title: Properties of the cytoplasmic progestin-binding protein in the rabbit uterus.
abstract: An exchange assay for the measurement of total cytoplasmic progestin binding sites has been developed on rabbit uterine cytosol using the highly potent progestin, R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione) labelled to a high specific activity. This compound has several advantages over progesterone: it is not bound by plasma corticosteroid binding globulin; it has high affinity for the progestin receptor; it binds virtually as fast as progesterone to the receptor, but the complex formed dissociated 8 times slower; its binding is not displaced by more than 2% by compounds devoid of progestational activity (estrogens, testosterone, dexamethasone, aldosterone). Bound endogenous progesterone was exchanged by tritiated R 5020 in a time compatible with receptor stability. At 0 C, total exchange of filled sites occurred in less than 4 h; at this temperature the R 5020-receptor complex was stable for at least 28 h. The conformation of the R 5020-receptor complex was investigated in sucrose density gradients under various experimental conditions. Unlike progesterone, it was possible to detect a 7S peak in uterine cytosol obtained from rabbits injected with a tracer dose of [3H]R 5020 1 h prior to sacrifice.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D003600:Cytosol / Q000378:metabolism; D004355:Drug Stability; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D006854:Hydrocortisone / Q000378:metabolism; D007700:Kinetics; D011374:Progesterone / Q000378:metabolism; D011817:Rabbits; D011980:Receptors, Progesterone / Q000187:drug effects / Q000378:metabolism*; D014599:Uterus / Q000378:metabolism*
chemical_list: D011980:Receptors, Progesterone; D011374:Progesterone; D004958:Estradiol; D006854:Hydrocortisone