BINDING OF AN EXTRACELLULAR STEROID-BINDING GLOBULIN TO MEMBRANES AND SOLUBLE RECEPTORS FROM HUMAN BREAST-CANCER CELLS (MCF-7 CELLS)

Studies of MCF-7 breast cancer cells demonstrated that sex hormone-binding globulin (SHBG) is internalized by receptor-mediated endocytosis. The present study demonstrated specific binding of SHBG to receptor on membranes isolated from MCF-7 cells. Scatchard analysis of these binding studies suggested that SHBG binds to a single class of sites on membranes. The analysis yielded a dissociation constant (K(d)) at 37 C of 3 x 10(-8) M and a binding capacity of 48 +/- 0.12 pmol/mg protein. A procedure for solubilizing the SHBG receptor from MCF-7 membranes used buffers containing protease inhibitors, 10% glycerol, and 10 mm 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate. Solubilization of the receptor resulted in a 5-fold increase in its binding capacity (246 +/- 14 pmol/mg protein) and a 10-fold decrease in binding affinity (K(d) at 37 C = 2 X 10(-7) M).