Estrogen-binding proteins of the human uterus

Abstract

1. 1. Sucrose gradient centrifugation analysis and agarose gel chromatography of the human uterine cytosols, equilibrated with [ 3H]estradiol, have demonstrated the presence of two specific estrogen-binding proteins. The endometrial cytosol contained estrogen-binding proteins which sediment in sucrose gradients at 8 S, with a secondary estrogen-binding protein sedimenting at 3 S, while the myometrial cytosol contained almost exclusively a 3-S (3.1 ± 0.1 S) estrogen-binding protein. A non-specific [ 3H]-estradiol-binding protein with a sedimentation coefficient of 4.6 S was shown to be serum albumin. 2. 2. The addition of diisopropylfluorophosphate (DFP) to the homogenization buffer resulted in the appearance of the 8-S and no 3-S estrogen-binding protein in the myometrial cytosol, suggesting that the 3-S species may be obtained from the 8-S estrogen-binding protein by limited proteolysis, but without loss of the estradiol-binding capacity. 3. 3. The myometrial 3-S estrogen-binding protein has a molecular Stokes radius of 26.7 (+ 0.4) Å, with a frictional ratio ( ƒ ƒ 0 ) of 1.20–1.25, and a molecular weight of 35 000–38 000 as approximated by agarose gel chromatography and sucrose gradient analysis. 4. 4. The apparent dissociation constant of the myometrial estrogen-binding protein was 1·10 −9 M and the binding capacity was 67 (± 10)·10 −15 mole of [ 3H]-estradiol bound per mg protein, with large variation among patients, 25·10 −15 140·10 −15 mole of estradiol bound per protein. Test compounds competed with the [ 3H]estradiol for binding by the myometrial estrogen-binding protein in the following sequence: 17β-estradiol > estrone > ethynylestradiol ≥ diethylstilbestrol > 17α-estradiol > estriol > CI-628 > U11, 100A > cis-clomiphene > 5-androsten-3β,17β-diol > 4-androsten-3β,17β-diol. Dihydrotestosterone, testosterone, androstenedione, progesterone or cortisol were not effective competitors of [ 3H]estradiol for the myometrial estrogen-binding protein.