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).