Purification and Characterization of an Estradiol-17 beta Binding Macromolecule in Rat Pancreas.

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Abstract
A high capacity estradiol-17.beta. binding protein was purified from rat pancreatic cytosol. The protein constitutes about 2% of the protein content in the cytosol. Using estradiol-17.beta. as ligand the Kd in vitro was calculated to be 2.0 .times. 10-7 M. Estrone and estriol also show affinity to the protein but testosterone, progesterone and dexamethasone do not. The protein was purified to homogeneity by chromatography on concanavalin-A and hydroxylapatite followed by preparative polyacrylamide gel electrophoresis. The purified protein formed a single protein band when analyzed in different systems. The partially purified [3H]estradiol-macromolecule complex formed a 2.6 S complex using sucrose gradient analysis, had a Stokes radius of 35 .ANG. when analyzed using gel chromatography and focused at pI [isoelectric point] 4.6. The complex did not bind to phosphocellulose or DNA-cellulose and showed no similarities to [3H]estradiol-receptor complexes. The biological function of this protein is unknown, but may include regulation of estrogen fluxes in pancreatic cells. The purified protein from rat pancreas is similar but not identical to the earlier purified estrogen binding protein from human pancreas. The presence and purification of the protein from rat pancreas will make it possible to use rat pancreas as a model system to study the biological function of a high capacity intracellular steroid binding protein.