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 K m for citrulline of 2 × 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 K m for citrulline of 2 × 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.
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This research was supported by a National Institutes of Health Training Grant (5-TO1-GM-0071) and NIH Program Project Grant (2-PO1-GM-15419).
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Lockridge, O., Spector, E.B. & Bloom, A.D. Argininosuccinate synthetase activity in cultured human lymphocytes. Biochem Genet 15, 395–407 (1977). https://doi.org/10.1007/BF00484469
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DOI: https://doi.org/10.1007/BF00484469