Functional characterization of an epsilon glutathione S-transferase (SfGSTe9) associated with insecticide detoxification in Spodoptera frugiperda

Abstract

The fall armyworm, Spodoptera frugiperda (J. E. Smith) is a notorious insect pest of poaceae and causes severe economic damage to major cereal crop. It has evolved with different levels of insecticide resistance. Insect cytosolic GSTs are a major class of detoxifying enzymes and have been implicated in metabolic resistance and protection against oxidative stress. In this study, an epsilon class GST (SfGSTe9) was identified from S. frugiperda and its transcript levels were highest in the fat bodies, second instar and pupae. Furthermore, SfGSTe9 is upregulated after exposure to beta-cypermethrin, lambda-cyhalothrin, chlorpyrifos, and malathion. The kinetic analysis suggests the purified recombinant proteins exhibit CDNB conjugating activity, with the V_max of 11.87 ± 0.25 μM·min⁻¹ ·mg⁻¹ protein and K_m of 0.22 ± 0.01 mM. The peroxidase activity assays indicate that SfGSTe9 has antioxidant activity against both CHP and H₂O_2. The inhibition assays infer that four types of insecticides have strong inhibition on the GST activity. In vitro metabolism assays with HPLC further suggest that SfGSTe9 could be able to deplete 11.2 %, 21.3 % and 19.3 % of beta-cypermethrin, lambda-cyhalothrin and chlorpyrifos within 2 h, respectively, with specific activity varying between 30 and 40 μM·min⁻¹ ·mg⁻¹ protein. However, no metabolites were identified in this case, indicating that SfGSTe9 probably involves in detoxification via binding and sequestration. 3D modeling and molecular docking analysis indicate that above three types of insecticide compounds fit nicely into the hydrophobic pocket in the active site of SfGSTe9. Our results definitely demonstrate that SfGSTe9 of S. frugiperda plays pivotal role in insecticide detoxification and serves in defence against oxidative stress.