Drug metabolism problems such as drug-drug interactions and production of toxic metabolites cause almost half of all drug candidate failures during clinical trials. Sulfation is an important type of conjugative drug metabolism that is also used to regulate the level and activity of endogenous hormones. For instance, sulfation regulates the activity of endogenous ligands for specific neurotransmitter receptors, nuclear receptors, and protein kinases that are drug targets for depression, breast cancer, and cardiovascular disease. Development of more selective therapies for these disorders by pharmaceutical companies is currently hampered a lack of high throughput screening assays for delineating the specificity of the 11 known human sulfotransferases for endogenous hormones and drug candidates. To address this need, we propose in Phase I to provide proof of principle for a fluorescence based homogenous assay method for sulfotransferases using estrogen sulfotransferase (SULT1E1) as a model. In Phase II, the novel assay methods will be optimized, a full panel of 11 human sulfotransferases will be purified and incorporated, and the assay will be validated for pharmaceutical HTS applications by screening a diverse library of bioactive chemicals for identification of SULT substrates and inhibitors.