Organophosphate polyneuropathy and neuropathy target esterase: studies with methamidophos and its resolved optical isomers

Environmental behavior of the chiral organophosphorus insecticide acephate and its chiral metabolite methamidophos: enantioselective transformation and degradation in soils

Acephate is a widely used organophosphorus insecticide globally, although there are some concerns about its usage with regard to acute consumer exposure and side-effects on nontarget organisms. These concerns are always attributed to the acephate metabolite methamidophos. In the many reports about the environmental behavior of acephate and its metabolite, none pay any attention to the chirality of them. In this study, the enantiomeric transformation and degradation of acephate was investigated in three soils under laboratory conditions using enantioselective GC-MS/MS. Racemic and enantiopure compounds were incubated in separate experiments. The degradation of racemates was shown to be enantioselective in unsterilized soils but not in the sterilized soils, thus confirming the enantioselectivity was microbially based. The priority of enantiomer degradation and transformation varied among soils and racemates. R-(+)-methamidophos was enriched in the Zhengzhou soil, but degraded faster in the Changchun and Nanchang soils than its antipode. For acephate, the Nanchang soil enriched R-(+)-acephate, and S-(-)-acephate accumulated in the other two soils. Acephate and methamidophos were both configurationally stable in soil, showing no interconversion of R-(+)- to S-(-)-enantiomers, or vice versa. The conversion of acephate to methamidophos proceeded with retention of configuration. Generally, the degradation followed approximate first-order kinetics, but showed significant lag phases.

Biochemical, histopathological and clinical evaluation of delayed effects caused by methamidophos isoforms and TOCP in hens: ameliorative effects using control of calcium homeostasis

This work evaluated the potential of the isoforms of methamidophos to cause organophosphorus-induced delayed neuropathy (OPIDN) in hens. In addition to inhibition of neuropathy target esterase (NTE) and acetylcholinesterase (AChE), calpain activation, spinal cord lesions and clinical signs were assessed. The isoforms (+)-, (±)- and (-)-methamidophos were administered at 50mg/kg orally; tri-ortho-cresyl phosphate (TOCP) was administered (500mg/kg, po) as positive control for delayed neuropathy. The TOCP hens showed greater than 80% and approximately 20% inhibition of NTE and AChE in hen brain, respectively. Among the isoforms of methamidophos, only the (+)-methamidophos was capable of inhibiting NTE activity (approximately 60%) with statistically significant difference compared to the control group. Calpain activity in brain increased by 40% in TOCP hens compared to the control group when measured 24h after dosing and remained high (18% over control) 21 days after dosing. Hens that received (+)-methamidophos had calpain activity 12% greater than controls. The histopathological findings and clinical signs corroborated the biochemical results that indicated the potential of the (+)-methamidophos to be the isoform responsible for OPIDN induction. Protection against OPIDN was examined using a treatment of 2 doses of nimodipine (1mg/kg, i.m.) and one dose of calcium gluconate (5mg/kg, i.v.). The treatment decreased the effect of OPIDN-inducing TOCP and (+)-methamidophos on calpain activity, spinal cord lesions and clinical signs.

Mechanisms for consideration for intervention in the development of organophosphorus-induced delayed neuropathy

Organophosphorus-induced delayed neuropathy (OPIDN) is a neurodegenerative disorder characterised by ataxia progressing to paralysis with concomitant central and peripheral distal axonopathy. Symptoms of OPIDN in people include tingling of the hands and feet. This tingling is followed by sensory loss, progressive muscle weakness and flaccidity of the distal skeletal muscles of the lower and upper extremities and ataxia, which appear about 8-14 days after exposure. Some organophosphorus compounds (OPs) that are still used in worldwide agriculture have potential to induce OPIDN, including methamidophos, trichlorfon, dichlorvos and chorpyrifos. This review summarizes experimental attempts to prevent and/or treat OPIDN and the different mechanisms involved in each approach. The initial mechanism associated with development of OPIDN is phosphorylation and inhibition of neuropathy target esterase (NTE). The phosphorylated enzyme undergoes a second reaction known as "aging" that results in the loss of one of the "R" groups bound to the phosphorus of the OP. A second mechanism involved in OPIDN is an imbalance in calcium homeostasis. This can lead to the activation of calcium-activated neutral protease and increases in calcium/calmodulin-dependent protein kinases. These events contribute to aberrant phosphorylation of cytoskeletal proteins and protein digestion in the terminal axon that can proceed similarly to Wallerian-type degeneration. Several experimental studies demonstrated alleviation of the signs and symptoms of OPIDN by restoring calcium balance. Other studies have used preadministration of NTE inhibitors, such as carbamates, thiocarbamates, sulfonyl fluorides and phosphinate to prevent OPIDN. Progress is being made, but there is yet no single specific treatment available for use in clinical practice to prevent or alleviate the severe effects of OPIDN.

Semipreparative enantioseparation of methamidophos by HPLC-UV and preliminary in vitro study of butyrylcholinesterase inhibition

Many chiral pesticides are introduced into the environment as racemates, although their pesticidal activity is usually the result of preferential reactivity of only one enantiomer, while the other enantiomer may have toxic effects against nontarget organisms. Methamidophos (O,S-dimethyl phosphoramidothioate), a chiral compound, is an insecticide widely used in agriculture in both developed and developing countries. However, this pesticide has a high toxicity not only to targeted insects but also to human and animals. In the present study, the enantiomers of methamidophos were enantiomerically separated by a semipreparative chiral liquid chromatography at the multimilligram scale on a polysaccharide-based chiral stationary phase and a preliminary evaluation of their in vitro inhibition of plasma butyrylcholinesterase (BChE) of hens was performed. In the present study, our first effort was to resolve the racemic mixture of methamidophos and to that end reversed-phase, normal-phase, and polar organic elution conditions were investigated in four different polysaccharide-based chiral phases. The best performance was achieved on a cellulose tris(3,5-dimethylphenylcarbamate) phase under normal phase. This chromatographic condition allowed the separation of 225 mg of methamidophos enantiomers with a high degree of chiral purity (>98%) in a short analysis time. Significant differences were found between the concentration that causes 50% of enzyme inhibition (IC50) of the three isoforms of methamidophos. (-)-Methamidophos showed an IC50 approximately three times larger than the (+)-enantiomer for plasma BChE of hens.

Screening for peripheral neuropathy in chemical warfare victims

We aimed to assess the prevalence of peripheral neuropathy in chemical warfare victims. We speculated that peripheral neuropathy is a late complication of exposure to chemical warfare agents. Late complications of exposure to chemical warfare agents are not well known and are poorly discussed in the existing literature. Scientific data regarding delayed complications are sparse, but this warrants recognition, especially when the clinician has to treat chemical warfare victims. The hazards of organophosphate pesticides and several toxins, although recognized to some extent, are, however, different from the hazards of chemical warfare agents which are far more serious. In this study, 100 chemical warfare patients, with varying degrees of exposure and an average age of 37.2+/-9.0 years, were examined clinically and studied electrodiagnostically from January 2002 to January 2003. Five of these patients proved to be suffering from axonal neuropathy. This rate was significantly higher than that found in the normal population. Our data indicate that chemical warfare agents may cause peripheral neuropathy in chemical warfare victims. In conclusion, organophosphorous agents used against Iranian troops during the war on Iran correlate with delayed neuropathy in these victims.

Peripheral nerve esterases and the promotion of organophosphate-induced neuropathy in hens

Several esterase inhibitors, not capable of causing peripheral neuropathy by themselves, exacerbate organophosphate-induced delayed polyneuropathy (OPIDP) and other axonopathies. This effect was called promotion of axonopathies and it was found not to be associated with inhibition of neuropathy target esterase (NTE), the molecular target of OPIDP. The search for an esterase as the target of promotion has started long ago, when an eterogeneous group of esterases-hydrolysing phenyl valerate (PV) was identified in hen's sciatic nerve by means of selective inhibitors. Correlation studies in vivo indicated that the target of promotion may have been among the proteins present in the soluble fraction. When this soluble PV-esterase activity was separated on a Sephacryl-S-300 column, correlation was found between promotion and its inhibition in vivo. The electrophoretic analysis of this fraction indicated the presence of several proteins. Subsequent ion-exchange chromatography identified a protein of about 80 kDa molecular weight that was associated with PV-esterase activity. The inhibition of this activity did also correlate with promotion. The sequence of this protein identified it as ovotransferrin, but commercial preparations of ovotransferrin were found to lack PV-esterase activity. Binding experiments on this purified PV-activity and on commercial ovotransferrin using radiolabelled promoters were inconclusive. Titration of this PV-activity showed that about 20-30% of it is resistant to high concentrations of several inhibitors, suggesting heterogeneity of the fraction. In fact, bi-dimensional electrophoresis indicated the presence of several proteins. Finally, in vivo correlation experiments with p-toluensulfonyl fluoride showed that whereas this chemical does not promote OPIDP induced by dibutyl dichlorovinyl phosphate, it does inhibit about 80% of this PV-activity. In conclusion, available data indicate that the target of promotion is unlikely to be ovotransferrin. However, all promoters identified so far are esterase inhibitors suggesting that the target of promotion might be, indeed, a protein with esteratic activity.

Late onset polyneuropathy due to organophosphate (DDVP) intoxication

BACKGROUND

Organophosphate intoxication can cause some well-known life threatening acute neurological complications such as seizures, paralysis, neuromuscular and cardiac conduction disorders. Less often, a predominantly motor and delayed axonal neuropathy can occur. This syndrome is due to inhibition of neuropathy target esterase.

CASE REPORT

A 30-year-old woman attempted suicide by drinking approximately 1,000mg/kg dimethyl-2,2-dichloro vinyl phosphate (DDVP). After a muscarinic and cholinergic syndrome lasting four days, she developed a purely motor distal axonal polyneuropathy on the fifth week after ingestion confirmed by electroneuromyography and sural nerve biopsy. Neurological examination and electroneuromyography revealed a slight recovery at the end of the 21st month.

CONCLUSION

This case of late onset polyneuropathy caused by organophosphate intoxication had unusual features such as intact sensory nerves and worse prognosis when compared to previously reported cases.

Promotion of organophosphate induced delayed polyneuropathy by certain esterase inhibitors

Organophosphate induced delayed polyneuropathy (OPIDP) is an axonopathy caused by single doses of some organophosphates (OPs). Other OPs, sulphonyl halides, carbamates, thiocarbamates and phosphinates do not cause axonopathy but elicit or intensify the clinical expression of OPIDP when given after a neuropathic OP (promotion of OPIDP). One enzymatic activity (M200) was identified by means of selective inhibitors in hen peripheral nerve crude homogenates. Promotion of OPIDP initiated with several OPs was found to correlate with inhibition of M200 when various promoters were given to hens. Most M200 is in the soluble fraction of peripheral nerves and was separated from other esterases by means of molecular exclusion chromatography. In a second series of experiments, inhibition of this fraction also correlated with promotion when induced by the same chemicals. Further ion exchange chromatography identified a protein (60 kDa MW): the inhibition of its enzymatic activity correlated with promotion in another series of in vivo experiments. Biochemical and structural analyses of this protein are underway. Several experiments indirectly suggest that promotion may be related to mechanisms of repair and/or compensation of peripheral nerves. These include the observation that promotion results in clinical expression of biochemical lesions that otherwise would be well compensated, that promotion is not specific because axonopathies of other etiology are also exaggerated, and that promoters are effective when given several days before the neuropathic insult. Moreover, developing animals are more resistant to promotion.

Age-related sensitivity of the nervous system to neurotoxic insults

Complex genomic activity and environmental factors regulate neuronal plasticity, which operates during pre- and postnatal development, can be reactivated after injury, and is impaired during aging. In these contexts, the effects of chemicals are often unpredictable because the developing and aging nervous system may or may not be equally susceptible as that of the adult. Thus, the developing central nervous system may be more susceptible to the acute toxicity of certain organophosphorus esters, whereas the developing peripheral nervous system is resistant to organophosphate-induced delayed polyneuropathy. Reasons for age-related susceptibility are manifold, including both differences in toxicokinetics and toxicodynamics and in intrinsic susceptibility of the nervous system, which is related to given physiological conditions. Therefore, the identification of genetic and environmental factors regulating neuronal plasticity becomes critical to the understanding of age-related sensitivity to chemicals. The example of age-related sensitivity to organophosphate-induced delayed polyneuropathy is illustrated together with that of the promotion of axonopathies by certain esterase inhibitors, which also seem modulated according to age. The identification of the molecular targets of both organophosphate neuropathy and promotion of neuropathy might allow the understanding of processes involved in the expression of peripheral neurotoxicities according to age.

Testing for organophosphate-induced delayed polyneuropathy

Organophosphorous compounds may cause two distinct types of toxicity: acute cholinergic toxicity and organophosphate-induced delayed polyneuropathy (OPIDP). The ability of a compound to cause OPIDP is assessed as described by administering the compound to hens and screening the brain, spinal cord, and peripheral nerves for neuropathy target esterase activity to detect OPIDP and acetylcholinesterase activity to rule out the acute toxicity. This assay can also be used as part of a screen for protective agents.

Organophosphorus pesticide-induced butyrylcholinesterase inhibition and potentiation of succinylcholine toxicity in mice

Succinylcholine is the most important rapid-acting depolarizing muscle relaxant during anesthesia. Its desirable short duration of action is controlled by butyrylcholinesterase, the detoxifying enzyme. There are two reported cases of prolonged paralysis from succinylcholine in patients poisoned with the organophosphorus insecticides parathion and chlorpyrifos. The present study examines the possibility that other organophosphorus and methylcarbamate pesticides might also prolong succinylcholine action by inhibiting butyrylcholinesterase using mice treated intraperitoneally as a model and relating inhibition of blood serum hydrolysis of butyrylthiocholine to potentiated toxicity (mouse mortality). The organophosphorus plant defoliant tribufos (4 h pretreatment, 160 mg/kg) and organophosphorus plant growth regulator ethephon (1 h pretreatment, 200 mg/kg) potentiate the toxicity of succinylcholine by seven- and fourfold, respectively. Some other pesticides or analogs are more potent sensitizers for succinylcholine toxicity with threshold levels of 0.5, 1.0, 1.7, 8, 10, and 67 mg/kg for phenyl saligenin cyclic phosphonate, profenofos, methamidophos, tribufos, chlorpyrifos, and ethephon, respectively. Enhanced mortality from succinylcholine is generally observed when serum butyrylcholinesterase is inhibited 55-94%. Mivacurium, a related nondepolarizing muscle relaxant also detoxified by butyrylcholinesterase, is likewise potentiated by at least threefold on 4 hour pretreatment with tribufos (25 mg/kg) or profenofos (10 mg/kg).

Promoters and promotion of axonopathies

Promotion is the exacerbation by certain esterase inhibitors (organophosphates, organophosphinates, sulfonyl halides, carbamates and thiocarbamates) of the clinical and morphological expression of toxic and traumatic axonopathies. Promotion is believed to interfere with mechanisms of compensation/repair of the nerves. The target of promotion is unknown but there are indications that it might be similar and/or linked to neuropathy target esterase (NTE), which is the molecular target of organophosphate-induced delayed polyneuropathy (OPIDP). OPIDP is the model axonopathy used to characterize promotion. NTE is defined as the activity resistant to paraoxon (40 microM) and sensitive to mipafox (50 microM). An esterase activity sensitive to higher concentrations (1 mM) of mipafox was identified in the nervous system homogenate, and its inhibition correlated with promotion. An activity with similar characteristics was present in the soluble fraction of peripheral nerves and could be physically separated (about 60 kDa). Identification and characterization of the target of promotion might be helpful in understanding the mechanism(s) of compensation and repair of the peripheral nervous system.

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.

Promotion of organophosphate induced delayed polyneuropathy by certain esterase inhibitors

Certain esterase inhibitors elicit or intensify the clinical expression of various insults to axons. This phenomenon was called promotion of axonopathies because these chemicals are not additive neurotoxicants nor do they interfere with the pharmacokinetics. Characterization of promotion was carried out by using organophosphate induced delayed polyneuropathy (OPIDP) as a model. The search for a physiological explanation of promotion has the following background: (1) Promotion expresses clinically the biochemical lesions which are otherwise well compensated (such as 30/40% neuropathy target esterase (NTE) inhibition by neuropathic organophosphates). (2) Promotion is not specific because axonopathies of different origin are affected. (3) Promoters are effective when given several days before the neuropathic insult. (4) Promotion is less effective in young animals as compared with adults. (5) Promotion occurs when axons, but not necessarily the cell body, are targeted by promoters. (6) Repeated dosing with a promoter failed to produce axonopathy. Based on this evidence it is suggested that promotion might interfere with a mechanism(s) of compensation and/or repair of long axons. The target of promotion of axonopathies is thought to be similar or linked to NTE which is defined as the phenyl valerate esterase activity (PVE) in nervous tissues resistant to paraoxon and sensitive to mipafox (40 and 50 microM, pH 8.0, 20 min, respectively). Mipafox (50 microM) resistant PVEs include some activity sensitive to the promoter phenylmethane sulfonylfluoride (PMSF) but no correlation was found between its inhibition and promotion. A complete titration curve of paraoxon-resistant PVEs by mipafox (0-1 mM) dissected, besides NTE (I50 about 10 microM), another PVE with an I50 of approximately 200 microM. This enzyme was present in hen brain, spinal cord and peripheral nerve, corresponding to about 10, 20 and 30% of NTE activity, respectively, and was sensitive both in vitro and in vivo to promoters and much less so to neuropathic NTE inhibitors. By means of chromatography, other workers have identified in soluble extracts of peripheral nerves two forms of mipafox-sensitive PVEs with different molecular weights and different sensitivity to mipafox. These might correspond to NTE and to the other enzyme. Inhibition in vivo of the latter also correlated with promotion.

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.

Inhibition and aging of neuropathy target esterase by the stereoisomers of a phosphoramidate related to methamidophos

Discrepancies in the aging reaction between neuropathy target esterase (NTE) inhibited in vitro and in vivo by racemic mixtures of O-alkyl O-2,5-dichlorophenyl phosphoramidates have been observed. It suggested the existence of differences in the interactions (inhibition and aging) between NTE and each stereoisomers of the above mentioned compounds. In order to verify this hypothesis, stereoisomers of O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) were isolated by chiral column chromatography, followed by the evaluation of NTE inhibition and aging for each stereoisomers. The loss of reactivation capacity by KF was used as criterion of aging. The stereoisomer S-(-)-HDCP inhibited hen brain NTE with an I50 of 7.6 nM for 30 min of incubation, this being similar to the value obtained for the racemic mixture (I50 = 6.2 nM), and much lower than that recorded for R-(+)-HDCP (I50 = 191 nM). NTE inhibited by HDCP racemic mixture and the stereoisomer S-(-)-HDCP was reactivated by KF after 20 h of incubation at 37 degrees C. The NTE inhibited by R-(+)-HDCP could not be fully reactivated after inhibition.

Mechanisms of toxicity and risk assessment

Incorporating mechanistic information into the risk assessment process is necessary because proliferation of in vitro and in vivo tests of uncertain significance has led to the realisation that the quantity of toxicological information may undermine its own value. Default options in risk assessment to be used in the absence of mechanistic data are mainly derived from extrapolations. Examples from mechanistic studies on organophosphate-induced delayed polyneuropathy (OPIDP) will illustrate 3 main areas of extrapolation where mechanistic data might allow meaningful conclusions for risk assessment: (i) from animal to humans; (ii) from high to low levels of exposure; (iii) from disaggregated systems to complex systems. The continuing effort to understand the mechanisms of toxicity will reduce uncertainty in these and other areas of the extrapolation processes. It could also lead to better appreciation of the significance of biomarkers (such as lymphocyte neuropathy target esterase (NTE) for OPIDP) to be used in biomonitoring programs.