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 Kms 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 Km for
citrulline; AS in the variant cells had apparent Kms for
citrulline of approximately 20 mM; and AS in the normal cell line and one clone had apparent Kms for
citrulline of 0.2 mM. The data suggest that the defect in the citrullinemic cell lines is due to a mutation in the structural gene coding for
argininosuccinate synthetase.