APPARENT THERMODYNAMIC PARAMETERS OF LIGAND-BINDING TO THE CLONED RATMU-OPIOID RECEPTOR

The apparent thermodynamic parameters of binding of ten ligands to the cloned rat mu-opioid receptor stably expressed in Chinese hamster ova ry (CHO) cells were investigated. For every ligand, the K-d or K-i val ues at 0 degrees C, 12 degrees C, 25 degrees C and 37 degrees C were d etermined, a van't Hoff plot was generated and Delta H degrees', Delta S degrees' and -T Delta S degrees' and Delta G degrees' were calculat ed. Changes in free energy (Delta G degrees') ranged from - 10.35 to - 15.65 kcal/mol. The binding of sufentanil, ohmefentanyl, diprenorphin e and D-Phe-Cys-Tyr-D-Trp-Arg-Tnr-penicillamine-Thr-NH2 (CTAP) was end othermic (Delta H degrees' > 0) and driven by an increase in entropy ( -T Delta S degrees' = -13.08 to -18.57 kcal/mol). The binding of naltr exone was exothermic (Delta H degrees' = -12.56 kcal/mol) and essentia lly enthalpy-driven. The binding of morphine, methadone, pentazocine, [D-Ala(2), NMePhe(4), Gly-ol]enkephalin (DAMGO) and Tyr-Pro-NMePhe-D-P ro-NH2 (PL017) was exothermic (Delta H degrees' = -3.53 to -9.95 kcal/ mol) and occurred with an increase in entropy (-T Delta S degrees' = - 2.48 to -7.92 kcal/mol). Plots of enthalpy versus entropy and enthalpy versus free energy were linear, although enthalpy-entropy compensatio n was not evident. The entropy changes were not correlated with appare nt lipophilicity of the compounds. These results suggest that: (1) opi oid ligands bind to the mu receptor by specific mechanisms, unrelated to lipid solubility; (2) the mechanism of binding is not universally d ifferent for peptide and non-peptide Ligands; (3) the nature of bindin g does not a priori determine intrinsic activity. The results reveal a novel differentiation of opioid ligands into two groups (group 1. ohm efentanyl, sufentanil, diprenorphine, CTAP and PL017; group 2. naltrex one, morphine, methadone, DAMGO, pentazocine), based on two distinct r elationships between enthalpy versus free energy of binding, the detai ls of which are yet to be elucidated. (C) 1998 Elsevier Science B.V. A ll rights reserved.