State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences , Beijing 100085, China.
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
Environ Sci Technol. 2016 Dec 20;50(24):13335-13341. doi: 10.1021/acs.est.6b04041. Epub 2016 Nov 29.
Microbial conversion of inorganic mercury (IHg) to methylmercury (MeHg) is a significant environmental concern because of the bioaccumulation and biomagnification of toxic MeHg in the food web. Laboratory incubation studies have shown that, despite the presence of large quantities of IHg in cell cultures, MeHg biosynthesis often reaches a plateau or a maximum within hours or a day by an as yet unexplained mechanism. Here we report that mercuric Hg(II) can be taken up rapidly by cells of Desulfovibrio desulfuricans ND132, but a large fraction of the Hg(II) is unavailable for methylation because of strong cellular sorption. Thiols, such as cysteine, glutathione, and penicillamine, added either simultaneously with Hg(II) or after cells have been exposed to Hg(II), effectively desorb or mobilize the bound Hg(II), leading to a substantial increase in MeHg production. The amount of thiol-desorbed Hg(II) is strongly correlated to the amount of MeHg produced (r = 0.98). However, cells do not preferentially take up Hg(II)-thiol complexes, but Hg(II)-ligand exchange between these complexes and the cell-associated proteins likely constrains Hg(II) uptake and methylation. We suggest that, aside from aqueous chemical speciation of Hg(II), binding and exchange of Hg(II) between cells and complexing ligands such as thiols and naturally dissolved organics in solution is an important controlling mechanism of Hg(II) bioavailability, which should be considered when predicting MeHg production in the environment.
无机汞（IHg）向甲基汞（MeHg）的微生物转化是一个重大的环境问题，因为有毒的 MeHg 在食物网中会发生生物累积和生物放大。实验室孵育研究表明，尽管细胞培养物中存在大量的 IHg，但由于尚未阐明的机制，MeHg 生物合成通常在数小时或一天内达到平台期或最大值。在这里，我们报告 Hg(II) 可以被脱硫弧菌 ND132 细胞迅速吸收，但由于强烈的细胞吸附，很大一部分 Hg(II) 无法用于甲基化。半胱氨酸、谷胱甘肽和青霉素胺等巯基化合物，无论是与 Hg(II) 同时添加还是在细胞暴露于 Hg(II) 之后添加，都能有效地解吸或移动结合的 Hg(II)，从而导致 MeHg 产量的大幅增加。解吸的巯基-Hg(II)的量与产生的 MeHg 量强烈相关(r = 0.98)。然而，细胞并不优先摄取 Hg(II)-硫醇配合物，而是这些配合物与细胞相关蛋白之间的 Hg(II)-配体交换可能限制了 Hg(II)的摄取和甲基化。我们认为，除了 Hg(II)的水相化学形态外，Hg(II)与细胞和络合剂（如硫醇和天然溶解有机物）之间的结合和交换也是 Hg(II)生物有效性的一个重要控制机制，在预测环境中 MeHg 的产生时应考虑这一机制。
