Progesterone receptor measurement by isoelectric focusing: a potential microassay

Sixty-two selected breast cancers were used to compare the conventional dextran-coated charcoal (DCC) assay with a new method of separating progesterone receptor by isoelectric focusing in flat beds of agarose gel. Ninety assays were performed. Isoelectric focusing indicated correctly the presence of receptor in 92% and the absence of receptor in 86% of assays, when compared with the DCC assay. The relationship between the results of the two methods was linear. Isoelectric focusing underestimated receptor to a variable extent, finding relatively less receptor at higher absolute levels of binding than at lower levels. The lower limit of sensitivity of isoelectric focusing was 30 fmol/ml cytosol. The protein concentrations of cytosols prepared from 46 needle biopsy samples (mean weight 25 mg) ranged from 0.5 to 30 g/l (median 4 g/l, 10th percentile 0.75 g/l). Isoelectric focusing is a satisfactory method of progesterone receptor measurement and can be applied to samples too small for conventional techniques.