Identification of Carboxylesterase Metabolites of Residual Malathion in Wheat Kernels Using Semi-Micro Radio Liquid Chromatography

摘要

Since residual malathion in wheat kernels is shown to be enzymatically decomposed during sample preparation using the Japanese official method for pesticide analysis, the decomposition products were identified by semi-micro radio liquid chromatography (LC). The reaction mixture of the supernatant of wheat kernel homogenate incubated with [~(14)C] malathion for 0-8 hr was fractionated, and the radioactive decomposition products in the fractions were identified by comparison with the retention time of fifteen reference standards. Twelve of these products occurring in the environment, diethyl fumarate, diethyl maleate, diethyl malate and diethyl succinate, and their hydrolysate and desmethyl malathion dicarboxylic acid were not detected in the reaction mixture, indicating no contribution of organophosphorus hydrolase or methyl transf erase. A trace amount of desmethyl malathion was observed, however, this seemed to be non-enzymatically produced. Malathion monocarboxylic acid and malathion dicarboxylic acid were identified as the major products by the LC condition with and without an ion pair reagent, respectively. The half life of malathion decomposition was 2.1 hr, and the reaction time course of these products demonstrated that malathion was decomposed to malathion monocarboxylic acid followed by malathion dicarboxylic acid. These results suggested that residual malathion in wheat kernels was mainly hydrolyzed to malathion carboxylic acids by wheat carboxylesterase during the sample preparation.