Characterization and Quantification of the Androgen and Glucocorticoid Receptors in Cytosol from Rat Skeletal Muscle

The binding of the radioactive synthetic hormonal steroids [3H]dexamethasone (9α‐fluoro‐11β, 17α, 21‐trihydroxy‐16α‐methyl‐1, 4‐pregnadiene‐3,20‐dione) and [3H]methyltrienolone (17β‐hydroxy17‐α‐methyl‐4,9,11‐estratrien‐3‐one) to cytosol from rat skeletal muscle was studied using dextrancoated charcoal to separate unbound and receptor‐bound steroid. The rates of association, dissociation, and degradation of the complexes of dexamethasone and methyltrienolone with receptor were highly dependent on temperature. The temperature dependence of association was greater for dexamethasone, and that of degradation was greater for methyltrienolone. Dissociation rates were insignificant for both steroid‐receptor complexes compared to association and degradation rates. The apparent equilibrium dissociation constants for the binding of dexamethasone and methyltrienolone to their receptor binding sites were about 7 and 0.3 nM, respectively, regardless of temperature (0, 15 or 23° C). The lack of influence of temperature on the equilibrium constants indicate that the binding was of hydrophobic character, and the corresponding free energy changes upon binding of dexamethasone and methyltrienolone to their respective binding sites were –41 and –49 kJ mol‐1 under equilibrium conditions at 0° C. The apparent maximum number of binding sites determined from Scatchard plots under these conditions was about 1900 fmol/g of tissue, 3500 fmol/mg of DNA or 30 fmol/mg of protein in the case of the dexamethasone receptor, and the corresponding figures for the methyltrienolone were about 100 fmol/g of tissue, 200 fmol/mg of DNA or 2 fmol/mg of protein. The ligand specificities of the binding sites for dexamethasone and methyltrienolone were typical of a glucocorticoid and an androgen receptor, respectively. Both steroid‐receptor complexes were retained on DNA‐cellulose columns, and were eluted by NaCl at an ionic strength of 0.1. The DNA‐cellulose step purified about 20 times, and was used to allow gel exclusion chromatography and electrofocusing. Both steroid‐receptor complexes were excluded from a column of Sephadex G‐150. Electrofocusing in preparative columns gave reproducible patterns consisting of three peaks for each receptor. The apparent isoelectric points were 5.4, 5.6 and 0.2 for the glucocorticoid receptor, and 5.9, 6.2 and 8.5 for the androgen receptor.