Studies on rat liver argininosuccinate synthetase. Inhibition by various amino acids

Inhibition studies of crystallized rat liver argininosuccinate synthetase [EC 6.3.4.5] are described. 1. L-Argininosuccinate, L-histidine, and L-tryptophan inhibited the enzyme activity at saturating amounts of the substrates. 2. L-Norvaline, L-argininosuccinate, L-arginine, L-isoleucine, and L-valine competitively inhibited the enzyme activity at a low concentration of L-citrulline, with Ki values of 1.3 x 10(4) M, 2.5 X 10(-4) M, 6.7 X 10(-4) M, 6.3 X 10(-4) M, and 6.0 x 10(-4) M, respectively. 3. L-Argininosuccinate and L-arginine competitively inhibited the enzyme activity at a low concentration of L-aspartate, with Ki values of 9.5 x 10(-4) M and 1.2 x 10(-3) M, respectively. 4. The modes of inhibition by L-histidine were mixed-noncompetitive, uncompetitive, and noncompetitive types with respect to L-citrulline, L-aspartate, and ATP, respectively. 5. When the enzyme was preincubated with L-citrulline, the enzyme activity was slightly increased in the presence of a low concentration of L-histidine in the assay mixture. 6. The conformation of the enzyme was markedly changed by the addition of L-histidine as judged from the CD spectrum. This change was partially reversed by incubation with L-citrulline.