Studies on Corticosterone–Receptor Complexes from Mouse Placenta

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Abstract
The in vitro binding of radioactive steroids to components of mouse placental nuclei and cytoplasm was investigated using Sephadex or charcoal to remove unbound steroid. Specificity was indicated in competition experiments using excess unlabelled competing steroids. Only the active glucocorticoids formed complexes that could be isolated from the nucleus. The binding properties of the cytoplasmic steroid–receptor complex were studied. From the time course of binding the complex was shown to be more stable at 0° than at 37°, and the distribution of receptors in the cytosol appeared to be homogeneous. The complex was labile to heat and to proteolytic digestion but did not appear to be affected by nucleases or sulfhydryl reagents. Kinetic analysis revealed the presence of high affinity specific binding sites with a dissociation constant of 17.5 nM and a receptor site concentration of 0.26 pmol/mg protein. The corticosterone isolated from nuclear complexes and dexamethasone from cytoplasmic complexes were identified by chromatography and by cocrystallization as the unchanged steroid in each case.

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