Progesterone-binding properties of microsomes from rat liver

Abstract

The progesterone binding properties of microsomal suspensions from rat liver have been studied using a Dextran-coated charcoal method for the determination of the progesterone binding capacity. Microsomes isolated from the livers of adult female, male and pregnant female rats all exhibited progesterone binding of high affinity and limited capacity ( K a ∼ 2 × 10 7M −1), in addition to non-specific binding of progesterone. The high affinity progesterone binding components of the hepatic microsomes were partly solubilized by treatment with the detergent Triton X-100. These solubilized progesterone binding components were progesterone saturable ( K a ∼ 3 × 10 7M −1) and specific for progesterone. Of the 11 steroids tested progesterone showed the highest competition for microsome-bound [ 3H]-progesterone. The progesterone binding capacity of female rat liver microsomes was decreased after protease and RNase treatment, suggesting that both protein and RNA were involved, directly or indirectly, in progesterone binding to the microsomes. Treatment of the liver mierosomes with EDTA (0–17 mM) or KC1 (0.4 M) had no effect on the binding of progesterone. The microsomal suspensions of rat liver resisted freezing without loss of their progesterone binding activity, whereas approximately one half of their progesterone binding capacity was lost after incubation at +37°C for 15 min.