AskDefine | Define ornithine

Dictionary Definition

ornithine n : an amino acid that does not occur in proteins but is important in the formation of urea

User Contributed Dictionary



  1. An amino acid, 2,5-diaminopentanoic acid, that is not present in protein, but is an intermediate in the biosynthesis of urea


amino acid
  • Spanish: ornitina

Extensive Definition

Ornithine is an amino acid, whose structure is:

Role in urea cycle

Ornithine is one of the products of the action of the enzyme arginase on L-arginine, creating urea. Therefore, ornithine is a central part of the urea cycle, which allows for the disposal of excess nitrogen.
Ornithine is not an amino acid coded for by DNA, and, in that sense, is not involved in protein synthesis. However, in mammalian non-hepatic tissues, the main use of the urea cycle is in arginine biosynthesis, so as an intermediate in metabolic processes, ornithine is quite important.


It is believed to not be a part of genetic code because polypeptides containing unprotected ornithines undergo spontaneous lactamization. This proved to be a problem when ornithine was artificially incorporated in 21st amino acid systems.

Other reactions

Ornithine, via the action of ornithine decarboxylase (E.C., is the starting point for the synthesis of polyamines such as putrescine.
In bacteria, such as E. coli, ornithine can be synthesized from L-glutamate.


ornithine in German: Ornithin
ornithine in Spanish: Ornitina
ornithine in French: Ornithine
ornithine in Italian: Ornitina
ornithine in Dutch: Ornithine
ornithine in Japanese: オルニチン
ornithine in Polish: Ornityna
ornithine in Russian: Диаминовалериановая кислота
ornithine in Thai: ออร์นิทีน
ornithine in Chinese: 鳥氨酸
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