title: Argininosuccinate synthetase activity in cultured human lymphocytes.
abstract: The activity of argininosuccinate synthetase (E.C. 6.3.4.5), a urea cycle enzyme, was measured in cultured human lymphocytes using a new radioactive assay. Control cells had a maximum specific activity of 15.7 +/- 8.7 nmoles per hour per milligram of protein and an apparent Km for citrulline of 2 X 10(-4) M, whereas cells derived from a patient with citrullinemia had no detectable activity. A nutritional variant, selected out of the citrullinemic lymphocyte population by ability to grow in citrulline, had a maximum specific activity of 10.7 +/- 3.8 nmoles/hr/mg and an apparent Km for citrulline of 2 X 10(-2) M. These measurements confirm the observation that citrullinemia is associated with a defect in argininosuccinate synthetase activity and provide further evidence that citrullinemia is expressed in cultured lymphocytes. The emergence of a nutritional variant with a partial defect in argininosuccinate synthetase enzyme suggests that this citrullinemic patient has a heterogeneous population of cells, some totally defective and others only partially defective in argininosuccinate synthetase. The new activity assay is described in detail.
mesh_terms: D000592:Amino Acid Metabolism, Inborn Errors / Q000201:enzymology; D001124:Argininosuccinate Synthase / Q000172:deficiency / Q000378:metabolism*; D002478:Cells, Cultured; D002956:Citrulline / Q000494:pharmacology; D014644:Genetic Variation; D006801:Humans; D007700:Kinetics; D008025:Ligases / Q000378:metabolism*; D008214:Lymphocytes / Q000201:enzymology*
chemical_list: D002956:Citrulline; D008025:Ligases; D001124:Argininosuccinate Synthase