Progesterne-binding properties of microsomes from pregnant rat uterus

Abstract

The progesterone binding characteristics of microsomes from pregnant and post partum rat myometrium were investigated using a competitive protein binding method. The myometrial microsomal preparations showed progesterone binding of high affinity ( K a ~ 6 × 10 7 M −1) and limited capacity in addition to low affinity binding of high capacity. The microsomal progesterone binding components of high affinity were characterized by rapid dissociation and exchange of bound steroid at + 6°C and 0°C. The progesterone binding activity of the rat myometrial microsomes was resistant to freezing. The microsomal progesterone binding components were shown to be specific for progesterone. Of 18 different steroids tested only testosterone, 5α-dihydrotestosterone (17β-hydroxy-5α-androstan-3-one) and 5αpregnane-3,20-dione exhibited moderate competition with [ 3H]-progesterone for the binding sites. Microsomal preparations from rat striated muscle and heart displayed only low concentrations of non-specific progesterone binding whereas rat liver microsomes exhibited progesterone binding resembling that of the myometrial microsomes. The myometrial microsomal progesterone binding capacity was lower post partum than one day before parturition. The microsomal progesterone binding components described in the present study are distinctly different from the cytosolic high affinity progesterone binding proteins found in rat myometrium.