Cytoplasmic inhibiting factors for 3H-dexamethasone binding to glucocorticoid receptors in rat tissues

Factors which specifically inhibit 3H-dexamethasone binding to unoccupied glucocorticoid receptors were examined in cytosols of the seminal vesicle and ventral prostate of rats. The inhibiting factors are mostly precipitated at 40-50% saturation of (NH4)2SO4 and eluted from a Sephacryl S-200 column immediately after the elution volume of bovine serum albumin. The inhibitory activity was relatively heat-stable and decreased to approximately 35% of the initial activity by trypsin digestion. These results suggest that the inhibiting factors are of a polypeptide-like nature. The inhibitory activity of the factors seems to be blocked by treatment with dithiothreitol and molybdate. The inhibiting factors act to decrease the binding affinity of glucocorticoid receptors to 3H-dexamethasone without altering the number of the available binding sites in cytosols. These results indicate that the binding of glucocorticoid receptors to ligands in vitro is influenced by a number of factors.