Binding of cortisol and 21-dehydrocortisol with transcortin.

Abstract

The interaction of human plasma transcortin (corticosteroid-binding globulin) with cortisol and 21-dehydrocortisol was studied under various experimental conditions. The amounts of protein-bound steroids were determined by gel filtration on Sephadex G-25. Both steroids interacted with bovine serum albumin and transcortin. The binding increased with increasing alkalinity and was low at pH 4. The patterns of binding in relation to pH values with the 2 steroids were different. When unlabeled cortisol was added to the assay mixture, the binding of labeled cortisol was blocked and little or no effect was observed with the binding of 21-dehydrocortisol. Upon heating, acidification or extraction with methylene chloride, practically all of the cortisol bound with transcortin was liberated, whereas significant amounts of 21-dehydrocortisol remained bound. The maximum binding of transcortin with cortisol occurred at 0–4 C and with 21-dehydrocortisol at 37 C. The results of this study indicate that cortisol and 21-d...