Estrogen Receptor of Cow Uterus 1

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Abstract
The estrogen receptor (ER) was separated from the cytoplasmic components of the cow uterus through successive gel filtrations of the cytosol in the presence of antipain, a protease inhibitor. The partially purified ER (designated as native “4S” ER) sedimented at 4.5S under both hypotonic (low salt) and hypertonic (0.4 M KCI) conditions. The molecular weight of native “4S” ER was estimated to be approximately 82,000 with a Stokes radius of 44 A. If purification was performed in the absence of antipain, native “4S” ER was very labile to a protease present in the cytoplasm. The modified ER (designated as modified “4S” ER) sedimented at 4.5S like native “4S” ER, but had a smaller Stokes radius (35 A) and a smaller molecular weight (65,000). In the presence of 0.4 M NaSCN, native “4S” ER remained intact, but modified “4S” ER dissociated and gave a fragment which retained the estradiol binding site. This fragment sedimented at 2.9S and had a Stokes radius of approximately 24 A (molecular weight, approximately 29,000). In the presence of Ca2+ ions and in the absence of antipain, the native ER was modified by the cytoplasmic protease to give ERs sedimenting at 3.8–4.5S. The presence of a cytoplasmic component which specifically binds with native “4S” ER to give “8S” ER under hypotonic conditions was indicated. Modified “4S” ER was unable to bind with this component.