THERMODYNAMICS OF A2B ADENOSINE RECEPTOR BINDING DISCRIMINATES AGONISTIC FROM ANTAGONISTIC BEHAVIOUR

Thermodynamic parameters ΔG°, ΔH° and ΔS° of the binding equilibrium of six agonists and six antagonists to the A2B adenosine receptor subtype have been evaluated by affinity measurements carried out on HEK 293 cells stably transfected with human A2B adenosine receptors at six different temperatures (4, 10, 15, 20, 25, 30 °C) and van't Hoff plot analysis have been performed. Affinity constants were obtained from saturation experiments of [3H]MRE 2029-F20 or by its displacement in inhibition assays for the other compounds. van't Hoff plots were essentially linear in the temperature range investigated, showing that the ΔCp°of the binding equilibrium is nearly zero. Thermodynamic parameters are in the range 7≤ ΔH° ≤ 23 kJ mol-1 and 123≤ΔS°≤219 JK-1mol-1 for agonists and -40≤ΔH°≤-20 kJ mol-1 and 10≤ΔS°≤91 JK-1mol-1 for antagonists indicating that agonistic binding is always totally entropy-driven while antagonistic binding is enthalpy and entropy-driven. In the - TΔS°versus ΔH° degrees plot the thermodynamic data are clearly arranged in separate clusters for agonists and antagonists, which, therefore, turn out to be thermodynamically discriminated according to the behavior of A1,1 A2A2 and A33 adenosine receptors. (1) Borea, P.A.; Varani, K.; Guerra, L.; Gilli, P.; Gilli, G. Binding thermodynamics of A1 adenosine receptor ligands. Mol. Neuropharmacol. 1992, 2, 273–81. (2) Borea, P.A.; Dalpiaz, A.; Varani, K; Guerra, L; Gilli, G. Binding thermodynamics of adenosine A2A receptor ligands.Biochem. Pharmacol. 1995, 49, 461–9. (3) Merighi, S.; Varani, K.; Gessi, S.; Klotz, K-N.; Leung, E; Baraldi, P.G.; Borea, P.A. Binding thermodynamics at the human A3 adenosine receptor. Biochem. Pharmacol. 2002, 63, 157–61