Evaluationof an Enzyme ImmunoassayKit for EstrogenReceptorMeasurement-Report II

We present evidence to show that monoclonal antibodies to estrogen receptors (ER) in solid phase recognize the sec-ondary estrogen binding sites with moderate to low affinity for estradiol (E2). An excellent quantitative agreement was found in five cytosols between the ER values obtained by the enzyme immunoassay (ER-EIA) and the amount of second-ary estrogen binding sites measured by the assay involving dextran-coated charcoal (Gun Chem 1986;32:1 496). The immunoreactive protein recognized by the antibody-coated beads,when allowed to react with ER(+) cytosols, is shown to bind [3H]estradiol only when the ligand concentration exceeds 8 nmol/L. Further biochemical and functional char-acterization of the immunoreactive protein is required to establish similarities/dissimilarities between this protein, high-affinity Type I ER sites, and the secondary sites such a