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

Progesterone induces maturation of the amphibian oocyte through its action on the plasma membrane. However, whether or not this action requires high-affinity binding to a specific receptor is unclear. In this study, the binding activity of progesterone was characterized in plasma membranes from whole ovaries or defolliculated oocytes of Xenopus laevis. Membrane-bound, radiolabeled progesterone was isolated by filtration of membrane suspensions and quantified by liquid scintillation. The association of progesterone to membrane preparations reached equilibrium within 15 min. Progesterone binding activity was directly proportional to the sample concentration, was significantly reduced by trypsin digestion, and was pH-dependent and temperature-sensitive. Also, binding activity was observed in membrane preparations from whole ovaries and defolliculated oocytes but not in diose from somatic cells, indicating 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 protein) binding sites for progesterone in atl oocyte membrane preparations tested. Progesterone binding activity was also detected in preparations from albino frog ovaries, indicating that the binding activity is not an artifact of melanin contamination. Competition studies showed the following order of affinities: progesterone > pregnenolone > 17alpha,20beta,21-trihydroxy-4-pregnen-3-one > 11-deoxy-corticosterone > 17alpha,20beta-dihydroxy-4-pregnen-3-one > 11-deoxycortisol > estradiol > R5020 > testosterone > corticosterone > aldosterone > cortisol > androstenedione > ouabain. Experiments using radiolabeled R5020 as primary ligand showed low levels of specific binding and a different pattern of steroid competition, indicating that the membrane receptor for progesterone in the Xenopus oocyte is different than the classical intracellular progesterone receptor. The density of the receptor for progesterone increased with oocyte development and after stimulation of intact follicles with gonadotropin.