Lipases sensitive to organophosphorus (OP) inhibitors play critical roles in cell regulation, nutrition, and disease, but little is known on the toxicological aspects in mammals. To help fill this gap, six lipases or lipase-like proteins are assayed for OP sensitivity in vitro under standard conditions (25 degrees C, 15 min incubation). Postheparin serum lipase, lipoprotein lipase (LPL) (two sources), pancreatic lipase, monoacylglycerol (MAG) lipase, cholesterol esterase, and KIAA1363 are considered with 32 OP pesticides and related compounds. Postheparin lipolytic activity in rat serum is inhibited by 14 OPs, including chlorpyrifos oxon (IC50 50-97 nM). LPL (bovine milk and Pseudomonas) generally is less inhibited by the insecticides or activated oxons, but the milk enzyme is very sensitive to six fluorophosphonates and benzodioxaphosphorin oxides (IC50 7-20 nM). Porcine pancreatic lipase is very sensitive to dioctyl 4-nitrophenyl phosphate (IC50 8 nM), MAG lipase of mouse brain to O-4-nitrophenyl methyldodecylphosphinate (IC50 0.6 nM), and cholesterol esterase (bovine pancreas) to all of the classes of OPs tested (IC50 < 10 nM for 17 compounds). KIAA1363 is sensitive to numerous OPs, including two O-4-nitrophenyl compounds (IC50 3-4 nM). In an overview, inhibition of 28 serine hydrolases (including lipases) by eight OPs (chlorpyrifos oxon, diazoxon, paraoxon, dichlorvos, and four nonpesticides) showed that brain acetylcholinesterase is usually less sensitive than butyrylcholinesterase, liver esterase, cholesterol esterase, and KIAA1363. In general, each lipase (like each serine hydrolase) has a different spectrum of OP sensitivity, and individual OPs have unique ranking of potency for inhibition of serine hydrolases.
Title: Organophosphate toxicology: safety aspects of nonacetylcholinesterase secondary targets Casida JE, Quistad GB Ref: Chemical Research in Toxicology, 17:983, 2004 : PubMed
Neuropathy target esterase (NTE) undergoes phosphorylation and aging as the initial steps in organophosphorus (OP)-induced delayed neuropathy (OPIDN). Localization of NTE is an important step in characterizing the mechanism of OPIDN. Earlier histochemical immunoreactivity or esterase assays localized NTE in areas of the brain and spinal cord rich in neuronal cell bodies and in the dorsal root ganglion. We use a more direct and quantitative autoradiographic approach of forming phosphorylated and aged [3H]octylphosphonyl-NTE on treatment with the highly potent [octyl-3H]octyl-4H-1,3,2-benzodioxaphosphorin 2-oxide to determine NTE as the labeling site resistant to the non-neuropathic paraoxon and sensitive to the neuropathic mipafox. NTE is observed in the cerebral cortical layer, some layers of the optic tectum, the gray matter of the spinal cord and the sensory neurons of the dorsal root ganglion to a higher extent than in adjacent areas.
Title: Organophosphorus neuropathy target esterase inhibitors selectively block outgrowth of neurite-like and cell processes in cultured cells Li W, Casida JE Ref: Toxicol Lett, 98:139, 1998 : PubMed
This study compares two direct-acting neuropathy target esterase (NTE) inhibitors (mipafox and 2-octyl-4H-1,3,2-benzodioxophosphorin 2-oxide (OBDPO)), a metabolic precursor to an NTE inhibitor (tri-o-cresyl phosphate or TOCP) and a potent acetylcholinesterase inhibitor (chlorpyrifos oxon or CPO) for their effects on outgrowth of neurite-like and cell processes and on viability in differentiated cultured cells (rat adrenal pheochromocytoma (PC-12) and brain glial tumor (C6)). The direct-acting NTE inhibitors block process outgrowth by 50% or more at 50-100 microM for OBDPO and 100-200 microM for mipafox, well below their cytotoxic levels (EC50 values, 445-474 microM for OBDPO and 1021-1613 microM for mipafox). In contrast, the effects on process development for TOCP and CPO parallel their cytotoxicity. These findings suggest that inhibition of neurite-like and cell process outgrowth by OBDPO and mipafox may be associated with NTE inhibition.
Title: Actions of two highly potent organophosphorus neuropathy target esterase inhibitors in mammalian cell lines Li W, Casida JE Ref: Toxicol Lett, 92:123, 1997 : PubMed
Neuropathy target esterase (NTE) is inhibited by many organophosphorus compounds that induce delayed neuropathy. This study examines two of the most potent NTE inhibitors, 2-octyl-4H-1,3,2-benzodioxaphosphorin 2-oxide (OBDPO) and ethyl octylphosphonofluoridate (EOPF), in cell lines with neural properties (PC-12 and NB41A3) and of nonneural origin (C6 and HeLa). NTE-like esteratic activity is higher in PC-12, HeLa and C6 cells than in NB41A3 cells and in each case is inhibited 50% by OBDPO and EOPF at 0.03-3.4 nM in vitro and by OBDPO at 0.080-36 nM in situ in culture. An NTE-like protein(s) of about 155 kDa is phosphorylated and labeled by [3H-octyl]OBDPO in these cell lines in the same order as their relative NTE esteratic activity. Cytotoxic levels of OBDPO and EOPF (300-500 microM) are generally 10(5) to > 10(7)-fold higher than required for NTE inhibition. PC-12 cells and OBDPO/[3H]OBDPO and EOPF are therefore suitable for research on non-lethal biochemical disruptions from NTE phosphorylation and aging.
Title: Subacute neurotoxicity induced in mice by potent organophosphorus neuropathy target esterase inhibitors Wu SY, Casida JE Ref: Toxicology & Applied Pharmacology, 139:195, 1996 : PubMed
The mouse is considered to be insensitive and the hen sensitive to clinical expression of organophosphorus-induced delayed neuropathy (OPIDN) which is associated with inhibition of neuropathy target esterase (NTE). This species difference is reevaluated with two optimized inhibitors of hen brain NTE by examining them for potential neurotoxic effects in mice. 2-Octyl-4H-1,3,2-benzodioxaphosphorin 2-oxide (OBDPO) and ethyl octylphosphonofluoridate (EOPF) inhibit mouse brain NTE in vitro by 50% at 0.12 and 0.02 nM and induce neurotoxic signs in mice at 10 and 5 mg/kg, respectively. The action of these compounds in both l- and 6-month-old mice, sometimes after early transient cholinergic signs, involves ataxia, paralysis, and death in 1 to 3 days and is accordingly referred to as subacute neurotoxicity. The neurotoxic signs are associated with brain edema and severe vacuolation in the grey matter of the brain and spinal cord, particularly the neuropile. Subacute neurotoxic signs are always associated with at least 80% inhibition of brain NTE activity 16-24 hr after treatment. Acetylcholinesterase and butyrylcholinesterase are much less sensitive than NTE to inhibition by OBDPO and EOPF both in vitro and in vivo. Selected carbamates, thiocarbamates, phosphinates, and sulfanyl fluorides are prophylactic agents and dipentyl 2,2-dichlorovinyl phosphate is a promoter for OBDPO-induced subacute neurotoxicity. Although this type of neurotoxicity in mice is similar to OPIDN in the correlation with NTE inhibition and the prophylactic action of reversible NTE inhibitors, it differs from OPIDN in the delay time prior to onset, the sensitivity of both young and old animals, and the high incidence of fatality. A full neuropathological study is desirable to further characterize this subacute neurotoxicity.
Title: A novel probe for characterisation of neuropathy target esterase Moretto A Ref: Hum Exp Toxicol, 14:930, 1995 : PubMed
Title: Neuropathy target esterase of hen brain: active site reactions with 2-[octyl-3H]octyl-4H-1,3,2-benzodioxaphosphorin 2-oxide and 2-octyl-4H-1,3,2-[aryl-3H]benzodioxaphosphorin 2-oxide Yoshida M, Tomizawa M, Wu SY, Quistad GB, Casida JE Ref: Journal of Neurochemistry, 64:1680, 1995 : PubMed
2-Octyl-4H-1,3,2-benzodioxaphosphorin 2-oxide (octyl-BDPO) is one of the most potent inhibitors known for neuropathy target esterase (NTE) of hen brain with 50% inhibition at 0.2 nM. Two NTE-like proteins, i.e., resistant to paraoxon and sensitive to mipafox, of approximately 155 and approximately 119 kDa (designated NTE-155 and NTE-119, respectively) are labeled by [octyl-3H]octyl-BDPO and separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Labeling with [aryl-3H]octyl-BDPO is only approximately 15% of that with [octyl-3H]octyl-BDPO, indicating that the majority of the phosphorylated NTE undergoes aging with only a small proportion of nonaged target or intramolecular group transfer ("alkylation"). NTE-155 and NTE-119 have the same kinetic constants and maximal number of phosphorylation sites, equivalent for each of them to 26 fmol/mg of protein and totaling at least 0.44-1.2 micrograms of NTE protein/g of brain. Structure-activity investigations involving 17 combinations of organophosphorus (OP) compounds of varied chemical type, stereochemistry, and concentration establish an excellent correlation (r = 0.95) between inhibition of NTE activity and protein labeling and thereby the toxicological relevance of these assays, which equally implicate NTE-155 and NTE-119 (probably an autolysis product of NTE-155) as target in OP-induced delayed neuropathy. [octyl-3H]-Octyl-BDPO is an improved probe for NTE in terms of its potency, reactivity, selectivity, and the formation of 3H-labeled NTE with a stable phosphorus-carbon bond.
