COMPARATIVE THERMODYNAMICS OF OPIOID RECEPTOR-LIGAND INTERACTION IN THE BOVINE ADRENAL-MEDULLA MEMBRANES - EVIDENCE OF OPIOID SITE HETEROGENEITY

1. A marked dependence on temperature of agonist binding delta, mu and kappa1-3 opioid sites in the bovine adrenal medulla was observed, at the range of 0 to 37-degrees-C. These changes concern kinetic (k1) and equilibrium constants (K(d)), but not binding capacities (B(max)). 2. These dependences are different for each ligand and each opioid receptor, suggesting their molecular heterogeneity. 3. The comparative thermodynamics indicates that the interaction of opioid agonists with their receptor is exergonic (DELTAG-degrees < 0) and entropy driven (DELTAS-degrees > 0). 4. The comparison of Van't Hoff and Arrhenius plots indicates a discrete mechanism in the binding of each opioid receptor.