3H-cyproterone acetate: binding characteristics to human uterine progestagen receptors

The availability of tritium labeled cyproterone acetate (CPA) facilitated the systematic investigation of the binding characteristics of this compound for human uterine progesterone receptors (PgR). The binding parameters of 3H-CPA are compared to those of 3H-R5020 and 3H-progesterone. The rate constants of association (k1M-1sec-1) to PgR were 7.8 X 10(3) for 3H-R5020, 4.5 X 10(4) for 3H-progesterone and 4.0 X 10(4) for 3H-CPA. The rate constants of dissociation (k-1, sec-1) were 3.6 X 10(-5) for 3H-R5020, 21.3 X 10(-5) for 3H-progesterone and 17.8 X 10(-5) for 3H-CPA. The Kd-values (M), as obtained by titration analysis and subsequent Scatchard plot analysis were 1.2 X 10(-9) for 3H-R5020, 6.0 X 10(-9) for 3H-progesterone and 5.2 X 10(-9) for 3H-CPA. On sucrose density gradient analysis binding in the 3.5, 5 and 8 S area could be observed using 3H-R5020. For 3H-progesterone and 3H-CPA binding was exclusively found in the 5 S area. The specificity of the steroid binding site of PgR is identical for 3H-R5020 and 3H-CPA. The order of potency of binding for various competitors decreases identically for both radioactive ligands: R5020 less than progesterone less than R1881 less than CPA less than dihydrotestosterone less than dexamethasone less than cyproterone less than 15 beta-OH-cyproterone. CPA resembles progesterone very closely in its binding characteristics to human uterine PgR.