The estrogen receptor in rat liver cytosol was analyzed as the [3H]estradiol-receptor complex by isoelectric focusing in slabs of polyacrylamide gel. The complex focused at pH 6.4. The radioactive peak became sharper and the recovery of receptor improved from 67% to 83% after limited trypsin digestion of [3H]estradiol-labeled cytosol before analysis. Using this technique, the estrogen receptor in rat liver cytosol was shown to have the expected ligand specificity. The[3H]estradiol-receptor complex had a dissociation constant of 7.6 x 10-10 M. Both male and female rat liver contained estrogen receptor. However, analysis of the receptor in male liver cytosol was seriously disturbed by an unspecific estrogen binder focusing at pH 5.3. This peak was eliminated when labeling of the estrogen receptor was performed in the presence of 0.1 mM unlabeled 5α-androstan-3α,17β-diol. Using this modified technique, no significant difference in receptor concentration was found between male and female rat liver cytosols (88 and 67 fmol/mg protein, respectively). The estrogen receptors present in cytosol from liver and uterus were found to have very similar isoelectric focusing profiles after treatment with increasing amounts of trypsin. Furthermore, the substrate specificities of these receptors were identical, and it was concluded that the hepatic and uterine estrogen receptors probably are identical. Rats ovariectomized for 29 days showed the same hepatic estrogen receptor concentration as rats ovariectomized for 2 days. Hypophysectomy reduced the receptor level to 11% of that in ovariectomized control rats. Insertion of a pituitary transplant beneath the kidney capsule partially restored the estrogen receptor level to 37% of the control level.
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