THERMODYNAMICS OF 5-HT3 RECEPTOR-BINDING DISCRIMINATES AGONISTIC FROMANTAGONISTIC BEHAVIOR

Thermodynamic parameters Delta G degrees, Delta H degrees and Delta S degrees of the binding equilibrium of eleven ligands (seven agonists a nd four antagonists) to the serotonin 5-HT3 receptor subtype have been determined by affinity measurements carried out on mt cortex membrane s at six different temperatures (0, 10, 20, 25, 30, 35 degrees C) and van't Hoff plots. Affinity constants were obtained from saturation exp eriments of .3.1]non-3-yl)-1-methyl-1-H-indazole-3-carboxamide ([H-3]B RL 43694, a selective 5-HT3 ligand) or by its displacement in inhibiti on assays for the other compounds. Van't Hoff plots were essentially l inear in the temperature range investigated, showing that the Delta C- p degrees of the binding equilibrium is nearly zero. Thermodynamic par ameters are in the range 18 less than or equal to Delta H degrees less than or equal to 53 kJ mol(-1) and 202 less than or equal to Delta S degrees less than or equal to 320 J K-1 mol(-1) for agonists and -16 l ess than or equal to H degrees less than or equal to 0 kJ mol(-1) and 70 less than or equal to Delta S degrees less than or equal to 179 J K -1 mol(-1) for antagonists indicating that agonistic binding is totall y entropy-driven while antagonistic binding is relatively less entropy - and more enthalpy-driven in the -T Delta S degrees versus Delta H de grees plot the thermodynamic data are clearly arranged in separate clu sters for agonists and antagonists, which, therefore, turn out to be t hermodynamically discriminated. Experimental results are discussed acc ording to the following main points: (i) the approximate linearity of the Delta H degrees versus Delta S degrees plot in terms of enthalpy-e ntropy compensation and (ii) the fact that Delta C-p degrees similar o r equal to 0 for practically all membrane receptors at variance with m ost reactions involving biomacromolecules in solution. Finally, the ph enomenon of thermodynamical discrimination is reviewed and found to oc cur in five distinct membrane receptorial systems.