Comparison of the Physicochemical Properties of Uterine Nuclear Estrogen Receptors Bound to Estradiol or 4-Hydroxytamoxifen*

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Abstract
We have compared the physicochemical properties of rat uterine nuclear estrogen receptors (ER) labeled with (E2) or the high affinity antiestrogen 4-hydroxytamoxifen [OH Tam; 1-[4-(2-dimethylaminoethoxy)phenyl]-1-(4-hydroxyphenyl)2-phenyl-but-1-(Z)ene]. Nuclear ER labeled in vivo with either ligand had sedimentation rates of 4.5-5S and mol wt (calculated from sedimentation coefficients and Stokes radii) of about 80,000. The 3S and 4S forms labeled by in vitro exchange with [3H]E2 or [3H]OH Tam, respectively, had mol wt of 31,000 and 50,000. Based on apparent Kd values, numbers of binding sites per uterus, and dissociation kinetics, the ligand-binding sites of the nuclear E2- and OH Tam-ER complexes could not be distinguished. An appreciable proportion (25-75%) of 4.5-5S estrogen- or antiestrogen-ER complexes were retained on DNA-cellulose columns and were eluted with 0.21 M KCl, while the 3S and 4S forms had lost the DNA-binding site. Nuclear ER bound to E2 or OH Tam were differentially sensitive to proteolysis with trypsin or .alpha.-chymotrypsin. Both in vivo and in vitro labeled [3H]OH Tam-ER complexes sedimented as discrete species (s20,w = 3.8-3.9; mol wt, .apprx. 40,000) after trypsin treatment (50 .mu.g/ml; 1 h at 23.degree. C); under the same conditions, peaks of [3H]E2 were obliterated. After digestion with .alpha.-chymotrypsin (10 .mu.g/ml; 1 h at 23.degree. C), nuclear ER labeled in vivo with [3H]E2 or [3H]OH Tam sedimented at 2.9S (mol wt, 29,000) or 4.0S (mol wt, 47,000), respectively; at 50 .mu.g/ml, [3H]E2-ER complexes were barely discernible, while binding of [3H]OH Tam was only partially decreased. Nuclear ER labeled with the nonsteroidal estrogen [3H]diethylstilbestrol resembled [3H]E2-ER complexes in sensitivity to proteolysis. These results suggest that nuclear estrogen-and antiestrogen-ER complexes may differ in conformation such that they are differentially susceptible to degradation. This may influence their interactions with chromatin or specific DNA sequences as well as their release from nuclear binding sites.

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