News from China

Insight into species difference in estrogenic activities of selected parabens

Species selectivity of environmental contaminants often presents difficulty in prediction and uncertainty in extrapolation when toxicity data from model organisms are used in risk assessment. Insight into the related molecular mechanism can help to reduce evaluation deviation resulting from species difference. A structure–activity relationship study on the intrinsic correlation between receptor polymorphism and species sensitivity may explain the differential preference of tested organisms, according to the research work published in the recent issue of Chinese Environmental Chemistry.¹

“There is no doubt that receptor polymorphism plays a pivotal role in species selectivity”, explains Prof. Aiqian Zhang, an environmental chemist at the Research Center for Eco-environmental Sciences (RCEES), Chinese Academy of Sciences. Recent results from two-hybrid yeast assays indicate that estrogen receptor subtype alpha from human (hERα) and medaka (Oryzias latipes) (medERα) sources exhibit differential ligand preference and binding affinity for parabens, but the inherent molecular mechanism behind such a species-specific difference remains unclear. The RCEES scientists carried out a structure–activity relationship study on the theoretical basis of a receptor-mediated endocrine disrupting process for selected parabens and their chlorinated derivatives. They generated a three-dimensional structure model of the ligand binding domain (LBD) for medERαby homology modeling using two hERα templates, 1QKU and 1ERE. Subsequently, they applied molecular docking to further analyze the difference between the two receptors in their binding modes with the parabens.

It turned out that hERα shares a common LBD structure feature with medERα, and the sequence homology between medERα and hERα is 65%. Moreover, both receptors have the same amino acids by which the ligand is surrounded within the distance of 7 Å with only one exception. LEU349 in hERα is replaced by MET353 in medERα. Such substitution leads to both shape adjustment and volume increase in the medERα hydrophobic cavity. Furthermore, the side chain of LEU349 in hERα points to the active pocket and may result in dramatic steric hindrance for ligand binding. By contrast, the bulky methylthio group in medERα stretches away from the pocket region. As a result, the solvent accessible surface area of the binding pocket in medERα is larger than hERα. Overall, the work has shown that the replacement of LEU349 in hERα by MET353 in medERα can explain the species difference in estrogenic activity of the two species. It also points to a direct link between key amino acid modification of hormone receptors and change in potential endocrine disrupting effects.

(Contributed by the editorial office of Chinese Environmental Chemistry)

Heterogeneous photochemical reactions of CS₂ on TiO₂ particles

Some mineral aerosols contain metal oxide semiconductor particles such as TiO₂ and Fe₂O₃. When a semiconductor particle absorbs a photon with an energy that exceeds its band gap, an electron is excited from the valence band into the conduction band. The generated exited electrons and holes are very reactive, and can participate in a variety of photochemical reactions at the atmospheric particulate/gas interface. These heterogeneous chemical reactions have been shown to play a key role in the transformation of pollutants in the atmosphere. CS₂ is a major sulfur-containing pollutant. The heterogeneous photochemical reactions of CS₂ on the surface of semiconductor particles TiO₂ were investigated in detail by Prof. Shicheng Zhang and Prof. Jianmin Chen's groups at the Department of Environmental Science and Engineering, Fudan University, China, as reported in the recent issue of Chinese Environmental Chemistry.²

The research showed that under the simulated sunlight irradiation, heterogeneous photochemical reactions of low concentration of CS₂ (8.1 mg m⁻³) occurred on the surface of TiO₂ particles. Using in-situ infrared spectroscopy and X-ray photoelectron spectroscopy, it was found that sulfate was generated on the particle surface. Photochemical reaction products COS, SO₂ and H₂S were detected by gas chromatography in the gas phase. By examining different types of TiO₂ photocatalysts, it was found that P25 produced the largest amount of sulfate ions, followed by anatase and rutile TiO₂. It was also demonstrated that the oxygen concentration had a significant impact on the photochemical reactions. Based on these results, it was proposed that upon light irradiation, hydroxyl radicals were generated on the semiconductor surface. CS₂ first reacted with the radical to generate the active species (SCS–OH), which then under the action of surface reactive oxygen species produced gaseous products COS, SO₂ and H₂S, and also generated sulfate on the surface of TiO₂.

(Contributed by the editorial office of Chinese Environmental Chemistry)

References

1. Y. Lin, J.-J. Fu, C.-A. Gao and A. Q. Zhang, Chinese Environmental Chemistry, 2011, 30, 399–404.
2. J.-D. Shen, L. Jiao, S.-M. Hong, L. Yang, C.-C. Wang, S.-C. Zhang and J.-M. Chen, Chinese Environmental Chemistry, 2011, 30, 378–385.

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