A simple glutathione transferase-based colorimetric endpoint assay for insecticide detection

The natural ability of the detoxification enzymes glutathione transferases (GSTs) to interact with xenobiotics can be used for the production of colorimetric assays. Detection is usually based on the inhibition of the GST-catalysed reaction, with detection achieved spectrophotometrically or electrochemically. Here we have adopted a chromogenic (visual) activity assay for screening GSTs with alkyltransferase activity for iodoalkene substrates for detection of insecticides. We screened a number of GSTs from insecticide resistant mosquito species for their ability to catalyse iodoalkane biotransformation reactions. AaGSTE2 was found to metabolise iodoethane with high turnover, which resulted in a dark blue colour in the enzymatic reaction. Following assay optimisation we exploited the high recognition affinity of the AgGSTE2 for insecticides to develop a novel colorimetric detection assay for organochlorine and pyrethroid quantification. Calibration curves were obtained for permethirn, deltamethrin, λ-cyhalothrin and DDT, with useful concentration ranges of 0–40 μg/ml (0–100 μM), 0–50 μg/ml (0–100 μM), 0–100 μg/ml (0–220 μM), and 0–50 μg/ml (0–140 μM), respectively. The assay was validated with extracts from insecticide sprayed surfaces and found to be reproducible and reliable compared with HPLC. The assay is therefore suitable for monitoring insecticide residues in insecticide treated materials, and therefore has potential for insect vector control operations.