Biochemical Comparison of Estrogen Receptors of the Hamster Hypothalamus and Uterus1

Comparison of the soluble estradiol receptor proteins in the hypothalamus and uterus of the golden Syrian hamster revealed that both have sedimentation coefficients of 8S in a low ionic strength buffer and migrate similarly on analytical disc-gel electrophoresis in both 5% and 7% acrylamide gels. The competition of [3H]-17beta-estradiol binding by unlabeled estrogenic as well as nonestrogenic compounds was similar with the receptors present in the two tissues. The affinity constants for the hypothalamic and uterine receptors were 4.3 X 10-9M-1 and 1.6 X 10-10M-1, respectively; the number of binding sites when expressed on an equivalent protein basis (i.e. 1 mg/ml) was 0.18 X 10-10M and 3.1 X 10-10M, respectively. Identical amounts of soluble receptors with similar sedimentation characteristics were found in hypothalami from male and female animals gonadectomized 64 h prior to use. A titration of binding sites in the uteri of intact and ovariectomized animals revealed decreased binding in the supernatant preparation from intact animals, probably due to the presence of bound endogenous unlabeled hormone.