Toxicological and therapeutic implications of interactions between polychlorinated biphenyl sulfates and human transthyretin

The displacement of L-thyroxine (T4) from binding sites on transthyretin (TTR) is considered a significant contributing mechanism in PCB-induced thyroid disruption. Previous research has discovered hydroxylated PCB metabolites (OHPCBs) as highaffinity ligands for TTR, but the potential for binding of conjugated PCB metabolites such as PCB sulfates has not been explored. We therefore evaluated the binding of five lower-chlorinated PCB sulfates to human TTR and compared their binding characteristics to those determined for their OHPCB precursors and for T4. To accomplish this objective, we utilized fluorescence probe displacement studies and molecular docking simulations to characterize the binding of PCB sulfates to TTR. The stability of PCB sulfates and the reversibility of these interactions were characterized by HPLC analysis of PCB sulfates following their binding to TTR. The ability of OHPCBs to serve as substrates for human sulfotransferase 1A1 (hSULT1A1) was assessed by OHPCB-dependent formation of adenosine-3’, 5’-diphosphate, an end product of the sulfation reaction. Our results indicated that all five PCB sulfates were able to bind to the high-affinity binding site of TTR with equilibrium dissociation constants (Kd values) in the low nanomolar range (4.8 – 16.8 nM), similar to that observed for T4 (4.7 nM). Docking simulations provided corroborating evidence for these binding interactions and indicated multiple high-affinity modes of binding. All OHPCB precursors for these sulfates were found to be substrates for hSULT1A1. Therefore, our findings show that PCB sulfates are high-affinity ligands for human transthyretin and therefore indicate, for the first time, a potential relevance for these metabolites in PCB-induced thyroid disruption. 1 Manuscript published in Environmental Health Perspectives 2013, 121(6): 657-62