EFFECT OF STEROIDS ON TRANSCORTIN BINDING TO HUMAN SYNCYTIOTROPHOBLAST

Effects of steroids on specific binding of transcortin (corticosteroid -binding globulin, CBG) to membrane microvesicles have been studied Tw o species of transcortin, nCBG and vCBG, were isolated from the blood of pregnant women. Microvesicles were obtained from the brush border m embranes of human placental syncytiotrophoblast Steroid-deficient inta ct species of CBG lost the capacity for interaction with their specifi c binding sites on the microvesicles. The binding, which increased wit h the proportion of cortisol-complexed CBG, attained its maximum at 5. 10(-8) to 5.10(-7) M cortisol (the saturating concentration for the fo rmation of CBG-cortisol complexes). The sites for specific CBG binding did not recognize nCBG complexes with cortisol, corticosterone, proge sterone, or testosterone. Conversely, the complex formed by vCBG and c ortisol was selectively bound to these sites.