DDT-resistance in Anopheles gambiae (Diptera: Culicidae) from Zanzibar, Tanzania, based on increased DDT-dehydrochlorinase activity of glutathione S-transferases

DDT-resistant Anopheles gambiae Giles from Zanzibar, Tanzania, had increased levels of DDT-dehydrochlorination compared to a DDT-susceptible strain. Glutathione S-transferases (GSTs) are responsible for conversion of DDT to DDE in both the susceptible and resistant strains. Sequential column chromatography, including Q-Sepharose, S-hexylglutathione agarose, hydroxylapatite and phenyl Sepharose, allowed the partial purification of seven GSTs. All seven GSTs possessed different degrees of DDTase activity. There was an eight-fold increase in total DDTase activity in the resistant compared to the susceptible enzymes. Characterization with three substrates, 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB) and DDT, revealed the different substrate specificity for each isolated GST indicating different isoenzymes. GST Va possessed 60% of total DDTase activity suggesting that it contributed most to DDT-metabolism in this insect species. The DDTase activity of the GSTs in both strains of A. gambiae were found to be correlated with the GST activities toward DCNB. Preliminary studies on DDT-resistant and susceptible A. gambiae showed that both DDT-resistance and the increased levels of GST activity were stage specific which suggested that different GSTs may be involved in DDT-resistance in adults and larvae of A. gambiae.