SUMMARY
The synthetic progestogen R5020 (17,21-dimethyl-19-norpregna-4,9-diene-3,20-dione) binds with high affinity (Ka = 8·8 × 108 1/mol at 0 °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 uterine cytosol. The affinity of the receptor for the natural hormone progesterone is remarkably low (Ka= 1 × 108−1·7 × 1081/mol at 0 °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 °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 (progesterone-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.
Metabolism of the synthetic progestogen norethynodrel by human ketosteroid reductases of the aldo–keto reductase superfamily
