Characterization and Properties of Progesterone-Binding Components in Human Endometrium1

Abstract

Macromolecular components of normal human endometrial cytosol which bind ³H-progesterone in vitro were identified and characterized by sucrose-glycerol gradient centrifugation, polyacrylamide gel electrophoresis and gel filtration on Agarose. The major binding component has a sedimentation coefficient of about 4S. A 7S component was observed occasionally. The 4S component binds both ³H-progesterone and ³H-cortisol, whereas the 7S component binds the former but not the latter. These binding components were not detected in two tumor tissues. The interaction of the binding components with ³H-progesterone has an apparent association constant of about 2.7 × 10⁹M⁻¹ for the high affinity binding sites. When the affinities of various steroids for all the progesterone-binding components were compared by gel filtration on Sephadex G-50 and charcoal adsorption, the following order was observed: progesterone > 5α-pregnane-3,20-dione > 5β-pregnane-3,20-dione > 20α-dihydroprogesterone > testosterone > estradiol-17β > cortisol. The rate of dissociation of the “receptor”-progesterone complexes in the presence of large amounts of unlabeled cortisol also indicates that cortisol does not compete with progesterone for the binding components. Under the same experimental conditions, the corticosteroid-binding globulin of human serum and the major progesterone-binding component of human endometrial cystosol both sediment at about the same rate, chromatograph as a single component on Agarose A-0.5m, and are not distinguishable by acrylamide gel electrophoresis. Temperature and N-ethylmaleimide had very different effects on binding of progesterone or cortisol by endometrial cytosol and serum proteins. The hormone-binding kinetics exhibited by endometrial cystosol and female serum are also markedly different. These findings are helpful in the differentiation of progesterone-binding by macromolecules from the two sources studied thus far.