Neuropathy target esterase in human lymphocytes

Neuropathy target esterase (NTE/PNPLA6) and organophosphorus compound-induced delayed neurotoxicity (OPIDN)

Systemic inhibition of neuropathy target esterase (NTE) with certain organophosphorus (OP) compounds produces OP compound-induced delayed neurotoxicity (OPIDN), a distal degeneration of axons in the central nervous system (CNS) and peripheral nervous system (PNS), thereby providing a powerful model for studying a spectrum of neurodegenerative diseases. Axonopathies are important medical entities in their own right, but in addition, illnesses once considered primary neuronopathies are now thought to begin with axonal degeneration. These disorders include Alzheimer's disease, Parkinson's disease, and motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Moreover, conditional knockout of NTE in the mouse CNS produces vacuolation and other degenerative changes in large neurons in the hippocampus, thalamus, and cerebellum, along with degeneration and swelling of axons in ascending and descending spinal cord tracts. In humans, NTE mutations cause a variety of neurodegenerative conditions resulting in a range of deficits including spastic paraplegia and blindness. Mutations in the Drosophila NTE orthologue SwissCheese (SWS) produce neurodegeneration characterized by vacuolization that can be partially rescued by expression of wild-type human NTE, suggesting a potential therapeutic approach for certain human neurological disorders. This chapter defines NTE and OPIDN, presents an overview of OP compounds, provides a rationale for NTE research, and traces the history of discovery of NTE and its relationship to OPIDN. It then briefly describes subsequent studies of NTE, including practical applications of the assay; aspects of its domain structure, subcellular localization, and tissue expression; abnormalities associated with NTE mutations, knockdown, and conventional or conditional knockout; and hypothetical models to help guide future research on elucidating the role of NTE in OPIDN.

Interindividual Variations in Enzymes Controlling Organophosphate Toxicity in Man

1 Interindividual variations in an unexposed population have been defined for five enzymes involved in organophosphate (OP) toxicity. The enzymes measured were: red blood cell acetylcholinesterase (AChE), lymphocyte neuropathy target esterase (NTE), serum cholinesterase (ChE), serum paraoxonase and serum arylesterase.

2 AChE and arylesterase were normally distributed in the population whilst the distribution of NTE, ChE and paraoxonase deviated significantly from normal.

3 Assay precision and intra-individual variability were measured for each of the enzymes; the effect on interindividual variation was assessed.

4 Variations in enzyme activities between individuals could have profound effects on susceptibility to OP toxicity. Prior determination of these enzymes may be predictive of susceptibility.

5 Lymphocyte NTE has some limitations as an indicator of exposure to neurotoxic OPs.

The patatin-like lipase family in Gallus gallus

Background

In oviparous species, genes encoding proteins with functions in lipid remodeling, such as specialized lipases, may have evolved to facilitate the assembly and utilization of yolk lipids by the embryo. The mammalian gene family of patatin-like phospholipases (PNPLAs) has received significant attention, but studies in other vertebrates are lacking; thus, we have begun investigations of PNPLA genes in the chicken (Gallus gallus).

Results

We scanned the draft chicken genome using human PNPLA sequences, and performed PCR to amplify and sequence orthologous cDNAs. Full-length cDNA sequences of galline PNPLA2/ATGL, PNPLA4, -7, -8, -9, and the activator protein CGI-58, as well as partial cDNA sequences of avian PNPLA1, -3, and -6 were obtained. The high degree of sequence identities (~50 to 80%) between the avian and human orthologs suggests conservation of important enzymatic functions. Quantitation by qPCR of the transcript levels of PNPLAs and CGI-58 in 21 tissues indicates that expression patterns and levels diverge greatly between species. A particularly interesting tissue in which certain PNPLAs may contribute to physiological specialization is the extraembryonic yolk sac.

Conclusion

Knowledge about the exact in-vivo functions of PNPLAs in any system is still sparse. Thus, studies about the temporal expression patterns and functions of the enzymes identified here, and of other already known extracellular lipases and co-factors, in the yolk sac and embryonic tissues during embryogenesis are called for. Based on the information obtained, further studies are anticipated to provide important insights of the roles of PNPLAs in the yolk sac and embryo development.

Biosensor assay of neuropathy target esterase in whole blood as a new approach to OPIDN risk assessment: review of progress

Organophosphates (OPs) that inhibit neuropathy target esterase (NTE) with subsequent ageing can produce OP-induced delayed neuropathy (OPIDN). NTE inhibition in lymphocytes can be used as a biomarker of exposure to neuropathic OPs. An electrochemical method was developed to assay NTE in whole blood. The high sensitivity of the tyrosinase carbon-paste biosensors for the phenol produced by hydrolysis of the substrate, phenyl valerate, allowed NTE activity to be measured in diluted samples of whole blood, which cannot be done using the standard colorimetric assay. The biosensor was used to establish correlations of NTE inhibitions in blood with that in lymphocytes and brain after dosing hens with a neuropathic OP. The results of further studies demonstrated that whole blood NTE is a reliable biomarker of neuropathic OPs for up to 96 hours after exposure. These validation results suggest that the biosensor NTE assay for whole blood could be developed to measure human exposure to neuropathic OPs as a predictor of OPIDN. The small blood volume required (100 microL), simplicity of sample preparation and rapid analysis times indicate that the biosensor should be useful in biomonitoring and epidemiological studies. The present paper is an overview of our previous and ongoing work in this area.

New mechanism of organophosphorus pesticide-induced immunotoxicity

Organophosphorus pesticides (OPs) are widely used throughout the world as insecticides in agriculture and as eradicating agents for termites around homes. The main toxicity of OPs is neurotoxicity, which is caused by the inhibition of acetylcholinesterase. OPs also affect the immune response, including effects on antibody production, interleukin-2 production, T cell proliferation, decrease of CD5 cells, and increases of CD26 cells and autoantibodies, Th1/Th2 cytokine profiles, and the inhibition of natural killer (NK) cell, lymphokine-activated killer (LAK) cell, and cytotoxic T lymphocyte (CTL) activities. However, there have been few studies of the mechanism of OP-induced immunotoxicity, especially the mechanism of OP-induced inhibition of cytolytic activity of killer cells. This study reviews new mechanisms of OP-induced inhibition of the activities of NK cells, LAK cells, and CTLs. It has been reported that NK cells, LAK cells, and CTLs induce cell death in tumors or virus-infected target cells by two main mechanisms. The first mechanism is direct release of cytolytic granules that contain the pore-forming protein perforin, several serine proteases termed granzymes, and granulysin by exocytosis to kill target cells, which is called the granule exocytosis pathway. The second mechanism is mediated by the Fas ligand (Fas-L)/Fas pathway, in which FasL (CD95 L), a surface membrane ligand of the killer cell cross links with the target cell's surface death receptor Fas (CD95) to induce apoptosis of the target cells. To date, it has been reported that OPs inhibit NK cell, LAK cell, and CTL activities by at least the following three mechanisms: 1) OPs impair the granule exocytosis pathway of NK cells, LAK cells, and CTLs by inhibiting the activity of granzymes, and by decreasing the intracellular levels of perforin, granzyme A, and granulysin, which were mediated by inducing degranulation of NK cells and by inhibiting the transcription of the mRNAs of perforin, granzyme A, and granulysin. 2) OPs impair the FasL/Fas pathway of NK cells, LAK cells, and CTLs, as investigated by using perforin-knockout mice, in which the granule exocytosis pathway of NK cells does not function and only the FasL/Fas pathway remains functional. 3) OPs induce apoptosis of immune cells.

Characterization of the human patatin-like phospholipase family

Several publications have described biological roles for human patatin-like phospholipases (PNPLAs) in the regulation of adipocyte differentiation. Here, we report on the characterization and expression profiling of 10 human PNPLAs. A variety of bioinformatics approaches were used to identify and characterize all PNPLAs encoded by the human genome. The genes described represent a divergent family, most with a highly conserved ortholog in several mammalian species. In silico characterization predicts that two of the genes function as integral membrane proteins and are regulated by cAMP/cGMP. A structurally guided protein alignment of the patatin-like domain identifies a number of conserved residues in all family members. Quantitative PCR was used to determine the expression profile of each family member. Affymetrix-based profiling of a human preadipocyte cell line identified several members that are differentially regulated during cell differentiation. Cumulative data suggest that patatin-like genes normally expressed at very low levels are induced in response to environmental signals. Given the observed conservation of the patatin fold and lipase motif in all human PNPLAs, a single nomenclature to describe the PNPLA family is proposed.

Biosensor detection of neuropathy target esterase in whole blood as a biomarker of exposure to neuropathic organophosphorus compounds

Neuropathy target esterase (NTE) is the target protein for neuropathic organophosphorus (OP) compounds that produce OP compound-induced delayed neurotoxicity (OPIDN). Inhibition/aging of brain NTE within hours of exposure predicts the potential for development of OPIDN in susceptible animal models. Lymphocyte NTE has also found limited use as a biomarker of human exposure to neuropathic OP compounds. Recently, a highly sensitive biosensor was developed for NTE activity using a tyrosinase carbon-paste electrode for amperometric detection of phenol produced by hydrolysis of the substrate, phenyl valerate. The I50 (20 min at 37 degrees C) for N,N'-di-2-propylphosphorodiamidofluoridate (mipafox) against hen lymphocyte NTE was 6.94 +/- 0.28 microM amperometrically and 6.02 +/- 0.71 microM colorimetrically. For O,O-di1-propyl O-2,2-dichlorvinyl phosphate (PrDChVP), the I50 against hen brain NTE was 39 +/- 8 nM amperometrically and 42 +/- 2 nM colorimetrically. The biosensor enables NTE to be assayed in whole blood, whereas this cannot be done with the usual colorimetric method. Amperometrically, I50 values for PrDChVP against hen and human blood NTE were 66 +/- 3 and 70 +/- 14 nM, respectively. To study the possibility of using blood NTE inhibition as a biochemical marker of neuropathic OP compound exposure, NTE activities in brain and lymphocytes as well in brain and blood were measured 24 h after dosing hens with PrDChVP. Brain, lymphocyte, and blood NTE were inhibited in a dose-responsive manner, and NTE inhibition was highly correlated between brain and lymphocyte (r = .994) and between brain and blood (r = .997). The results suggest that the biosensor NTE assay for whole blood could serve as a biomarker of exposure to neuropathic OP compounds as well as a predictor of OPIDN and an adjunct to its early diagnosis.

Bioelectrochemical analysis of neuropathy target esterase activity in blood

Bioelectrochemical analysis of neuropathy target esterase (NTE) and its inhibitors is based on the combination of the NTE-catalyzed hydrolysis of phenyl valerate and phenol detection by a tyrosinase carbon-paste electrode. The use of the tyrosinase electrode improves 10-fold the sensitivity of NTE detection in comparison with a spectrophotometric method. The tyrosinase electrode was found to be suitable for measurements in whole human blood where spectrophotometric detection is considerably restricted. The specificity of NTE in blood for mipafox and di-2-propyl phosphorofluoridate was close to that for neuronal NTE. The NTE-like activity in blood was determined to be 0.19 +/- 0.02 nmol/min/mg of protein.

Biological Monitoring of Pesticide Exposure: a review. Introduction

Pesticides are used worldwide in agriculture, industry, public health and for domestic applications: as a consequence, a great part of the population may be exposed to these compounds. In spite of this extensive use, knowledge on the health risks associated with prolonged exposure is rather poor, and major uncertainties still exist. Epidemiological observations in man have so far produced little conclusive information, mainly because of weaknesses in exposure assessment. Therefore, information on the type and levels of exposure is fundamental in order to better understand and characterize risk to human health. Exposure assessment can be carried out via measurement of environmental concentrations, as well as via determination of the chemical or its metabolites in body tissues (biological monitoring). Besides indices of internal dose, biological monitoring also includes measurements of early effects attributable to interaction between the chemical agent and the human body. Biological monitoring has the advantage, over environmental monitoring, of determining the dose actually absorbed via any possible route: differences in absorption can be taken into account. whether they are due to biological variability or to use of protective equipment. When, in some cases, a combination of occupational and non-occupational exposure occurs, this also can be taken into consideration by biological monitoring. Few reference documents have been published on biological monitoring of pesticides. For this reason, the Office of Occupational Health of the World Health Organization gave ICPS a mandate to prepare a monograph specifically addressed to reviewing methods for biological monitoring of pesticide exposure. This review is based on more than 300 studies published over the period 1980-1999. For the most representative chemical classes, the available biological exposure indices are reported. Both indices of internal dose and. when available, of early effects are discussed. The reported tests were used to monitor exposure of pesticide applicators in agriculture and public health, manufacturing and formulating workers. subjects poisoned after accidental exposure or attempted suicide, volunteers involved in pharmacokinetic studies, as well as sub-groups of the general population exposed to environmentally persistent pesticides. Single chapters deal with organophosphorus insecticides, carbamate pesticides, dithiocarbamates, phenoxyacids, quaternary ammonium compounds. coumarin rodenticides, synthetic pyrethroids, organochlorine pesticides, chlorotriazines, and pentachlorophenol.

A new approach for determination of neuropathy target esterase activity

Neuropathy target esterase (NTE) was shown to be an excellent biochemical marker for screening of organophosphates (OPs) with respect to their ability to result in organophosphate induced delayed neurotoxicity (OPIDN). This paper describes a new biosensor approach to the analysis of NTE and its inhibitors. The method is based on the combination of NTE enzymatic hydrolysis of phenyl valerate (PV) with phenol detection by the Clark-type oxygen electrode modified by immobilized tyrosinase. The validity of this biosensor method is confirmed by the facts that the calibration curves for NTE obtained by colorimetric and flow-through electrochemical methods were nearly identical and the titration of NTE by test inhibitor mipafox was shown to yield the same pI50 values. The developed electrochemical methods can be considered as a promising approach both for serial express NTE analysis and for kinetic characteristics of NTE.

Poisoning by organophosphorus insecticides and sensory neuropathy

OBJECTIVES

Poisoning by organophosphate insecticides causes cholinergic toxicity. Organophosphate induced delayed polyneuropathy (OPIDP) is a sensory-motor distal axonopathy which usually occurs after ingestion of large doses of certain organophosphate insecticides and has so far only been reported in patients with preceding cholinergic toxicity. Surprisingly, it was recently reported by other authors that an exclusively sensory neuropathy developed in eight patients after repeated unquantified exposures to chlorpyrifos, which did not cause clear-cut cholinergic toxicity. The objective was to assess whether an exclusively sensory neuropathy develops in patients severely poisoned by various OPs.

METHODS

Toxicological studies and electrophysiological measurements were performed in peripheral motor and sensory nerves in 11 patients after acute organophosphate poisoning among which two subjects were poisoned with chlorpyrifos.

RESULTS

Three patients developed OPIDP, including one poisoned by chlorpyrifos. Exclusively sensory neuropathy was never seen after either single or repeated acute organophosphate poisoning. A mild sensory component was associated with a severe motor component in two of the three cases of OPIDP, the other was an exclusively motor polyneuropathy.

CONCLUSION

A sensory-motor polyneuropathy caused by organophosphate insecticides might occur after a severe poisoning and the sensory component, if present, is milder than the motor one. Bearing in mind the toxicological characteristics of these organophosphate insecticides, other causes should be sought for sensory peripheral neuropathies in patients who did not display severe cholinergic toxicity a few weeks before the onset of symptoms and signs.

A comparative study of inhibition of acetylcholinesterase, trypsin, neuropathy target esterase, and spleen cell activation by structurally related organophosphorus compounds

Organophosphorus (OP) compounds can bind to and inactivate several target molecules other than acetylcholinesterase (AChE). In the present study, five sets of structurally related organophosphorus compounds were used to evaluate the relationships between organophosphorus binding sites of AChE, neuropathy target esterase (NTE), trypsin, and the target molecule(s) involved in inhibition of splenocyte activation by OP compounds. The concentration of each OP compound required to inhibit enzyme activity or splenocyte activation by concanavalin A by 50% was determined. The pattern of IC50 values indicated that AChE, trypsin, NTE, and the molecule(s) involved in inhibition of splenocyte activation are distinct with regard to patterns of inhibition by OP compounds. However, there was a striking similarity in the patterns of inhibition for trypsin and NTE with substantial differences for only 2 of 20 compounds. This pattern suggests similarity in the active sites of these molecules. There were also similarities in the IC50 patterns for lymphocyte activation and trypsin or NTE activity. However, the correlation was not as strong as between NTE and trypsin, and the data suggested the possibility of multiple target molecules for inhibition of splenocyte activation by OP compounds. More importantly, there was essentially no correlation between the pattern of IC50 values for AChE and splenocyte activation. This strongly suggests that acetylcholine and AChE of the type found in the brain are not important in the regulation of splenocyte activation by concanavalin A.

Neurological aspects of organophosphate poisoning

Besides their well-known anticholinesterase action resulting in a typical acute cholinergic crisis, organophosphorus (OP) agents are capable of producing several subacute or chronic neurological syndromes. The acute over-stimulation at the neuromuscular junction results in muscle fiber necrosis. The significance of this OP-induced myopathy in human intoxication is unknown. Organophosphate-induced delayed neuropathy (OPIDN) arises 1-3 weeks after exposure to some OP compounds all capable of remarkably inhibiting a distinct esterase called neuropathy target esterase (NTE) during a critical time period. An experimental hen model has been designed to screen new OP compounds as to their delayed neurotoxic effects. The recently described intermediate syndrome emerges 1-4 days after an apparently well-treated cholinergic crisis. It main clinical features are sudden respiratory paralysis, cranial motor nerve palsies, and proximal limb muscle and neck flexor weakness. Whether or not this is a separate entity in OP agent toxicology remains to be seen. Further studies are required to further determine its clinical and paraclinical characteristics and the actual type of underlying neuromuscular dysfunction involved.

The pathogenesis of organophosphate polyneuropathy

This review discusses the facts regarding organophosphate-induced delayed polyneuropathy (OPIDP) as they are related to its pathogenesis rather than being a comprehensive review of all available data. Neuropathy target esterase (NTE) is considered to be the molecular target for OPIDP which is affected by several esterase inhibitors. Such inhibitors are ranked according to their toxicological effects as follows: 1. Phosphates, phosphoroamidates, and phosphonates cause OPIDP when high amounts of NTE are inhibited. In most cases 70 to 80% inhibition is enough, whereas in others much more is required. 2. Phosphinates, carbamates, and sulfonyl halides cause either protection from or promotion of OPIDP when given before or after a neuropathic OP, respectively. Both effects are related to doses that inhibit NTE. Neuropathy is also caused by the combined treatment with a carbamate and a sulfonyl fluoride. The potency of a given NTE inhibitor to cause OPIDP is related to the chemistry of the residue left attached to NTE, in addition to its affinity for the enzyme. The capability of inhibited NTE to undergo the aging process distinguishes inhibitors with high from those with negligible or very low potency to cause OPIDP. Therefore, protection from neuropathic doses of effective OPs is obtained when NTE is mostly inhibited with nonageable inhibitors. Promotion of OPIDP is likely to involve another site besides NTE because it might occur when almost all NTE is affected. Promotion affects either progression or expression of OPIDP after the initial biochemical lesion on NTE. Since only NTE inhibitors have been proven to be promoters, it is possible that this site is made available after the initiation of OPIDP and that it may have biochemical properties indistinguishable from those of NTE of naïve birds. Age-related resistance to OPIDP also seems to be related to either progression or expression of OPIDP and/or to the different physiology of NTE at a given age. Previously reported resistance of rats to clinical OPIDP seems also to be age-dependent. The physiological function(s) of NTE is unknown, but some practical gains have been obtained from its identification, including OPIDP risk assessment and biomonitoring.

Acute poisoning with methidathion: a case

An acute poisoning in a 50-year-old man who ingested approximately 6.2 g of the phosphorus ester methidathion is described. The patient was treated with three haemoperfusions 23, 44 and 115 h after ingestion, with continuous gastric lavage, atropine and pralidoxime administration and with prolonged mechanical ventilation. Haemoperfusion was an ineffective epuration technique since it removed only 0.22% of the ingested methidathion. The clinical course wavered because of a probable redistribution of phosphorus ester from fat to blood. A plasma level higher than 100 micrograms l-1 was associated with the most serious phases. Methidathion was present in the plasma until the sixth day, in the urine until the seventh and in the gastric juice until the eighth. Its absence in the fat biopsy made on the tenth day was an aid to therapy. The phosphorus ester did not inhibit lymphocyte neuropathy target esterase (NTE), neither did it induce development of delayed polyneuropathy.

Neuropathy target esterase in hens after sarin and soman

To estimate the potential of small doses of sarin (types I and II) and soman to cause delayed neuropathic effects, 400, 200, 61, and 0 micrograms/kg of sarin-I, 280, 140, 70, and 0 micrograms/kg of sarin-II, and 14.2, 7.1, 3.5, and 0 micrograms/kg of soman by gavage were compared with 510 mg/kg tri-o-cresyl phosphate (TOCP) in 14- to 18-month-old SPF white leghorn hens (4/dose) protected with atropine (100 mg/kg). The neuropathy target esterase (NTE) activity 24 hr after dosing was determined in brain, spinal cord, and lymphocytes and in plasma and brain for cholinesterase and carboxylesterase. None of the compounds showed statistically significant NTE decreases. Sarin-II showed a dose-related trend in the lymphocyte NTE (to 33% of control at 280 micrograms/kg), suggesting that longer exposure to lower doses might cause a cumulative neurotoxic insult. All of the agents decreased the activity of plasma and brain cholinesterase and carboxylesterase. Using more than 70% inhibition of brain NTE as a biochemical predictor of delayed neuropathy, sarin and soman appear unable to cause delayed neuropathy at nonlethal doses within this protocol.

Immunotoxicity of organophosphorus compounds. Modulation of cell-mediated immune responses by inhibition of monocyte accessory functions

Several organophosphorus compounds (OP) used commercially as flame retardants and plasticizers and related chemicals were evaluated for their effects on human in vitro cell-mediated immune responses. At nontoxic concentrations ranging from 0.1 to 20 microM, two of the tested compounds, triphenylphosphine oxide (TPPO) and tetra-o-cresylpiperazinyl diphosphoamidate (TCPD) caused significant suppression of antigen-specific lymphocyte proliferation (P less than 0.01). Mitogenesis was less sensitive to OP treatment and was affected only by TCPD. When monocytes and lymphocytes were treated separately with OP, washed, and recombined, it appeared that these OP mediated their suppressive effects by interfering with a monocyte function rather than acting directly on lymphocytes. Further, triphenyl phosphate (TPP), triphenyl thiophosphate (TPTP) as well as TPPO and TCPD were tested for direct inhibition of monocyte antigen presentation, and all four compounds were found to cause significant inhibition at concentrations as low as 1 microM (P less than 0.001).

A review of organophosphate poisoning

Many organophosphate compounds are pesticides widely used for the control of insect vectors. They are not ideal agents because they lack target vector selectivity, and have caused severe toxicity and even death in humans and domestic animals. Their toxicity has been recognised since the 1930s, when they were also developed for use as chemical warfare agents. The mechanism of action of organophosphates has been determined in some depth; the understanding of the toxic effects resulting from the inhibition of cholinesterase activity, causing accumulation of acetylcholine at nerve endings has played a major part in providing a rationale for specific antidote treatment using atropine and oximes. However, the most suitable oxime for reactivation of cholinesterases has still not been established with certainty, although pralidoxime is widely recommended. Chronic toxicity, particularly the neuropathic effects, merits further study because it contributes substantially to the long term morbidity in cases of severe acute, or chronic, exposure. Prevention of potentially toxic organophosphate exposure, particularly amongst employees in industries manufacturing or using the compounds and in the most susceptible groups of the population, such as the young and the elderly, should be sought wherever possible. Government authorities should be encouraged to control organophosphate product licensing, manufacture, storage, import, methods of use and delivery, food contamination and disposal.

Inhibition of lymphocytic neuropathy target esterase predicts the development of organophosphate-induced delayed polyneuropathy

Neuropathy Target Esterase (NTE) is the molecular target in the nervous system for organophosphorus esters (OP) when they cause delayed polyneuropathy. Some NTE activity was recently found also in blood lymphocytes. An unsuccessful suicide attempt with the widely used pesticide chlorpyrifos (0,0-diethyl-0-3,5,6,-trichloro-2-pyridyl phosphorothioate) is reported, where prior inhibition of lymphocytic NTE correlates with the delayed development of polyneuropathy. A 42-year-old man drank approximately 300 mg/kg chlorpyrifos. The subsequent severe cholinergic syndrome lasted for 17 days with varying degrees of severity. Thirty days after intoxication the clinical and electrophysiological examination of the peripheral nervous system was normal but lymphocytic NTE was about 60% inhibited. On day 43 the patient began to complain of paresthesia and leg weakness. Clinical examination, electrophysiology and a nerve biopsy revealed signs of a peripheral polyneuropathy, axonal in type. This case report indicates that measurement of lymphocytic NTE might be used as a clinical test to predict the development of OP-induced delayed polyneuropathy.

Biological monitoring for organophosphate-induced delayed polyneuropathy

Certain organophosphate (OP) pesticides cause a delayed polyneuropathy. The two-step initiation mechanism for this toxicity involves the phosphorylation and subsequent 'aging' of a protein in the nervous system called Neuropathy Target Esterase (NTE). The observation of this enzyme activity in peripheral blood lymphocytes led to several studies to verify whether its measurement after OP exposures might be used as a biological monitoring test for the OP-induced delayed polyneuropathy (OPIDP). The evidence, so far, on the use of this biochemical test in man is discussed together with the need for further research.

Lymphocyte and brain neurotoxic esterase: dose and time dependence of inhibition in the hen examined with three organophosphorus esters

Certain organic phosphorus esters produce sensorimotor axonopathy in man and other species. There is an excellent correlation between the capacity of an organophosphorus compound to produce axonopathy and its ability to inhibit brain neurotoxic esterase (NTE) in hens. Because NTE is present in peripheral lymphocytes of both hen and man, it has been suggested that the lymphocyte enzyme might be useful both in experimental and clinical situations as an indicator of exposure to organophosphorus compounds producing axonopathy. Diethyl 4-nitrophenyl phosphate (paraoxon), tri-2-cresyl phosphate (TOCP), methyl 2,5-dichloro-4-bromophenyl phenylphosphonothionate (leptophos), and di-n-butyl-2,2-dichlorovinyl phosphate (di-n-butyl dichlorvos, DBDCV) were used to examine the relationship between lymphocyte and brain NTE inhibition in hens. As expected, paraoxon (0.75 mg/kg) did not inhibit NTE in brain or lymphocytes. TOCP (10 to 100 mg/kg), leptophos (25 to 150 mg/kg), and DBDCV (1.0 to 4.0 mg/kg) inhibited both brain and lymphocyte NTE activity in a dose-related manner with good correlation of inhibition between tissues taken 24 hr after exposure (r2 = 0.53 to 0.67; p less than 0.020 to 0.001). However, correlation of inhibition between tissues taken from animals killed 48 hr after exposures was poor (r2 = 0.15 to 0.30; p less than 0.10 to 0.05), with consistently less inhibition of lymphocyte NTE relative to brain NTE. This study indicates that assay of lymphocyte NTE can provide a good monitor of exposure to axonotoxic organophosphorus compounds within 24 hr between exposure and measurement.