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| Names | |
|---|---|
| IUPAC name (2S)-2,5-diaminopentanoic acid | |
| Other names (+)-(S)-2,5-Diaminovaleric acid (+)-(S)-2,5-Diaminopentanoic acid | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChEMBL | |
| ChemSpider | |
| DrugBank | |
| ECHA InfoCard | 100.000.665 |
| EC Number |
|
| KEGG | |
| MeSH | Ornithine |
| UNII | |
| |
| |
| Properties[1] | |
| C5H12N2O2 | |
| Molar mass | 132.16 g/mol |
| Melting point | 140 °C (284 °F; 413 K) |
| Soluble | |
| Solubility | Soluble inethanol |
| Acidity (pKa) | 1.94 |
Chiral rotation ([α]D) | +11.5 (H2O,c = 6.5) |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Ornithine is anon-proteinogenic α-amino acid that plays a role in theurea cycle. It is not incorporated into proteins during translation. Ornithine is abnormally accumulated in the body inornithine transcarbamylase deficiency, a disorder of the urea cycle. Themoiety derived from ornithine is calledornithyl.[2]
L-Ornithine is one of the products of the action of the enzymearginase onL-arginine, creatingurea. Therefore, ornithine is a central component of theurea cycle, which enables the disposal of excessnitrogen. Ornithine itself is recycled and, in a sense, acts as a catalyst.
First, ammonia is converted intocarbamoyl phosphate (H
2NC(O)OPO2−
3) bycarbamoyl phosphate synthetase.Ornithine transcarbamylase then catalyzes the reaction between carbamoyl phosphate and ornithine to formcitrulline and phosphate (Pi). Anotheramino group is contributed byaspartate, leading to the formation of arginine and the byproductfumarate. The resulting arginine, aguanidinium compound, is subsequently hydrolyzed by arginase to regenerate ornithine and release urea.
The two nitrogen atoms in urea are derived from ammonia and aspartate, while the nitrogen atoms in ornithine remain unchanged.
Ornithine is not an amino acid directly coded for byDNA; that is, it is not aproteinogenic amino acid. However, in mammalian non-hepatic tissues, the primary role of the urea cycle is often the biosynthesis of arginine. As an intermediate in metabolic pathways, ornithine is thus quite important.[4]
Ornithine, through the action ofornithine decarboxylase (EC 4.1.1.17), serves as the starting point for the synthesis ofpolyamines such asputrescine.
In bacteria such asE. coli, ornithine can be synthesized fromL-glutamate.[5]
L-Ornithine supplementation has been shown to attenuate fatigue in subjects in placebo-controlled studies using a cycle ergometer. The results suggest thatL-ornithine may exert an antifatigue effect by increasing the efficiency of energy consumption and promoting the excretion of ammonia.[6][7]
Amino acid supplements, includingL-ornithine, are frequently marketed to bodybuilders and weightlifters with claims of increasing levels ofhuman growth hormone (HGH), muscle mass, and strength. A short, four-day clinical study conducted in 1993 reported thatL-ornithine, in combination withL-arginine andL-lysine at 2 g/day each, did not increase HGH levels.[8] A review published in 2002 concluded, "The use of specific amino acids to stimulate GH release by athletes is not recommended."[9]
L-ornithineL-aspartate (LOLA), a stable salt of ornithine and aspartic acid, has been used in the treatment ofcirrhosis[10] andhepatic encephalopathy.[11]
