Biochim Biophys Acta 1982 Feb 02;7142:200-8.

The steroid ligands of estrogen binding proteins in Xenopus laevis liver cytosol are primarily metabolites 17beta-estradiol.

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The interactions in vitro between [3H]estradiol and liver proteins from Xenopus laevis have been examined to determine if the binding reaction meets criteria of steroid-receptors which may function in the induction of vitellogenesis. Estrogenic hormones associated with proteins is serum and liver cytosol from Xenopus laevis. However, the interactions between soluble liver proteins and estrogens apparently do not result from serum contamination of liver as specific binding was distinguishable by ligand affinity and by differential mobility on polyacrylamide gels. Steroid ligands bound by liver proteins during incubation in vitro were examined by solubility and by thin-layer chromatography. Only a small percentage (13%) of the bound radioactive ligand was recovered as the original tritium-labeled steroid, 17beta-estradiol. The major ligand was recovered as a water-soluble metabolite of estradiol which was identified tentatively as an estradiol-glucoside. To investigate whether the protein-bound estradiol metabolite(s) merely masks a small amount of authentic estradiol-receptor complexes or if the metabolite could be an intermediate in estrogen function, isolated liver nuclei were incubated with liver cytosol containing 3H-labeled steroid-protein complexes or with serum protein-steroid complexes relative to [3H]estradiol. Nuclei preferentially accumulated 3H-labeled steroids from liver cytosol protein-steroid complexes relative to [3H[estradiol from serum proteins. However, analysis of the steroids recovered in the nuclei after incubation with liver cytosol revealed that both 17beta-[3H]estradiol and the 3H-labeled water-soluble metabolite were retained in vitro by nuclei.U

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