An analysis of the decrease in the assayed level of charged bovine estrogen receptor observed at physiological ionic...

Lower assayed levels of heifer uterine estrogen receptor (ER) occur at physiologic ionic strength when ER is separated from [3H]estradiol by Dextran-coated charcoal treatments, or by gel filtration on Sephadex or polyacrylamide resins. The assayed level of charged ER in buffers containing 150-200 mM ionic strength is approximately one-half that of ER levels assayed in buffers either at 0-50 or 400-450 mM ionic strength. Treatment of ER with trypsin or molybdate eliminates this observed reduction. Evidence is presented that the decrease results from a preferential adsorption of ER to the assay resins at 150-200 mM ionic strength. This adsorption is likely to be mediated by a hydrophobic region of the ER, which is removed by trypsin cleavage.