Ornithine is an intermediate nonprotein-forming amino acid of the urea cycle. Arginine is converted to ornithine via the arginase enzyme, with urea as a byproduct. Ornithine combined with carbamoyl phosphate is then converted into citrulline via the ornithine transcarbamylase (OTC) enzyme. The contribution of carbamoyl phosphate results from the metabolism of ammonia by the enzyme carbamoyl phosphate synthase, and if this magnesium-dependent process is impaired, ammonia buildup, or hyperammonemia can occur.
Ornithine can also form polyamines including putrescine via the ornithine decarboxylase (ODC) enzyme, which requires pyridoxal-5-phosphate (vitamin B6) as a cofactor. Putrescine and other polyamines are crucial to the growth and proliferation of cells. Ornithine forms glutamate via ornithine aminotransferase (OAT), requiring pyridoxine (vitamin B6) as a cofactor. OAT deficiency is a rare congenital disorder characterized by gyrate atrophy of the choroid and retina, and is treated with vitamin B6 to prevent vision loss.
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Ornithine
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