Qualitative and quantitative changes in glutathione S-transferases in the mosquito Anopheles gambiae confer DDT-resistance.

Glutathione S-transferases (GSTs; EC 2.5.1.18) are a multigene family of dimeric multifunctional proteins that play a central role in detoxication of xenobiotic compounds including drugs, herbicides and insecticides [ 11. Based on extensively characterized physicochemical and immunological properties, the mammalian GSTs have been grouped into at least four distinct classes: Alpha, Mu, Pi and Theta [2,3]. In contrast, the insect GSTs have not been well characterized. Multiple forms of these enzymes have been reported for house fly and Drosophila [4,5]. There are at least two forms present in the mosquito, Aedes aegypti [6]. The GSTs in insects are of interest because of their role in insecticide resistance. Several insecticide-resistant strains of housefly have been reported to have elevated GST activity in crude extracts [7-91. The different forms of GSTs exhibited different specificities for the insecticides studied. The goal of this study was to identify and characterize the GST involvement with DDT-resistance in the malaria vector, Anopheles gambiae. Therefore it was necessary to perform a comparative study of the GST activity of a DDT-resistant strain versus a DDT-susceptible strain of this mosquito. However, initially only small differences in GST activity were detectable with general substrates such as CDNB and DCNB. This contrasts with previous reports of GST involvement in insecticide resistance where significant changes in CDNB activity have been observed [7,8]. Characterizing the GSTs from both resistant and susceptible strains clearly illustrates the qualitative changes in the studied enzymes. This is shown by differences in substrate specificity, especially an increased activity for DDT as well as the changes observed for the other substrates (TABLES 1 & 2). Three strains of housefly were shown to have multiple forms of GST, with the enzymes in each strain present in varying amounts [a]. It was suggested that resistance was due to differing degrees of synthesis of enzyme rather than enhancement of a single enzyme or appearance of a different form. In An. gambiae DDTresistance is conferred by qualitatively different GSTs being present in greater quantities.