Study of enzyme defect in a case of ornithine transcarbamylase deficiency

Activity of liver ornithine transcarbamylase was measured in a biopsy obtained from a seven years old girl, suffering from chronic hyperammonemia and orotic aciduria. The activity of the defective enzyme was only 17% of that of a control. pH optimum was 8.1 in the patient and the control. However, the pH curves were different between 7.0 and 8.1. Km (ornithine) of the patient's ornithine transcarbamylase was within the normal range (0.41 nM), but the Km (carbamyl phosphate) was low (0.18 mM). The girl seems to be a heterozygote carrier of ornithine transcarbamylase deficiency due to an abnormal liver enzyme.