activity assay
MPH purification was performed following the method described by Zhao et al. (2019). Briefly, the cells harboring MPH expression cassette were grown in 2×SR medium for 36 h at 37 ℃. The supernatant of the cultures was used to purify C-terminal His-tagged MPH through Ni-NTA affinity chromatography according to the manufacturer’s instructions (Sangon Biotech Co., Ltd., Shanghai, China). Protein concentration was measured by the BCA Kit (Shanghai, Sangon Biotech Co., Ltd., China). Target proteins were analyzed by SDS-PAGE and the gel was stained with Coomassie brilliant blue R250. The activity of MPH toward chlorpyrifos was measured as described previously (Xie et al. 2014).