Ornithine δ-aminotransferase (OAT, E.C. 2.6.1.13) catalyzes the transfer of the δ-amino group from ornithine (Orn) to α-ketoglutarate (aKG), yielding glutamate-5-semialdehyde and glutamate (Glu), and vice versa. In mammals, OAT is a mitochondrial enzyme, mainly located in the liver, intestine, brain, and kidney. In general, OAT serves to form glutamate from ornithine, with the notable exception of the intestine, where citrulline (Cit) or arginine (Arg) are end products. Its main function is to control the production of signaling molecules and mediators, such as Glu itself, Cit, GABA, and aliphatic polyamines. It is also involved in proline (Pro) synthesis. Deficiency in OAT causes gyrate atrophy, a rare but serious inherited disease, a further measure of the importance of this enzyme.

Ornithine Aminotransferase, an Important Glutamate-Metabolizing Enzyme at the Crossroads of Multiple Metabolic Pathways

Ninety-four cases of a hereditary retinal degeneration in household Abyssinian cats were found in Sweden, mainly during a 3-year period. The disease was investigated by ophthalmoscopy, fluorescein angiography, electroretinography, and light microscopy. A bilateral retinopathy was usually first seen in affected cats at the age of 1.5-2 years. Fluorescein angiography did not demonstrate abnormalities of etiological significance to the disease process. A reduction mainly of a- and b-wave amplitudes in the ERG indicated a generalized photoreceptor disease. Light microscopy showed that the photoreceptor layer was primarily affected, while other retinal layers were mainly normal. The midperipheral and peripheral retina was affected more severely than the retina of the posterior pole until late stages of disease, when there was a generalized loss of photoreceptors. The clinical and laboratory findings suggest that PRA in these Abyssinian cats is a heritable photoreceptor degenerative disease with a fairly slow rate of progression.

Progressive retinal atrophy in the Abyssinian cat. Clinical characteristics.

Gyrate atrophy (GA) of the choroid and retina is a blinding chorioretinal degeneration caused by deficiency of ornithine delta-aminotransferase (OAT) The phenotype of GA is characterized by progressive concentric reduction of the visual fields and ornithine accumulation To understand better the pathogenesis of GA and to develop a model to test therapeutic strategies, we produced an OAT-deficient mouse by gene targeting Like human GA patients, adult OAT-deficient mice exhibit chronic hyperornithinemia to levels 10-15-fold above normal and massive ornithinuria Slowly progressive retinal degeneration is reflected by a gradual decline in electroretinogram amplitudes over the first 12 mo of life At 2 mo, the retinal pigment epithelium is histologically normal, but electron microscopy reveals sporadic degeneration of scattered pigment epithelial cells By 6 mo there are more diffuse abnormalities of the pigment epithelium with accumulation of large phagosomes and crystalloid inclusions Although morphologically normal at 2 mo, the photo-receptor outer segments become highly disorganized and shortened to 60% of control length by 10 mo Additionally, there is cumulative loss of the photoreceptor cells, which reaches 33% by 10 mo and is most pronounced in the central region of the retina Our results indicate that retinal pigment epithelial cells are the initial site of insult in GA and that the OAT-deficient mouse is an excellent animal model of GA in human patients

/pdf/a-mouse-model-of-gyrate-atrophy-of-the-choroid-and-retina-4lv95gn7h7.pdf

A mouse model of gyrate atrophy of the choroid and retina. Early retinal pigment epithelium damage and progressive retinal degeneration.

Eight cases of hereditary progressive retinal atrophy in Abyssinian cats in Denmark are reported. Pedigree studies indicate direct lineage to affected cats of the same breed in Sweden. The disease is bilateral, progressive, and of the generalized type, and ultimately leads to blindness.

Progressive retinal atrophy in the Abyssinian cat.

Tissue culture of cat retinal pigment epithelium

An adult male, domestic short-haired cat with generalized retinal atrophy was found to have a 60-fold increase in plasma ornithine and ornithinuria. Ornithine-8-aminotransferase activity was undetectable in its tissues and in its cultured skin fibroblasts. This feline condition is thus analogous to gyrate atrophy of the choroid and retina in humans.

Gyrate atrophy of the choroid and retina in a cat.

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Gyrate atrophy of the choroid and retina in a cat.