Permit (O,O-diethyl phosphorothioate), FMC F 8801, is a candidate agrochemical to be used as cotton seed protector against damage that may be caused by pre-emergence herbicides, such as clomazone. Because it is an organophosphorus ester, Brazilian environment and health authorities required additional tests for its registration purposes. The objective of this study was to evaluate Permit ability of inhibiting house fly head acetylcholinesterase “in vitro”as compared to dichlorvos (dimethyl 2,2-dichlorovinyl phosphate) used as standard. A potenciometric method based on the hydrolysis of acetylcholine and the libaration of acetic acid in a phosphate buffer (pH=6,0) was developed in order to determine the I50 values of both compounds. Experiments were performed with different amounts of the agrochemicals (0.02-200 µg active ingredient/tube). After 30 minutes incubation period, a 1 M acetylcholine chloride solution was added to each vial, and the variation of pH inside the tubes were checked for a period of 150 minutes. The percentage inhibition in all treatments were calculated based on pH variation in the control treatment. Regression analyses were conducted in order to estimate the I50 values. The results showed that dichlorvos is a powerful acetylcholinesterase inhibitor (I50=1.85 x 10-6 M) and that Permit is not at all, up to 200 µg/tube at experimental conditions.
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