Characterization of a triphenylethylene-antiestrogen-binding site on rat serum low density lipoprotein

Several laboratories have reported the presence of an intracellular, high affinity [dissociation constant (Kd), 1-2 nM] antiestrogen-specific binding site in estrogen target tissues. In this report, we describe the binding properties and characteristics of an additional triphenylethylene antiestrogen-binding site in the serum of rats. By using saturation and competitive binding analyses with [3H]tamoxifen as the radio-labeled ligand, we have determined that rat serum contains a relatively high affinity (Kd, 28 nM), protease-sensitive binding site that is specific for antiestrogens of the triphenylethylene type (clomiphene and nafoxidine). Since this serum site has little affinity for the benzythiophene antiestrogens (LY 117018 and 156758), we have chosen the name of triphenylethylene-antiestrogen binding site (TABS). The concentration of serum TABS (picomoles of [3H]tamoxifen bound per ml serum) is roughly the same in males and females but is significantly greater in both sexes at days 5, 10, and 15 of age (400-500 pmol/ml) than in newborn or adult animals (100-200 pmol/ml). Preliminary characterization studies indicated that the serum TABS is probably a serum lipoprotein. Separation of rat serum lipoproteins on potassium bromide density gradients revealed that the serum TABS migrated with rat low density lipoprotein (LDL). Furthermore, rat LDL purified by density gradient centrifugation has an affinity and specificity for [3H]tamoxifen that is similar to the TABS found in whole serum. In contrast, the rat liver intracellular TABS does not have these density gradient characteristics. These data suggest that the rat serum TABS is LDL; however, the role of this serum lipoprotein in the mechanism of action of antiestrogens remains to be determined.