Sulfation of Environmental Estrogens by Human Cytosolic Sulfotransferase (SULT) 1C4

Sulfation plays an important role in the metabolism of drugs and hormones, the bioactivation of carcinogens, and the detoxification of xenobiotics. Human cytosolic sulfotransferase (SULT) 1C4, a Phase II conjugating enzyme, catalyzes the transfer of the sulfonate group from 3′‐phosphoadensosine 5′‐phosphosulfate (PAPS) to the hydroxyl or amine group of the substrate. SULT1C4 expression has been reported to be highest in fetal lung and fetal kidney, with lower expression in the fetal heart and adult kidney, ovary, and spinal cord. SULT1C4's expression suggests a role in detoxication defenses across the maternal‐fetal barrier and integration of hormonal and metabolic signals that are generated during organ development. Unlike many of the other cytosolic SULTs, the role of SULT1C4 in endogenous metabolism and physiology remains poorly understood; however, previous studies have shown that SULT1C4 can sulfate environmental estrogen‐like compounds, such as bisphenol A (BPA). Numerous studies have shown that exposure to environmental estrogens can have harmful effects on fetal development. The objective of this study is to characterize the sulfation of environmental estrogens by SULT1C4 using molecular modeling and kinetic analysis. Native SULT1C4 was bacterially expressed with the pKK233‐2 bacterial expression vector and purified using affinity chromatography (Ni‐NTA) and DEAE‐ion exchange chromatography. To identify environmental estrogens sulfated by SULT1C4, we performed a combination approach, including modeling, pharmacophore analysis, and kinetics. Pharmacophore analysis helps to initially screen potential substrates by fitting possible substrates into a defined catalytic pocket; lead compounds were tested for sulfation by SULT1C4 using a [ 35 S]PAPS thin layer chromatography assay. Daidzein and BPA appear to be better substrates for SULT1C4 than genistein. In silico substrate docking into the SULT1C4 active site did not identify daidzein as a substrate suggesting that the current model for SULT1C4 (PDB 2GWH) may not be predictive of substrate binding, or daidzein binds only to the unliganded (open) enzyme. Inhibition assays using [ 14 C]‐1‐naphthol identified apigenin as an inhibitor of SULT1C4. Our data suggests that SULT1C4 sulfates selected environmental estrogen‐like compounds, including phytoestrogens and industrial chemicals, although some of these compounds may be non‐catalytic competitive inhibitors. Further characterization of SULT1C4's substrate reactivity is needed to gain a better understanding of its role in physiology and metabolism. Support or Funding Information This work was supported by NIH ES022606 and GM038953.