Thermodynamic parameters for [D-Pen^2,5]enkephalin at δ-opioid receptors in the mouse isolated vas deferens

Dissociation constants (K_A) for [D-Pen^2,5]enkephalin (DPDPE) inhibition of the electrically evoked twitch of the mouse isolated vas deferens preparations (MVD) were calculated at five temperatures (25, 30, 34, 37 and 40°C). These values were determined from the equiactive concentrations obtained before (A) and after (A′) partial irreversible blockade of a fraction of the receptor population by β-chlornaltrexamine (β-CNA) plotted as ( 1 A) against ( 1 A′) where K_A = slope - 1)/intercept. The values of K_A tended to increase (approximately doubled) over this temperature range, indicating that the affinity of DPDPE for the opioid δ receptor is an inverse function of temperature. From these results, thermodynamic parameters were calculated from a Van 't Hoff plot of 1n(K_A) against 1 T. The relative magnitudes of the change in enthapy (ΔH°′ = -6.67 kcal mol^-1), the change in entropy (ΔS°′ = +0.009 kcal mol^-1 °K^-1) and the change in free energy (ΔG°′ = -9.43 kcal mol^-1) suggest that the interaction between DPDPE and the δ-opioid receptor in MVD is an exothermic exergonic reaction, predominantly enthalpy-driven and results in an increase in the entropy of the system.