Characterization and assay of the progesterone receptor in rat uterine cytosol

The synthetic progestogen R5020 (17,21-dimethyl-19-norpregna-4,9-diene-3,20-dione) binds with high affinity (Ka = 8.8 X 10(8) l/mol at 0 degree C) to the progesterone receptor from rat uterine cytosol. At nanomolar concentrations, equilibrium is attained in less than 90 min. R5020 has a very low affinity for other specific steroid-binding proteins (corticosteroid-binding globulin and oestrogen receptors) present in relatively high concentrations in the urine cytosol. The affinity of the receptor for the natural hormone progesterone is remarkably low (Ka = 1 X 10(8)-1.7 X 10(8) l/mol at 0 degree C) which explains the instability of progesterone-receptor complexes. Advantage may be taken of this property to remove endogenous progesterone easily by charcoal treatment at 0 degree C, a treatment which does not modify the concentration of receptors. A method based on these characteristics is described for the assay of the total number (progesteron-bound and unbound) of receptor sites in uterine cytosol. This assay may be used in various physiological situations where endogenous progesterone is present at unknown concentrations.