Progesterone-binding protein in uterine, endometrial and myometrial cytosol of normal and estrogen (diethylstilbestrol)-primed, immature rabbits has been characterized using sucrose gradient centrifugation and charcoal adsorption techniques. Estrogen priming daily for five days increased the concentration of uterine progesterone receptors 32-fold as compared to progesterone binding in serum. The number of receptor binding sites in the uterus reached a maximum on day ten of 51,700 per cell. This result compares favorably with the best progestational response, which occurs on days 10 and 11 in McPhail's bioassay. The mean dissociation constant for progesterone and progesterone receptor complexes was estimated at 3.3 × 10-10M (n = 18). We observed a higher concentration of progesterone receptors in myometrial than in endometrial cytosol; both levels were less than that in uterine cytosol. In addition to quantitative changes, we observed a qualitative change in the sedimentation constant of progesterone-binding protein from a 4-5S to a 5-6S complex after ten days of estrogen administration; the complex changed irreversibly to the 4-5S form in the presence of 0.4 M KCl. Among the natural steroids tested, the highest relative binding affinity was observed with 5α-pregnan-3,20-dione (15%) when progesterone was 100%. Synthetic progestational compounds showed a wide range of binding affinities for progesterone receptors.
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