Affinity of five progestin derivatives to progesterone receptors

The progesterone receptors in rabbit endometrium were studied by sucrose density gradient ultracentrifugation using [3H]R5020 as a ligand. The receptors subsided at about 7 S in a low salt medium with a Kd of 0.34nM as determined by saturation analysis and Scatchard plot. The progesterone receptors had a high affinity for [3H]R5020. The relative binding affinities of 5 progestin derivatives for the receptors were determined by competitive binding assay and shown to be in the following order: megestrol acetate＞norethindrone＞progestin I (megestrol 3-cyclopentyl propionate, a contraceptive designed and synthesized in China)＞progesterone＞danazol. The analysis of sucrose density gradient ultracentrifugation revealed that progestin I inhibited the binding of [3H]R5020 to progesterone teceptors but did not compete with [3H]E2 for estrogen receptors.