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

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 in 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, 17β-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-bound ³H]estradiol. Nuclei preferentially accumulated ³H-labelea 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 17β-³H]estradiol and the ³H-labeled water-soluble metabolite were retained in vitro by nuclei.