Zm. Liu et R. Patino, HIGH-AFFINITY BINDING OF PROGESTERONE TO THE PLASMA-MEMBRANE OF XENOPUS-OOCYTES - CHARACTERISTICS OF BINDING AND HORMONAL AND DEVELOPMENTALCONTROL, Biology of reproduction, 49(5), 1993, pp. 980-988
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.