THERMODYNAMIC ANALYSIS OF AGONIST AND ANTAGONIST BINDING TO MEMBRANE-BOUND AND SOLUBILIZED-A1 ADENOSINE RECEPTORS

The thermodynamic properties of the agonist [Adenine-2,8-H-3, ethyl-2( 3)-H]-N6-phenylisopropyladenosine ([H-3]R-PIA) and the antagonist 8-Cy clopentyl-1,3-[H-3]dipropylxanthine ([H-3]DPCPX) binding to membrane-b ound and lammoniol-1-propanesulfonate/digitonin-solubilized A1 adenosi ne receptors from pig brain cortex were evaluated. Rate constants for [H-3]R-PIA and [H-3]DPCPX association (k+1) and dissociation (k-1) pro cesses to this receptor subtype were measured from association-dissoci ation experiments at six different temperatures. The values for equili brium association constant (K(A) = 1/K(D)) were derived from rate cons tant values (k+1/k-1). The antagonist binding to membrane-bound recept ors, the agonist binding to fast kinetic component membrane-bound rece ptors and the agonist binding to soluble receptors showed a linear tem perature-dependence of the standard free-energy change. The first two processes are enthalpy- and entropy-driven, and the third process is e nthalpy-driven with entropy working against it. On the other hand, a c urvilinear temperature-dependence appears in the agonist binding to sl ow kinetic component membrane-bound receptors and in the antagonist bi nding to soluble receptors, but analyzing the semireactions (associati on-dissociation) involved in each case reveals that the thermodynamic behavior is very different. The thermodynamic similarities and differe nces are discussed in terms of receptor - G protein interaction.