Ornithine-ketoacid Transaminase Research Articles

Gyrate Atrophy of the Choroid and Retina: A Review.

Gyrate atrophy (GA) of the choroid and retina is a rare autosomal recessive genetic condition characterized by elevation of the plasma level of the amino acid ornithine due to deficiency of the enzyme ornithine ketoacid aminotransferase. Accumulation of ornithine occurs in various body tissues but leads primarily to characteristic ophthalmic manifestations including myopia, cataract, progressive chorioretinal atrophy, and macular changes. Patients usually present with night blindness that starts in the first decade of life followed by visual field constriction and eventually diminution of the central visual acuity and blindness. The condition has been reported worldwide and its differential diagnosis is broad and includes choroideremia and retinitis pigmentosa. Treatment currently depends on life-long dietary modifications including restriction of the amino acid arginine in diet. This article describes in detail the pathogenesis, clinical features, multimodal imaging findings, and treatment options for GA of the choroid and retina and its complications.

Regulation of Arginine Metabolism in Germinating Peanut ( Arachis hypogea L.) Seeds

Summary Regulation of arginase (EC 3.5.3.1), ornithine ketoacid aminotransferase (EC 2.6.1.13), and ornithine carbamyltransferase (EC 2.1.3.3) was investigated by measuring the specific activities of these enzymes in the cotyledons and the embryo axis of germinating peanut ( Arachis hypogea L.) seeds grown under various conditions. It was observed that the activities of arginase and ornithine ketoacid aminotransferase of peanut cotyledons and embryo axis were reduced by auxins, ethrel, and ethylene chlorohydrin at early stages of germination. In the cotyledons, the activities of these enzymes returned to the control level as seedling growth progressed. However, in case of the embryo axis, the activities of these enzymes increased to more than the control levels. Exogenous polyamines increased the activities of these enzymes in the embryo axis and this encrease in the activities was prevented by cycloheximide. The activity of ornithine carbamyltransferase of the cotyledons and the embyo axis was not affected by any of these compounds. That the regulatory control on the embryo axis arginase and ornithine ketoacid aminotransferase was not influenced by the accompanying cotyledons, was demonstrated by cultivating the embryo axis in a synthetic medium.

Heterogeneity and complementation analysis of fibroblasts from vitamin B6 responsive and non‐responsive patients with gyrate atrophy of the choroid and retina

Fibroblasts from four pyridoxine responsive and three non-responsive patients with gyrate atrophy of the choroid and retina have been examined. Responsive patients had higher activity of ornithine ketoacid transaminase (OKT) in cell homogenates and greater incorporation of radioactivity from 14C-ornithine into protein in cultured cells in situ compared to non-responsive patients. Complementation analysis of the cells from these seven patients was performed, based on the ratio of incorporation of 14C/3H into protein in fused cells incubated in 14C-ornithine and 3H-leucine. Lack of positive complementation in these crosses suggests that pyridoxine responsive and non-responsive patients with gyrate atrophy represent different allelic mutations of the same genetic locus coding for OKT.

Gyrate atrophy of the choroid and retina. Approaches to therapy.

Gyrate atrophy of the choroid and retina is caused by deficient activity of ornithine ketoacid aminotransferase, a pyridoxal phosphate dependent enzyme. Besides the typical eye findings, abnormalities have been found on muscle biopsy, electro-encephalography, electromyography and electrocardiography, establishing this as a generalized disorder. Ornithine is markedly elevated in plasma and other body fluids. Plasma lysine, glutamate, glutamine and creatine are reduced. The possible contributions of these biochemical disturbances to the pathogenesis of gyrate atrophy are discussed. The disease is one of the few examples of an inherited chorioretinal dystrophy whose underlying biochemical defect is known. It therefore offers a unique opportunity to develop and test rational approaches to therapy. These include lowering of the abnormally high ornithine by dietary restriction of its precursor arginine, facilitation of ornithine excretion by administration of alpha-aminoisobutyric acid, replacement of deficient products such as lysine or creatine, or increasing residual enzyme activity by high levels of cofactor (vitamin B6). The results of several studies employing such approaches to therapy are presented as well as preliminary indications of possible benefit in a few patients.

Reduction of hyperornithinemia with a low protein, low arginine diet and pyridoxine in patients with a deficiency of ornithine-ketoacid transaminase (OKT) activity and gyrate atrophy of the choroid and retina

Five patients with gyrate atrophy of the choroid and retina showed a 60% or greater decline in plasma ornithine levels during a five week trial of a low protein (10–15 g/day), low arginine (0.50–0.75 g/day) diet supplemented with essential amino acids and pyridoxine administration. These declines in plasma ornithine levels were seen in Patients 1–4 with the pyridoxine non-responsive variant and in Patient 5 with the pyridoxine responsive variant. No harmful systemic side effects were noted. Patients 1–4 continued on a modified low protein (20–35 g/day), low arginine (1.25–1.75 g/day) diet as tolerated and Patient 5 on pyridoxine alone. After one year no improvement was observed in visual acuities, visual fields, final dark adapted thresholds and full field electroretinograms for four patients. In contrast to the other four patients, Patient 3, with relatively poor control of plasma ornithine levels, showed signs of progression of the chorioretinal atrophy and further reduction of electroretinographic responses. Patients 1–4 continue on the dietary regimen and Patient 5 on pyridoxine alone to determine whether any lowering of plasma ornithine levels will modify the course of their ocular disease.

Hyperornithinemia with gyrate atrophy of the choroid and retina: A disturbance in de novo formation of proline.

Two patients with hyperornithinemia and gyrate atrophy of the choroid and retina, the first report in Japan, were described. Serum ornithine was increased 10- to 15-fold and serum lysine was slightly decreased in the affected patients. Urinary excretion of ornithine was also markedly increased. There was no increase in serum proline after the oral loading of ornithine in the patients, whereas serum proline increased after the loading in normal controls. Ornithine-ketoacid transaminase (OKT) activity was markedly reduced in phytohemagglutinin transformed lymphocytes from the patients. In all of their parents and one of the siblings, OKT activity was found to be decreased to about half of the normal control values. These findings indicate that the pathway from ornithine to proline is a major route of ornithine catabolism in man. A possible pathogenesis of ocular disturbance in the patients with OKT deficiency was postulated.

Atypical gyrate atrophy of the choroid and retina and iminoglycinuria.

A 44-year-old woman with atypical gyrate atrophy and iminoglycinuria was described. The serum ornithine level and ornithine-ketoacid transaminase (OKT) activity were both normal. Urinary excretion of proline, hydroxyproline and glycine was markedly increased. This finding, together with the existence of gyrate atrophy with hyperornithinemia due to OKT deficiency, suggests that proline deficiency in the chorioretinal tissues may concern the development of gyrate atrophy.

Amino-acid metabolism and liver ultrastructure in hyperornithinemia with gyrate atrophy of the choroid and retina

Three patients with the rare hyperornithinemia with gyrate atrophy of the choroid and retina (HOGA) syndrome were studied to elucidate the metabolic derangement and its pathologic concomitants. Tenfold elevations of blood ornithine levels, decreases in lysine levels, and hitherto unreported decreases in blood glutamate and glutamine concentration were observed. The output of ornithine from muscle kidney and splanchnic beds was curtailed or reversed after intravenous glucose. Levels of ornithine in venous blood declined after oral glucose, and rose after intravenous arginine. Increased amounts of 3-amino-2-piperidone were found in the urine, but these did not increase after the arginine-induced increase in ornithine levels. Liver biopsies in two patients revealed a marked alteration in mitochondrial ultrastructure. These studies extend the knowledge of the metabolic and pathologic derangements in HOGA. These findings are consistent with a disorder of ornithine-ketoacid transaminase, but such a disorder might not account for all the observations.

Hyperornithinemia and Gyrate Atrophy of the Choroid and Retina

The diagnosis of hyperornithinemia and gyrate atrophy (HOGA) depends upon the presence of five characteristic features: (1) typical chorioretinal lesions, (2) high myopia, (3) cataracts, (4) hyperornithinemia, and (5) autosomal recessive inheritance. We have seen three patients and described four new findings: (1) decreased whole blood glutamic acid, (2) low normal intelligence, (3) hepatic mitochondrial changes, and (4) urinary excretion of ornithine methyl ester. Investigations of amino acid metabolism in vivo are consistent with the presence of a defect in ornithine keto-acid transaminase.

144: Gyrate atrofy of the choroid and retina and ornithine-ketoacid aminotransferase deficiency

In a 10 years old girl with gyrate atrofy of the choroid and retina plasma ornithine concentration was consistently elevated ranging from 1000-1250 μmol/1. The ornithine concentration of the spina fluid was similary increased. The concentration of the other amino-acids was normal in all samples. The urine showed a typical overflow aminoaciduria, with an increase in ornithine only The prenatal and postnatal history was generally uneventful. Nc clinical abnormality could be found; liver was not enlarged. Intelligence was normal. The following laboratory tests gave normal results: haematological examination, serum proteins and their electrophoretic pattern: SGOT, SGPT, LAP, γGT, bilirubin, ureum, creatinin, blood ammonia. A deficiency of ornithine keto-acid aminotransferase was found in cultured fibroblasts by radiochemical method. Pharmologic doses of Vit. B6 for 7 days and restriction of protein intake to 0.8 g/kg for 2 months did not result in normal plasma ornithine concentration. Normal ornithine concentrations were found in plasma of the parents and the healthy sibs.

HYPERORNITHINEMIA WITH GYRATE ATROPHY OF THE CHOROID AND RETINA

The constant combination of 10-20 times larger as normal plasma concentration of ornithine with gyrate atrophy of the choroid and retina was seen in 18 patients, 4 of which were children. The ornithine concentration of urine, spinal fluid and aqueous humour was similarly increased. The height and weight of the patients averaged normal for the Finnish people. Liver and spleen were regularly not enlarged. Hyperammonemia was never found. The fundus of the eye was typically involved containing sharply de-fined areas of choroidal atrophy. Electroretinogram was extinguished and dark adaptation delayed, and the visual fields became extremely narrow with increasing age. All patients were high grade myops. 11 out of the 18 had slightly subnormal intelligence.The metabolic abnormality is suggested to be decreased activity of the enzyme ornithine ketoacid amino-transferase. Autosomal recessive inheritance of the syndrome is obvious.

Extractability and Intracellular Localisation of Urea Cycle Enzym es from Rat Liver

1. Urea cycle enzymes and ornithine ketoacid transaminase were extracted from rat liver using various buffer systems and different procedures for the mechanical disintegration of the cells. Of the buffers tested, phosphate and glutathipne gave optimal results, whereas significant differences were found when other extracting media were used: activities of ornithine ketoacid transaminase and ornithine carbamyl transferase were significantly decreased in triethanolamineand in tris-[hydroxymethyl]-amiiiomethane (tris)-buffer when compared with phosphate. Arginase activity remains unchanged in phosphate and in tris-buffer, but it decreased significantly in triethanolamine-buffer. With the exception of ornithine ketoacid transaminase, no significant increase of enzyme activities were found when livers were extracted with 0.1 % cetylammonium bromide and distilled water respectively.

RAISED PLASMA-ORNITHINE AND GYRATE ATROPHY OF THE CHOROID AND RETINA

In nine patients with gyrate atrophy of the choroid and retina, plasma, urinary, cerebrospinal-fluid, and aqueous-humour ornithine concentrations were ten to twenty times higher than in controls. The nine patients came from seven families. It is suggested that the basic metabolic abnormality in patients with gyrate atrophy of the choroid and retina is decreased activity of the enzyme ornithine-ketoacid aminotransferase.

Functional properties of ornithine-ketoacid aminotransferase from rat liver

Functional properties of rat liver ornithine ketoacid aminotransferase ( l-ornithine:2-oxoacid aminotransferase, EC 2.6.1.13) were examined by two procedures: (A) The overall enzyme reaction was separated into its component half-reactions and the capacity of the enzyme to react with each of its substrates individually was investigated under different conditions. (B) The enzyme was exposed to pyridoxal phosphate, and resultant effects on enzyme activity and pyridoxal phosphate binding were analyzed. The following properties of the enzyme were disclosed by these studies. 1. 1. The reaction of the enzyme with ornithine alone is maximal between pH 8 and 10 whereas its reaction with α-ketoglutarate is maximal between pH 6 and 8. The pH optimum for the overall reaction (pH 8.15) coincides with the point of intersection of the curves for the half reactions. 2. 2. The pyridoxamine form of the enzyme, produced by the reaction of the pyridoxal enzyme with ornithine, reverts to the pyridoxal form in the absence of the second substrate. The rate of this reversion is lower in Tris than in phosphate buffer, although the catalytic activity of the enzyme is identical in both buffers. The lability of pyridoxamine at the active site is a consequence of its attachment to the apoenzyme, since free pyridoxamine phosphate does not release its amino group under the same conditions. It is suggested that this destabilizing effect of the apoenzyme is important to its role in the catalysis of the amino-transfer reaction. 3. 3. Exposure of the enzyme to excess pyridoxal phosphate is accompanied by the partial inactivation of the enzyme. This partial inactivation coincides with the binding of a small amount of pyridoxal phosphate to the enzyme. The further binding of pyridoxal phosphate produces no additional inactivation. Since pyridoxal phosphate reacts specifically with lysine residues in proteins the results suggest that certain of the lysine residues are involved in maintaining the enzyme at full catalytic capacity while others are functionally inert.

Effect of hydrazine on urea cycle enzymes in vitro and in vivo

The effect of hydrazine on urea cycle enzymes was studied in adult male rats. Addition of different concentrations of hydrazine to homogenates of rat liver produced a gradual inhibition of citrulline and urea synthesis. A daily injection of subconvulsive doses (32 mg/kg) of this toxic substance for 4 days provoked an increase of citrulline and urea contents in different tissues. The activities of urea cycle enzymes were not affected by such a treatment except for argininosuccinase where an increase was noted. The activity of ornithine-ketoacid transaminase was greatly inhibited. The results suggest that the inhibition of ornithine-ketoacid transaminase provokes an accumulation of ornithine. The presence of a high concentration of this amino acid together with an increased ammonia production stimulates urea synthesis. Under these physiological conditions, the condensation reaction of citrulline with aspartic acid may become ratelimiting in the operation of the urea cycle with the resultant accumulation of citrulline.

Effect of Vitamin E on the Urea Cycle Enzymes, Ornithine-keto Acid Transaminase, and Isocitric Dehydrogenase in Rat Liver

Albino female rats were fed for 4 to 6 months on either a vitamin E-supplemented or on a vitamin E-unsupplemented diet.Of the urea cycle enzyme activities, carbamyl phosphate synthetase and ornithine transcarbamylase were significantly higher in the vitamin E-supplemented group than in the vitamin E-unsupplemented one. Reversely, the activity of ornithine-keto acid transaminase was lower in the former group.Urea level in the serum and liver was lower in the vitamin E-supplemented group, but urea excretion was accelerated in this group.Activity of isocitric dehydrogenase was significantly higher in the vitamin E-supplemented group. From these results, it is suggested that vitamin E is efficient for the de-toxication of ammonia in the animal body.

Inhibition of pyridoxal enzymes by l-canaline

The effect of l-canaline, a structural analogue of l-ornithine, was studied on several mammalian enzymes in vitro. The results obtained with three ornithine metabolizing enzymes indicated that canaline is not an effective competitor of ornithine in these reactions. However, canaline strongly inhibited the activity of all seven pyridoxal-dependent enzymes studied, including amino acid decarboxylases [ornithine decarboxylase (EC 4.1.1.17), 5-hydroxytryptophan decarboxylase (EC 4.1.1.28)], aminotransferases [ornithine-ketoacid aminotransferase (EC 2.6.1.13), tyrosine aminotransferase (EC 2.6.1.5)], ornithine transcarbamylase (EC 2.1.3.3) and plasma diamino-oxidase (EC 4.1.3.6). The reversibility of this inhibition by excess pyridoxal phosphate, as well as a strong interaction between canaline and pyridoxal phosphate in aqueous solution, support the view that canaline inhibition is due to complex formation between canaline and the pyridoxal coenzyme. l-canaline is one of the most potent inhibitors of pyridoxal enzymes. Ornithine-ketoacid amino-transferase, for example, was inhibited by 50% in the presence of 3 ° 10 −6 M l-canaline.

Effect of Vitamin E on the Urea Cycle Enzymes, Ornithine-keto

Albino female rats were fed for 4 to 6 months on either a vitamin E-supplemented or on a vitamin E-unsupplemented diet. Of the urea cycle enzyme activities, carbamyl phosphate synthetase and ornithine transcarbamylase were significantly higher in the vitamin E-supplemented group than in the vitamin E-unsupplemented one. Reversely, the activity of ornithine-keto acid trans-aminase was lower in the former group. Urea level in the serum and liver was lower in the vitamin E-supplemented group, but urea excretion was accelerated in this group. Activity of isocitric dehydrogenase was significantly higher in the vitamin E-supple-mented group. From these results, it is suggested that vitamin E is efficient for the de-toxication of ammonia in the animal body.

The catabolism of ornithine after intravenous loading in normal subjects and two patients with hyperprolinaemia type II

The results of studies in which intravenous ornithine or proline loadings were administered to normal subjects suggest a rate-limiting function of ornithine-ketoacid aminotransferase in the conversion of ornithine to glutamate, and of proline oxidase in the conversion of proline to glutamate. The failure of Δ 1-pyrroline-5-carboxylic acid to appear in the plasma and urine during ornithine-proline loading indicates the high capacity of Δ 1-pyrroline-5-carboxylic acid dehydrogenase in the conversion of its substrate to glutamate. In two patients with hyperprolinaemia type II, the urinary excretion of Δ 1-pyrroline-5-carboxylic acid increased slightly during ornithine loading. This suggests that this inborn metabolic disease exhibits a defect in the conversion of Δ 1-pyrroline-5-carboxylic acid to glutamate and that ornithine may be converted to this substance.

Ornithine-ketoacid transaminase activity in human skin and amniotic fluid cell culture

l-Ornithine-2-oxoacid aminotransferase activity was detected in human diploid cells cultured from control skin biopsies, from amniotic fluids, and from the skin biopsy of a patient with the type of hyherornithinemia associated with homocitrullinuria and hyperammonemia. The potential usefulness of finding this enzyme activity in cultured human cells is discussed.