Isolation and Preliminary Characterization of a Progestogen Specific Binding Macromolecule from the 273,000 g Supernatant of Rat and Rabbit Uteri

Abstract

Two different progesterone binding macromolecules have been identified in the cytosol fraction of rat and rabbit uteri. One is apparently the binder earlier reported by Milgrom and Baulieu. It is not specific in its binding. The second macromolecule is apparently that which has been reported by McGuire and DeDella and Milgrom et al. It apparently binds only to progestogens and has been termed a progestogen receptor. These two binders can be separated on the basis of binding specificity and by ammonium sulfate fractionation. Kinetic studies indicate the presence of two binders. Attempts to characterize the progestogen specific binder demonstrate that the binder is a protein with a molecular weight of about 85,000. Tissue specificity studies indicate that this binder exists in the cytosol fraction of uterus and vagina but not in the cytosol fraction of spleen, stomach, liver, kidney, intestine, leg muscle, or arm muscle. No evidence was found for its presence in blood. Optimal progesterone binding occurs between pH 8.0 and 9.0. This binder is relatively stable to both heating and lyophilization. A Kd for the reaction between receptor and hormone has been calculated to be ∧10-10, a value which is in fairly good agreement with findings for the estrogen receptor and progesterone binder found in chick oviduct. (Endocrinology90: 496, 1972)