[Kinetics of interaction of [3H]16alpha,17alpha-cyclohexanoprogesterone with progesterone receptor in the rabbit uterus].

A discrepancy between high biological activity of 16 alpha,17 alpha-cyclohexanoprogesterone (pentarane) and its relatively low affinity for progesterone receptor was demonstrated previously; the present study revealed that this is not true and that the discrepancy was due to the overestimation of the effective ligand concentration during calculations of the Kd or relative binding affinity (RBA) because of high absorption of pentarane on the surface of test tubes; up to 70% of added ligand can thus be absorbed. Corrected RBAs of pentarane versus progesterone in progesterone receptor-binding assay are from 7 to 10 and do not depend on [3H]-labeled ligand used. Unlabeled steroids competitively inhibit binding of [3H]progesterone and [3H]pentarane with progesterone receptor. The Kd values are 27 and 2.5 nM at 0-4 degrees, respectively. Pentarane affinity to serum proteins is lower and its metabolism in the liver homogenate is more slow versus progesterone characteristics. D'-ring unsaturated pentarane analog 16 alpha,17 alpha-cyclohex-23-enoprogesterone completely inhibited specific binding of [3H]progesterone and [3H]pentarane to a cytosolic protein with similar efficiencies which were about 0.1 of pentarane efficiency. Thus, progesterone receptor is the only protein in the soluble fraction of rabbit uterus homogenate that specifically binds pentarane.