Bisphenol S (4-hydroxyphenyl sulfone, BPS) is increasingly used as a bisphenol A (BPA) alternative. The global usage of BPS and its analogues (BPSs) resulted in the frequent detection of their residues in multiple environmental media. We investigated their potential endocrine-disrupting effects toward thyroid hormone receptor (TR) β. The molecular interaction of BPSs toward TRβ ligand binding domain (LBD) was probed by fluorescence spectroscopy and molecular dynamics (MD) simulations. BPSs caused the static fluorescence quenching of TRβ LBD. The 100 ns MD simulations revealed that the binding of BPSs caused significant changes in the distance between residue His435 at helix 11(H11) and residue Phe459 at H12 in comparison to no ligand-bound TRβ LBD, indicating relative repositioning of H12. The recombinant two-hybrid yeast assay showed that tetrabromobisphenol S (TBBPS) and tetrabromobisphenol A (TBBPA) have potent antagonistic activity toward TRβ, with an IC of 10.1 and 21.1 nM, respectively. BPS and BPA have the antagonistic activity with IC of 312 and 884 nM, respectively. BPSs significantly altered the expression level of mRNA of TRβ gene in zebrafish embryos. BPS and TBBPS at environmentally relevant concentrations have antagonistic activity toward TRβ, implying that BPSs are not safe BPA alternatives in many BPA-free products. Future health risk assessments for TR disruption and other adverse effects should focus more on the structure-activity relationship in the design of environmentally benign BPA alternatives.

Parabens have been widely used in packaged foods, pharmaceuticals, and personal-care products. Considering their potential hydrolysis, we herein investigated structural features leading to the disruption of human androgen receptor (AR) and whether hydrolysis could alleviate such effects using the recombinant yeast two-hybrid assay. Parabens with an aryloxy side chain such as benzyl paraben and phenyl paraben have the strongest antiandrogenic activity. The antiandrogenic activity of parabens with alkyloxyl side chains decreases as the side chain length increases from 1 to 4, and no antiandrogenic effect occurred for heptyl, octyl, and dodecyl parabens with the number of alkoxyl carbon atoms longer than 7. The antiandrogenic activity of parabens correlates significantly with their binding energies (R = 0.84, p = 0.01) and were completely diminished after the hydrolysis, particularly for parabens with aryloxy side chains. The K for the hydrolysis of parabens with aromatic moiety side chain is 1 order of magnitude higher than that of the parabens with alkyl side chains. Both in vitro and in silico data, for the first time, suggest parabens with aromatic side chains are less prone to hydrolysis. Our results provide an insight into risk of various paraben and considerations for design of new paraben-related substitutes.

Pentabromoethylbenzene (PBEB), as one of the novel brominated flame retardants (NFBRs), has caused increasing public concern for health risks. Till now, information regarding potential effects of PBEB on thyroid function remains unclear. Herein, we investigated thyroid disruption of PBEB in vitro and in silico and evaluated thyroid dysfunction induced by PBEB using Sprague–Dawley rats. PBEB showed thyroid receptor (TR) β antagonistic activity with IC50 of 9.82 × 10–7 M in the dual-luciferase reporter gene assay and induced relative reorientation of helix 11 (H11) and H12 of the TR ligand binding domain as revealed by molecular dynamics simulations. PBEB (0.2, 2, 20 mg/kg BW/d) markedly altered the transcriptome profile of thyroid with induction of 17, 42, and 119 differentially expressed genes (DEGs) involved in thyroid hormone signaling and synthesis pathway, of which transthyretin and albumin are common DEGs. The 28-d exposure to PBEB significantly decreased the triiodothyronine level (from 7.23 to 5.67 ng/mL) and increased the thyrotropin level (from 7.88 to 12.86 mU/L) for female rats. PBEB consequently reduced thyroid weight and altered its morphology with more depleted follicles. Overall, our study provides the first account of evidence on PBEB exerted thyroid disruption, transcriptome aberration, and morphological alteration, facilitating health risk assessment of PBEB and structurally related NBFRs.

Cytochrome P450 3A4 (CYP3A4) is a promiscuous enzyme, mediating the biotransformations of ∼50% of clinically used drugs, many of which are chiral molecules. Probing the interactions between CYP3A4 and chiral chemicals is thus essential for the elucidation of molecular mechanisms of enantioselective metabolism. We developed a stepwise-restrained-molecular-dynamics (MD) method to model human CYP3A4 in a complex with cis-metconazole (MEZ) isomers and performed conventional MD simulations with a total simulation time of 2.2 μs to probe the molecular interactions. Our current study, which employs a combined experimental and theoretical approach, reports for the first time on the distinct conformational changes of CYP3A4 that are induced by the enantioselective binding of cis-MEZ enantiomers. CYP3A4 preferably metabolizes cis-RS MEZ over the cis-SR isomer, with the resultant enantiomer fraction for cis-MEZ increasing rapidly from 0.5 to 0.82. cis-RS MEZ adopts a more extended structure in the active pocket with its Cl atom exposed to the solvent, whereas cis-SR MEZ sits within the hydrophobic core of the active pocket. Free-energy-perturbation calculations indicate that unfavorable van der Waals interactions between the cis-MEZ isomers and the CYP3A4 binding pocket predominantly contribute to their binding-affinity differences. These results demonstrate that binding specificity determines the cytochrome P450 3A4 mediated enantioselective metabolism of cis-MEZ.

Thioxanthones (TXs) are photoinitiators widely used in UV curable resins and food packaging, and their residues have been frequently detected in human bodies. Our current understanding of the susceptibility of residual TXs to metabolism and their effects on human health is very limited. The in vitro metabolism of TXs and its toxic effects on cytochrome P450 (CYP) (the key xenobiotic metabolizing enzymes) were examined in this study. 2-Chlorothioxanthone (2-Cl-TX) significantly inhibited the enzymatic activities of CYP1A2 and CYP3A4 with IC 50 of 8.36 and 0.86 μM, respectively. The exposure to 2-Cl-TX at 2.5 μM up-regulated the mRNA expression of CYP1A2 and CYP3A4 in human hepatocellular carcinoma cells to 3.03-fold and 2.02-fold, respectively. 2-Cl-TX at 2.5 μM caused 2.19-fold and 1.98-fold overexpression of CYP1A2 and CYP3A4, respectively. In vitro studies revealed that 2-Cl-TX was biotransformed into two metabolites through the sulfoxidation of the sulfur atom, or via the hydroxylation of aromatic carbon. Results from this study, including the metabolic susceptibility of residual 2-Cl-TX, the proposed metabolites and the significant toxic effect on the activities, mRNA, and protein expression of CYP1A2 and CYP3A4, are vital to the human health and safety risk assessment from this ubiquitous xenobiotic.