Specific progesterone receptors in hepatic microsomes of the female rat

Microsomes isolated from the liver of female rats bind [3H] progesterone. The progesterone binding is highly specific, estradiol, cortisol, testosterone, 5 alpha-pregnane-3 beta ol-20-one or 16 alpha-hydroxyprogesterone show very low affinity. The binding is sulfhydryl group dependent, p-hydroxymercury benzoate or N-ethylmaleimide inhibit progesterone-microsome association but the inhibition can be partially reversed by the addition of glutathione. Progesterone-microsomal receptor complex formation is altered by Ca2+ and Mg2+, these cations block progesterone binding in concentration ranging from 2 to 5 mmol.