Progesterone “Receptor” in Rabbit Uterus.

Sucrose gradient centrifugation, gel nitration, and charcoal adsorption techniques have been used successfully to isolate and partially characterize progesterone—binding proteins from rabbit uterine cytosol and nuclei. Dissociability in high ionic strength buffers and precipitability in 30% saturated ammonium sulfate differentiated the cytosol binding component from a CBG—like binding protein present in rabbit serum. Progesterone was bound to a 4S complex in uterine cytosol following injection of 3H—progesterone to castrate rabbits; hormone binding increased quantitatively after pretreatment of castrate animals with estradiol– 17β and a qualitative change in the binding pattern became apparent with the appearance of an 8S binding complex in addition to the 4S unit. 0.4M KC1 extraction of the nuclear pellet obtained from estrogen—treated castrates yielded only a 4S binding component. Rabbit uterine cytosol prepared from either castrate or estrogen—treated animals in vitro, when equilibrated with 3Hprogesterone, formed the 4S binding component but not the 8S component. Estrogen—treatment of castrate animals increased the concentration of the binding component measured in vitro from 550 molecules per uterine cell to 3,500. Binding characteristics define the rabbit uterine cytosol binding component as a hormone receptor and differentiate it from progesterone binding proteins present in rabbit serum. (Endocrinology92: 1229, 1973)