| Abstract
| - Serine hydrolase KIAA1363 is highly expressed in invasive cancer cells and is the major protein inmouse brain diethylphosphorylated by and hydrolyzing low levels of chlorpyrifos oxon (CPO) (the activatedmetabolite of a major insecticide). It is also the primary CPO-hydrolyzing enzyme in spinal cord, kidney,heart, lung, testis, and muscle but not liver, a pattern of tissue expression confirmed by fluorophosphonate-rhodamine labeling. KIAA1363 gene deletion using homologous recombination reduces CPO binding,hydrolysis, and metabolism 3−29-fold on incubation with brain membranes and homogenates determinedwith 1 nM [3H-ethyl]CPO and the inhibitory potency for residual CPO with butyrylcholinesterase as abiomarker. Studies with knockout mice further show that KIAA1363 partially protects brain AChE andmonoacylglycerol lipase from CPO-induced in vivo inhibition. Surprisingly, mouse brain KIAA1363and AChE are similar in in vitro sensitivity to seven methyl, ethyl, and propyl but not higher alkyl OPinsecticides and analogues, prompting structural comparisons of the active sites of KIAA1363 and AChErelative to OP potency and selectivity. Homology modeling based largely on the Archaeoglobus fulgidusesterase crystal structure indicates that KIAA1363 has a catalytic triad of S191, D348, and H378, aGDSAG motif, and an oxyanion hole of H113, G114, G115, and G116. Excellent selectivity for KIAA1363is achieved on OP structure optimization with long alkyl chain substituents suggesting that KIAA1363has larger acyl and leaving group pockets than those of AChE. KIAA1363 reactivates faster than AChEpresumably due to differences in the uncoupling of the catalytic triad His upon phosphorylation. Thestructural modeling of KIAA1363 helps us understand OP structure−activity relationships and thetoxicological relevance of this detoxifying enzyme.
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