Summary Misclassification of the oestrogen status of a human breast tumour cytosol, arising from different sources and magnitudes of error in the dextran-coated charcoal (DCC) method, have been investigated using both practical and computer simulated data analysed by Scatchard (Haybittle et al., 1982). A recent gathering of experts (Consensus Meeting, 1980), has, however, confirmed the effectiveness of the dextran-coated charcoal (DCC) receptor assay for the determination of receptor levels and it now forms the basis on which prognostic investigations are currently being established.This report evaluates the effect of certain practical inefficiencies in this methodology, such as the presence of non-specific binding which can affect quantitation (Wilson et al., 1971;Chamness & McGuire, 1975). By using the appropriate model for curve-fitting of binding data, the study provided estimates of oestrogen receptor concentrations that can be distinguished from zero, based on either the probability of detection, using practical laboratory data, or criteria derived from computer simulation studies. The data generated are of interest in relation to the establishment of the hormone sensitivity of tumours with low cytosol receptor levels and may be useful in assessing the possibly improved procedures for receptor measurement using monoclonal antibodies to receptor protein. Materials and methodsOestradiol-17/B receptor assay Breast tumour tissue was stored in liquid N2 and assayed within 2-3 weeks after surgery. Tissue (%0.5g) was cut into small pieces, pulverised into a fine powder in an all-glass homogeniser in 3 ml buffer (1OmM TRIS; 1 mM EDTA; 10% v/v glycerol; 5mM dithiothreitol; pH = 7.4). The high speed supernatant (100pl; 105,000 g for 60 min) was incubated for 16h at 4°C with lOOul of one of 10 concentrations of [3H] oestradiol (Sp. act. -100 Ci mmol-1), ranging from 0.2-5.0 nmol 1 buffer. Aliquots of these solutions were taken for counting and from a knowledge of the counting efficiency and the specific activity a "better" estimate of oestradiol mass added to the incubation medium was calculated. Similar incubations were established in the absence of cytosol to assess the inefficiency of the procedure for separating free from bound hormone, which used DCC (200 gl; 0.5% gelatin, 0.05% dextran; 0.5% charcoal in TRIS buffer; 30 min at 4°C). Parallel incubations contained 100-fold excess of diethylstilboestrol at each oestradiol concentration
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