High-affinity binding to the estrogen receptor of [3H]4-hydroxytamoxifen, an active antiestrogen metabolite.

The tritiated 4-hydroxytamoxifen (OHT), an active metabolite of tamoxifen found in estrogen target nuclei, was prepared in vitro and its interaction with the cytosol estrogen receptor (R) of uterus and chick oviduct was specified. OHT bound to the 8S R with the same affinity and the same kinetic parameters as estradiol (E2). Its ability to protect the E2-binding sites and to stabilize the R--DNA interaction was also similar to that of E2. We conclude that the binding characteristics in vitro of ligands on the cytosol R do not presently allow us to discriminate between agonist and antagonist ligands.