Evaluation of the role of the glutathione redox cycle in Cu(II) toxicity to green algae by a chiral perturbation approach.

The effect of heavy metal toxicity on the environment is usually linked to changes in the glutathione redox cycle and oxidative damage as causative events. However, it is unknown whether changes in the glutathione redox cycle are a cause or result of Cu(II) toxicity. Herein, a new chiral perturbation strategy involving a chiral herbicide, dichlorprop (DCPP), as a perturbation factor was used. According to the dose-response fitting curve of DCPP and the combination with Cu(II), 40 μM (R)-DCPP and (S)-DCPP, whose toxicities were low enough to not significantly perturb the Cu(II) toxicity, were selected as the chiral perturbation factor. When Scenedesmus obliquus was incubated with the chiral perturbation factor and 10 μM Cu(II), chiral perturbation was observed in the chlorophyll content and the PAM chlorophyll fluorescence. Then, the role of the glutathione redox cycle in the toxicity of Cu(II) was evaluated with the chiral perturbation approach. The results revealed that the GSH differences in algae cells exposed to (R)-DCPP or (S)-DCPP were well correlated with the differences in the production of reactive oxygen species (ROS) after exposure to the two enantiomers. When (R)-DCPP or (S)-DCPP was added with Cu(II) to the algae culture, treatment with (R)-DCPP-Cu resulted in a decrease in the GSH content in algae cells compared to the control, whereas treatment with (S)-DCPP-Cu resulted in an increase in the GSH. The GSH/GSSG ratio and GR activity also showed similar enantioselectivities. The enantioselectivities would not exist if the changes of in glutathione redox cycle were the cause. Therefore, these data provide indirect evidence that ROS induced cell toxicity of Cu is a causative event, which results in the response of the glutathione redox cycle. These results also provided an implication that before sustainable detoxification strategies for heavy metal pollutants were proposed, it is better that the roles of ROS production and glutathione redox cycle are elucidated. In this case, the chiral perturbation strategy may be a good choice.