organisms, or ammonotelic organisms, excrete ammonia without converting it. Ammonia is toxic, but upon excretion from aquatic species, it is diluted by the water outside the organism. Organisms that cannot easily and safely remove nitrogen as ammonia convert it to a less toxic substance such as urea or uric acid. The urea cycle mainly occurs in the liver. The urea produced by the liver is then released into the bloodstream where it travels to the kidneys and is ultimately excreted in urine. In species including birds and most insects, the ammonia is converted into uric acid or its urate salt, which is excreted in solid form. Source description: [https://en.wikipedia.org/wiki/Urea_cycle Wikipedia] Proteins on this pathway have targeted assays available via the [https://assays.cancer.gov/available_assays?wp_id=WP497 CPTAC Assay Portal]. L-Glutamate tautomer of (S)-1-Pyrroline-5-carboxylate Urea Putrescine L-Ornithine Creatine L-Proline L-Citrulline L-Arginine Formaldehyde Sarcosine Spermidine Spermine Creatinine Phosphocreatine Tautomer of L-glutamate 5-semialdehyde d28 d28 a0f a39 a39

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