Estrogen-binding properties of cytoplasmic and nuclear estrogen receptors in the presence of Triton X-100

The effect of the nonionic detergent Triton X-100 on cytoplasmic and nuclear estrogen receptors from rat uterus was determined. The cytoplasmic estrogen receptor was deaggregated from the 8S aggregate to a 4S form on sucrose gradients which contained low concentrations of Triton X-100. At concentrations of detergent great than 0.1% a partial loss of binding was observed. Estradiol dissociated more rapidly from the extracted nuclear receptor and as a consequence in the presence of detergent, only centrifugation runs of abbreviated duration (4 h) allowed the nuclear receptor to be resolved on sucrose gradients. The dissociation rate constant, k_-1., of the cytoplasmic receptor was not significantly affected at 0-4°C by ionic strength or by activation of cytoplasmic receptor. Triton X-100 increased the rate of ligand dissociation from cytoplasmic estrogen receptor ~ 1.3-2.0-fold and from nuclear receptor ~ 4.5-fold. In a homogeneous environment obtained by combining cytosol and nuclear extracts, ligand dissociation from nuclear and cytoplasmic receptors proceeded at a similar velocity and this velocity was faster than previously determined for the cytoplasmic receptor under a variety of conditions. The data reported support the hypothesis that the estrogen receptor may be modified by certain factors contained in the crude nuclear pellet so that in the presence of Triton X-100 the nuclear receptor appears less able to bind estradiol.