Progesterone binding in uterine cytosols of the guinea pig.

Guinea pig uterine cytosols have been prepared from normal, ovariectomized, and ovariectomized, estrogen-primed animals. The cytosols were incubated with [3H]progesterone and analyzed by sucrose density gradient ultracentrifugation and equilibrium dialysis. A progesterone receptor, which has a sedimentation coefficient of about 7.6 S, was found in the cytosols but not in plasma. The receptor was dependent on in vivo estrogen. The intrinsic association constant, estimated from Scatchard plots, was about 5 X lo* M-' when the concentration of cytosol protein was 5 mg per ml, and about 2 X log M-~ when the concentration was 2 mg per ml. Scatchard plots showed a maximum value of B/F at low ligand concentrations, indicating “cooperativity,” similar to that seen in plots of the estrogen receptor. Exposure of the receptor to 0.3 M KC1 resulted in inactivation of binding sites rather than dissociation into 3.5 to 5.0 S subunits. There is evidence for the existence of a 5.5 to 6.0 S receptor after exposure to 0.3 M KC1 which may have been derived from the 7.6 S receptor.