Transcortin does not restrict the transmembrane transfer of cortisol.

We have studied the specific binding of both free and transcortin-bound cortisol to the microvesicles derived from the brush border of the plasma membrane of human placental syncytiotrophoblast. Kinetics of the steroid binding to these microvesicles was found to be independent on cortisol being complexed with transcortin. Both cortisol and transcortin were accumulated in the inner space of the microvesicles. This suggests that transcortin-cortisol complex penetrates the plasma membrane and the transcortin-bound steroid can thus enter syncytiotrophoblast and exert its hormonal effects on this tissue.