title: Apparent thermodynamic parameters of ligand binding to the cloned rat mu-opioid receptor.
abstract: The apparent thermodynamic parameters of binding of ten ligands to the cloned rat mu-opioid receptor stably expressed in Chinese hamster ovary (CHO) cells were investigated. For every ligand, the Kd or Ki values at 0 degrees C, 12 degrees C, 25 degrees C and 37 degrees C were determined, a van't Hoff plot was generated and deltaH degrees' , deltaS degrees' and -TdeltaS degrees' and deltaG degrees' were calculated. Changes in free energy (deltaG degrees') ranged from -10.35 to -15.65 kcal/mol. The binding of sufentanil, ohmefentanyl, diprenorphine and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-penicillamineThr-NH2 (CTAP) was endothermic (deltaH degrees' > 0) and driven by an increase in entropy (-TdeltaS degrees' = -13.08 to -18.57 kcal/mol). The binding of naltrexone was exothermic (deltaH degrees' = -12.56 kcal/mol) and essentially enthalpy-driven. The binding of morphine, methadone, pentazocine, [D-Ala2, NMePhe4, Gly-ol]enkephalin (DAMGO) and Tyr-Pro-NMePhe-D-Pro-NH2 (PL017) was exothermic (deltaH degrees' = -3.53 to -9.95 kcal/mol) and occurred with an increase in entropy (-TdeltaS degrees' = -2.48 to -7.92 kcal/mol). Plots of enthalpy versus entropy and enthalpy versus free energy were linear, although enthalpy-entropy compensation was not evident. The entropy changes were not correlated with apparent lipophilicity of the compounds. These results suggest that: (1) opioid ligands bind to the mu receptor by specific mechanisms, unrelated to lipid solubility; (2) the mechanism of binding is not universally different for peptide and non-peptide ligands; (3) the nature of binding does not a priori determine intrinsic activity. The results reveal a novel differentiation of opioid ligands into two groups (group 1: ohmefentanyl, sufentanil, diprenorphine, CTAP and PL017; group 2: naltrexone, morphine, methadone, DAMGO, pentazocine), based on two distinct relationships between enthalpy versus free energy of binding, the details of which are yet to be elucidated.
authors.affiliation: Department of Pharmacology, School of Medicine, Temple University, Philadelphia, PA 19140, USA.
authors.forename: J G
authors.initials: JG
authors.lastname: Li
authors.forename: R B
authors.initials: RB
authors.lastname: Raffa
authors.forename: P
authors.initials: P
authors.lastname: Cheung
authors.forename: T B
authors.initials: TB
authors.lastname: Tzeng
authors.forename: L Y
authors.initials: LY
authors.lastname: Liu-Chen
mesh_terms: D000700:Analgesics / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D016466:CHO Cells; D006224:Cricetinae; D004174:Diprenorphine / Q000378:metabolism; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004745:Enkephalins / Q000494:pharmacology; D005283:Fentanyl / Q000031:analogs & derivatives / Q000378:metabolism; D008024:Ligands; D008691:Methadone / Q000378:metabolism; D009020:Morphine / Q000378:metabolism; D009271:Naltrexone / Q000378:metabolism; D009292:Narcotic Antagonists / Q000494:pharmacology; D010423:Pentazocine / Q000378:metabolism; D010446:Peptide Fragments; D010455:Peptides / Q000494:pharmacology; D011869:Radioligand Assay; D051381:Rats; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013004:Somatostatin; D017409:Sufentanil / Q000378:metabolism; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D000700:Analgesics; C529311:CTAP octapeptide; D004745:Enkephalins; D008024:Ligands; D009292:Narcotic Antagonists; D010446:Peptide Fragments; D010455:Peptides; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004174:Diprenorphine; D013004:Somatostatin; D009271:Naltrexone; D009020:Morphine; C037016:F 7302; D017409:Sufentanil; D010423:Pentazocine; D008691:Methadone; D005283:Fentanyl