The binding of progesterone, R-5020 and ORG-2058 to progesterone receptor

Abstract

The binding of two synthetic progestins, R- 5020 and ORG- 2058 and a natural progestin, progesterone to progesterone receptors of human myometrium is examined. The receptor, which sediments at 4S in a sucrose density gradient, binds all three progestins with high affinity ( K D values for R- 5020, ORG- 2058 and progesterone are 4.1 × 10 −10 , 1.2 × 10 −10 and 3.8 × 10 −10 mole/1 respectively). However, competition studies using various concentrations of unlabelled steroids indicate that the synthetic steroids bind to the progesterone receptor with higher specificity than does progesterone itself. It is suggested that because of its high specific activity, its similar receptor binding kinetics to the natural hormone and its higher specificity for the progesterone receptor, R- 5020 is the most suitable of the three progestins for use in progesterone receptor assays.