Mechanism of the estrogen receptor interaction with 4-hydroxytamoxifen

The binding mechanism of the estrogen receptor with 4-[³H]hydroxytamoxifen was investigated. The equilibrium binding analysis with 4-[³H]hydroxyta-moxifen indicated a positive cooperative interaction: the Scatchard plot was convex and the Hill coefficient was 1.4-1.5. This binding appears similar to the positively cooperative interaction of the estrogen receptor with [³H]estradiol. However, a competitive binding assay with a saturating concentration of [³H] estradiol and variable concentrations of 4-hydroxy-tamoxifen produced nonparallel displacement curves indicating that the binding mechanism of the receptor with these two ligands is different. The competitive binding assay with [³H]estradiol and 4-hydroxytamoxifen at constant molar ratios demonstrated that the receptor’s affinity for estradiol was reduced and the receptor preferentially bound 4-hydroxytamoxifen. These data suggest that 4-hy-droxytamoxifen interacts with the receptor differently than estradiol; it antagonizes the binding of estradiol when these two ligands are simultaneously present.