Assay of ornithine carbamoyltransferase activity in human liver using carbon-labeled ornithine and thin-layer chromatography

Prenatal diagnosis of ornithine transcarbamylase deficiency: results in Spfash mice

Ornithine transcarbamylase (OTC) deficiency is a human X-chromosome-linked disease (McKusick 31125). The presence of OTC activity in the human placenta encouraged us to examine the possible diagnosis of the disease in an animal model (Spfash mice) by enzymatic assay on placental samples. A significant positive correlation (p < 0.02) was found between placental and hepatic activities; the pH dependence of OTC was similar in the placenta and liver when compared within normal or homozygous mutant mice. The apparent Km (ornithine) and Km (carbamoylphosphate) values of the enzyme did not show any significant differences when compared in both placentae and livers of normal fetuses. The use of OTC assay in the placenta for prenatal diagnosis of OTC deficiency in mouse fetuses obtained by the crossbreeding of Spfash/+ with +/Y has shown that our method has good diagnostic value. We made the diagnosis of OTC deficiency in male fetuses with a sensitivity and a specificity of 1.0. The + gene from the father in Spfash/+ animals is preferentially inactivated in extraembryonic tissues, explaining why very low placental OTC activity was observed in 12 of the 18 female fetuses studied. Because these 12 females have variable OTC activity in their livers, it is not possible to appreciate the true residual activity in their livers by measuring this activity in the placenta.

N-acetylglutamate synthetase deficiency: clinical and laboratory observations

Two male siblings presented in the first 6 weeks of life with emesis, diarrhoea, metabolic acidosis and lethargy. A male sibling had previously died at 14 months of age from liver failure of unknown aetiology. Both of the current cases had mild hyperammonaemia with normal orotic acid, organic acid and argininosuccinic acid levels. Citrulline and arginine levels were normal or mildly decreased. One of the brothers was biopsied and had no detectable N-acetylglutamate synthetase activity and normal values for other enzymes of the urea cycle in liver. Treatment with a low-protein diet and sodium benzoate/sodium phenylacetate resulted in near normal blood ammonia levels, except during viral illness. Subsequent neurological development has been normal to mildly delayed. These patients differ from those previously described with N-acetylglutamate synthetase deficiency in that their presentation and subsequent course were relatively benign.

Partial ornithine transcarbamylase deficiency associated with recurrent hyperammonemia, lethargy and depressed sensorium

A 6-year-old boy presented with recurrent coma associated with hyperammonemia and infection is reported. A liver biopsy revealed decreased ornithine transcarbamylase (OTC) activity (16% of normal). The enzymatic abnormality in the child is supported by the finding of elevated orotic acid excretion in his mother following a protein load, compatible with an X-linked pattern of inheritance. Since initiation of a dietary arginine supplement, the child has not had recurrent episodes of hyperammonemia.