Abstract
We have attempted to delineate some salient features of the progesterone binding site of the cytosol progesterone receptor ( Rp) in rat placenta by studying the binding profile of various chemical modifications of the progesterone molecule (P). The relative competition ratio ( RCR) was used to calculate the relative affinity of P and the modified ligand for receptor ( K a prog/ K a inh). Cortisol exhibited no appreciable binding. Other corticoids (corticosterone, deoxycorticosterone, 11β-hydroxyprogesterone) had relative affinities 10–30-fold lower than P. Alterations in the structure of P which caused extensive declines in relative binding affinity (i.e. ⩾ 100-fold) include: reduction of A-ring to the Sa-stereoisomere (A/B trans), introduction of a 17α-hydroxyl group > removal of C17 side chain > reduction of C20 carbonyl. The binding profile of the rat placental Rp was similar to that described for uterine Rp from other species indicating a high degree of conservation of molecular structure for the progesterone receptor binding site from species to species.
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