Effect of copper exposure on GST activity and on the expression of four GSTs under oxidative stress condition in the monogonont rotifer, Brachionus koreanus

Glutathione S-transferases (GSTs; EC 2.5.1.18) are major enzymes that function in Phase II detoxification reactions by catalyzing the conjugation of reduced glutathione through cysteine thiol. In this study, we cloned and sequenced four GST genes from the monogonont rotifer Brachionus koreanus. The domain regions of four Bk-GSTs showed a high similarity to those of other species. In addition, to evaluate the potential of GST genes as an early warning signal for oxidative stress, we exposed sublethal concentrations of copper (Cu) to B. koreanus and measured glutathione (GSH) contents and several antioxidant enzymes such as glutathione S-transferase (GST), glutathione peroxidase (GPx; EC 1.11.1.9), and glutathione reductase (GR; EC 1.8.1.7). The reactive oxygen species (ROS) at 12 h and 24 h after copper exposure increased significantly. GSH contents however did not increase significantly and even it decreased at 0.24 mg/L at 12 h. The activities of several antioxidant enzymes, particularly GPx and GR, showed a dramatic increase in 0.24 mg/L of CuCl₂. Messenger RNAs of each Bk-GST showed different patterns of modulations according to GST types, and particularly, Bk-GST-omega, Bk-GST-sigma, and Bk-GST zeta genes were highly sensitive to Cu. These results indicate that Bk-GSTs, functioning as one of the enzymatic defense mechanisms particularly in the early stage of oxidative stress response, were induced by Cu exposure. This also suggests that these genes and related enzymes have a potential as biomarkers for a more sensitive initial stress response.