Isolation and characterization of binder IIIA, a new protein which binds corticosteroid anions.

A new protein binding corticosteroid metabolites has been purified over 300-fold from liver cytosols of adrenalectomized rats, treated for 45 min in vivo with [1,2-3H]cortisol. Purification was accomplished by column chromatography on Sephadex G-25, DEAE-Sephadex A-50, Sephadex G-75, and hydroxylapatite. The protein has a Stokes radius of 2.27 nm by gel filtration and an apparent sedimentation coefficient of 3.0 S by sucrose gradient centrifugation. The calculated molecular weight is 30,700. The bound steroid was extracted and has been shown by Sephadex LH-20 chromatography to be a monosulfate derivative of cortisol. Using liver cytosol from adrenalectomized rats pretreated in vivo for 45 min with [1,2-3H]cortisol plus 1000-fold excess competing steroid, cortisol derivatives and progesterone were shown to be the most active competitors. Testosterone and 17 beta-estradiol were least active as competitors. The synthetic steroids, dexamethasone and triamcinolone, produced little or no competition. The protein has been named corticosteroid-anion binder IIIA in keeping with its elution position from a DEAE-Sephadex A-50 column, compared to other binding proteins. Binder IIIA has been separated chromatographically from the glutathione S-transferases (including ligandin) and protein z described by Arias [Levi, A.J., Gatmaitan, Z. and Arias, I.M. (1969) J. Clin. Invest. 48, 21856-21866], both of which have been shown to bind anionic metabolites. It has been resolved from the activities of transcortin, cortisone 5 beta-reductase, and 3 alpha-hydroxysteroid dehydrogenase.