Relationship between progesterone receptor binding and progestin biological activity

Abstract The binding affinities of a series of steroidal compounds for the hamster uterine progesterone receptor were determined using two sets of incubation conditions. These competitive binding conditions were designed to deduce the relative rates of ligand dissociation from the progesterone receptor. The progestin activity of these compounds was also determined in a bioassay employing the measurement of diamine oxidase in the traumatized hamster uterus. Steroids could be classified into two categories based on either an increase or decrease in relative binding affinity (RBA) with increasing time of competitive incubation. The mean ( ± SEM) progestin biopotency for the compounds having an increase in RBA was 120 ± 18 (progesterone = 100), while the biopotency for compounds having a decrease in RBA was only 44 ±17. This difference was significant ( P