Temperature dependence of the dissociation rate constants for the nonactivated (molybdate-stabilized) and activated progesterone receptor-hormone complexes from the chick oviduct

Abstract

Both the nonactivated and activated forms of the chick oviduct cytosol progesterone receptor-hormone complexes displayed first-order dissociation kinetics at temperatures between 0 and 25°C. The rate constant was always 2—3-times greater for the nonactivated than for the activated complex. The thermodynamic parameters calculated from the Eyring plot for the nonactivated and activated forms, respectively, were: ΔH † = 28.6 and 29.9 ± 1.5 kcal/mol; −T†S † = 7.4 ± 0.6 and 7.7 ± 1.6 kcal/mol; and ΔG † = 21.3 ± 0.5 and 22.1 ± 0.1 kcal/mol . These values suggest that activation results in an increase in enthalpy of the ligand-receptor interaction, thus stabilizing the complex. The dissociation rate constants for the native complex obtained by two different experimental approaches, namely, isotope dilution (‘chase’) and dissociation against charcoal, indicated the absence of cooperativity in the receptor-ligand binding.