Characterization of Activation Metabolism Activity of Indoxacarb in Insects by Liquid Chromatography-Triple Quadrupole Mass Spectrometry

Abstract A triple-quadrupole LC-MS/MS method was developed for the investigation of indoxacarb N -decarbomethoxyllation activity catalyzed by insect crude enzymes. The metabolite, N -decarbomethoxyllated indoxacarb (DCJW), was determined using electrospray ionization in positive ion mode. The quantitative and qualitative analyses were performed in multiple reaction monitoring (MRM) mode. The relative standard deviation (RSD, n = 5) of DCJW was 2.1%, indicating the metabolite had a good chemical stability in 30 h after the sample preparation. Five injections of DCJW standard solution in acetonitrile, five injections of the same sample in one incubation (substrates plus enzyme samples), and five injections of 5 separate metabolic reactions with the whole body of a susceptible strain of Plutella xylostella (L.) were individually injected into LC-MS/MS system. The RSD was 0.7%, 1.1% and 2.7%, respectively. The average recoveries for 440, 880 and 2200 pg of DCJW added to incubation mixtures were 96.1%–102.9%, and the RSD of recoveries of three added levels ranged from 4.8% to 9.4%. The limits of quantification and detection were 0.1 and 0.01 pg, respectively. A good linearity was achieved in the range of 46–2310 pg ( R 2 = 0.9996). This method with high sensitivity and simplicity was applicable to the assay of indoxacarb N -decarbomethoxyllation activity of insect metabolic enzymes. A comparison of indoxacarb N -decarbomethoxyllation activity of an avermectin-resistant strain and a susceptible strain of Plutella xylostella (L.) was carried out using this novel LC-MS/MS method. The results showed that the indoxacarb N -decarbomethoxyllation activity in the resistant strain was 3.4-fold of that in the susceptible strain, implicating that negative cross-resistance might exist between indoxacarb and avermectin in Plutella xylostella (L.).