Abstract
A steroid-binding protein was examined in human placental cytosol and nuclei which had several characteristics of an androgen receptor. It binds R 1881 (methyltrienolone), a steroid specific for androgen receptors, with high affinity (dissociation constant = 1.3 to 50 nM) and low capacity (n = 1.2 to 21 fmoles/mg of protein). The steroid-binding protein is found in the nuclei as well as the cytosol of the placenta but is not found in either the cytosol or the nuclei of fetal membranes; thus, it is tissue-specific. Among the natural steroids, 5α-dihydrotestosterone competes most efficiently with R 1881 for binding to the protein. Testosterone is the next best competitor. When the ranges of dissociation constants and concentrations of binding sites for placentas of various gestational periods are compared, no significant differences are observed between period of gestation and magnitude of binding constants.
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