Deficient formation of GAA in gyrate atrophy
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.