Glutathione Transferase Isozymes of Diamondback Moth Larvae and Their Role in the Degradation of Some Organophosphorus Insecticides

Glutathione transferase (GST) of diamondback moth larvae was purified >30-fold by affinity chromatography following ammonium sulfate fractionation. Subsequent cation-exchange chromatography yielded two electrophoretically homogenous isozymes, GST-2 (p l 8.2, subunit MA 23.6 kDa) and GST-3 (p l > 8.2, subunit MA 26.5 kDa); a third isozyme, GST-1 (p l < 5.1, subunit MA 27.1 kDa), was only partially purified. GST-3 exhibited apparent substrate preference for 1,2-dichloro-4-nitrobenzene, methyl parathion, parathion, and paraoxon, while GST-2 had much higher activity toward 1-chloro-2,4-dinitrobenzene; both GST-1 and GST-3 showed significantly higher activity toward 4-nitrophenyl acetate than GST-2. All three GST isozymes appeared to be aryltransferase for the only product observed in parathion, methyl parathion, and paraoxon conjugation was S-4-nitrophenyl glutathione. Both susceptible and methyl parathion-resistant diamondback moth larvae appeared to have the same profile but different proportions of these three GST isozymes.