Serine Hydrolase KIAA1363:  Toxicological and Structural Features with Emphasis on Organophosphate Interactions

Abstract

Serine hydrolase KIAA1363 is highly expressed in invasive cancer cells and is the major protein in mouse brain diethylphosphorylated by and hydrolyzing low levels of chlorpyrifos oxon (CPO) (the activated metabolite 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-to 29-fold on incubation with brain membranes and homogenates determined with 1 nM [ 3 H-ethyl]CPO and the inhibitory potency for residual CPO with butyrylcholinesterase as a biomarker. Studies with knockout mice further show that KIAA1363 partially protects brain AChE and monoacylglycerol lipase from CPO-induced in vivo inhibition. Surprisingly, mouse brain KIAA1363 and AChE are similar in in vitro sensitivity to seven methyl, ethyl and propyl but not higher alkyl OP insecticides and analogs, prompting structural comparisons of the active sites of KIAA1363 and AChE relative to OP potency and selectivity. Homology modeling based largely on the Archaeoglobus fulgidus esterase crystal structure indicates that KIAA1363 has a catalytic triad of S191, D348 and H378, a GDSAG motif and an oxyanion hole of H113, G114, G115 and G116. Excellent selectivity for KIAA1363 is achieved on OP structure optimization with long alkyl chain substituents suggesting that KIAA1363 has larger acyl and leaving group pockets than those of AChE. KIAA1363 reactivates faster than AChE presumably due to differences in the uncoupling of the catalytic triad His upon phosphorylation. The structural modeling of KIAA1363 helps understand OP structure-activity relationships and the toxicological relevance of this detoxifying enzyme.

Review of the Toxicology of Chlorpyrifos With an Emphasis on Human Exposure and Neurodevelopment

“…On the other hand, KIAA1363 may serve as an important detoxication mechanism for chlorpyrifos-oxon (much like serum and liver paraoxonase-1 and carboxylesterase) in brain. Indeed, as mentioned earlier, the IC 50 for chlorpyrifos-oxon for mouse AChE inhibition was sixfold higher in brain from wildtype mice than from KIAA1363 knockout mice, indicating a protective role for this enzyme (Nomura et al, 2006).…”

“…However, this result may be due to a differential contribution of extrinsic factors (that may bind to and/or inactivate chlorpyrifos-oxon), present in the crude homogenate, and displaying age-related differences (Padilla et al, 2000;Kousba et al, 2007). For example, in brain membranes from wild type mice, the IC 50 for AChE inhibition was reported to be 4 nM; however, it was 0.7 nM in brain tissue from mice lacking KIAA1363, a serine hydrolase that binds chlorpyrifos-oxon with relatively high affinity (Nomura et al, 2006).…”

“…One in particular, the serine hydrolase KIAA1363, was inhibited by chlorpyrifos-oxon with an IC 50 of 8 nM, well within the range of AChE inhibition. However, upon in vivo administration of chlorpyrifos-oxon to mice, a 39% inhibition of brain KIAA1363 was associated with a 98% inhibition of AChE, and lethality (Nomura et al, 2006). On the other hand, KIAA1363 may serve as an important detoxication mechanism for chlorpyrifos-oxon (much like serum and liver paraoxonase-1 and carboxylesterase) in brain.…”

The Critical Role of Neutral Cholesterol Ester Hydrolase 1 in Cholesterol Removal From Human Macrophages

“…Indeed, KIAA1363, an ortholog of NCEH1, is potently inhibited by many of organophosphates (OP) including paraoxon. 16,41 There is a great difference in the ability to mobilize cholesterol between different types of macrophages. For example, THP-1 cells are known to be relatively ineffective at mobilizing cholesterol, primarily owing to slow hydrolysis of CE.…”

Neurotoxic Effects Associated with Current Uses of Organophosphorus Compounds

“…96,152 The possibility of the relationship between the inhibition of esterases and the toxicological effects of OPs have been studied in the last years by several groups. [153][154][155][156][157][158][159][160][161] This interactions have been experimentally observed and quantified by enzymatic activity assays on specific substrates, 160,161 by activitybased protein profiling studies, 162,163 by exhaustive kinetic characterization with OPs models and phenyl valerate as a non-specific substrate for esterases [164][165][166][167][168][169] and through further separation and fractionation protein studies in chicken brain. 165 A recent study, using high performance liquid chromatography tandem mass spectrometry (LC-MS/MS) proteomic identification, pointed butyril cholinesterase (BuChE) as an esterase highly sensitive to paraoxon and mipafox that hydrolyzes phenyl valerate in chicken brain.…”