SPECIFIC BINDING OF [H-3] 17-ALPHA,20-BETA-DIHYDROXY-4-PREGNEN-3-ONE TO OOCYTE CORTICES OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Specific binding of [H-3] 17alpha,20beta-dihydroxy-4-pregnen-3-one (17 alpha,20beta-DP) to plasma membranes prepared from defolliculated oocy tes of rainbow trout (Onchorhynchus mykiss) was identified and charact erized. Binding was rapid and reached equilibrium in 30 min. 17alpha,2 0beta-DP strongly inhibited [H-3] 17alpha,20beta-DP binding in a compe titive manner. Scatchard analysis revealed two different binding sites : a high affinity binding site with a K(d) of 18 nM and a B(max) of 0. 2 pmoles/mg protein; and a low affinity binding site with a K(d) of 0. 5 muM and a B(max) of 1 pmoles/mg protein. This binding activity was s uccessfully solubilized with n-heptyl-beta-D-thioglucoside. [H-3] 17al pha,20beta-DP binding to solubilized preparations reached equilibrium in 1 h, and was competitively inhibited with 17alpha,20beta-DP and 17a lpha,20beta,21-trihydroxy-4-pregnen-3-one. However, Scatchard analysis showed a single binding site with a K(d) of 0.3 muM. The reason for t he disappearance of the high affinity binding site in solubilized prep arations remains unclear. These results demonstrate that a specific bi nding site for 17alpha,20beta-DP exists in the plasma membrane of rain bow trout oocytes.