Suitability of human HepaRG cells and liver spheroids as in vitro model to investigate the bioactivation of the organothiophosphate pesticide parathion.

Organophosphorus compounds (OP) constitute a large group of chemicals including pesticides and nerve agents. Organothiophosphate pesticides require cytochrome P450-mediated oxidative desulphuration in the liver to form corresponding oxons, which are potent inhibitors of the enzyme acetylcholinesterase (AChE). Human HepaRG cells are a promising tool to study liver-specific functions and have been shown to maintain drug metabolizing enzymes. This research describes for the first time the in vitro metabolic activation of an organothiophosphate to its active oxon by two different HepaRG cell-based models. Monolayer cultures and liver spheroids were exposed to the model OP parathion and the quantification of the corresponding oxon was performed with an AChE inhibition assay. Our results showed a time- and dose-dependent cytochrome P450 catalyzed bioactivation and a superior metabolism capacity of the monolayer HepaRG model in comparison with the liver spheroids. Finally, HepaRG cells can be assessed as a metabolically competent cell model intermediate between cell-free preparations and intact animals and as suitable to study OP metabolism in the human liver.

The impact of solvents on the toxicity of the banned parathion insecticide.

The aerial crop dusting and spraying of fields with the phosphorothioate insecticide parathion in the late 1900s, significantly improved crop yields but resulted in high levels of occupational toxicity in handlers and agricultural workers, as well as cases of intentional self-harm poisoning, culminating in its banning in many western countries by early 2000s. However because of the low solubility and volatility of parathion, most available products were formulated using organic solvents e.g. xylene, to increase the efficacy of the aerosols and dusts. In the present study, the toxicity of parathion was assessed when formulated in an aqueous solvents (ethanol/PBS (1:9)), and delivered to macaques as an aerosol. Doses of 780 µg/kg and 1.56 mg/kg were delivered one day apart, using a modified nebulizer calculated to result in lung deposition of ∼480 µg/kg with a similar or larger amount being swallowed; these doses being similar to the estimated lethal oral dose 286ug/kg - 1.43 mg/kg of formulated parathion in humans. Surprisingly, this dose (a combined amount of ∼14 mg) caused only low AChE inhibition and moderate BChE inhibition with no clinical symptoms, indicating that the use of organic solvents may have previously played a critical role in the severity of parathion toxicity following inhalation exposure. In addition, unlike constitutively toxic OPs, which are highly toxic when inhaled, these results are consistent with the idea that phosphorothioate insecticides appear to be more intoxicating following oral than inhalation exposure. However, this still remains uncertain because the presence of organic solvents in the ingested parathion studies was not always known.

Cellular and molecular responses to ethyl-parathion in undifferentiated SH-SY5Y cells provide neurotoxicity pathway indicators for organophosphorus impacts.

High-fidelity nonanimal screening methods are needed that can rapidly and accurately characterize organophosphorus compound (OP)-induced neurotoxicity. Herein, the efficacy of human neuroblastoma cell line (SH-SY5Y) to provide molecular and cellular responses characteristic of the OP neurotoxicity pathway was investigated in response to the OP-model compound, ethyl-parathion. Undifferentiated SH-SY5Y cells were exposed to ethyl-parathion for 30 min at 0 (control), 0.5, 2.5, 5, 10, and 25 µg/ml. Dose-responsive reductions in cell viability were observed with significant reductions at ≥10 µg/ml. From these results, ethyl-parathion exposures of 0 (control), 5, and 10 µg/ml were selected to examine bioindicators underlying the OP neurotoxicity pathway including: reactive oxygen species (ROS), cell membrane peroxidation, mitochondrial membrane potential (MMP), and apoptosis. Ethyl-parathion elicited highly significant increases in ROS relative to controls (p < .01) at both exposure concentrations, confirmed using N-acetyl cysteine (NAC) as a ROS quencher which alleviated ROS increases. A response characteristic of increased ROS exposure, cell membrane-lipid peroxidation, significantly increased (p < .05) at the highest ethyl-parathion exposure (10 µg/ml). As a likely consequence of membrane-lipid peroxidation, ethyl-parathion-induced reductions in MMP were observed with significant effects at 10 µg/ml, reducing MMP by 58.2%. As a culmination of these cellular-damage indicators, apoptosis progression was investigated by phosphatidylserine translocation where ethyl-parathion-induced dose-responsive, highly significant (p < .01) increases at both 5 and 10 µg/ml. Overall, the mechanistic responses observed in undifferentiated SH-SY5Y cells corresponded with in vivo mammalian results demonstrating potential for this nonanimal model to provide accurate OP neurotoxicology screening.

Chlorpyrifos and parathion regulate oxidative stress differentially through the expression of paraoxonase 2 in human neuroblastoma cell.

Organophosphate (OP) compounds are frequently linked to both chronic and acute forms of nervous system disorders. Chlorpyrifos (CPF) and parathion (PA) are two of the most widely used OP insecticides throughout the world. These compounds are acetylcholinesterase inhibitors and cause a cholinergic crisis. However, there are other non-cholinergic effects of the OP compounds as well. The role of Paraoxonase 1 (PON1) in the metabolism of OP compounds is well established owing to its significant organophosphatase activity. Since PON2 has no paraoxonase activity and the level of its expression is 20-40 fold higher in the brain, in this article the role of PON2 in response to CPF and PA exposure concerning both cholinergic and non-cholinergic effects are explored. The effect of these OPs on cell viability, reactive oxygen species (ROS), PON2 gene expression, and function was studied. Glutathione level, esterase activity, and paraoxonase activity were also measured in CPF- and PA-treated IMR-32 cells. At these levels, both CPF and PA showed different impacts on IMR-32 cells. PA at higher concentrations (50-200 µM) proved to be less toxic than CPF. Interestingly, induction of ROS was also lower in the case of PA-treated cells as compared to the CPF. However, PON2 protein expression was increased with the increasing concentration of PA and decreased with the increasing concentration of CPF. To explore the possible mechanism of the differential regulation of PON2 gene expression by CPF and PA, we investigated the possible binding and signaling through the human M2 muscarinic acetylcholine receptor (M2AChR). Since M2AChRs are similar to G-protein coupled receptors and function through cAMP signalling, we measured the cAMP level after CPF and PA treatment. CPF- and PA-treated IMR-32 cells can be used as a model to study the mechanism by which PON2 acts as a ROS scavenger in response to xenobiotics stimulation in the brain.

Genomic comparisons between hepatocarcinogenic and non-hepatocarcinogenic organophosphate insecticides in the mouse liver.

Short-term biomarkers of toxicity have an increasingly important role in the screening and prioritization of new chemicals. In this study, we examined early indicators of liver toxicity for three reference organophosphate (OP) chemicals, which are among the most widely used insecticides in the world. The OP methidathion was previously shown to increase the incidence of liver toxicity, including hepatocellular tumors, in male mice. To provide insights into the adverse outcome pathway (AOP) that underlies these tumors, effects of methidathion in the male mouse liver were examined after 7 and 28 day exposures and compared to those of two other OPs that either do not increase (fenthion) or possibly suppress liver cancer (parathion) in mice. None of the chemicals caused increases in liver weight/body weight or histopathological changes in the liver. Parathion decreased liver cell proliferation after 7 and 28 days while the other chemicals had no effects. There was no evidence for hepatotoxicity in any of the treatment groups. Full-genome microarray analysis of the livers from the 7 and 28 day treatments demonstrated that methidathion and fenthion regulated a large number of overlapping genes, while parathion regulated a unique set of genes. Examination of cytochrome P450 enzyme activities and use of predictive gene expression biomarkers found no consistent evidence for activation of AhR, CAR, PXR, or PPARα. Parathion suppressed the male-specific gene expression pattern through STAT5b, similar to genetic and dietary conditions that decrease liver tumor incidence in mice. Overall, these findings indicate that methidathion causes liver cancer by a mechanism that does not involve common mechanisms of liver cancer induction.

Signs of carcinogenicity induced by parathion, malathion, and estrogen in human breast epithelial cells (Review).

Cancer development is a multistep process that may be induced by a variety of compounds. Environmental substances, such as pesticides, have been associated with different human diseases. Organophosphorus pesticides (OPs) are among the most commonly used insecticides. Despite the fact that organophosphorus has been associated with an increased risk of cancer, particularly hormone­mediated cancer, few prospective studies have examined the use of individual insecticides. Reported results have demonstrated that OPs and estrogen induce a cascade of events indicative of the transformation of human breast epithelial cells. In vitro studies analyzing an immortalized non­tumorigenic human breast epithelial cell line may provide us with an approach to analyzing cell transformation under the effects of OPs in the presence of estrogen. The results suggested hormone­mediated effects of these insecticides on the risk of cancer among women. It can be concluded that, through experimental models, the initiation of cancer can be studied by analyzing the steps that transform normal breast cells to malignant ones through certain substances, such as pesticides and estrogen. Such substances cause genomic instability, and therefore tumor formation in the animal, and signs of carcinogenesis in vitro. Cancer initiation has been associated with an increase in genomic instability, indicated by the inactivation of tumor­suppressor genes and activation of oncogenes in the presence of malathion, parathion, and estrogen. In the present study, a comprehensive summary of the impact of OPs in human and rat breast cancer, specifically their effects on the cell cycle, signaling pathways linked to epidermal growth factor, drug metabolism, and genomic instability in an MCF­10F estrogen receptor­negative breast cell line is provided.

In vivo efficacy of the Reactive Skin Decontamination Lotion (RSDL®) kit against organophosphate and carbamate pesticides.

In this study, we assessed the efficacy of the Reactive Skin Decontamination Lotion (RSDL®) Kit against parathion and aldicarb pesticide dermal exposure in a guinea pig model. The pesticides inhibit acetylcholinesterase (AChE) leading to signs and symptoms of hyperactivity of organs due to accumulation of acetylcholine. The RSDL Kit has been shown to physically remove and chemically degrade chemical warfare agents. Degradation occurs from a nucleophilic substitution reaction between an active ingredient in the RSDL lotion, potassium 2,3-butanedione monoximate (KBDO), with susceptible sites in these compounds. In the present study, guinea pigs dermally exposed to parathion and aldicarb were decontaminated with RSDL to mitigate the toxic effects of the pesticides. It is observed that animals exposed to 749 mg/kg of parathion (n = 3) died within 24 h without RSDL decontamination; however, RSDL-treated animals (n = 3) showed only mild signs of neurotoxicity. The RSDL-treated animals had an AChE inhibition of 0-58% while the untreated animals had up to 86% inhibition. Similarly, RSDL has been demostrated to prevent aldicarb neurotoxicity effects. The percent inhibition of AChE activity during the 24 h post challenge of 9 mg aldicarb/kg of animal weight ranged from 25% to 61% with severe signs of intoxication while only up to 5% with mild or no signs of intoxication in the case of RSDL-decontaminated animals. Generally, it has been shown that the toxic effects of the organophosphate and carbamate pesticides can be prevented via decontamination using the RSDL Kit.

Effect of prenatal exposure to combined immunosuppressive agrochemicals in a mouse model of allergic airway inflammation.

The aim of this study is to identify the combined effect of multiple chemicals to the development of allergy. In this study, the effect of prenatal exposure to an organochlorine agent methoxychlor (MXC) and/or an organophosphate agent parathion (PARA) on trimellitic anhydride-induced allergic airway inflammation was examined in mice. Eosinophil infiltration in the bronchoalveolar lavage fluid (BALF) was significantly enhanced by MXC + PARA exposure compared to that of the control, MXC, and PARA groups. In the hilar lymph node, only slight increases in B-cell infiltration, as well as IL-6 and IL-9 secretions were observed in MXC + PARA group, and no effect was observed in the individual treatment groups. Our findings imply that prenatal exposure to some combinations of multiple chemicals may exacerbate the allergic inflammatory responses including eosinophils and cytokine production.

Gelatin hydrogel containing cerium oxide nanoparticles covered by interleukin-17 aptamar as an anti- inflammatory agent for brain inflammation.

The purpose of this study was to evaluate the anti-inflammatory property of gelatin hydrogel containing cerium oxide nanoparticles coated with interleukin-17 Aptemer ([CeON@IL-17]). Here, the brain inflammation model was induced by both proteolipid protein (PLP) and parathion. Then, the expression of some inflammatory genes and the serum level of related interleukins were evaluated. This study showed that the expression of IL-17, IL-10, and IL-6 genes and their serum levels were significantly decreased (P < .05) by administration of gelatin hydrogel containing [CeON@IL-17].

Interspecific differences in biochemical and behavioral biomarkers in endogeic earthworms exposed to ethyl-parathion.

Earthworms are common organisms in the soil toxicity-testing framework, and the epigeic Eisenia andrei and E. fetida are the recommended species. However, Eisenia species are rarely found in agricultural soils and recent studies have pointed out endogeic species are more sensitive to pesticides than Eisenia. Allolobophora chlorotica and Aporrectodea caliginosa are two endogeic soil-dwelling species that are abundant in the agroecosystem. However, knowledge on pesticide impact on this ecological group of earthworms is still incipient. Herein, we compared the biochemical (acetylcholinesterase [AChE] and carboxylesterase [CbE] activities) and behavioral (burrowing, casting and feeding) biomarker responses of these two endogeic earthworm species exposed for 7 days to soils contaminated with 0.1, 1 and 10 mg kg-1 ethyl-parathion. The results showed marked species-specific differences in both groups of biomarkers, suggesting A. caliginosa the most sensitive species to this organophosphorus pesticide under the exposure conditions in this study. Moreover, an in vitro inhibition trial with ethyl-paraoxon evidenced a higher sensitivity of A. caliginosa AChE activity compared with that of A. chlorotica. This finding suggested that this molecular target endpoint could contribute to the interspecific differences of behavioral responses rather than CbE activity; this latter considered a potent mechanism of OP removal. Our results suggest the inclusion of more than one endogeic earthworm species to assess toxicity from organophosphorus insecticides. However, the use of A. caliginosa in the environmental risk assessment framework of organophosphorus contamination is highly recommended because of its higher sensibility to this class of pesticides, in addition to its abundance in the agroecosystem.

Diazinon and parathion diverge in their effects on development of noradrenergic systems.

Organophosphate pesticides elicit developmental neurotoxicity through mechanisms over and above their shared property as cholinesterase inhibitors. We compared the consequences of neonatal exposure (postnatal days PN1-4) to diazinon or parathion on development of norepinephrine systems in rat brain, using treatments designed to produce equivalent effects on cholinesterase, straddling the threshold for barely-detectable inhibition. Norepinephrine levels were measured throughout development from the immediate posttreatment period (PN5), to early adolescence (PN30), young adulthood (PN60) and full adulthood (PN100); we assessed multiple brain regions containing all the major noradrenergic synaptic projections. Diazinon elicited a significant overall deficit of norepinephrine, whereas parathion produced a net increase. The effects were not immediately apparent (PN5) but rather emerged over the course of development, indicating that the organophosphate effects represent alteration of the trajectory of development, not just continuance of an initial injury. There were no comparable effects on ß-adrenergic receptors, indicating that the presynaptic changes were not an adaptation to an underlying, primary effect on postsynaptic receptor signaling. Because we used the cholinesterase inhibition benchmark, the absolute dose of diazinon was much higher than that of parathion, since the latter is a more potent cholinesterase inhibitor. Our results are consistent with the growing evidence that the various organophosphates can differ in their impact on brain development and that consequently, the cholinesterase benchmark is an inadequate predictor of adverse neurodevelopmental effects.

Evaluation of absorbent materials for use as ad hoc dry decontaminants during mass casualty incidents as part of the UK's Initial Operational Response (IOR).

The UK's Initial Operational Response (IOR) is a revised process for the medical management of mass casualties potentially contaminated with hazardous materials. A critical element of the IOR is the introduction of immediate, on-scene disrobing and decontamination of casualties to limit the adverse health effects of exposure. Ad hoc cleansing of the skin with dry absorbent materials has previously been identified as a potential means of facilitating emergency decontamination. The purpose of this study was to evaluate the in vitro oil and water absorbency of a range of materials commonly found in the domestic and clinical environments and to determine the effectiveness of a small, but representative selection of such materials in skin decontamination, using an established ex vivo model. Five contaminants were used in the study: methyl salicylate, parathion, diethyl malonate, phorate and potassium cyanide. In vitro measurements of water and oil absorbency did not correlate with ex vivo measurements of skin decontamination. When measured ex vivo, dry decontamination was consistently more effective than a standard wet decontamination method ("rinse-wipe-rinse") for removing liquid contaminants. However, dry decontamination was ineffective against particulate contamination. Collectively, these data confirm that absorbent materials such as wound dressings and tissue paper provide an effective, generic capability for emergency removal of liquid contaminants from the skin surface, but that wet decontamination should be used for non-liquid contaminants.

A method for quantitative risk appraisal for pesticide risk assessments.

Pesticide risk assessments are fraught with uncertainties that arise from the process of estimating exposure to and toxicity of chemicals. Regulatory agencies resolve those uncertainties in a health-protective (conservative) manner, typically acknowledging only inter- and intraspecies uncertainties quantitatively. Other uncertainties may be acknowledged qualitatively, but those safety factors (SF) are not enumerated. Quantitative risk appraisal may be used to enumerate the multiplicative SF generated by conservative assumptions regarding uncertainties. The magnitude of SF derived from decision points dealing with historically unquantified uncertainty may far exceed explicit SF used to gauge acceptable margins of exposure (MoE). Examination of the basis for some previously unenumerated SF may justify potential changes in regulatory practices and policies. Using past risk assessments of 3 pesticides (mevinphos, parathion, and methyl iodide) for which the California Department of Pesticide Regulation found unacceptable risk as examples, the previously unquantified SF ranged from 47 to 1 × 106 for scenarios involving handlers, reentry workers, and bystanders.

Dermostyx (IB1) - High efficacy and safe topical skin protectant against percutaneous toxic agents.

Prevention of the penetration of toxic agents through the skin is crucial for both military troops and civilian populations. We have developed a novel topical skin protectant (TSP), coded as IB1 and commercially available as Dermostyx protective solution (Rekah Pharm, Israel). The formulation afforded significant protection against chemical warfare agents such as sulfur mustard (SM) and VX (2LD50), pesticides such as parathion and irritants such as acrolein. The efficacy of the protectant was evaluated in the pig model using clinical, histological and biochemical monitoring. A single topical application prior to exposure to the toxic agents reduced significantly the size and severity of skin lesions and ameliorated or prevented systemic clinical symptoms. The barrier properties of IB1 are immediate upon application and remain effective for at least 12 h. It is absorbed into the stratum corneum of the skin and remains there until rinsing with water, yet the ingredients are not absorbed into the body. The formulation is a hydrophilic water-based solution, composed of magnesium sulfate and glycerin that are widely used in cosmetic and medicine, and was shown to be safe in preclinical and in Phase I clinical studies. The suggested mode of action is based on the unique interaction of glycerin with the stratum corneum, changing its properties to hydrophilic and on the "salting out" effect of magnesium sulfate. The expected use of the TSP is by application on exposed skin areas and sensitive skin sites (e.g. armpits, groin, waist), when necessary. A quantity of 10 ml is sufficient for one application covering approximately 20% of the body surface area. The formulation was approved for human use by the Israel Ministry of Health and a CE mark certificate in Europe has been recently issued (Class I). Dermostyx has been adopted by the IDF and first responders as a skin protectant for special needs.

Novel approaches to mitigating parathion toxicity: targeting cytochrome P450-mediated metabolism with menadione.

Accidental or intentional exposures to parathion, an organophosphorus (OP) pesticide, can cause severe poisoning in humans. Parathion toxicity is dependent on its metabolism by the cytochrome P450 (CYP) system to paraoxon (diethyl 4-nitrophenyl phosphate), a highly poisonous nerve agent and potent inhibitor of acetylcholinesterase. We have been investigating inhibitors of CYP-mediated bioactivation of OPs as a method of preventing or reversing progressive parathion toxicity. It is well recognized that NADPH-cytochrome P450 reductase, an enzyme required for the transfer of electrons to CYPs, mediates chemical redox cycling. In this process, the enzyme diverts electrons from CYPs to support chemical redox cycling, which results in inhibition of CYP-mediated biotransformation. Using menadione as the redox-cycling chemical, we discovered that this enzymatic reaction blocks metabolic activation of parathion in rat and human liver microsomes and in recombinant CYPs important to parathion metabolism, including CYP1A2, CYP2B6, and CYP3A4. Administration of menadione to rats reduces metabolism of parathion, as well as parathion-induced inhibition of brain cholinesterase activity. This resulted in inhibition of parathion neurotoxicity. Menadione has relatively low toxicity and is approved by the Food and Drug Administration for other indications. Its ability to block parathion metabolism makes it an attractive therapeutic candidate to mitigate parathion-induced neurotoxicity.

A new post-intoxication treatment of paraoxon and parathion poisonings using an evolved PON1 variant and recombinant GOT1.

Organophosphate (OP) based pesticides are highly toxic compounds that are still widely used in agriculture around the world. According to World Health Organization (WHO) data, it is estimated that between 250,000 and 370,000 deaths occur yearly around the globe as a result of acute intoxications by pesticides. Currently available antidotal drug treatments of severe OP intoxications are symptomatic, do not reduce the level of intoxicating OP in the body and have limited ability to prevent long-term brain damage. Pesticide poisonings present a special therapeutic challenge since in many cases, such as with parathion, their toxicity stems from their metabolites that inhibit the essential enzyme acetylcholinesterase. Our goal is to develop a new treatment strategy for parathion intoxication by combining a catalytic bioscavenger that rapidly degrades the intoxicating parathion-metabolite (paraoxon) in the blood, with a glutamate bioscavenger that reduces the elevated concentration of extracellular glutamate in the brain following OP intoxication. We report on the development of a novel catalytic bioscavenger by directed evolution of serum paraoxonase 1 (PON1) that effectively detoxifies paraoxon in-vivo. We also report preliminary results regarding the utilization of this PON1 variant together with a recombinant human enzyme glutamate oxaloacetate transaminase 1 (rGOT1), suggesting that a dual PON-GOT treatment may increase survival and recovery from parathion and paraoxon intoxications.

Environmental pollutants parathion, paraquat and bisphenol A show distinct effects towards nuclear receptors-mediated induction of xenobiotics-metabolizing cytochromes P450 in human hepatocytes.

Environmental pollutants parathion, bisphenol A and paraquat were not systematically studied towards the effects on the expression of phase I xenobiotics-metabolizing cytochromes P450 (CYPs). We monitored their effects on the expression of selected CYPs in primary cultures of human hepatocytes. Moreover, we investigated their effects on the receptors regulating these CYPs, particularly arylhydrocarbon receptor (AhR), pregnane X receptor (PXR) and glucocorticoid receptor (GR) by gene reporter assays. We found that parathion and bisphenol A are the activators of AhR. Moreover, they are the inducers of CYP1A1 mRNA in hepatoma cells HepG2 as well as in human hepatocytes by AhR-dependent mechanism via formation of AhR-DNA-binding complex, as revealed by gel shift assay. All three compounds possessed anti-glucocorticoid action as revealed by GR-dependent gene reporter assay and a decline in tyrosine aminotransferase (TAT) gene expression in human hepatocytes. Moreover, parathion and bisphenol A are the activators of PXR and inducers of CYP3A4 mRNA and protein in the primary cultures of human hepatocytes. In conclusion, the studied compounds displayed distinct activities towards nuclear receptors involved in many biological processes and these findings may help us to better understand their adverse actions in pathological states followed after their exposure.

The Pesticide Metabolites Paraoxon and Malaoxon Induce Cellular Death by Different Mechanisms in Cultured Human Pulmonary Cells.

Organophosphorus (OP) pesticides are known to induce pulmonary toxicity in both humans and experimental animals. To elucidate the mechanism of OP-induced cytotoxicity, we examined the effects of parathion and malathion and their respective metabolites, paraoxon and malaoxon, on primary cultured human large and small airway cells. Exposure to paraoxon and malaoxon produced a dose-dependent increase in cytotoxicity following a 24-hour exposure, while treatment with parathion or malathion produced no effects at clinically relevant concentrations. Exposure to paraoxon-induced caspase activation, but malaoxon failed to induce this response. Since caspases have a major role in the regulation of apoptosis and cell death, we evaluated OP-induced cell death in the presence of a caspase inhibitor. Pharmacological caspase inhibition protected against paraoxon-induced cell death but not malaoxon-induced cell death. These data suggest that caspase activation is a key signaling element in paraoxon-induced cell death, but not malaoxon-induced cellular death in the pulmonary epithelium.

Vitamin K3 (menadione) redox cycling inhibits cytochrome P450-mediated metabolism and inhibits parathion intoxication.

Parathion, a widely used organophosphate insecticide, is considered a high priority chemical threat. Parathion toxicity is dependent on its metabolism by the cytochrome P450 system to paraoxon (diethyl 4-nitrophenyl phosphate), a cytotoxic metabolite. As an effective inhibitor of cholinesterases, paraoxon causes the accumulation of acetylcholine in synapses and overstimulation of nicotinic and muscarinic cholinergic receptors, leading to characteristic signs of organophosphate poisoning. Inhibition of parathion metabolism to paraoxon represents a potential approach to counter parathion toxicity. Herein, we demonstrate that menadione (methyl-1,4-naphthoquinone, vitamin K3) is a potent inhibitor of cytochrome P450-mediated metabolism of parathion. Menadione is active in redox cycling, a reaction mediated by NADPH-cytochrome P450 reductase that preferentially uses electrons from NADPH at the expense of their supply to the P450s. Using human recombinant CYP 1A2, 2B6, 3A4 and human liver microsomes, menadione was found to inhibit the formation of paraoxon from parathion. Administration of menadione bisulfite (40mg/kg, ip) to rats also reduced parathion-induced inhibition of brain cholinesterase activity, as well as parathion-induced tremors and the progression of other signs and symptoms of parathion poisoning. These data suggest that redox cycling compounds, such as menadione, have the potential to effectively mitigate the toxicity of organophosphorus pesticides including parathion which require cytochrome P450-mediated activation.