The influence of synthetic anti-estrogens on the binding of tritiated estradiol-17β by cytosols of human uterus and human breast carcinoma

Abstract The cytosol fractions of human uterus and human breast carcinoma were incubated with tritiated estradiol-17β alone or in the presence of non-steroid anti-estrogens. Some anti-estrogens (CN-55945, clomiphene, ICI-46474, U-11100A) inhibited the binding of estradiol-17β by the tissue fractions if they were present in at least 20-fold excess over the steroid. Other antiestrogens tested (ICI-47699, MRL-37, U-11555A, dl-norgestrel, d-norgestrel, MER-25, RMI-4613) did not markedly influence the binding of estradiol even if they were present in 1000-fold excess. Evidence is presented which may indicate that in contrast to estriol which acts as competitive inhibitor for the estradiol-17β-binding, the anti-estrogens CN-55945, clomiphene, ICI-46474 and U-11100A influence the estradiol-17β-receptor interaction by allosteric (apparent) competition.