Binding interactions of progesterone and other C21 steroids with rat uterine cytosol

The binding of progesterone and other C21 steroids by rat uterine cytosol preparations was studied by equilibrium dialysis. Scatchard plots and preliminary computer-assisted analysis indicated at least two types of binding sites for progestrone. Individual Scatchard plots for cortisol and corticosterone binding were linear with Ka's 0.4 and 1.4 times 10(9) M-1, respectively. Inhibition analyses and modeling of competition experiments suggest, however, that the situation is more complex. There was no binding of labeled dexamethasone.