In vitro fluorescence displacement investigation of thyroxine transportdisruption by bisphenol A

Bisphenol A (BPA) is a chemical with high production volume and wide applications in many industries. Although BPA is knownas an endocrine disruptor, its toxic mechanisms have not been fully characterized. Due to its structural similarity to thyroid hormonesthyroxine (T4) and triiodothyronine (T3), one possible mechanism of BPA toxicity is disruption of hormone transport by competitivebinding with the transport proteins. In this study, the binding interactions of BPA, T4, and T3 with three thyroid hormone transportproteins, human serum albumin (HSA), transthyretin (TTR), and thyroxine-binding globulin (TBG) were investigated by fluorescencemeasurement. Using two site-specific fluorescence probes dansylamide and dansyl-L-proline, the binding constants of BPA with HSAat drug site I and site II were determined as 2.90 104 and 3.14 104 L/mol, respectively. By monitoring the intrinsic fluorescenceof tryptophan, a binding constant of 4.70 103 L/mol was obtained. Similarly, by employing 8-anilino-1-naphthalenesulfonic acid asfluorescence probe, the binding a nity of BPA with TTR and TBG was measured to be 3.10 105 and 5.90 105 L/mol, respectively.In general, BPA showed lower binding a nity with the proteins than T3 did, and even lower a nity than T4. Using these bindingconstants, the amount of BPA which would bind to the transport proteins in human plasma was estimated. These results suggest thatthe concentrations of BPA commonly found in human plasma are probably not high enough to interfere with T4 transport.