Biochemical Characteristics of Microsomal and Cytosolic Glutathione S-Transferases in Larvae of the Fall Armyworm, Spodoptera frugiperda (J. E. Smith)

Abstract

Glutathione S-transferase (GST) from midgut microsomes of fall armyworm larvae metabolized a variety of model substrates such as 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene, p-nitrophenyl acetate, and p-nitrobenzyl chloride but had no activity toward 1,2-epoxy-3-(p-nitrophenoxy) propane, 4-nitropyridine-N-oxide, bromosulfophthalein, and α,β-unsaturated carbonyl compounds (e.g., trans-4-phenyl-3-buten-2-one, trans-2-octenal, and trans,trans-2,4-decadienal). Microsomal GST activity (toward CDNB) was generally less sensitive to inhibition by different inhibitors than the cytosolic GSTs. Unlike cytosolic GSTs, microsomal GST was not induced by xanthotoxin and indole 3-acetonitrile. The enzyme was not activated by the treatment of microsomes with N-ethylmaleimide. A single GST isozyme was affinity-purified 22-fold from midgut microsomes, which had a subunit molecular weight of 27,000 Da. The transferase had an apparent Km value of 0.91 mM and a Vmax of 6.67 μmol/min/mg protein (toward CDNB). In comparison with microsomal GST, midgut cytosolic GSTs showed a broader substrate specificity and were active toward various α,β-unsaturated carbonyl compounds. Two affinity-purified GST isozymes, GST-1 and GST-2, from the midgut cytosol exhibited the same substrate specificity as the cytosol except that DCNB did not serve as substrate for the enzymes. The purifications were 5- to 133-fold, depending on the substrates used. Both isozymes were heterodimers with subunit molecular weights of 26,700 and 28,000 Da. GST-1 had an apparent Km value of 0.91 mM and a Vmax of 2.35 μmol/min/mg protein, whereas GST-2 showed an apparent Km of 2.26 mM and a Vmax of 3.00 μmol/min/mg protein (toward CDNB). GST-2 was not immunologically related to microsomal GST. Both microsomal and cytosolic GST isozymes possessed cumene hydroperoxide peroxidase activity, indicating the antioxidant nature of the enzymes.