Analysis and properties of the cytosolic estrogen receptor from rat thymus

The equilibrium association constant, KA, and the number of receptor binding sites, Bmax, that characterize the estrogen receptor present in rat thymus were determined using five different methods of data analysis on binding data obtained using a dextran-coated-charcoal assay. The methods of data analysis consisted of fitting the binding data directly to the hyperbolic binding function, fitting the data to the Lineweaver-Burk, Scatchard, and Woolf transforms of the hyperbolic function, and the direct linear plot method of Eisenthal--Cornish-Bowden. The Woolf and Eisenthal--Cornish-Bowden methods gave identical results indicating that added estradiol caused a decrease in KA (competitive inhibition) and a marked decrease in Bmax (noncompetitive inhibition). The Scatchard analyses gave only a very qualitative indication of this trend, while the Lineweaver-Burk analyses gave no indication of any such systematic trend. The results from the computationally more difficult fits to the hyperbolic binding equation were in agreement with the Woolf and Eisenthal--Cornish-Bowden results. Thus, the cytosolic thymus estrogen receptor from the rat shows both competitive and noncompetitive inhibition in the presence of endogenous or exogenous estradiol.