L-Ornithine-ketoacid-transaminase deficiency in cultured fibroblasts of a patient with hyperornithinaemia and gyrate atrophy of the choroid and retina

Ocular genetics: current understanding

Over the past decade, there has been an exponential increase in our knowledge of heritable eye conditions. Coincidentally, our ability to provide accurate genetic diagnoses has allowed appropriate counseling to patients and families. A summary of our current understanding of ocular genetics will prove useful to clinicians, researchers, and students as an introduction to the subject.

Characteristics of L-ornithine: 2-oxoacid aminotransferase and potential prenatal diagnosis of gyrate atrophy of the choroid and retina by first trimester chorionic villus sampling

A deficiency of the mitochondrial matrix enzyme L-ornithine: 2-oxoacid aminotransferase causes gyrate atrophy of the choroid and retina with hyperornithinemia (MIM 258870), a blinding degenerative disease, which is inherited as an autosomal recessive trait. We have developed a sensitive microradioisotopic method for enzyme assay by using 2-oxo-[5-14C] glutarate as the substrate and performing the separation of the product, [5-14C] glutamate from the substrate on a cation-exchange column. The enzyme activity was determined in human and rat tissues and in cultured cells. The enzyme activity in fibroblasts from a patient was deficient and that of the parents ranged between 25 and 60% of the control values. In addition we have found the enzyme expressed in native and cultured chorionic villi indicating a potential detection of the disease during the first trimester of pregnancy.

Creatine corrects muscle 31P spectrum in gyrate atrophy with hyperornithinaemia

BACKGROUND

Eye fundus destruction and type II muscle fiber atrophy in gyrate atrophy of the choroid and retina with hyperornithinaemia (GA) may be mediated by elevated ornithine concentrations which strongly inhibit creatine biosynthesis. This results in deficiency of creatine phosphate (PCr), a key intracellular energy source, as we have demonstrated in skeletal muscle of the patients by 31P magnetic resonance spectroscopy (31P MRS).

MATERIALS AND METHODS

Possible correction of the relative PCr deficiency by long-term daily exogenous supplementation of creatine or its precursors was investigated in four GA patients receiving creatine and in five patients treated with guanidinoacetic acid-methionine combination. The relative PCr concentration, expressed as PCr/Pi (Pi; inorganic phosphate) or as PCr/ATP ratios, was compared with the values of untreated GA patients, and matched healthy volunteers.

RESULTS

Muscle PCr/Pi ratios (mean +/- SD) of the untreated and creatine supplemented GA patients and controls were 4.9 +/- 1.4, 7.9 +/- 0.4 and 8.4 +/- 1.3. Guanidinoacetate-methionine combination was similarly effective (respective PCr/Pi ratios: 4.9 +/- 0.7, 6.3 +/- 1.1 and 10.7 +/- 2.8).

CONCLUSION

Supplementation with creatine or creatine precursors almost normalised low muscle PCr/Pi ratios of patients with GA.

Gyrate atrophy of the choroid and retina with hyperornithinaemia, cystinuria and lysinuria

The case of a 45-year-old woman with gyrate atrophy of the choroid and retina is documented. Additional features in this case, to the authors' knowledge not previously described in gyrate atrophy, are massive cystinuria, massive lysinuria, axial hypermetropia and diabetes. Gyrate atrophy is a rare autosomal recessive degenerative disease of the choroid and retina and is accompanied by defective ornithine metabolism. Simell and Takki demonstrated the association with hyperornithinaemia in 1973. The main metabolic features are those of hyperornithinaemia and ornithuria caused by a deficiency of the mitochondrial matrix enzyme, ornithine aminotransferase (OAT). The responsible human gene has been localised to chromosome 10. Despite the generalised deficiency of OAT, the literature indicates significant pathological involvement of the eye only. Ophthalmological features of the disease are myopia (up to 10-20 dioptres), night blindness, constricted visual fields and complicated cataracts. The clinical picture has been detailed previously by various authors. The case of a 45-year-old woman with gyrate atrophy and hyperornithinaemia is documented here. She has been followed up for 12 years and fully investigated. Additional features in this case, to our knowledge not previously described in gyrate atrophy, are massive cystinuria, massive lysinuria, axial hypermetropia and diabetes.

Some biochemical and pathophysiological aspects of long-term elevation of brain ornithine concentrations

Mice and chicken were given 5-fluoromethylornithine (5FMOrn), a selective inactivator of ornithine aminotransferase (OAT) over extended periods of time. This treatment allowed us to maintain elevated concentrations of ornithine in all tissues. Since gyrate atrophy, an autosomal recessive human disease, is characterized by the absence of OAT, special emphasis was put on the study of the visual system. Ophthalmoscopic and histologic examinations of the eye as well as electroretinograms and locomotor behaviour demonstrated an unimpaired visual system and brain. No toxic effects were observable in the treated mice. Likewise, chick embryo development was normal in spite of highly elevated brain and tissue ornithine concentrations. A likely explanation for the absence of toxic effects of 5FMOrn treatment, disregarding the non-toxicity of ornithine, is the fact that 10-20% of tissue OAT is refractory to inactivation by 5FMOrn. This residual activity may be sufficient to maintain vital functions.

A 15-bp deletion in exon 5 of the ornithine aminotransferase (OAT) locus associated with gyrate atrophy

Gyrate atrophy of the choroid and retina (GA) is an autosomal recessive disorder in which a deficiency of the mitochondrial matrix enzyme ornithine aminotransferase (OAT) leads to progressive blindness. Previously, we and others have reported a number of missense mutations and splice defects in the OAT gene associated with GA. In the present case, through sequencing of the PCR amplified cDNA products, we have detected a novel 15-bp deletion within exon 5 of the OAT gene which retains the original reading frame. The deleted PCR product is the only one produced from the patient's mRNA, while mRNA from the patient's mother yields both deleted and normal length PCR products. The alternate, apparently nonexpressing OAT allele in this patient was inherited from the father, who displays only the normal length PCR product. The codon at the deletion joint remains unaltered, predicting the loss of the pentapeptide Tyr-Thr-Val-Lys-Gly without any other amino acid change. The breakpoints are adjacent to or within two copies of a 4-bp direct repeat, which may have implications for the mechanism of deletion.

Retinitis pigmentosa

Retinitis pigmentosa is a clinically and genetically heterogeneous group of hereditary disorders in which there is progressive loss of photoreceptor and pigment epithelial function. The prevalence of retinitis pigmentosa is between 1/3000 and 1/5000 making it one of the most common causes of visual impairment in all age groups. The natural history, differential diagnosis, diagnostic clinical and electrophysiologic findings are reviewed. Generalization about the different genetic subtypes of retinitis pigmentosa are reviewed along with the uses of DNA probes for linkage studies. Syndromes in which retinitis pigmentosa is a manifestation are summarized.

Accumulation of ornithine by chick retinal pigment epithelium cells in culture

This study was designed to measure the accumulation of ornithine in retinal pigment epithelium cells grown in culture. Ornithine accumulated in retinal pigment epithelium cells in which the ornithine aminotransferase activity was inhibited with L-canaline. The effect of L-canaline was eliminated by the concomitant presence of methionine.

Tuberous sclerosis: aberrant metabolism of ornithine, proline and glutamate in cultured fibroblasts

To investigate aberrant metabolism of proline (Pro) and its precursors in tuberous sclerosis (TS), 6, 7 and 5 strains of control, TS (normal skin) and TS (tumor) fibroblasts, respectively, were cultured in Eagle's MEM containing dialyzed fetal bovine serum with or without 0.1 mM ornithine (Orn). Ornithine aminotransferase (OAT) activity was decreased in TS, especially in TS (tumor) after mild sonication treatment. The yield of the OAT protein was inhibited in TS (tumor) when cultured without Orn. Free glutamate (Glu) in the medium was significantly increased in TS (tumor). Free proline (Pro) in cells was significantly decreased in TS (tumor) when cultured with Orn, but protein-bound Pro was not. The relative concentration of free Glu to glutamine (Gln) in the medium and that of free Glu to Pro in cells cultured with Orn were increased in TS (tumor). These results suggest that the requirement for Orn, increased turnover of Pro to Glu and increased elimination of Glu into the medium occur in TS (tumor). Aberrant regulation or turnover of Pro and Glu metabolism may occur in TS, especially in tumor cells.

Comparison of ornithine aminotransferase activities in the pigment epithelium and retina of vertebrates

1. The specific activities of ornithine aminotransferase (OAT) in the pigment epithelia, retinas, and livers from several classes of vertebrates were assayed. 2. The specific activities of OAT were much higher in the pigment epithelia from mammals and birds than in their respective retinas or livers. 3. Pigment epithelium from porcine eyes had the highest specific activity measured. The specific activity of OAT in the pigment epithelium from the pig was five times higher than the OAT activity in its retina and 13 times higher than the OAT activity in its liver.

Molecular cloning of human ornithine aminotransferase mRNA

The isolation and characterization of a cDNA clone for the mRNA of human ornithine aminotransferase (OATase; ornithine-oxo-acid aminotransferase; L-ornithine:2-oxo-acid aminotransferase, EC 2.6.1.13), a nonabundant mitochondrial matrix enzyme that is severely deficient in a hereditary chorioretinal degenerative disease (gyrate atrophy), is described. Human liver, retina, and retinoblastoma (Y79) mRNAs were prepared and tested for the OATase mRNA content by in vitro translation, immunoprecipitation, and NaDodSO4/PAGE. The retinoblastoma cells were found to be expressing this enzyme at a relatively high level. The primary translation product of the OATase mRNA is larger than the pure OATase protein on NaDodSO4/PAGE by approximately equal to 4 kDa, suggesting a precursor protein. lambda gt11 cDNA libraries were prepared from the human mRNAs, and the recombinant clones were immunoscreened as plaques with two different preparations of rabbit anti-human OATase antibodies. A clone (lambda gtRB315) was isolated from the retinoblastoma library that reacts with both of the antibody preparations, and the DNA sequence of its 2.1-kilobase-pair cDNA insert was obtained. An open reading frame consisting of 1371 nucleotides is present in the sequence, and a putative translational initiation methionine codon is identified at position 55. A putative leader sequence consisting of 32 amino acid residues is identified, resulting in a precursor protein of 439 amino acid residues and a molecular mass of 48,534 Da and a mature protein of 407 residues and 45,136 Da. The amino acid sequences of seven tryptic peptides (115 amino acid residues) of the pure human OATase were obtained by microsequencing. When the tryptic peptide and cDNA-derived amino acid sequences were compared, homologies in 111 of 115 residues, including a match of 20 consecutive residues, were observed. An RNA blot hybridization of 32P-labeled OATase cDNA to normal human retina and retinoblastoma mRNAs demonstrated an OATase mRNA species of approximately equal to 2.2 kilobases. The level of OATase mRNA in the normal human retina is approximately equal to 1/100th the level of rhodopsin mRNA and 1/5th to 1/10th the level present in the retinoblastoma cells.

Hyperprolinaemia and gyrate atrophy of the choroid and retina in members of the same family

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Gyrate atrophy of the choroid and retina. Early findings

Examination of two sisters ages 2 years 10 months and 6 years four months with gyrate atrophy of the choroid and retina provided an opportunity for detailed clinical investigation. Although the chorioretinal lesions were confined to the peripheral retina in the older case and were quite minimal in the younger case, there was electroretinographic evidence of marked involvement of the cone and rod systems. These cases offer an opportunity to assess an arginine restricted diet in preventing the progress of the disease.

Pyrroline-5-carboxylate in human plasma

Pyrroline-5-carboxylate is the intermediate in the interconversions of proline, ornithine, and glutamate. Recent studies have suggested that pyrroline-5-carboxylate has regulatory properties in a wide range of tissues. Physiologic studies in humans have been limited by the unavailability of a sufficiently sensitive assay for this compound. Until now pyrroline-5-carboxylate has not been detectable in plasma of normal humans or even in plasma of patients with type II hyperprolinemia. We now report a method for measuring pyrroline-5-carboxylate made possible by a preparation of purified Escherichia coli pyrroline-5-carboxylate reductase. This method is more sensitive than currently available methods by two or three logs and is applicable for all biologic fluids. We have quantitated pyrroline-5-carboxylate in normal plasma, urine, and saliva. In addition, we have found pyrroline-5-carboxylate levels 10x to 20x normal in two patients with type II hyperprolinemia. The ability to measure physiologic concentrations of pyrroline-5-carboxylate now enables further characterization of its role as a regulatory molecule.

Gyrate atrophy of the choroid and retina: decreased ornithine aminotransferase concentration in cultured skin fibroblasts from patients

A sensitive and convenient radioisotopic assay for ornithine aminotransferase (OAT) and an enzyme immunoassay for human ornithine aminotransferase were developed for studying decrease in activity of this enzyme in gyrate atrophy of the choroid and retina with hyperornithinemia. Picogram amounts of human OAT could be detected by the enzyme immunoassay. The OAT activity in cultured fibroblasts from two patients was 6% and 2% of that in control fibroblasts, and did not increase on addition of a high concentration of vitamin B6. The decreased activity, assayed by enzyme immunoassay, was associated with decreased enzyme concentration, suggesting that deficiency of OAT in these patients is not due to production of a structurally altered enzyme lacking catalytic activity, but to decreased production of enzyme protein. When cells from patients were repeatedly subcultured, selective loss of both the activity and concentration of OAT was observed, without changes in activities of several other enzymes tested.

A lethal neonatal variant of carbamoyl-phosphate synthetase deficiency in combination with an intermediate activity of L-ornithine: 2-oxoglutarate amino-transferase

Shortly after birth, a newborn girl developed anorexia, hypotonia, apneic attacks and seizures. After 61 h the child died in coma. Biochemically, a highly elevated blood ammonia level was found together with an increased plasma level of the amino acids mainly involved in ammonia detoxication. Enzyme studies in post-mortem liver tissue material revealed a deficiency of carbamoyl-phosphate synthetase (0.9% of the mean value in controls) in combination with an intermediate activity of L-ornithine: 2-oxoglutarate aminotransferase (40% of the mean value in controls).

Pathology of iridectomy specimens in gyrate atrophy of the retina and choroid

Gyrate atrophy of the retina and choroid is an autosomal recessive disease characterized by progressive retinal degeneration and ornithine aminotransferase deficiency. We report here the new histological findings and ultrastructural changes in 3 iridectomy specimens from 2 Finnish patients with gyrate atrophy. The iridectomy specimens were removed during routine cataract extraction and studied with a transmission electron microscope. The dilator muscle showed atrophy, abnormal mitochondria, and tubular aggregate type structures similar to those found in skeletal muscle. Degenerative changes such as extracted cellular matrix, dropout of cellular organelles, and dilated intercellular spaces were observed in the pigmented posterior epithelium and the anterior iris epithelium.

Metabolic tapetoretinal degenerations

There are a number of metabolic diseases which cause tapetoretinal degeneration, suggesting that pure pigmentary retinopathy may also be metabolic in nature. On the other hand tapetoretinal degenerations may have various modes of inheritance, so we may conclude that the metabolic disorder at the basis of these diseases is not unique and that tapetoretinal degenerations are heterogenic. In this article, some 450 published reports on tapetoretinal degenerations are reviewed. Based on these reports, the clinical and ocular manifestations, laboratory and histopathological findings, inheritance patterns, and treatments of various syndromes characterized by tapetoretinal degenerations are described. It is hoped that the gathering together of this information in one source will acid in the future understanding of metabolically based eye disease.

Defective ornithine metabolism in cultured skin fibroblasts from patients with the syndrome of hyperornithinemia, hyperammonemia and homocitrullinuria

The syndrome of hyperornithinemia, hyperammonemia, and homocitrullinuria (HHH) is a metabolic disorder resulting in protein intolerance and mental retardation. The primary metabolic defect has yet to be determined. We studied some aspects of ornithine metabolism in cultured skin fibroblasts from two patients from two patients with the HHH syndrome. The fibroblasts failed to incorporate 14C-label from ornithine into protein, a defect also observed in fibroblasts from patients with gyrate atrophy of the choroid and retina and a deficiency of ornithine aminotransferase activity. The defect can be corrected in heterokaryons formed between these two types of fibroblasts. These findings indicate that fibroblasts are suitable for further studying the underlying metabolic defect in HHH syndrome. The combination of the ornithine incorporation assay and genetic complementation analysis provide a confirmatory test for the diagnosis of this syndrome.

Ornithine oxoacid aminotransferase found in AH 130 ascites hepatoma cells

We found heterogenous ornithine oxoacid aminotransferase (L-ornithine: 2-oxo-acid aminotransferase, EC 2.6.1.1.3) in rat ascites hepatoma AH 130 cells. Compared with enzymes from normal rat tissues, this heterogenous enzyme showed larger Km values for 2-oxoglutarate, a different elution-profile upon affinity chromatography with 2-oxoglutarate, more anionic mobility upon polyacrylamide gell electrophoresis, and a clearly different salting-out property upon ammonium sulfate fractionation. Similar heterogeneity of this aminotransferase was found in human cancer cells.

Decreased transport of ornithine across the inner mitochondrial membrane as a cause of hyperornithinaemia

Hyperornithinaemia due to a transport of ornithine across the inner mitochondrial membrane was demonstrated in three patients by measuring ornithine uptake by fibroblast mitochondria. Particulate compartments and soluble cytoplasm of fibroblasts were separated by a slight modification of the digitonin method of Zuurendonk and Tager. Patients' fibroblast pellet fraction contained significantly less radioactivity than control fibroblast pellet fraction after incubation of fibroblasts with [14C]-ornithine. Since neither of the patients was deficient in ornithine-delta-oxoacid aminotransferase, we concluded that in these hyperornithinaemia patients a defect exists for transport of ornithine across the inner mitochondrial membrane. The exact nature of this transport defect remains to be elucidated.

Biochemical and therapeutical studies in a case of atrophia gyrata

A thirty-six years old man from an inbred family with the typical clinical picture of Atrophia gyrata chorioideae et retinae was found to have hyperornithinemia and a partial deficiency of ornithin-ketoacid-transaminase activity. The residual activity was stimulated in vitro by high concentrations of pyridoxal phosphate. We have initiated a therapeutic study with vitamin B6 per os accordingly. Comparitively low doses may be sufficient for long term treatment. The necessity to start therapy early in life is emphazised. Possible mechanisms of the pathogenesis of Atrophia gyrata are discussed.

Regulation of proline biosynthesis: the inhibition of pyrroline-5-carboxylate synthase activity by ornithine

Mammalian cells have the capacity for proline biosynthesis from ornithine or glutamic acid. Using a radioisotopic assay, we have studied the regulation by ornithine of delta 1-pyrroline-5-carboxylate synthase, the enzyme that catalyzes the first step of proline biosynthesis from glutamic acid. In homogenates from Chinese hamster ovary cells, ornithine was found to be a potent inhibitor of pyrroline-5-carboxylate synthase activity(50% inhibition at 0.37 mM). The effect was reversible and did not occur with amino acids other than ornithine. Preliminary findings suggest that the inhibition does not result from altered requirements for the cofactors NADPH and ATP. Significant inhibition was observed in four different Chinese hamster cell lines. Ornithine was also shown to inhibit the conversion of 3H-glutamic acid to 3H-proline in intact human skin fibroblasts. Cells from patients with a rare ocular disease, gyrate atrophy of the choroid and retina, were used for these studies since they lack interfering ornithine aminotransferase activity. We conclude that ornithine may be a physiologic regulator of the rate of proline formation from glutamic acid. This information allows us to construct an hypothetical model for the overall regulation of proline biosynthesis and also to suggest a pathophysiologic mechanism for the disease gyrate atrophy.

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% of 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.

Gyrate atrophy of the choroid and retina with hyperornithinemia: characterization of mutant liver L-ornithine:2-oxoacid aminotransferase kinetics

Deficient activity of L-ornithine:2-oxoacid aminotransferase is associated with gyrate atrophy of the choroid and retina with hyperornithinemia, an autosomal recessive disease leading to blindness. Liver tissue from two patients contained trace activity of the enzyme. The Michaelis (Km) value of the mutant enzyme for ornithine was 200 mM, 50-fold higher than normal, but increasing the concentrations of alpha-oxoglutarate and pyridoxal 5'-phosphate to 10 times those giving maximal activity of the normal enzyme had no effect on the mutant enzyme. Substrate inhibition of the mutant could not be demonstrated at 1,000 mM ornithine concentration, whereas ornithine concentrations above 70 mM inhibited the normal enzyme. The data suggest that the abnormal L-ornithine:2-oxoacid aminotransferase in the two patients studied has an altered binding site for ornithine.

Systemic manifestations of gyrate atrophy of the choroid and retina

In ten patients with gyrate atrophy (GA) and hyperornithinemia, the head hair was fine, straight, and sparse. On microscopic examination, both scalp and pubic hair contained intermittent dark cores within the medullary zone, which was not a deposit, but appeared to be due to the unusual refractive properties of loosely formed macrofilaments amidst wide spaces containing a structureless electron lucent but compact substance, which was insoluble in both water and solvents. Seven of the ten patients had abnormal wave forms on electroencephalography. Three of the five patients who underwent muscle biopsy had tubular aggregates. Of particular interest was the toxicity of exogenous ornithine added to muscle cell culture from GA patients as compared with the lack of toxicity in muscle from control patients. What specific role the hyperornithinemia and absence of OAT is playing in the histopathology of hair and muscle and the EEG changes awaits further biochemical investigation.

Gyrate atrophy of the choroid and retina. Biochemical considerations and experience with an arginine-restricted diet

Ornithine-delta-aminotransferase deficiency is the primary biochemical defect in gyrate atrophy of the choroid and retina and results in the characteristic accumulation of ornithine. An additional consequence of this inborn error is that arginine, the precursor of ornithine, becomes an essential amino acid. Therefore, to reduce the accumulated ornithine, we placed nine gyrate atrophy patients on an arginine-restricted diet. Plasma ornithine decreased by 50 to 85% within one month. Orally administered, alpha-aminoisobutyric acid facilitated the reduction in ornithine by augmenting renal losses. Over the long term, three patients have maintained near normal plasma ornithine concentrations from 4 to 32 months. Two patients have maintained less striking reductions in ornithine, and four have either been poorly controlled or have terminated the diet. Urinary losses of arginine and ornithine in gyrate atrophy patients with high or low plasma ornithine concentrations are less than 50% of the estimated arginine intake. This observation suggests that the bulk of ingested arginine is somehow metabolized despite the severe reduction in ornithine-delta-aminotransferase activity.

Experimental model of gyrate atrophy in animals

Intravitreal injection of L-ornithine hydrochloride in physiologic saline solution caused marked edema specifically in the pigment epithelium of Sprague-Dawley strain albino and Evans black hooded rats and rhesus and cynomologus monkeys. Swelling of the pigment epithelial cells, which was most prominent four hours after the injection, disappeared by 24 hours. However, many pigment epithelial cells gradually degenerated resulting in patches of denuded areas. The photoreceptor cells overlying the damaged pigment epithelium degenerated secondarily.

Gyrate atrophy of the choroid and retina: improved visual function following reduction of plasma ornithine by diet

In a patient with gyrate atrophy of the choroid and retina, an arginine-deficient diet has reduced plasma ornithine concentration fivefold during the past 20 months. Subjective improvement in her visual function was noted approximately 15 months after institution of her diet. This has been documented by improvements in the electroretinogram, dark-adaptation, and color vision. The improvement involves rod and, to a lesser extent, cone function. The results, although preliminary and limited to a single patient, suggest that reduction of plasma ornithine with a low arginine diet is beneficial in this disease.

Gyrate atrophy of the choroid and retina with hyperornithinemia. Deficient formation of guanidinoacetic acid from arginine

Patients with gyrate atrophy of the choroid and retina have 10- to 20-fold increased ornithine concentrations in body fluids and significantly reduced activity of ornithine aminotransferase in lymphocytes and cultured fibroblasts. We administered intravenous arginine to six patients and six controls to study in vivo inhibition by high ornithine concentrations of arginine-glycine transamidinase, the rate-limiting enzyme in creatine synthesis. Serum arginine concentrations curves after administration were similar for the two groups. The increment in serum ornithine was more than three times as great in patients as in controls. The mean half-times in plasma ornithine were 360 and 97 min, respectively. In the patients, the metabolic clearance of ornithine from the extracellular fluid was significantly delayed. Urinary guanidinoacetate excretion rose markedly in all controls, the excretion rate being higher in females. The patients always excreted less than the controls, the differences within the sexes being highly significant. Differences in creatine excretion after administration were less marked. We conclude that in gyrate atrophy patients, formation of guanidinoacetate, creatine, and possibly phosphocreatine is inhibited at the transaminidation step by the high concentrations of ornithine. Deficiency of the high-energy phosphates may underlie the pathogenesis of the eye and muscle atrophies.

Inhibition of arginine-glycine amidinotransferase by ornithine. A possible mechanism for the muscular and chorioretinal atrophies in gyrate atrophy of the choroid and retina with hyperornithinemia

The inhibitory effect of ornithine on L-arginine:glycine amidinotransferase (EC 2.1.4.1) was studied in crude rat kidney homogenates. The enzyme activity was linear with time up to 45 min and with protein up to 200 microgram. The apparent Km and V of amidinotransferase were 9.21 mM and 1.53 mu mol/g protein per min, respectively. The enzyme was competitively inhibited by ornithine, with a Ki of 0.253 mM. Kidney arginase was inhibited only slightly and non-competitively. The inhibition of amidinotransferase by ornithine may thus be important in creatine biosynthesis. In gyrate atrophy of the choroid and retina with hyperornithinemia, a human autosomal recessive disease caused by decreased ornithine aminotransferase activity, plasma ornithine concentrations are elevated 10-20-fold (0.65-1.35 mM during fasting). In consequence endogenous creatine production probably is severely decreased because of inhibition of the rate-limiting transamidination step by ornithine. The deficiency of creatine and further of readily available energy in the form of phosphocreatine is suggested to be involved in the pathogenesis of the choroidal, retinal and type II muscle fiver atrophies in gyrate atrophy.

Gyrate atrophy of the choroid and retina: amino acid metabolism and correction of hyperornithinemia with an arginine-deficient diet

Four patients with gyrate atrophy of the choroid and retina were studied, all of whom exhibited the hyperornithinemia characteristic of this disorder. Elevated plasma histidine and diminished plasma lysine and branched-chain amino acids were also noted. The renal clearances of these four amino acids were not sufficiently elevated to explain their low plasma levels. In one subject, an arginine-deficient diet led to progressive reduction in plasma ornithine from 13 times normal to the upper limits of normal, along with the disappearance of ornithinuria and lysinuria. Orally administered alpha-aminoisobutyric acid facilitated the fall in plasma ornithine by increasing renal losses of ornithine. It also increased the clearances of most other amino acids. When plasma ornithine approached normal (less than 200 microM), plasma lysine became normal, plasma arginine became subnormal, and renal clearances of basic amino acids decreased. Long-term (1.5 yr) maintenance with a diet containing 10-20 g of protein plus essential amino acids served to keep plasma ornithine at between 55-355 microM; chorioretinal degeneration did not progress and vision apparently improved.

Clinical and biochemical heterogeneity in gyrate atrophy

Two patients had milder variants of the usual form of gyrate atrophy. Although the appearance of the chorioretinal degeneration was identical, the progression of the clinical signs and symptoms were slower with night blindness, cataracts, and decreased vision occurring at an older age. Additionally, the level of hyperornithinemia, although still increased above normal, was lower than that usually found in such patients (between 448 and 676 microM). Additionally, one of our patients responded to high dosage vitamin B6 with a 27% reduction in plasma ornithine. The difference in the quantitation of the increase in plasma ornithine, the difference in responsiveness to vitamin B6, and the difference in the clinical consequences of this underlying biochemical abnormality, indicated a variant form of gyrate atrophy, suggesting the presence of genetic heterogeneity.

The importance of ornithine as a precursor for proline in mammalian cells

Ornithine aminotransferase catalyzes the reversible transamination of L-ornithine to delta1-pyrroline-5-carboxylate, the immediate precursor of proline. The direction and flux through this pathway in mammalian cells has not been established. Glutamate has generally been considered to be the most important precursor for proline biosynthesis, but recent studies in xiphoid cartilage indicate that a significant fraction of cellular proline is derived from ornithine. Using newly isolated mutant Chinese hamster ovary cells with defined defects in the proline biosynthetic pathways, we now have established that cells can grow at a maximal rate with ornithine as the sole source of proline. Furthermore, we have measured the rate of proline formation from ornithine (1.6 nmol/h/10(6) cells); Future studies with these mutant Chinese hamster ovary cells may offer insight into the regulatory mechanism which coordinates proline biosynthesis from ornithine and glutamate.