Heterotropic effects of dipolar amino acids on the activity of the anionic amino acid transport system X-AG in rabbit jejunal brush-border membrane vesicles.

D-Aspartic acid was used as a specific substrate to evaluate the effects of dipolar amino acids on the high affinity anionic amino acid transport system X-AG in rabbit jejunal brush-border membrane vesicles. At pH 6, increasing L-phenylalanine concentrations caused a saturable activation of 0.05 mM D-aspartic acid uptake (Ka = 2.4 mM), and a saturating concentration of effector increased the Vmax of transport (2.6-fold) without any significant effect on the Km. At pH 8, however, a complex activation/inhibition curve was obtained with increasing L-phenylalanine concentrations, and a saturating concentration of effector increased both the Vmax (1.5-fold) and Km (2.1-fold) for transport. Increasing concentrations of L-valine, L-isoleucine, L-methionine, and L-threonine also showed complex activation/inhibition curves of D-aspartic acid uptake at both pH 6 and 8. The maximum level of activation, the plateau reached at saturating concentrations, and the concentration of effector producing either maximum activation or inhibition were, however, different at these two pH values. By using an optimum concentration of 10 mM L-valine at pH 6, the absence of trans-activation and of further activation by a cis-gradient of effector could be demonstrated. These results suggest that two allosteric sites directly accessible from the external medium are responsible for the heterotropic activation of intestinal system X-AG by dipolar amino acids and that, under physiological conditions, such effects might compensate for the lack of specificity of the neutral brush-border system.