Thermodynamic Parameters of Opioid Binding in the Presence and Absence of G-Protein Coupling

Abstract

We have investigated the thermodynamic parameters of various opioid ligands interacting with their receptors in rat brain membranes. Affinity constants (K_a), enthalpy and entropy values were obtained from homologous displacement experiments performed at 0, 24 and 33°C. It was found that all the opioid agonists tested ([³H]dihydromorphine (DHM) μ alkaloid; [³H]DAMGO μ peptide; [³H]deltorphin-B δ peptide) display endothermic binding accompanied with a large entropy increase, regardless of their chemical structure (alkaloid or peptide), or of their μ or δ receptor selectivity. In contrast, binding of the antagonist naloxone is exothermic, mainly enthalpy driven. Na⁺ or Mg²⁺ results only in quantitative changes of the thermodynamic parameters. In the presence of the GTP-analog Gpp(NH)p; or Gpp(NH)p + Na⁺; or Gpp(NH)p + Na⁺ + Mg²⁺ the affinity of DHM binding dramatically decreases which might reflect functional uncoupling of the receptor-ligand complex and G-proteins. This altered molecular interactions are also indicated by curvilinear van't Hoff plot and entropy increase. It is concluded that the thermodynamic analysis provides means of determining the underlying driving forces of ligand binding and helps to delineate its mechanism.

KEY WORDS:

• opioid binding
• thermodynamic analysis
• G-protein coupling
• van't Hoff analysis