ENDOMETRIAL NUCLEAR ESTRADIOL RECEPTOR OF THE PREGNANT COW HAS A MOLECULAR-WEIGHT OF 53-000 IN 6-M GUANIDINE-HC1

Cell nuclei isolated from the endometrium of a 3 mo. pregnant cow were dialyzed against 6 M guanidine-HCl + 0.1 M 2-mercaptoethanol. The resulting material (after eliminating the bulk of DNA by centrifugation) was filtered on a Sepharose 6/b column equilibrated in the above denaturing solvent. The MW of the denatured nuclear estradiol receptor was about 53,000. This value is similar to that of the cytoplasmic receptor (55,000) but dissimilar to that of the cytoplasmic receptor transformed by the action of the Receptor Transforming Factor (35,000) estimated under the same experimental conditions, suggesting that the latter form of the receptor is not the biological precursor of the nuclear receptor. According to indirect estimates, probably not more than 10% of the nuclear receptor was renatured in these experiments. Therefore the alternative that the results obtained are not representative for the nuclear receptor, cannot be excluded.