Urea Cycle


The mammalian urea cycle, discovered by Krebs and Henseleit in 1932, takes place in the liver. It is the main chemical pathway used by ureotelic species to dispose of ammonium nitrogen by converting it into urea, which is much less toxic and efficiently excreted by the kidneys.

Keywords: urea cycle; ammonia; hyperammonaemia; amino acid; arginine

Figure 1.

The mammalian urea cycle. Enzymes are shown in red, transporters in green. AL, argininosuccinate lyase; ARG, arginase; AS, argininosuccinate synthetase; ATP, adenosine triphosphate; CITR, citrin mitochondrial glutamate–aspartate transporter; CoA, coenzyme A; CPS, carbamyl phosphate synthetase; Y LAT‐1, dibasic amino acid transporter; NAGS, N‐acetylglutamate synthetase; OTC, ornithine transcarbamylase; ORNT, mitochondrial ornithine transporter.


Further Reading

Batshaw ML, Bachmann C and Tuchman M (1998) Advances in inherited urea cycle disorders. Journal of Inherited Metabolic Disease 21(supplement 1): 1–160.

Brusilow SW and Horwich AL (2001) Urea cycle enzymes. In: Scriver CR, Beaudet AL, Sly WS and Valle D (eds) The Metabolic and Molecular Bases of Inherited Disease, pp. 1909–1963. New York: McGraw‐Hill

Caldovic L and Tuchman M (2003) N‐acetylglutamate and its changing role through evolution. Biochemical Journal 372: 279–290.

Cooper AJL and Lai JCK (1987) Cerebral ammonia metabolism in normal and hyperammonemic rats. Neurochemical Pathology 6: 67–95.

Holmes FL (1980) Hans Krebs and the discovery of the ornithine cycle. Federation Proceedings 39: 216–225.

Jackson MJ, Beaudet AL and O'Brien WE (1986) Mammalian urea cycle enzymes. Annual Reviews in Genetics 20: 431–464.

Meijer AJ, Lamers WH and Chamuleau RAFM (1990) Nitrogen metabolism and ornithine cycle function. Physiological Reviews 70: 701–748.

Powers‐Lee SG and Meister A (1988) Urea synthesis and ammonia metabolism. In: Arias IM, Jakoby WB, Popper H, Schachter D and Shafritz DA (eds) The Liver: Biology and Pathobiology, pp. 313–329. New York: Raven Press

Summar M and Tuchman M (2001) Proceedings of the consensus conference for the management of patients with urea cycle disorders. Journal of Pediatrics 138: S6–S10.

Walsh PJ and Wright P (1995) Nitrogen Metabolism and Excretion. Boca Raton, FL: CRC Press.

Withers PC (1998) Urea: diverse functions of a waste product. Clinical and Experimental Pharmacology and Physiology 25: 722–777.

Wright PA (1995) Nitrogen excretion: three end products, many physiological roles. Journal of Experimental Biology 198: 273–281.

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How to Cite close
Tuchman, Mendel(May 2005) Urea Cycle. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0003907]