β-Endorphin: Thermodynamics of the binding reaction with rat brain membranes

Abstract

The binding of human β-endorphin to rat brain membranes was studied at various temperatures in the absence and presence of sodium. Between 0 and 30 °C, 100 m m sodium depresses the binding of β h-endorphin by reducing its affinity for binding sites 7- to 10-fold but has no effect on the total number of binding sites present in the membranes preparation. Kinetic studies show that the decrease in affinity in sodium is largely accounted for by a 10-fold decrease in the association rate constant within the whole range of temperatures examined. High versus low temperatures markedly enhance the binding of β h-endorphin by increasing its affinity for receptors, both in the presence and absence of sodium, without change in the number of receptors. This enhancement in affinity is due to an increase in the association rate constant between 0 and 30 °C, coupled with a slight increase in the respective dissociation rate constants. Temperature variations do not interact with the influence of sodium on the β-endorphin receptor. The integrity of the tritiated peptide was assessed at each temperature used. No more than 6% of degraded material was found at the highest temperature of incubation examined. Large positive values of the standard enthalpy and entropy changes were observed both in the absence and presence of sodium indicating an entropically driven binding reaction in both cases. The dependence of binding affinity upon sodium and temperature strongly suggests that hydrophobic interactions play a major role in stabilizing the hormone-receptor complex while electrostatic and/or hydrogen-bonded interactions might play a more specific role in recognition processes.