THERMODYNAMIC PARAMETERS OF FROG BRAIN KAPPA-OPIOID RECEPTORS

The thermodynamic parameters for [H-3]ethylketocyclazocine binding in frog (Rana esculenta) brain membranes have been examined. Computer-bas ed nonlinear regression analysis of the untransformed equilibrium disp lacement data showed that this ligand bound to two sites with differen t affinities and capacities in this tissue. K-A values derived from eq uilibrium displacement curves have been used for calculating the chang es in the standard Gibbs energy, enthalpy, and entropy during the bind ing process. Van't Hoff plots are bipartite, with transitions occurrin g at 18 degrees C for both the high- and the low-affinity sites. For t he high-affinity site, the reaction appears to be associated with a de crease in enthalpy below the transition temperature and a significant gain in entropy above this temperature. The reverse appears to be true for the low-affinity site. We conclude that this profile fairly appro ximates the mixed agonist-antagonist nature of this ligand and surmise that thermodynamic analysis could be a very useful tool for character ization of the nature of cloned opioid receptors in vitro.