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 those from somatic cells, indicating that progesterone binding is restricted to the oocyte membrane. Scatchard analysis indicated a single class of high-affinity (average KD, 10(-9) M), low-capacity (average concentration, 10(-12) mol/mg protein) binding sites for progesterone in all 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 > 17 alpha, 20 beta, 21-trihydroxy-4-pregnen-3-one > 11-deoxycorticosterone > 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one > 11-deoxycortisol > estradiol > R5020 > corticosterone > aldosterone > cortisol > androstenedione > ouabain.(ABSTRACT TRUNCATED AT 250 WORDS)