Reversible Inhibition of Cytosolic Glucocorticoid Receptors by Mercurial Agents. Application in the Measurement of Total and Unoccupied Receptors

Abstract

Recently developed “exchange assays” have been used to measure total cytosolic glucocorticoid receptor (GR) binding activity as compared to standard GR assays which measure unoccupied receptor. In the current study we modified these methods and extended the applications of such assays. Experiments defined the conditions whereby two sulfhydryl-binding agents, p-hydroxymercuribenzoate (PHMB) and mersalyl, completely inhibited binding of the glucocorticoid receptor to ligand in mouse renal cytosol. Reactivation of steroid-binding activity was restored by addition of dithiothreitol. The present study demonstrates 12% higher GR binding activity when this exchange assay is performed using saturated glucocorticoid-receptor complexes, rather than standard cytosol. Combining the data from the standard and exchange mouse renal cytosolic GR assays, it was determined that, at physiologic tissue corticosterone levels, the respective mean concentrations of unoccupied, occupied, and total GR were 467, 89, and 556 fmol/mg cytosol protein. Measurement of receptor concentrations by the use of these methods permits precise experimental differentiation of factors which affect total, as well as unoccupied GR.

KEY WORDS:

• Glucocorticoids
• cytosolic receptor measurement
• mouse kidney