The size and/or configuration of the cycloalkane D′ ring in pentacyclic progesterone derivatives are crucial for their high-affinity binding to a protein in addition to progesterone receptor in rat uterine cytosol☆

Abstract

[ 3H]labeled progesterone and a number of its 16α,17α-cycloalkano derivatives with an additional three to six-membered D′ ring were investigated for mutual competition and equilibrium binding to proteins from rat uterine cytosol. The interaction of all studied [ 3H]ligands with proteins was characterized by comparable affinity ( K d in nM region) and apparent homogeneity in terms of affinity. At the same time, the concentrations of binding sites for ligands bearing 16α,17α cyclopentano, cyclohexano, or cyclohexeno substituents were several-fold higher than those for progesterone or 16α,17α-cyclopropanoprogesterone. In mutual competition experiments, when [ 3H]progesterone or [ 3H]16α,17α-cyclopropanoprogesterone were used, the curves of ‘bound radioactivity—log of competitor concentration’ for all compounds studied were parallel and corresponded to a model of ‘one protein—two ligands.’ However, when [ 3H]ligands with bulky 16α,17α-substituents (with the possible exception of cyclohexene derivative) were used, competitive curves for various ligands had different appearances and fell into two groups. Parallel curves for derivatives with 5 or 6 carbons in D′ ring described by a model of ‘one protein—two ligands’ formed the 1st group. The 2nd group comprised curves for progesterone or 16α,17α-cyclopropanoprogesterone that had lower slopes and could be described by a model of ‘two proteins—two ligands.’ Taken together, the results suggest the presence in rat uterine cytosol, of a protein in addition to progesterone receptor capable of discriminating between ligands with no or small 16α,17α-cycloalkano substituents and ligands with more bulky substituents.