Substrate specificity of glutathione S-transferases from the fall armyworm

Abstract

Ten cytosolic glutathione S-transferase (GST) isozymes isolated from midguts and fat bodies of control and allelochemical-induced fall armyworm ( Spodoptera frugiperda) larvae were tested for their activity toward 13 model substrates belonging to halogenated compounds, nitro compounds, α,β-unsaturated carbonyl compounds, and organic hydroperoxides. Based on the pattern of activity toward these substrates, these GST isozymes exhibited different but overlapping substrate specificities. With a few exceptions, 1-chloro-2,4-dinitrobenzene (CDNB) was the best substrate for these isozymes. The isozymes were active toward numerous toxic α,β-unsaturated carbonyl allelochemicals including trans-2-octenal, trans-2-nonenal, 2,4-hexadienal, trans, trans-2,4-heptadienal, trans, trans-2,4-nonadienal, and trans, trans-2,4-decadienal, suggesting that GSTs play an important role in the feeding strategies of lepidopterous insects. These GSTs also possessed glutathione peroxidase activity toward cumene hydroperoxide and conjugating activity toward 4-hydroxy nonenal, a lipid peroxidation product, and therefore they are antioxidant enzymes. Microsomal glutathione S-transferase from fat bodies of fall armyworm larvae metabolized a variety of model substrates such as CDNB, 1,2-dichloro-4-nitrobenzene (DCNB), p-nitrophenyl acetate, and cumene hydroperoxide, but had no activity toward α,β-unsaturated carbonyl compounds. With the exception of ethacrynic acid, glutathione S-transferase activities toward these substrates were all inducible by allelochemicals such as xanthotoxin and indole 3-acetonitrile in midguts and fat bodies of fall armyworm larvae. Induction ranged from 1.3- to 20.2-fold for midgut GSTs and 1.4- to 48.8-fold for fat body GSTs, depending on the inducer and substrate used. In all instances, DCNB-conjugating activity was most inducible based on percentage of control.