Earthworms Produce phytochelatins in Response to Arsenic

Phytochelatins are small cysteine-rich non-ribosomal peptides that chelate soft metal and metalloid ions, such as cadmium and arsenic. They are widely produced by plants and microbes; phytochelatin synthase genes are also present in animal species from several different phyla, but there is still little known about whether these genes are functional in animals, and if so, whether they are metal-responsive. We analysed phytochelatin production by direct chemical analysis in Lumbricus rubellus earthworms exposed to arsenic for a 28 day period, and found that arsenic clearly induced phytochelatin production in a dose-dependent manner. It was necessary to measure the phytochelatin metabolite concentrations directly, as there was no upregulation of phytochelatin synthase gene expression after 28 days: phytochelatin synthesis appears not to be transcriptionally regulated in animals. A further untargetted metabolomic analysis also found changes in metabolites associated with the transsulfuration pathway, which channels sulfur flux from methionine for phytochelatin synthesis. There was no evidence of biological transformation of arsenic (e.g. into methylated species) as a result of laboratory arsenic exposure. Finally, we compared wild populations of earthworms sampled from the field, and found that both arsenic-contaminated and cadmium-contaminated mine site worms had elevated phytochelatin concentrations.植物螯合肽（Phytochelatins）是一类富含半胱氨酸的小型非核糖体肽类，可螯合软金属及类金属离子，如镉（Cadmium）与砷（Arsenic）。该类物质广泛分布于植物与微生物体内；尽管多个不同门类的动物物种中均存在植物螯合肽合酶基因（phytochelatin synthase genes），但目前仍不清楚这些基因在动物体内是否具有功能，若具有功能，其表达是否受金属诱导调控。本研究通过直接化学分析法，对暴露于砷环境中28天的赤子爱胜蚓（Lumbricus rubellus）的植物螯合肽生成情况进行了检测，发现砷会以剂量依赖的方式显著诱导植物螯合肽的合成。由于暴露28天后植物螯合肽合酶的基因表达并未出现上调，因此必须通过直接检测来测定植物螯合肽代谢物的浓度——这表明动物体内的植物螯合肽合成似乎并不受转录水平调控。进一步的非靶向代谢组学分析还发现，转硫通路相关代谢物发生了显著变化；该通路负责将甲硫氨酸中的硫源定向输送至植物螯合肽合成途径。实验中的砷暴露未引发砷的生物转化（例如生成甲基化砷物种）。最后，本研究对比了野外采集的蚯蚓野生种群，发现来自砷污染与镉污染矿区的蚯蚓体内的植物螯合肽浓度均显著升高。