| Abstract: | The characteristics of binding of radiolabeled progesterone, promegestone [17 alpha,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione (R5020)], medroxyprogesterone acetate (4-pregnen-6 alpha-methyl-17 alpha-ol-3,20-dione acetate), and methyltrienolone [17 beta-hydroxy-17 alpha-methyl-4,9,11-estratriene-3-one (MT)] to the progesterone receptor in human prostatic cytosol have been compared. MT binds to both androgen and progesterone receptors with high affinity (Kd = 0.9 and 0.6 nM, respectively). The binding of MT to the progesterone receptor can be blocked by adding an excess of unlabeled triamcinolone acetonide [9 alpha-fluoro-11 beta, 16 alpha, 17 alpha,21-tetrahydroxy-1,4-pregnadiene-3,20-dione-16,17-acetonide (TAC)]. The difference between the binding of [3H]MT in the absence and presence of TAC (i.e. [MT - (MT + TAC)] represents specific binding of MT to the progesterone receptor. Ligand specificity studies demonstrated that this binding was typical of a progesterone receptor. Furthermore, progesterone receptor levels measured in this way were comparable to those obtained using progesterone, R5020, or medroxyprogesterone acetate as labeled ligands. Progesterone receptor quantitation from the difference MT - (MT + TAC) is of particular advantage when simultaneous quantitation of progesterone and androgen receptors is desired in small tissue specimens since only three sets of incubations are required: [3H]MT, [3H]MT plus unlabeled TAC, and [3H]MT plus unlabeled MT (to measure nonspecific binding). Conditions are described for the application of this methodology to a microassay. A marked underestimate of progesterone receptor content was observed when incubation was terminated with hydroxylapatite compared to that measured when dextran-coated charcoal was used. The presence of comparable amounts of progesterone and androgen receptors in human prostatic cytosol deserves further investigation. |
