Ornithine transcarbamylase deficiency: enzyme studies on a further case and a method of diagnosis using plasma enzyme ratios.

The activities of the urea cycle enzymes in the liver of a female patient with hyperammonemia were determined (Table 1). Ornithine transcarbamylase (OTC, EC. 2.1.3.3) was reduced to 5-10% of normal and the residual enzyme showed an apparent Kmorn of 0.69 (normal 0.37 +/- 0.10) mmol liter. The pH dependence was normal. The patient's mother also showed hyperammonemia but was not clinically affected. Consideration of the genetics of the disease suggested that many female patients should have a mixture of normal and mutant enzymes. Electrophoresis of the patient's liver extract showed an additional band of OTC activity probably due to this mutant enzyme. The ratio of plasma glutamate-pyruvate transaminase to OTC was abnormal in four clinically affected patients with OTC deficiency (Fig. 4B) but not in two of their mothers without clinical signs.