Citrullinemia: enzymatic evidence for genetic heterogeneity.

The specific activity of argininosuccinate synthetase (micromoles of 14CO2 per milligram of protein per hour) was 0.00104 and 0.00087 in fibroblasts derived from two patients with citrullinemia, and was undetectable in both fibroblasts and cultured lymphocytes from a third patient. In five obligate heterozygotes the specific activity in fibroblasts was 0.012-0.029 and in nine control subjects was 0.058 +/- 0.014 (0.030-0.076). In both control and patient cells, the maximum activity was obtained at pH 8.5 and there was no inhibition of normal argininosuccinate synthetase by any of the mutant cells.