Environmental Behavior of the Chiral Organophosphorus Insecticide Acephate and Its Chiral Metabolite Methamidophos: Enantioselective Transformation and Degradation in Soils

Abstract

Acephate is a widely used organophosphorus insecticide globally, although there are some concerns about its usage with regard to acute consumer exposure and side-effects on nontarget organisms. These concerns are always attributed to the acephate metabolite methamidophos. In the many reports about the environmental behavior of acephate and its metabolite, none pay any attention to the chirality of them. In this study, the enantiomeric transformation and degradation of acephate was investigated in three soils under laboratory conditions using enantioselective GC-MS/MS. Racemic and enantiopure compounds were incubated in separate experiments. The degradation of racemates was shown to be enantioselective in unsterilized soils but not in the sterilized soils, thus confirming the enantioselectivity was microbially based. The priority of enantiomer degradation and transformation varied among soils and racemates. R-(+)-methamidophos was enriched in the Zhengzhou soil, but degraded faster in the Changchun and Nanchang soils than its antipode. For acephate, the Nanchang soil enriched R-(+)-acephate, and S-(-)-acephate accumulated in the other two soils. Acephate and methamidophos were both configurationally stable in soil, showing no interconversion of R-(+)- to S-(-)-enantiomers, or vice versa. The conversion of acephate to methamidophos proceeded with retention of configuration. Generally, the degradation followed approximate first-order kinetics, but showed significant lag phases.

A review on sample preparation and chromatographic determination of acephate and methamidophos in different samples

“…This information cannot be obtained from conventional achiral analysis. However, to our knowledge, there was the only one published information available on the transformation and degradation of the enantiomers of acephate and its chiral metabolite methamidophos in soils (Wang et al, 2013). Study highlighted that degradation of racemates is enantioselective in unsterilized soils but not in the sterilized soils, thus confirming that enantioselectivity is microbial based.…”

“…The enantioselective bioactivity of enantiomers of acephate and methamidophos has been observed during past years. The R-(+)-enantiomers of acephate and methamidophos were found to be more potent to houseflies than the optical antipodes and racemates, whereas the S-(À)-enantiomers were more toxic for German cockroaches soils (Miyazaki et al, 1988;Wang et al, 2006Wang et al, , 2013. Another experiment showed that (À)-methamidophos was about 8.0À12.4 times more potent to the bovine erythrocytes and Electrophorus electricus than its (+)-form, but the (+)-enantiomer was 7.0 times more toxic to D. magna in 48 h tests (Lin et al, 2006).…”

“…The toxicity of acephate and methamidophos varies with application of enantiomeric compound of them (Wang et al, 2013). Both acephate and its metabolite methamidophos have the stereogenic phosphorus atom.…”

“…Methamidophos is the major metabolite of acephate. It is toxic, not only for insects but also in various components of the environment such as nontarget animals, plants, waters, and soils (Wang et al, 2013) (see Tables 2-4).…”

“…Acephate leads to contamination of groundwater much more readily than methamidophos under normal environmental conditions. Studies highlight that wet soils are more conducive for the contamination of the aquatic environment with acephate and its metabolites (Chai et al, 2009;Wang et al, 2013).…”