Selective complexing of the "nuclear" 5S estradiol receptor by a serum component, 5S-CA.

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Abstract
A component present in several sera interacts selectively with the 5S "nuclear" form of estradiol receptor but not with the 4S "cytosol" form. This component is therefore referred to as "5S-complexing activity (5S-CA)." This selective interaction occurs with 5S receptor from mouse, calf, or rat. In addition, 5S-CA recognizes the nuclear form of the receptor from hypothalamic as well as uterine tissue. 5S-CA indicates a general similarity in the chemical nature of 5S nuclear receptors prepared from several sources. Using 5S-CA as a chemical probe for the nuclear form of the estradiol receptor, the chemical as well as physical similarity of 5S receptors prepared in vivo (by injection of [3H]estradiol and subsequent isolation from nuclear/myofibrillar extracts) and in vitro (by labeling of cytoplasmic extracts and subsequent chromatography on columns of DNA-cellulose) was shown. These results indicate a distinctive chemical property of nuclear 5S forms of estradiol receptor. The data are interpreted with regard to models for 5S receptor formation. 5S-CA was found in sera from animals immunized against various antigens. The importance of testing for 5S-CA in antisera directed against steroid derivatives, steroid receptors, and other steroid-binding proteins is discussed.