Progesterone in Uterus and Plasma: V. An Assay of the Progesterone Cytosol Receptor of the Guinea Pig Uterus

Abstract

The concentration of progesterone receptor sites in uterine cytosol can be measured by microcolumn Sephadex G-25 chromatography under very strict experimental conditions. The ligand concentration must be kept in the 0.1–10 nM range and the protein concentration must be sufficiently low (=1 mg/ml) to minimize nonspecific binding. Under these conditions the saturation of the receptor is obtained and its apparent affinity for progesterone (K = 1.5 109 M1 at 0 C) and the binding sites concentration can be measured. When endogenous progesterone is present the technique can be used providing the time of incubation is shown to allow complete exchange between endogenous unlabelled and exogenous radioactive ligand. The apparent affinity for radioactive progesterone is then decreased but the concentration of the binding sites can still be measured. The proposed technique thus meets the necessary requirements for measuring receptor concentration in various physiological situations where endogenous progesterone varies. (Endocrinology90: 1064, 1972)