A model for the effect of estrogen antagonists on cooperative estradiol binding

Partial agonists such as estriol and estrone have been reported to diminish or even eliminate the upward convexity of the Scatchard plot of the binding of labeled estradiol to estrogen receptor. This has been interpreted as agonist interference with the receptor dimerization induced by estradiol. In order to investigate how a partial agonist or antagonist might interfere with dimerization we have developed a theoretical mass-action law model, where soluble receptors can dimerize and bind to two different ligands. Special attention was devoted to manifestations of positive cooperativity to determine whether they could be modified by competition with a second ligand. This was done using a computer program that evaluated a large set of combinations of affinity constants in an effort to explore all possible situations. The model could reproduce the effect of a second ligand on the cooperative binding of estradiol to the estrogen receptor but only if the second ligand was anticooperative, which is not the case of estriol, estrone and tamoxifen. Furthermore, even when the Scatchard plot was linear, the model still required dimerization of the receptor in most of the cases, showing that the addition of an antagonist may eliminate the upward curvature of the Scatchard without truly eliminating dimerization or cooperativity. We conclude that the effect of a second ligand on the binding of labeled estradiol to estrogen receptor is not necessarily due to interference with dimerization and/or cooperativity. The inability of this model to fully explain the published data for estriol, estrone, clomiphene, and tamoxifen suggests that a more complex mechanism is involved.