Thermodynamics and kinetics of mouse prolactin-hepatic receptor interaction.

Kinetic and thermodynamic parameters associated with the binding of secreted mouse prolactin (smPRL) to mouse hepatic receptors were investigated. When the reaction temperature was increased from 8 degrees C to 37 degrees C, the association rate constant k+1, increased approximately 5-fold, from 2.3 X 10(4) M-1 . S-1 to 12.6 X 10(4) M-1 . S-1. An Arrhenius plot indicated that there was a linear relationship between ln (k+1) and 1/T. When the reaction temperature was increased from 8 degrees C to 37 degrees C, the equilibrium binding constant, Ka, decreased approximately 1.5-fold, from 2.8 X 10(8) M-1 to 1.9 X 10(8) M-1. When the pH of the binding reaction was lowered from 9.0 to 6.2, Ka decreased approximately 3-fold, from 2.6 X 10(8) M-1 to 0.9 X 10(8) M-1. Transition state thermodynamic parameters for the formation of the smPRL-receptor complex, represented by delta G+', delta H+' and delta S+', were +45.7 kJ/mol, +41.2 kJ/mol and -15.1 J/(mol . K), respectively. Parameters for the equilibrium reaction, described by delta G0', delta H0' and delta S0', were -47.6 kJ/mol, -10.6 kJ/mol and +124 J/(mol . K), respectively. Over the temperature range studied, a Van't Hoff plot of the binding constants demonstrated a linear relationship between ln (ka) and 1/T, indicating that changes in enthalpy for the binding reaction were temperature independent. The binding reaction was largely entropically driven (delta S0' greater than 0), suggesting that hydrophobic interactions are involved in forming the smPRL-receptor complex.