L-Ornithine is one of the products of the action of the enzymearginase onL-arginine, creatingurea. Therefore, ornithine is a central part of the urea cycle, which allows for the disposal of excessnitrogen. Ornithine is recycled and, in a manner, is a catalyst. First, ammonia is converted intocarbamoyl phosphate (H 2NC(O)OPO2− 3) bycarbamoyl phosphate synthetase.Ornithine transcarbamylase catalyzes the reaction between carbamoyl phosphate and ornithine to formcitrulline and phosphate (Pi). Anotheramino group is added fromaspartate, producing arginine and denitrogenatedfumarate. The resulting arginine (aguanidinium 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:. Theside-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 (Pi),right.[3]
Ornithine is not an amino acid coded for byDNA, that is, notproteinogenic. 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]
L-Ornithine supplementation attenuated fatigue in subjects in a placebo-controlled study using a cycle ergometer. The results suggested thatL-ornithine has an antifatigue effect in 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 for increasing levels ofhuman growth hormone (HGH), muscle mass, and strength. A 1993 short 4-day clinical study reported thatL-ornithine in combination withL-arginine andL-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]
^"Ornithine Biosynthesis". School of Biological and Chemical Sciences, Queen Mary, University of London. Archived fromthe original on 2012-04-14. Retrieved2007-08-17.
^Sugino T, Shirai T, Kajimoto Y, Kajimoto O (November 2008). "L-ornithine supplementation attenuates physical fatigue in healthy volunteers by modulating lipid and amino acid metabolism".Nutrition Research.28 (11):738–43.doi:10.1016/j.nutres.2008.08.008.PMID19083482.
^Fogelholm GM, Näveri HK, Kiilavuori KT, Härkönen MH (September 1993). "Low-dose amino acid supplementation: no effects on serum human growth hormone and insulin in male weightlifters".International Journal of Sport Nutrition.3 (3):290–7.doi:10.1123/ijsn.3.3.290.PMID8220394.
