Inhibitory characteristics of methamidophos on house fly head acetylcholinesterase.

Abstract

The acetylcholinesterase (AChE) purified from housefly (Musca domestica L.) head of a TARI's colony has been tested for its sensitivity to methamidophos, a phosphoramidate that is highly water-soluble but with low sensitivity to AChE. The reaction of methamidophos to AChE was analyzed in vitro in several insecticide concentrations (0.3, 0.6, 1.2, 2.3 and 4.6 ppm) at different incubation times. In general, the activity of AChE remains unchanged in room temperature for 30 min, and all the results confirm that action of methamidophos to AChE is slow but stable. By increasing the incubation time, strong inhibition can be resulted from low Concentration of methamidophos. For example, 0.6 ppm results in 37% inhibition in 30 min, while 4.6 ppm only cause 36% inhibition in the 5 min treatment i.e., the 25 min difference in incubation time can cause 8-fold increase in insecticide sensitivity. By utilizing this character, the housefly head AChE can be used to detect the methamidophos residue in plant material simply by increasing the incubation time. In the actual measurement, almost 100% methamidophos residue was recovered by phosphate buffer extraction instead of methanol/ethanol extraction, and the sensitivity of AChE to methamidophos residue can extend to 2-4 ppm level when testing the marketable size cabbage. We conclude that by modifying the testing procedures, the housefly head AChE is feasible for monitoring methamidophos residue on plant material.