Citrulline metabolism in normal and citrullinemic human lymphocyte lines

Citrullinemia is one of the five aminoacidurias associated with the Krebs-Henseleit urea cycle. A long-term lymphocyte line (UM-21) derived from a patient with this disease and nine of ten clones of this line were found to have no activity for the enzyme argininosuccinate synthetase (AS), as demonstrated by their inability to grow in medium in which citrulline had been substituted for arginine, by their inability to incorporate arginine-C¹⁴ derived from citrulline-C¹⁴ into cellular protein, and by direct enzyme assay. One clone had normal or nearly normal argininosuccinate synthetase activity, as demonstrated by the same criteria. Nutritional "variants" able to grow logarithmically in medium containing citrulline were isolated from UM-21 and three clones. The apparent K_ms of AS for citrulline in UM-21, the ten clones, the variant lines, and a normal line were measured and fell into three groups: AS in UM-21 and nine clones had no measurable apparent K_m for citrulline; AS in the variant cells had apparent K_ms for citrulline of approximately 20 m m; and AS in the normal cell line and one clone had apparent K_ms for citrulline of 0.2 m m. The data suggest that the defect in the citrullinemic cell lines is due to a mutation in the structural gene coding for argininosuccinate synthetase.