Oestrogen receptor assay—Limitation of the method

The variability of estimates of K_d and binding capacity (fmol oestrogen bound/mg cytosol protein) of oestrogen receptors (RC) which may arise from different cytosol protein concentrations (0.25–5 mg/ml), different charcoal concentrations (0.25–5 mg/ml) and cytosols from different, histologically comparable portions of the same tumour has been studied using a dextran-coated charcoal (DCC) assay. K_d value correlated positively to protein concentration and negatively to charcoal concentration, whereas the binding capacity remained constant over a wide range of protein concentrations. In receptor-poor tumours, low cytosol protein contents precluded a distinction between specific and non-specific oestrogen binding. Both in receptor-rich and receptor-poor tumours, increasing charcoal concentrations added to the incubation mixture decreased the cytosol protein concentration and thus removed RC complexes as well as excess of oestrogen. Both in receptor-rich and receptor-poor tumours, a wide variation in recptor content was observed within histologically comparable portions of the same tumour (from 0 to 300 fmol/mg protein).