Glucocorticoid-binding Proteins of Rat Liver Cytosol

Abstract

Abstract The specific interaction of glucocorticoid hormones with liver cytosol proteins of adrenalectomized rats was studied by an adsorbent technique for the separation of free and protein-bound steroid. By this rapid procedure a high affinity binding site for glucocorticoids can be detected which is usually overlooked when the slower technique of equilibrium dialysis is used. This metastable binding protein can be quantitatively estimated in whole cytosol preparations because of its high affinity not only for natural glucocorticoids but also for [3H]dexamethasone, which does not bind to the other two more stable glucocorticoid-binding proteins previously reported in rat liver cytosol, the A and B proteins. It can also be distinguished from these two binding proteins for natural glucocorticoids on the basis of its differential fractionation and physical properties. Unlike the A and B proteins, which in sucrose gradients sediment at 4 S independently of ionic strength, the dexamethasone-binding protein sediments in low salt sucrose gradients largely as a heavy complex, 7 S, which reverts to a lighter form, 4 S, in the presence of 0.3 m KCl. A similar salt-dependent behavior is observed upon chromatography of the dexamethasone-binding protein through Sephadex G-200 columns. The fact that specific dexamethasone binding can only be detected in the cytosol of glucocorticoid target cells, in conjunction with previously presented evidence on its relation to hormonal enzyme induction in vivo, makes it reasonable to assume that this binding protein, which we will call the G protein, may indeed represent the physiologically significant hepatic glucocorticoid receptor.