Different affinities of tamoxifen binding sites in the estrogen receptor of breast cancer

We have subjected cytosol, isolated from hormone-dependent tumour tissue fr om women with breast cancer, to saturation analysis over a wide range of [H -3-] estradiol and tamoxifen concentrations. This analysis revealed differe nt binding sites to tamoxifen on the estradiol receptor. The union specific values analysed by the Scatchard method were in the range 175-350 fmol/mg protein. When cytosol was pre-treated with 0.5, 1.0 and 5 mu M tamoxifen th ere was 50%, 70% and 94% inhibition of the estradiol-receptor complex respe ctively. The inhibition was significantly different when individual cytosol s were incubated with tamoxifen at 5 CIM. At levels of estradiol receptor < 50 fmol/mg protein, the inhibition by tamoxifen was 50-60%, while a levels > 50 fmol/mg protein it was 85-95%. The different affinities of these bind ing sites to tamoxifen may thus provide additional information about the se nsitivity of breast cancer. Med Sci Res 26:829-830. (C) 1998 Lippincott Wil liams & Wilkins.