Ornithine

l-Ornithine
Names
	IUPAC name L-Ornithine
	Other names (+)-(S)-2,5-Diaminovaleric acid ; (+)-(S)-2,5-Diaminopentanoic acid
Identifiers
CAS Number	70-26-8 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:15729 ;
ChEMBL	ChEMBL446143 ;
ChemSpider	6026 ;
DrugBank	DB00129 ;
ECHA InfoCard	100.000.665
EC Number	200-731-7;
IUPHAR/BPS	725;
KEGG	D08302 ;
MeSH	Ornithine
PubChem CID	389;
UNII	E524N2IXA3 ;
CompTox Dashboard (EPA)	DTXSID00883219 ;
	InChI InChI=1S/C5H12N2O2/c6-3-1-2-4(7)5(8)9/h4H,1-3,6-7H2,(H,8,9)/t4-/m0/s1 Key: AHLPHDHHMVZTML-BYPYZUCNSA-N ; InChI=1/C5H12N2O2/c6-3-1-2-4(7)5(8)9/h4H,1-3,6-7H2,(H,8,9)/t4-/m0/s1; InChI=1/C5H12N2O2/c6-3-1-2-4(7)5(8)9/h4H,1-3,6-7H2,(H,8,9)/t4-/m0/s1 Key: AHLPHDHHMVZTML-BYPYZUCNBZ;
	SMILES O=C(O)[C@@H](N)CCCN;
Properties
Chemical formula	C5H12N2O2
Molar mass	132.16 g/mol
Melting point	140 °C (284 °F; 413 K)
Solubility in water	soluble
Solubility	soluble in ethanol
Acidity (pKa)	1.94
Chiral rotation ([α]D)	+11.5 (H2O, c = 6.5)
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Ornithine is a non-proteinogenic α-amino acid that plays a role in the urea cycle. Ornithine is abnormally accumulated in the body in ornithine transcarbamylase deficiency. The radical is ornithyl.^[2]

Role in urea cycle

L-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 recycled and, in a manner, is a catalyst. First, ammonia is converted into carbamoyl phosphate (H
₂NC(O)OPO²⁻
₃) by carbamoyl phosphate synthetase. Ornithine transcarbamylase catalyzes the reaction between carbamoyl phosphate and ornithine to form citrulline and phosphate (P_i). Another amino group is added from aspartate, producing arginine and denitrogenated fumarate. The resulting arginine (a guanidinium compound) is hydrolysed back to ornithine, producing urea. The amino groups of urea come from the ammonia and aspartate, and the nitrogen in ornithine remains intact.

**Reaction mechanism:**. The side-chain amino group of ornithine (Orn) attacks the carbonyl carbon of carbamoyl phosphate (CP) nucleophilically, *left*, to form a tetrahedral transition state, *middle*. Charge rearrangement releases citrulline (Cit) and phosphate (P_i), *right*.^[3]

Ornithine is not an amino acid coded for by DNA, that is, not proteinogenic. 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.^[4]

Other reactions

Ornithine, via the action of ornithine decarboxylase (E.C. 4.1.1.17), 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.^[5]

Research

Exercise fatigue

L-Ornithine supplementation attenuated fatigue in subjects in a placebo-controlled study using a cycle ergometer. The results suggested that L-ornithine has an antifatigue effect in increasing the efficiency of energy consumption and promoting the excretion of ammonia.^[6]^[7]

Weightlifting supplement

Amino acid supplements, including L-ornithine, are frequently marketed to bodybuilders and weightlifters with claims for increasing levels of human growth hormone (HGH), muscle mass, and strength. A 1993 short 4-day clinical study reported that L-ornithine in combination with L-arginine and L-lysine at 2 g/d each did not increase HGH.^[8] A review from 2002 on the topic concluded "The use of specific amino acids to stimulate GH release by athletes is not recommended."^[9]

Cirrhosis

L-Ornithine L-aspartate (LOLA), a stable salt of ornithine and aspartic acid, has been used in the treatment of cirrhosis^[10] and hepatic encephalopathy.^[11]