Interaction of the serine hydrolase KIAA1363 with organophosphorus agents: Evaluation of potency and kinetics

• IC50s and apparent bimolecular rate constant (kinact/Ki) for reactions of chlorpyrifos oxon and paraoxon with human KIAA1363 were determined.
• Human KIAA1363 was potently inhibited by both chlorpyrifos oxon and paraoxon, with IC50s in the nM range.
• Bimolecular rate constants for reactions of chlorpyrifos oxon and paraoxon with human KIAA1363 should prove useful in PBPK/PD modeling efforts.

Oxons are bioactive metabolites of organophosphorus insecticides (OPs) that covalently inactivate serine hydrolases. KIAA1363 is one of the most abundant serine hydrolases in mouse brain. Although the physiological consequences related to the inhibition of KIAA1363 due to environmental exposures to OPs are poorly understood, the enzyme was previously shown to have a role in the detoxification of oxons. Here, we overexpressed human KIAA1363 and CES1 in COS7 cells and compared the potency of inhibition (IC50s, 15 min) of KIAA1363 and CES1 by chlorpyrifos oxon (CPO), paraoxon (PO), and methyl paraoxon (MPO). The order of potency was CPO > PO >> MPO for both enzymes. We also determined the bimolecular rate constants (kinact/Ki) for reactions of CPO and PO with KIAA1363 and CES1. KIAA1363 and CES1 were inactivated by CPO at comparable rates (4.4 × 106 s−1 M−1 and 6.7 × 106 s−1 M−1, respectively), whereas PO inactivated both enzymes at slower rates (0.4 × 106 s−1 M−1 and 1.5 × 106 s−1 M−1, respectively). Finally, the reactivation rate of KIAA1363 following inhibition by CPO was evaluated. Together, the results define the kinetics of inhibition of KIAA1363 by active metabolites of agrochemicals and indicate that KIAA1363 is highly sensitive to inhibition by these compounds.

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