HIGH-AFFINITY BINDING OF PROGESTERONE TO THE PLASMA-MEMBRANE OF XENOPUS-OOCYTES - CHARACTERISTICS OF BINDING AND HORMONAL AND DEVELOPMENTALCONTROL

Progesterone induces maturation of the amphibian oocyte through its ac tion on the plasma membrane. However, whether or not this action requi res high-affinity binding to a specific receptor is unclear. In this s tudy, the binding activity of progesterone was characterized in plasma membranes from whole ovaries or defolliculated oocytes of Xenopus lae vis. Membrane-bound, radiolabeled progesterone was isolated by filtrat ion of membrane suspensions and quantified by liquid scintillation. Th e association of progesterone to membrane preparations reached equilib rium within 15 min. Progesterone binding activity was directly proport ional to the sample concentration, was significantly reduced by trypsi n digestion, and was pH-dependent and temperature-sensitive. Also, bin ding activity was observed in membrane preparations from whole ovaries and defolliculated oocytes but not in diose from somatic cells, indic ating that progesterone binding is restricted to the oocyte membrane. Scatchard analysis indicated a single class of high-affinity (average K(D), 10(-9) M), low-capacity (average concentration, 10(-12) mol/mg p rotein) binding sites for progesterone in atl oocyte membrane preparat ions tested. Progesterone binding activity was also detected in prepar ations from albino frog ovaries, indicating that the binding activity is not an artifact of melanin contamination. Competition studies showe d the following order of affinities: progesterone > pregnenolone > 17a lpha,20beta,21-trihydroxy-4-pregnen-3-one > 11-deoxy-corticosterone > 17alpha,20beta-dihydroxy-4-pregnen-3-one > 11-deoxycortisol > estradio l > R5020 > testosterone > corticosterone > aldosterone > cortisol > a ndrostenedione > ouabain. Experiments using radiolabeled R5020 as prim ary ligand showed low levels of specific binding and a different patte rn of steroid competition, indicating that the membrane receptor for p rogesterone in the Xenopus oocyte is different than the classical intr acellular progesterone receptor. The density of the receptor for proge sterone increased with oocyte development and after stimulation of int act follicles with gonadotropin.