Progesterone-binding protein in human myometrium. Binding site concentration in relation to endogenous progesterone and estradiol-17β levels

Abstract

The concentration of specific progesterone-binding sites in myometrial eytosol from 25 normal menstruating women was measured. Estradiol-17β and progesterone concentrations in the uterine cytosol and peripheral plasma of these patients were also determined by radioimmunoassay. The specific progesterone-binding capacity ranged from 63 to 722 fmol/mg of myometrial cytosolic protein (mean 335 ± 181 fmol/mg protein) with an equilibrium association constant of 4.2 ± 1.3 × 10 9 1/mol. Density gradient centrifugation showed that [ 3H]-progesterone was bound to a macromolecule with a sedimentation rate of 3.6 ± 0.3 S. The concentration of cytosolic progesterone-binding sites was inversely correlated to both the plasma and myometrial cytosol progesterone levels. There was also a direct correlation between the concentration of progesterone-binding sites and cytosol estradiol-17β levels. Progesterone binding showed no correlation with plasma estradiol-17β levels. The highest progesterone-binding capacity was found in myometrial cytosol during the late proliferative phase of the menstrual cycle.