Kinetic evidence for isomerization of the dopamine receptor-raclopride complex

The binding kinetics of the specific dopamine D-2 antagonist [H-3]raclopride to dopamine D-2 receptors in rat neostriatum were studied. The pseudo-first-order rate constants of [H-3]raclopride binding with these membranes revealed a hyperbolic dependence upon the antagonist concentration, indicating that the reaction had at least two consecutive and kinetically distinguishable steps. The first step was fast binding equilibrium, characterized by the dissociation constant K-A = 12+/-3 nM. The following step corresponded to a slow isomerization of the receptor-antagonist complex, characterized by the isomerization equilibrium constant K-i = 0.11. The dissociation constant K-d = 1.3 nM, calculated from these kinetic data, was similar to K-d = 2.4 nM, determined from equilibrium binding isotherm for the radioligand. Implications of the complex reaction mechanism on dopamine D-2 receptor assay by [H-3]raclopride were discussed. Copyright (C) 1996 Elsevier Science Ltd.