Evidence and Characterization of the Binding of Two3H-Labeled Androgens to the Estrogen Receptor*

Abstract

The effects of high doses of androgens on the uterus appear to be mediated by the estrogen receptor. To determine the mechanism of these effects, we studied the interaction of 5α-[³H]androstane-3β,17β-diol and 5-[³H]androstene-3β, 17β-diol in calf and rat uterine cytosol using dextran-coated charcoal assay and sucrose gradient ultracentrifugation. These two androgens bind with a high affinity (K_d = 10 nM) to a class of saturable proteins which is neither the androgen receptor nor the progesterone receptor but is the estrogen receptor. This conclusion is based on the binding stereospecificity, the number of binding sites, and the sucrose gradient pattern. The binding of these androgens is rapidly dissociated at 2 C and does not protect estrogen receptor sites against heat inactivation. These androgens bind to 8S receptor protein(s). The 4S binding observed after sucrose gradient centrifugation is due to the dissociation during the run of ³H-labeled androgens from the 8S receptor and further association to nonspecific binders.