Kinetic analysis of argininosuccinate synthetase in a variant form of citrullinaemia.

Argininosuccinate (ASA) synthetase in the liver from a patient with a variant form of citrullinaemia was analysed. Serum citrulline level was approximately 40 times higher than the control level, and the ASA synthetase activity was approximately 10% that of the control. Other urea cycle enzymes were within normal ranges. The specific activity (enzyme activity/enzyme protein, measured by immunochemical method), pH optimum, Michaelis constants, and thermal denaturation in the presence or absence of ASA were similar to those in enzymes obtained from the normal and the patient's liver cells. No in vitro effect of the patient's liver homogenate on ASA synthetase in normal liver was observed. Serum citrulline levels of the patient's parents and young brother were at the maximal level of control at fasting and definitely elevated 4 h after citrulline loading (100 mg/kg).