Kinetics and in vitro origin of the temperature-dependent transition of the estrogen receptor monomer

Partitioning of estrogen receptors in aqueous two-phase polymer systems has provided the basis for a detailed kinetic analysis of the effects of temperature on estrogen receptor (ER) structure in vitro. Exposure to temperatures of 0-30 degrees C increased the rate of change in ER partition coefficients by up to 100-fold but did not affect the final extent of the process. The temperature-dependent change in ER partition coefficients was characterized by a linear Arrhenius plot and an activation energy of 25 kcal/mol. The rate of the temperature-dependent ER transition (28 degrees C) was found to be unaffected by greater than 50-fold changes in receptor concentration, which indicates that the temperature-dependent change in partition coefficients reflects a first-order process. The partition coefficients of heated ER were unaffected by subsequent 18-h incubations at 0 degree C, indicating that the temperature-dependent ER transition is irreversible in vitro. Direct heating of the unoccupied ER resulted in both a change in ER partition coefficients and a loss of ER binding sites. The temperature-dependent change in unoccupied ER partition coefficients was complete within 30 min at 28 degrees C and yielded a first-order rate constant that was the same as that obtained for heating the receptor-estradiol complex at 28 degrees C. In contrast, the loss of unoccupied ER binding sites that occurred during 28 degrees C incubations did not reach completion after 150 min of heating and was found to behave as a second-order process.(ABSTRACT TRUNCATED AT 250 WORDS)