Interaction of the selective progesterone 16a, 17a-cyclohex-3'-enoprogesterone agonist with the rat uterus progesterone receptor

Binding of [3H]-16a-cyclohex-3'-enoprogesterone (I) and [3 H]-progesterone (II) with soluble fraction proteins of the rat uterus is compared. I specifically reacts with progesterone receptor but not with other proteins. Its affinity to the receptor assessed from Kd (11.6+2.0 nM), Kj (15 nM) values and relative competitive'activity (0.89+0.35 and 0.33+0.04 with [3H]-I and [3H]-II, respectively) is just negligibly inferior to the affinity of the natural hormone (Kd=6.9+2.6 nM, Kj=8.5 nM, relative competitive activity =1). The dissociation of [3H]-I and [3H]-II complexes with the receptor was biphasic with similar constants of dissociation rate (kfI=1.8x.lO'5C'1; кД=4.5*10'4 C1). Specific features of [3H]-I reaction with the receptor in comparison with [3H]-II are a lesser share of rapidly dissociating [3H]-I complexes and a lower capacity of [3H]-I to prevent the receptor degradation. It is probable that the detected features reflect the differences in the receptor conformation and are associated with the selectivity of I action.