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Ornithine

Ornithine is an amino acid that plays a role in the urea cycle. Ornithine is abnormally
accumulated in the body in ornithine transcarbamylase deficiency.

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 (phosphate-CONH2).

Ornithine is converted into a urea derivative at the δ (terminal) nitrogen by carbamoyl phosphate.

Another nitrogen is added from aspartate, producing the denitrogenated fumarate, and
the resulting arginine (a guanidinium compound) is hydrolysed back to ornithine,
producing urea.

The nitrogens of urea come from the ammonia and aspartate, and the
nitrogen in ornithine remains intact.

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.

 

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-glutamine.

Ornithine is also the starting point for cocaine biosynthesis when decarboxylated, then
modified greatly by cytochrome P450.
Potential Medical Uses

  • 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 anti-fatigue effect in increasing the efficiency of
    energy consumption and promoting the excretion of ammonia.
  • Cirrhosis – L-Ornithine L-aspartate (LOLA), a stable salt of
    ornithine and aspartic acid have been used in the treatment of cirrhosis.
  • Weightlifting Supplement – Amino acid supplements, including L-ornithine,
    are frequently marketed to bodybuilders and weightlifters through the claim that it
    will increase levels of human growth hormone (HGH). However, clinical study
    has shown that these supplements do not increase levels of HGH with low dose
    (2 grams per day divided into two doses) supplementation.
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