THERMODYNAMIC ANALYSIS OF THE INTERACTION OF PROLACTIN WITH ITS RECEPTOR IN THE RABBIT MAMMARY-GLAND MICROSOMES

The interaction of prolactin (PRL) with its membrane receptor depends markedly on temperature. Thermodynamic parameters for this reaction have been evaluated from data for time-course kinetics and equilibrium binding at multiple temperatures between 19 and 31 °C. The free-energy change with temperature and the van't Hoff plot were found to be linear. These suggest that there are minimal structure changes at the PRL-receptor contact site over this temperature range. The positive signs of the entropy and enthalpy of reaction, and of the entropy of activation (ΔS) for association, indicate that the hydrophobic bonding is the most significant force involved in PRL-receptor formation. The ΔS for dissociation was negative, and the enthalpy of activation for dissociation was about 20.3 kJ·mol-1 larger than that for association, indicating that the PRL-receptor complex is further stabilized by contributions of hydrogen bonds and van der Waals contacts after the initial interaction. The free energy of activation for dissociation, at 25°C, was about 2.5-fold larger than that for association. This would cause slow dissociation of PRL from its receptor.