Abstract
Multienzyme inhibition assay for residue analysis of insecticidal organophosphates and carbamates
A recently developed spectrophotometric assay for the detection of organophosphorus and carbamate insecticides by means of cutinase inhibition has been successfully extended to two esterases derived from Bacillus subtilis (BS2) and rabbit liver. These esterases were selected because of their high sensitivity to the examined insecticide classes and their pronounced inhibition profile. With inhibition constants (ki) of 2.0 x 10(7) and 2.6 x 10(6) L/(mol.min) for rabbit liver esterase and BS2, respectively, chlorpyrifos oxon proved to be the strongest inhibitor directly followed by paraoxon. As compared to choline esterases and the recently studied cutinase, both esterases are surprisingly strongly inhibited by organophosphorus thions, showing k i in the range of 5.3 x 10(2) to 2.3 x 10(4) L/(mol.min). All tested insecticidal carbamates were also inhibitors of BS2 and rabbit liver esterase, albeit in a rather uniform manner. Generally, both enzymes were found to be about two orders of magnitude more sensitive on the studied insecticides than cutinase even with an enhanced sensitivity against plant matrix effects. Plant extracts, obtained according to the QuEChERS method, were subjected to solid-phase extraction (SPE) using a mixed mode strong anion exchanger/primary secondary amine sorbent and C18endcapped cartridges for superior cleanup. With spiked samples of apple juice, best recoveries of 73 % (±61 %), 94 % (±25 %), and 134 % (±17 %) were obtained for chlorpyrifos, parathion-methyl, and paraoxon, respectively. Results of exemplarily performed liquid chromatography-mass spectrometry control measurements were well in accordance with measurements obtained by enzyme inhibition.