Lysophosphatidylcholine hydrolases as targets of organophosphorus toxicants

Organophosphorus (OP) toxicants are of two principal types: insecticides and chemical warfare agents. Both act primarily as inhibitors of acetylcholinesterase to block hydrolysis of acetylcholine. The most important OP secondary effect is delayed neurotoxicity, occurring 1 to 3 weeks after exposure and sometimes resulting in permanent paralysis. The target for OP neurotoxicants is neuropathy target esterase (NTE), which is an endoplasmic reticulum-bound enzyme in neurons. The proposed endogenous substrate of NTE is lysophosphatidylcholine (LPC), and the biological function of NTE is only partially characterized. Blood cholinesterase inhibition is used for monitoring OP pesticide exposure, and there is no marker for OP delayed neurotoxicants. Lymphocyte NTE activity is a proposed, but not validated indicator of brain NTE inhibition.;The goal of this study was to evaluate erythrocyte LPC hydrolase (LPCH) activity as a marker of OP exposure and a guide to the endogenous function of NTE. The inhibitor sensitivity and specificity profiles of human erythrocyte, lymphocyte and brain LPCH activities were surprisingly similar to each other and to recombinant NTE (rNTE), indicating the enzyme(s) involved have the same binding site specificity and possibly a similar structure. Delayed neurotoxicants were the most potent LPCH inhibitors (IC50 values < 1 nM), while widely-used OP insecticides were less potent inhibitors. Erythrocyte LPCH activity was not different between mothers and children, and independent of paraoxonase I (an OP detoxifier) genotype. In mice, erythrocyte LPCH activity was inhibited by a potent delayed neurotoxicant and was reflective of brain LPCH and NTE inhibition. At high doses, OP pesticides inhibited less than a quarter of erythrocyte LPCH activity. Thus, erythrocyte LPCH activity is a predictor of OP neurotoxicant exposure, but not of OP insecticide exposure.;rNTE was used to evaluate the endogenous substrate of NTE. Cells expressing rNTE were more resistant than controls to LPC-induced cytotoxicity and contain lower LPC levels, higher glycerophosphorylcholine levels, and unchanged phosphatidylcholine levels (measured by reverse phase-liquid chromatography-mass spectrometry) than those without rNTE. Thus, rNTE appears to hydrolyze exogenous and endogenous LPC. NTE and LPCH inhibition are likely contributers to the delayed neurotoxicity associated with some OPs.