Chlorpyrifos is bioactivated in the liver to chlorpyrifos-oxon by cytochrome P-450-
dependent desulfuration. This oxon is then rapidly hydrolysed to 3,5,6-trichloro-2-
pyridinol (TCP) via micosomal esterase, which includes paraoxanase and chlorpyrifos
oxonase, or via a non-enzymatic process. Alternatively, chlorpyrifos is dearylated to form
diethyl thiophosphoric acids and TCP in a reaction catalysed by microsomal enzymes.