Estrogen Cytosol Binding Proteins in Bovine Endometrium and Corpus Luteum1

Estrogen binding by endometrial and luteal cytosol proteins was determined by a dextran—charcoal adsorption method and correlated with plasma hormone levels in the same animals. Cytosol preparations from both tissues demonstrated saturable high affinity (10−9M) binding of estradiol. Inhibition of endometrial protein binding of [3H]estradiol-17β was obtained with estradiol-17β> estrone> estradiol-17α> diethylstilbestrol at 0°C. Binding by luteal protein could be inhibited with estradiol-17β> estrone. Estradiol-17β binding could not be inhibited with progesterone or cortisol. The concentration (pmole/mg protein) of estrogen-binding protein increased (p < .025) in the endometrium between the fifteenth and twenty-first days of the estrous cycle when compared to the second, fifth, and tenth days. The concentration of binding protein (on Day 15) was highly correlated with the estradiol-17β and estrone plasma levels found at that time (p < 0.01). Luteal estrogen-binding protein concentration increased from Day 5 to Day 10 (p < 0.01) and remained at the same level through Day 15. There was a significant decline in binding protein concentration at Days 17–18, at the initiation of luteal regression, followed by an increase in concentration of luteal-binding protein at Day 21 to a level higher than at any other time during the cycle. Plasma progesterone and estrone levels declined at Days 17–18, while plasma estradiol levels increased. Ovariectomy resulted in an increase in the concentration of endometrial estrogen binding protein over that in intact heifers. Estrogen treatment of ovariectomized heifers caused a decrease in the concentration and content of binding protein 3 h after treatment. Eighteen hours after treatment the concentration was depressed but the total uterine content was increased.