title: Specific binding of estrogen and estrogen-receptor complex by microsomes from estrogen-responsive tissues of the rat.
abstract: Rat uterine and anterior pituitary microsomes each contain a population of specific estrogen-binding sites. Saturation binding of estradiol is demonstrable, with an affinity similar to that of the cytosol estrogen receptor (Ka = 1-2 X 10(10) M-1). Dissociation rate kinetic determinations, however, revealed that estrogen-microsomal complexes are 4 times as stable as cytosol estrogen-receptor complexes. Sedimentation properties in sucrose gradients were salt-dependent, yielding values of 10S in KCl-free buffer and 5.5S in the presence of 0.4 M KCl. The concentration of microsomal sites varies in proportion to the level of cytosol estrogen receptor, such that microsomal binding constitutes a consistent 20% of the total extranuclear binding capacity. Binding is sensitive to pronase, but not to ribonuclease or deoxyribonuclease; steroidal specificity differs from cytosol receptor only with respect to a greater extent of competition by progesterone. Microsomal binding sites are readily extractable with KCl-free hypotonic buffer or with 0.4 M KCl, but are resistant to extraction by 0.15 M KCl. The presence of estradiol lends stability to the microsomal binding sites, while high salt has a deleterious effect on their longevity. After exhaustive extraction of binding sites, microsomes are capable of accepting cytosol estradiol-receptor complexes to a level corresponding to the concentration of depleted binding sites; microsomes from nontarget tissue do not manifest such capability. However, the original microsomal estrogen-binding sites are not simply cytosol receptor contaminants, as evidenced by the observations that the microsomal binding site concentration is independent of the volume of tissue homogenate (indicating that a trapping phenomenon is not operative) and that nonextracted microsomes are not potential acceptor sites for cytosol estradiol-receptor complexes. In considering total cellular dynamics of estrogen and estrogen receptor turnover, it thus becomes important to explore the role of the microsomal compartment, since it functions as a repository of specific estrogen-binding sites and may have significant acceptor capability for the cytosol estrogen-receptor complex.
mesh_terms: D000818:Animals; D001943:Breast Neoplasms / Q000378:metabolism; D002499:Centrifugation, Density Gradient; D004958:Estradiol / Q000378:metabolism; D004967:Estrogens / Q000378:metabolism*; D005260:Female; D006801:Humans; D007700:Kinetics; D008168:Lung / Q000378:metabolism / Q000648:ultrastructure; D008861:Microsomes / Q000378:metabolism*; D009130:Muscle, Smooth / Q000378:metabolism / Q000648:ultrastructure; D010903:Pituitary Gland, Anterior / Q000378:metabolism / Q000648:ultrastructure*; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D011960:Receptors, Estrogen / Q000378:metabolism*; D014599:Uterus / Q000378:metabolism / Q000648:ultrastructure*
chemical_list: D004967:Estrogens; D011960:Receptors, Estrogen; D004958:Estradiol; D011189:Potassium Chloride