Qualitative Abnormality of Liver Argininosuccinate Synthetase in a Patient with Citrullinemia

Abstract

The enzymes involved in the urea cycle in normal liver and in the liver of a patient with citrullinemia were compared. The activities of carbamoyl-phosphate synthetase, ornithine-carbamoyl transferase, argininosuccinate synthetase, argininosuccinate lyase, and arginase in the patient’s liver were normal under standard assay conditions. The properties of argininosuccinate synthetase of the patient were compared with those of the enzyme from normal liver. The enzyme from normal liver showed normal Michaelis-Menten kinetics and its Km values for ATP, L-aspartate, and L-citrulline were 1.8 × 10⁻⁴ M, 2.9 × 10⁻⁵ M, and 3 × 10⁻⁵ M, respectively. The enzyme from the patient with citrullinemia also gave hyperbolic curves with respect to the concentrations of ATP and L-aspartate and the Km values for ATP and L-aspartate were 1.8 x 10⁻⁴ M and 2.9 × 10⁻⁵ M, respectively. However, it gave a sigmoidal curve with respect to the concentration of citrulline, and Hill’s coefficient was 1.66. The molecular weight of argininosuccinate synthetase from both normal liver and the patient’s liver was estimated to be 185,000 by sucrose density gradient centrifugation both in the presence and absence of citrulline. Argininosuccinate stabilized the enzyme preparations from both livers against heat treatment, but the enzyme from the patient’s liver was less stable than the normal enzyme against heat treatment without added argininosuccinate.