Malathion Residue in Wheat Kernels is Degraded by Thion Organophosphorus Pesticide-Specific Carboxylesterase

Abstract

We earlier reported that malathion residue in wheat kernels is enzymatically eliminated by adding water to swell the sample as pretreatment in the official method set forth in the Food Sanitation Law of Japan. We examined the activities of monooxygenase, organophosphorus hydrolase, methyltransferase, and carboxylesterase as possible malathion-degradable enzymes in wheat kernels. GC/MS analysis resulted in no activity of monooxygenase in wheat kernels, because malaoxon was not produced in the reaction mixture of the homogenate incubated with malathion for 4 hr. When five organophosphorus pesticides that have a thioether group (P-S-C) were reacted with the wheat kernel homogenate, no formation of thiol groups was detected with the 5,5′-Dithiobis(2-nitro-benzoic acid) (DTNB) reagent, indicating that there was no activity of organophosphorus hydrolase in the kernels. Pesticides that do not have a carboxylester group but do have a dimethyl thio-or dimethyl dithio-phosphate group were not decomposed by the homogenate, suggesting that there is no contribution of methyltransferase to malathion degradation. Among the organophosphorus pesticides with a carboxylester group, only the thion compounds malathion, phenthoate, and methacrifos were enzymatically decomposed by the homogenate. Malathion was also demonstrated to be the substrate of carboxylesterase, as the pesticide competitively inhibited the activity with p-nitrophenyl acetate as the typical substrate. Conversely, oxon organophosphorus pesticides were not degraded in the homogenate, the carboxylesterase of which was noncompetitively inhibited. These results suggest that malathion degradation in wheat kernels is caused by the thion organophosphorus pesticide-specific carboxylesterase.

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

“…This is corresponding to the possibility as previously mentioned that malathion is degraded by carboxylesterase in wheat supernatant because the pesticide competitively inhibits against carboxylesterase activity in the supernatant. 17) Therefore, the present results strongly support the possibility.…”

“…10) However, thiomalic acid and its ethyl esters were not covered as targets in this study, because there was no production of these sulfhydryl compounds from malathion added to wheat supernatant in our previous report. 17) Some organophosphorus hydrolase also produces diethyl malate by hydrolysis of site C. 11) Hydrolysis of site D by carboxylesterase leads to malathion monocarboxylic acid and malathion dicarboxylic acid. We previously demonstrated that malathion could be a substrate of carboxylesterase, since the pesticide competitively inhibited the activity with p-nitrophenyl acetate as the typical substrate.…”

“…[20][21][22] We previously demonstrated that most malathion residue in their kernels could be detected by analysis with direct organic solvent-extraction, and that homogenizing the kernels with water before the extraction resulted in degradation of the residue. 6,17) In conclusion, malathion residue in cereal kernels such as wheat, barley and rye, on which this insecticide was sprayed as a postharvest pesticide, is expected to be hydrolyzed by their carboxylesterases when their homogenates are prepared with water for the analysis according to the Japanese official method. Therefore, the official method for the analysis of such residue in cereal kernels should immediately be improved.…”

Characterization and Malathion Degradability of Carboxylesterase in Wheat Kernels

“…7) Additionally, the supernatant of this wheat kernel homogenate did not decompose only malathion but other thion organophosphorus pesticides with carboxylester and p-nitrophenyl acetate as a typical substrate for carboxylesterase (3.1.1.1, CE). 8) Since the enzymatic hydrolysis of p-nitrophenyl acetate in the supernatant was inhibited by addition of malathion, the involvement of CE was suspected to cause the malathion degradation in the supernatant. 8) Malathion α-monocarboxylic acid and malathion β-monocarboxylic acid were identified as degradation products of malathion in the supernatant.…”

Investigation of Combined Pollution between Malathion and a Variety of Pollutants Based on the Fractional Factorial Design (Resolution IV)

“…Organophosphorus pesticides have the advantages of being economic, effective, and convenient, and they are widely used in prevention and control of plant diseases and pests of agriculture [22,23]. Some kinds of the non-durable organophosphorus pesticides also have a longer survival period in certain environmental conditions and would accumulate in the animal body [24].…”