A recognition system for sex-hormone-binding protein-estradiol complex in human decidual endometrium plasma membranes.

We have studied the specific binding of human serum sex-hormone-binding protein and its complexes with estradiol and testosterone to an enriched membrane fraction isolated from human decidual endometrium. The specific binding of sex-hormone-binding protein to these membranes has been found to be dependent on it being bound with estradiol. When estradiol concentration in the medium is high enough to provide for the saturation of sex-hormone-binding protein with the steroid, the protein exhibits specific binding to the membranes with an apparent equilibrium constant, Kd = (3.5 +/- 2.0) X 10(-12) M. In the absence of estradiol, the binding protein devoid of steroid or complexed with testosterone does not interact with the membranes. At low concentrations of sex-hormone-binding protein and estradiol, when their complex is almost completely dissociated in solution, it can be reconstituted on the membranes with the optimum estradiol to binding protein ratio being 1:1 (mol/mol). It is proposed that, in plasma membranes of the estradiol target cells, there is a recognition system for the sex-hormone-binding protein-estradiol complex which may allow these cells to take up from blood not only free estradiol, but also estradiol complexed with the binding protein.