Induction of New Glutathione S-Transferase Isozymes by Allelochemicals in the Fall Armyworm

The induction of glutathione S-transferases in different tissues of fall armyworm (Spodoptera frugiperda) larvae was studied using the allelochemical xanthotoxin as a model inducer. Newly molted sixth instars were fed an artificial diet containing 0.01% xanthotoxin for 2 days prior to enzyme preparation and characterization. Xanthotoxin treatment resulted in significant increases in glutathione S-transferase activity (DCNB) in midguts (76-fold), fat bodies (59-fold), and Malpighian tubules (32-fold) compared with the respective controls. Nondenaturing gel electrophoresis of glutathione-agarose affinity-purified preparations from control larvae showed that the midgut possessed six heterodimeric isozymes with subunit molecular weights of 26,700 to 30,000. The fat body contained three homodimeric isozymes with subunit molecular weights of 20,100 to 29,000. The Malpighian tubules had five heterodimeric isozymes with subunit molecular weights of 26,700 to 30,000. Induction of glutathione S-transferases by xanthotoxin in the larvae resulted in production of two new isozymes in fat bodies but had no effect on isozyme composition in midguts and Malpighian tubules. Both induced isozymes were homodimers with a subunit molecular weight of 28,000. The allelochemicals, indole 3-acetonitrile and flavone, also induced these new isozymes in fat bodies but, unlike xanthotoxin, both inducers decreased levels of the constitutive isozymes. Using isozyme-specific antisera as probes, one of the induced isozymes was immunologically related to two midgut isozymes. Actinomycin D at 0.1 µg/larva completely blocked the xanthotoxin induction of new isozymes in fat bodies.