INSECT GLUTATHIONE S-TRANSFERASES

Glutathione S-transferases (GSTs) have been purified from more than 24 insect species, including Lepidoptera, Diptera, Coleoptera, Dictyopte ra, and Hymenoptera. These transferases exist in multiple forms; as ma ny as eight isozymes were found in midguts and fat bodies of fall army worm larvae. Molecular weights of insect GSTs are within the range of 35 000-63 000. They consist of two subunits (homodimers and heterodime rs) of molecular weight between 19 000 and 35 000. Insect GSTs metabol ize various electrophilic xenobiotics, including halogenated compounds , nitro compounds, alpha,beta-unsaturated compounds, isothiocyanates, organothiocyanates, organophosphates, and oxides. At least 55 chemical s have served as substrates for GSTs in insects. GSTs have been induce d by numerous xenobiotics, including insecticides, drugs, host plants, and allelochemicals in over 21 species of insects. Among these induce rs, insecticides (organochlorines), host plants (umbellifers and cruci fers), and allelochemicals (furanocoumarins, indoles, and flavonoids) are the most potent inducers of the enzymes. Species differences in en zyme inducibility were observed in Lepidoptera. GST induction in insec ts was associated with increased GST mRNA levels indicating de novo sy nthesis of the enzyme. GSTs have been implicated in the resistance to insecticides and allelochemicals in insects. The high GST activity fou nd in insecticide-resistant insects was associated with increased leve l of specific mRNA.