Simultaneous measurement of estrogen and progesterone receptors in tumor cytosols with use of ¹²⁵ I-labeled estradiol and of ³H-R5020

We describe a dual-isotope assay for measuring the concentration of estrogen receptors and progesterone receptors in tumor cytosols. The concentrations of these receptors are derived from separate Scatchard-plot analyses of a single dextran-coated charcoal assay that incorporates both radioiodinated estradiol and tritiated R5020 as the labeled ligands. The two isotopes, radoiodine and tritium, are easily measured with a liquid-scintillation counter. The concentrations of estrogen and progesterone receptors derived from the dual-label assay are identical to values derived from the respective single-label assays. Moreover, values obtained from an assay with dextran-coated charcoal that incorporates radoiodinated estradiol are identical to those obtained from an assay in which tritiated estradiol is used as the labeled ligand. The dual-isotope assay requires both less time and less tissue.