Ultrafiltration tandem mass spectrometry of estrogens for characterization of structure and affinity for human estrogen receptors

Although hormone replacement therapy (HRT) is used by post-menopausal women for the relief of menopausal symptoms and the potential reduction of osteoporosis, HRT also increases their risk of Alzheimer's disease, stroke, breast cancer, and endometrial cancer. Since the majority of these effects are associated primarily with estrogen binding to only one of the estrogen receptors (ER), new assays are needed that can more efficiently evaluate ER-binding and identify ligands selective for ER-α and ER-β. High performance liquid chromatography-tandem mass spectrometry (LC-MS-MS) was combined with ultrafiltration as a new method to investigate the relative binding of compounds to the ERs and to evaluate the structures of these estrogens. Mixtures of estradiol and six equine estrogens, including equilin, equilenin, 8,9-dehydroestrone, and their 17β-hydroxyl derivatives, were assayed simultaneously to determine their relative binding to human ER-α and ER-β. Estrogens containing a 17β-OH group were found to have higher relative affinities for the estrogen receptors than their ketone analogs. In addition, 17β-EN showed selectivity for binding to ER-β over ER-α. The results were compared to the IC50 values obtained by using a conventional radiolabled estradiol competitive binding assay. Finally, the utility of negative ion electrospray tandem mass spectrometry for the unambiguous identification of these estrogen isomers was investigated. Several characteristic recyclization pathways during tandem mass spectrometry were identified, which might be useful for distinguishing related estrogens.