A rapid, convenient method for the quantitative determination of estrogen receptors (ER) under high salt (0.6 m KCI) conditions (such as in extracts of nuclear pellets from human breast cancer biopsies) using a commercially available kit [estrogen receptor enzyme immunoassay (ER-EIA) Monoclonal; Abbott] is described. This assay has been validated using breast tumor cytosol ER preparations. It determines total (both occupied and unoccupied) ER, it is insensitive to KCI at concentrations up to 0.8 m, and it can be used with ER preparations having very low protein concentrations. Results obtained using the ER-EIA method for breast tumor nuclear extracts have been compared to those obtained using the hydroxylapatite method, and higher values have been found using the ER-EIA method. The reasons for this discrepancy may be due to: (a) the sensitivity of ER binding to hydroxylapatite in high concentrations of KCI; (b) the temperature dependent degradation of the receptor complex at 30°C,the temperature commonly used to achieve exchange between radioactive estradiol and the endogenously bound estradiol; and (c) possible detection of immunoreactive, but non-ligand-binding forms of ER. The possibility that occurrence of “free” receptors in high salt extracts from nuclear pellets may be an artifact is discussed.

The availability of this ER-EIA suitable for nuclear ER determinations opens the possibility of extending correlations between the clinical course of breast cancer and the levels of the ER form (nuclear) that are thought to be of greatest physiological significance.

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Presented at the Symposium on “Estrogen Receptor Determination with Monoclonal Antibodies,” December 14, 1984, Monte Carlo.

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