Characterization by photoaffinity labeling of a steroid binding protein in rat liver plasma membrane

The mechanism of steroid uptake by the cell remains controversial. [3H]R5020 was utilized to characterize by photoaffinity labeling the steroid binding site in plasma membrane. This binding was saturable, reversible and had one type of binding site (Kd = 33 +/- 4 nM, Bmax = 32 +/- 2 pmol/mg). [3H]R5020 could be prevented from binding by a variety of steroids (cortisol, progesterone, deoxycorticosterone, and levonorgestrel); estradiol did not have affinity for this binding site. The kinetics of R5020 photoactivation was time dependent and saturable. SDS-PAGE showed a specific band which corresponded to a 53-kDa peptide. The sucrose density gradient analysis has revealed the existence of a protein with a sedimentation coefficient of 3.6 +/- 0.2 S. This polypeptide shows different characteristics than cytosolic steroid receptor or serum steroid binding proteins. This binding protein could correspond to the steroid binding site previously found in the plasma membrane.