Progesterone-binding properties of the microsomal fraction from chick oviduct

Progesterone binds to specific high-affinity and limited-capacity binding sites of chick-oviduct microsomes of estrogen-primed chicks. The dissociation constant is 2 x 10(-9) M (range 1.6--3.0) and the number of binding sites 500 femtomoles/mg microsomal protein (range 464--551). Treatment of the estrogen-primed chicks by progesterone had no apparent effect on the progesterone-binding capacity or affinity. Competition studies showed that testosterone, R-5020, Org-2058, D-norgestrel, 5 alpha-dihydrotestosterone and R-2323 were effective competitors of progesterone, in that order, whereas cortisol, estradiol and estrone exhibited only minimal displacement. No displacement of microsome-bound [3H]progesterone was found with fenoterol or prostaglandin F2 alpha. No high-affinity progesterone-binding sites were found in the microsomal fractions of liver, muscle, intestine or brain. On the basis of steroid-binding affinity and steroid-specificity determinations, the microsomal progesterone-binding components seem to be different from the progesterone receptor previously described in chick oviduct cytosol.