The phosphylated butyrylcholinesterase-derived tetrapeptide GlyGluSerAla proves exposure to organophosphorus agents with enantioselectivity

We herein present for the first time the phosphylated (*) tetrapeptide (TP)-adduct GlyGluSer198*Ala generated from butyrylcholinesterase (BChE) with proteinase K excellently suited for the verification of exposure to toxic organophosphorus nerve agents (OPNA). Verification requires bioanalytical methods mandatory for toxicological and legal reasons. OPNA react with BChE by phosphonylation of the active site serine residue (Ser198) forming one of the major target protein adducts for verification. After its enzymatic cleavage with pepsin, the nonapeptide (NP) PheGlyGluSer*AlaGlyAlaAlaSer is typically produced as biomarker. Usually OPNA occur as racemic mixtures of phosphonic acid derivatives with the stereocenter at the phosphorus atom, e.g. (±)-VX. Both enantiomers react with BChE, but the adducted NP does not allow their chromatographic distinction. In contrast, the herein introduced TP-adducts appeared as two peaks when using a stationary reversed phase (1.8 µm) in micro-liquid chromatography-electrospray ionisation tandem-mass spectrometry (µLC-ESI MS/MS) analysis. These two peaks represent diastereomers of the (+)- and (-)-OPNA adducted to the peptide that comprises chiral L-amino acids exclusively. Concentration- and time-dependent effects of adduct formation with (±)-VX and its pure enantiomers (+)- and (-)-VX as well as with (±)-cyclosarin (GF) were investigated in detail characterising enantioselective adduct formation, stability, ageing and spontaneous reactivation. The method was also successfully applied to samples from a real case of pesticide poisoning as well as to samples of biomedical proficiency tests provided by the Organisation for the Prohibition of Chemical Weapons.

The prediction of acute toxicity (LD50) for organophosphorus-based chemical warfare agents (V-series) using toxicology in silico methods

Nerve agents are organophosphate chemical warfare agents that exert their toxic effects by irreversibly inhibiting acetylcholinesterase, affecting the breakdown of the neurotransmitter acetylcholine in the synaptic cleft. Due to the risk of exposure to dangerous nerve agents and for animal welfare reasons, in silico methods have been used to assess acute toxicity safely. The next-generation risk assessment (NGRA) is a new approach for predicting toxicological parameters that can meet modern requirements for toxicological research. The present study explains the acute toxicity of the examined V-series nerve agents (n = 9) using QSAR models. Toxicity Estimation Software Tool (ver. 4.2.1 and ver. 5.1.2), QSAR Toolbox (ver. 4.6), and ProTox-II browser application were used to predict the median lethal dose. The Simplified Molecular Input Line Entry Specification (SMILES) was the input data source. The results indicate that the most deadly V-agents were VX and VM, followed by structural VX analogues: RVX and CVX. The least toxic turned out to be V-sub x and Substance 100A. In silico methods for predicting various parameters are crucial for filling data gaps ahead of experimental research and preparing for the upcoming use of nerve agents.

The suitability of a polydimethylsiloxane-based (PDMS) microfluidic two compartment system for the toxicokinetic analysis of organophosphorus compounds

Organ-on-a-chip platforms are an emerging technology in experimental and regulatory toxicology (species-specific differences, ethical considerations). They address gaps between in vivo and in vitro models. However, there are still certain limitations considering material, setup and applicability. The current study examined the suitability of a commercially available polydimethylsiloxane-based (PDMS) organ-chip for the toxicokinetic characterization of the highly toxic nerve agent VX and the organophosphate pesticide parathion. The respective concentrations of 1000 µmol/L and 100 µmol/L VX and parathion were chosen deliberately high in order to study concentrations even if high compound absorption by PDMS might occur. Neuronal and liver spheroids, totaling 2 × 106 cells were used to study concentration changes of VX and parathion. In addition, VX enantiomers were quantified. The current study suggests a significant absorption of VX, respectively parathion by PDMS. This might require future investigation of alternative materials or coatings to limit absorption for organophosphorus compounds in toxicokinetic studies.

"Catch-up" therapy: combining antidotal treatment with dermal application of AHA following percutaneous VX poisoning in the domestic swine

Low-volatility organophosphorus chemical warfare agents (OP CWAs) are cholinesterase inhibitors which easily absorb into the skin, leading to the formation of a dermal depot from which they slowly enter the bloodstream. This leads to sustained cholinergic hyperstimulation, which if untreated may lead to death. However, current available countermeasures are not adequate to neutralize the agent residing in the dermal depot. Accordingly, we evaluated the efficacy of the potassium salt of acetohydroxamic acid (880 mg/ml in DMSO/H2O 1:4, AHAK), as a potential "catch-up" therapy lotion intended to neutralize the dermal depot, by penetrating the skin and decomposing it before it reaches the bloodstream. To that end, we compared the clinical outcome following skin surface decontamination combined with antidotal treatment, to that following the same antidotal treatment combined with dermal application of AHAK at the site of VX exposure, against percutaneous poisoning by a lethal neat dose (4 mg/kg) of the low-volatility nerve agent VX, in an unanesthetized swine model. Following skin surface decontamination and antidotal treatment, recurrence of intoxication signs and a prolonged recovery time were observed. In contrast, similar antidotal treatment combined with dermal application of AHAK significantly reduced intoxication signs recurrences and accordingly medical supervision duration needed, paralleled by a significantly faster recovery of whole blood cholinesterase activity. An initial evaluation demonstrated the safety of prolonged whole-body AHAK application. Hence, the AHAK lotion may act as an efficient "catch-up" therapy against percutaneous poisoning by low-volatility OP CWAs, improving the clinical outcome and reducing the burden on medical staff.

The effectiveness of vitamin C on quinalphos ileal toxicity: a study of histological, ultrastructural, and oxidative stress markers

There is a significant hazard of human exposure to the organophosphates which is a constant threat, and they are responsible for numerous cases of poisoning and mammalian toxicity annually in non-target wildlife. The antioxidants, including the vitamin C (Vit C), have a protective effect on some organophosphorus compounds-induced organ damage. Quinalphos (QP) is one of these compounds. The investigation's objective is to see if there was any effect of QP on the rat ileum which could be rectified by using Vit C. Three groups of 24 animals were created. As a control, the first group was given pure water. Second group subjected to oral gavages of QPs. Third group rats were given oral gavages of Vit C plus QPs for 10 days. The reaction of ileal enterocytes to food-borne QPs was marked by poorly organized microvilli, numerous vacuoles within them, disrupted nuclei with chromatin margination, disoriented mitochondria, and an expanded intercellular space. The absorptive columnar cell illustrated many vacuoles inside with herniation of microvilli, and normal goblet cells were also seen. Many Paneth cells towards the lumen of intestinal gland contained secretory granules of different sizes and shapes. The histological architecture of the ileal mucosa in the QP plus Vit C group was found to be close to those of healthy controls. The outcomes of this study suggest that administering Vit C in rats treated with QPs protects them from ill dysfunction caused by QP.

Acute and long-term effects of VX in rat brain cell aggregate culture

The contact poison VX (O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate) is a chemical warfare agent that is one of the most toxic organophosphorus compounds known. Its primary mechanism of toxic action is through the inhibition of acetylcholinesterase and resultant respiratory paralysis. The majority of work on VX has thus concentrated on its potent anticholinesterase activity and acute toxicity, with few studies investigating potential long-term effects. In this report we describe the effects of VX in aggregating rat brain cell cultures out to 28 days post-exposure. Cholinesterase activity was rapidly inhibited (60 min IC50 = 0.73 +/- 0.27 nM), but recovered towards baseline values over the next four weeks. Apoptotic cell death, as measured using caspase-3 activity was evident only at 100 μM concentrations. Cell type specific enzymatic markers (glutamine synthase, choline acetyltransferase and 2',3'-cyclic nucleotide 3'-phosphodiesterase) showed no significant changes. Total Akt levels were unchanged, while an increased phosphorylation of this protein was noted only at the highest VX concentration on the first day post-exposure. In contrast, significant and delayed (28 days post-exposure) decreases were noted in vascular endothelial growth factor (VEGF) levels, a protein whose reduced levels are known to contribute to neurodegenerative disorders. These observations may indicate that the long-term effects noted in some survivors of nerve agent intoxication may be due to VX-induced declines in brain VEGF levels.

Protective effects of pretreatment with Fe2+, Cu2+, and Rb+ on phoxim poisoning in silkworm, Bombyx mori

BACKGROUND

Phoxim is a widely used organophosphorus pesticide in agriculture. People are paying more and more attention to its toxicity. At present, there is no appropriate way to solve the phoxim poisoning of silkworm, which severely affected the development of sericulture. Fe²⁺, Cu²⁺, Rb⁺ exerted their biological effects through various forms in vivo.

METHODS

To evaluate the effect of Fe²⁺/Cu²⁺/Rb⁺ on phoxim poisoning in silkworm, Bombyx mori were treated with fresh mulberry leaves soaked in 2.5 mg/L phoxim for 2 min with 50 mg/L FeCl₂, 150 mg/L CuCl₂, or 0.5 mg/L RbCl from 5 days of the fifth-instar silkworm.

RESULTS

Fe²⁺, Cu²⁺, and Rb⁺ pretreatments significantly inhibited the phoxim-induced reduction of survival rate and alleviated the phoxim-induced poisoning symptoms. The protective effects of Fe²⁺, Cu²⁺, and Rb⁺ on phoxim poisoning might be due to their enhancement of superoxide dismutase (SOD), catalase (CAT), and carboxylesterase (CarE) in the hemolymph and fat body of silkworm. This enhancement might reduce reactive oxygen species (ROS) accumulation and oxidative stress (OS) caused by phoxim poisoning. Thereby it reduced the damage to silkworm tissues and cells.

CONCLUSIONS

These results showed that Fe²⁺, Cu²⁺, and Rb⁺ treatments protected the silkworm from phoxim poisoning by directly enhancing the activity of SOD, CAT, and CarE enzymes and reducing oxidative stress, but not dependent on the high expression of CYP genes. The use of Fe²⁺, Cu²⁺, and Rb⁺ to enhance the activity of SOD, CAT, and CarE enzymes may be an underlying effective way to solve phoxim poisoning in the silkworm industry.

A delayed treatment model for the evaluation of scopolamine for VX nerve agent intoxication

Most research on medical countermeasures for nerve agent exposure assumes a military scenario, in which (autoinjector) treatment is envisaged to be available immediately. In a civilian setting however, treatment is delayed until arrival of first-aid responders. This may significantly affect treatment efficacy and the requirements for secondary intensive care. The aim of the current study was to develop a guinea pig model to evaluate the efficacy of delayed treatment following nerve agent exposure. We identified a trigger-to-treat based on a progressive stage of the toxidrome following VX exposure, which was associated with the subsiding of clonic movements. This paradigm resulted in treatment consistently being administered between 15 and 25 min post-exposure. Using the model, we investigated the potential for the anticholinergic scopolamine to act as a delayed treatment either as a standalone treatment, or as an adjunct to delayed treatment with Standard of Care (SOC), containing atropine, 2-PAM, and midazolam. The study provides a framework for a small animal model for evaluating the efficacy of treatment administered at a specific stage of the toxidrome, when immediate treatment is absent. As an adjunct, scopolamine treatment did not result in improved survival, but did show a beneficial effect on recovery, in terms of general posture. As a standalone treatment, scopolamine showed a significant, dose-responsive, beneficial effect on survival and recovery. These promising results warrant additional studies to investigate which observed physiological improvements are relevant for the recovery process and residual injury.

Efficacy of atropine and scopolamine on airway contractions following exposure to the nerve agent VX

Nerve agents are highly toxic organophosphorus compounds that inhibit acetylcholinesterase resulting in rapid accumulation of the neurotransmitter acetylcholine (ACh) causing a cholinergic syndrome including respiratory failure. In the present study, respiratory responses and antimuscarinic treatment efficacy was evaluated ex vivo using rat precision-cut lung slices (PCLS) exposed to the nerve agent VX. The respiratory effects were evaluated either by adding exogenous ACh directly to the culture medium or by applying electric-field stimulation (EFS) to the PCLS to achieve a release of endogenous ACh from neurons in the lung tissue. The airway contraction induced by both methods was enhanced by VX and resulted in lingering airway recovery, in particular when airways were exposed to a high VX-dose. Both contractions induced by EFS and exogenously added ACh were significantly reduced by administration of the antimuscarinic drugs atropine or scopolamine. Two additions of atropine or scopolamine after maximal ACh-induced airway response was demonstrated effective to reverse the contraction. By adding consecutive doubled doses of antimuscarinics, high efficiency to reduce the cholinergic airway response was observed. However, the airways were not completely recovered by atropine or scopolamine, indicating that non-muscarinic mechanisms were involved in the smooth muscle contractions. In conclusion, it was demonstrated that antimuscarinic treatment reversed airway contraction induced by VX but supplemental pharmacological interventions are needed to fully recover the airways. Further studies should therefore clarify the mechanisms of physiological responses in lung tissue following nerve agent exposures to improve the medical management of poisoned individuals.

Effect of Terbufos (Organophosphate) on the Cadaveric Colonization Process: Implications for Postmortem Interval Calculation

The determination of necrophagous fly specie's development time is considered an accurate method for estimating postmortem interval (PMI). However, pesticides and other chemicals can alter the flies' life cycle, inducing errors in PMI estimation. Thus, this work aimed to evaluate the effect of different doses of Terbufos (Organophosphates) on the temporal dispersion pattern and development of immature dipterans associated with decaying rat carcasses. For this, 150 g female Wistar rats received, via gavage, 200 µl of Terbufos (5 or 10 mg/kg) or distilled water (control) and, after 30 min of administration, the animals were euthanized and distributed in suspended traps to decompose under environmental conditions. The dispersing immatures were collected daily, and their development time was monitored until the emergence of adult flies. After data analysis, it was observed that Terbufos altered 1) the temporal pattern of larval dispersion; 2) the composition and structure of the colonizing assemblage (emerged adults); 3) species' development time, accelerating or delaying their cycle, depending on the dose used; and 4) the califorids and sarcophagids emergence rate, increasing the mortality of pupae from intoxicated carcasses. Thus, this work demonstrates experimentally that Terbufos directly influences the development of flies with forensic potential and discusses the implications for PMI estimation, which can assist in future investigative processes with suspected poisoning by this organophosphate.

Evaluation of Sensitivity to Phoxim and Cypermethrin in an Endoparasitoid, Meteorus pulchricornis (Wesmael) (Hymenoptera: Braconidae), and Its Parasitization Efficiency Under Insecticide Stress

Insecticides can have consequences for beneficial arthropods. Insect parasitoids can contact insecticides through direct exposure spray droplets or residues on crop foliage. Here, we focus on better understand the response of Meteorus pulchricornis (Wesmael), a parasitoid wasp of lepidopteran pests, and its detoxification mechanisms on stress caused by phoxim and cypermethrin. Hence, we determined the dose-mortality curves and estimating the sublethal concentrations (LC30 and LC50). Then, we applied the sublethal concentrations against adult parasitoids to assess its survival, parasitism efficacy, and also developmental and morphometric parameters of their offspring. Simultaneously, we check the activities of glutathione S-transferase (GST), acetylcholinesterase (AChE), and peroxidase (POD) after sublethal exposure of both insecticides, which has measured until 48 h after treatment. Overall, phoxim and cypermethrin exhibited acute lethal activity toward the parasitoid with LC50 values 4.608 and 8.570 mg/liter, respectively. Also, we detect that LC30 was able to trigger the enzymatic activity of GST, AChE, and POD, suggesting a potential detoxification mechanism. However, even when subjected to sublethal exposure, our results indicate strong negatives effects, in particular for phoxim, which has affected the parasitism efficacy and also the developmental and morphometric parameters of M. pulchricornis offspring. Therefore, it can be concluded that both phoxim and cypermethrin have negative impacts on M. pulchricornis and we suggest cautioning their use and the need for semifield and field assessments to confirm such an impact.

Simultaneous degradation of triazophos, methamidophos and carbofuran pesticides in wastewater using an Enterobacter bacterial bioreactor and analysis of toxicity and biosafety

Triazophos (TAP), methamidophos (MAP) and carbofuran (CF) pesticides are highly toxic, soluble and absorbable. Efficient co-degradation of multi-pesticides is rare reported. The objectives of this study were to investigate TAP, MAP and CF co-degradative ability of Enterobacter sp. Z1 and study the degradation mechanisms. Strain Z1 was shown to efficiently co-degrade TAP, MAP and CF when they were used as primary carbon sources. The degradation occurred over a wide range of temperatures, pH values and pesticide concentrations and followed first-order kinetics. Under the optimum conditions (37 °C, pH 7 and 100 mg/L of each pesticide), the degradation efficiencies were 100%, 100%, and 95.3% for TAP, MAP and CF, respectively. In addition, strain Z1 could simultaneously degrade TAP, MAP, CF and total nitrogen in wastewater in a batch bioreactor, with high removal efficiencies of 98.3%, 100%, 98.7% and 100%, respectively. Genomics, proteomics, qRT-PCR and gene overexpression analyses revealed that the degradation mechanisms involved the activities of multiple proteins, among which, organophosphorus hydrolase (Oph) and 3-hydroxyacyl-CoA dehydrogenase (PaaC) are primarily responsible for TAP and MAP degradation, while carbofuran hydrolase (Mcd) and amidohydrolase (RamA) primarily degrade CF. Among these enzymes, PaaC and RamA are newly identified pesticide-degrading enzymes. Toxicity assays of strain Z1 using reporter recombinase gene (recA) and zebrafish showed that there was no accumulation of toxic metabolites during the degradation process. Biosafety test using zebrafish showed that the strain was nontoxic toward zebrafish. Strain Z1 provides a good purification effect for pesticides-containing wastewater and novel microbial pesticide-degrading mechanisms were discovered.

Pharmacokinetics and efficacy of atropine sulfate/obidoxime chloride co-formulation against VX in a guinea pig model

Nerve agent exposure is generally treated by an antidote formulation composed of a muscarinic antagonist, atropine sulfate (ATR), and a reactivator of acetylcholinesterase (AChE) such as pralidoxime, obidoxime (OBI), methoxime, trimedoxime or HI-6 and an anticonvulsant. Organophosphates (OPs) irreversibly inhibit AChE, the enzyme responsible for termination of acetylcholine signal transduction. Inhibition of AChE leads to overstimulation of the central and peripheral nervous system with convulsive seizures, respiratory distress and death as result. The present study evaluated the efficacy and pharmacokinetics (PK) of ATR/OBI following exposure to two different VX dose levels. The PK of ATR and OBI administered either as a single drug, combined treatment but separately injected, or administered as the ATR/OBI co-formulation, was determined in plasma of naïve guinea pigs and found to be similar for all formulations. Following subcutaneous VX exposure, ATR/OBI-treated animals showed significant improvement in survival rate and progression of clinical signs compared to untreated animals. Moreover, AChE activity after VX exposure in both blood and brain tissue was significantly higher in ATR/OBI-treated animals compared to vehicle-treated control. In conclusion, ATR/OBI has been proven to be efficacious against exposure to VX and there were no PK interactions between ATR and OBI when administered as a co-formulation.

Assessment of lethal and sublethal effects of imidacloprid, ethion, and glyphosate on aversive conditioning, motility, and lifespan in honey bees (Apis mellifera L.)

Honeybees (Apis mellifera) play an important role in agriculture worldwide. Several factors including agrochemicals can affect honey bee health including habitat fragmentation, pesticide application, and pests. The growing human population and subsequent increasing crop production have led to widespread use of agrochemicals and there is growing concern that pollinators are being negatively impacted by these pesticides. The present study compares acute exposure to imidacloprid (0.2 and 0.4 mgL-1), ethion (80 and 106.7 mgL-1) or glyphosate (0.12 and 0.24 mgL-1) on aversive learning and movement, to chronic exposure at these and higher concentrations on movement, circadian rhythms, and survival in honey bee foragers. For acute learning studies, a blue/yellow shuttle box experiment was conducted; we observed honey bee choice following aversive and neutral stimuli. In learning studies, control bees spent >50% of the time on yellow which is not consistent with previous color bias literature in the subspecies or region of the study. The learning apparatus was also used to estimate mobility effects within 20 min of exposure. Chronic exposure (up to 2 weeks) with the above metrics was recorded by an automated monitoring system. In chronic exposure experiments, RoundUp®, was also tested to compare to its active ingredient, glyphosate. We found that imidacloprid and ethion have negative impacts on aversive learning and movement following a single-dose and that chronic exposure effects were dose-dependent for these two insecticides. In contrast, glyphosate had no effect on learning and less of an effect on movement; RoundUp® showed dose-dependent results on circadian rhythmicity. Overall, the results suggest that short-term exposure to imidacloprid and ethion adversely affect honey bee foragers and chronic exposure to glyphosate may affect pollination success.

Transcriptome analysis of Penaeus vannamei hepatopancreas reveals differences in toxicity mechanisms between phoxim and prometryne

Due to overuse and terrestrial input, there are large quantities of phoxim and prometryne residues in some aquatic environments. In the present study, the effects of these compounds on Penaeus vannamei hepatopancreas were analysed at the transcriptome level to investigate toxicity in this nontarget aquaculture organism. Twelve normalised cDNA libraries were constructed using RNA from phoxim and prometryne treatment groups, and an untreated control group. A total of 667,750,902 clean reads were obtained. Analysis of differentially expressed genes (DEGs) identified 449 in control vs phoxim groups, 185 in control vs prometryne groups, and 183 in prometryne vs phoxim groups. In the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, arachidonic acid metabolism, pancreatic secretion, linoleic acid metabolism, and beta-alanine metabolism pathways were significantly enriched in control vs phoxim groups. In control vs prometryne groups, lysosome, pentose and glucuronate interconversion, antigen processing and presentation, and glycosaminoglycan degradation pathways were significantly enriched. In prometryne vs phoxim groups, protein digestion and absorption, extracellular matrix (ECM)-receptor interaction, PI3K-Akt signalling, cell adhesion molecule (CAM), AGE-RAGE signalling related to diabetic complications, focal adhesion, and renin secretion pathways were significantly enriched. In further detailed analysis, glutathione S-transferase (GST), glutathione peroxidase and basic phospholipase A2 were downregulated in the phoxim treatment group, indicating that phoxim damaged hepatopancreas. Upregulation of phospholipase A2 (secretory phospholipase A2-like) indicates possible inflammatory pathological injury to hepatopancreas caused by phoxim. Meanwhile, downregulation of CD63 indicates that prometryne affect the immune system.

Enantioselective disposition and metabolic products of isofenphos-methyl in rats and the hepatotoxic effects

Isofenphos-methyl (IFP), a chiral organophosphorus pesticide, is one of the main chemicals used to control underground insects and nematodes. Recently, the use of IFP on vegetables and fruits has been prohibited due to its high toxicity. In this study, we investigated the enantioselective distribution and metabolism of IFP and its metabolites, namely, isofenphos-methyl oxon (IFPO) and isocarbophos oxon (ICPO), in male Sprague Dawley (SD) rats. Forty eight hours (48 h) after exposure, ICPO was the main detectable compound in blood (up to 75%) and urine (up to 77%), and we found that (S)-ICPO was significantly more stable than (R)-ICPO (p < 0.05). Therefore, (S)-ICPO was proposed as a suitable candidate biomarker for the biomonitoring of IFP in human urine and blood. After 48 h exposure, 21.2-41.0%, 4.1-15.1%, and 8.6-18.7% of dosed IFP was detected in the liver of racemic, R and S enantiomer-exposed rats, respectively, and R-IFP and R-IFPO showed a faster degradation (p < 0.05). Our results showed that after one week of consecutive exposure to IFP, ICPO was accumulated in the liver of rats in both racemic and enantiopure groups (no difference between the groups, p > 0.05). We found that cytochrome P450 (CYP) (i.e. CYP2C11, CYP2D2 and CYP3A2 enzymes and carboxylesterases) is responsible for the enantioselective metabolism of IFP in liver. In addition, rats exposed to (S)-IFP exhibited hepatic lipid peroxidation, liver inflammation and hepatic fibrosis. This study provides useful information and a reference for the biomonitoring and risk assessment of IFP and organophosphorus pesticide exposure.

Side effects of chlorantraniliprole, phosalone and spinosad on the egg parasitoid, Trichogramma brassicae

The combined use of chemicals and biological control is not always a successful strategy owing to the potential side effects on biocontrol agents. Lethal and sublethal effects of three commonly used insecticides were assessed on adult and immature stages of the egg parasitoid Trichogramma brassicae Bezdenko (Hymenoptera: Trichogrammatidae). Recommended field concentrations of chlorantraniliprole, phosalone and spinosad caused mortality on preimaginal stages by 24, 87, and 98%, respectively. Lethal effects on parasitoid adults exposed to the insecticide dry residues were estimated as median lethal concentrations (LC₅₀) that were 13.28, 0.25, and 0.03 µg a.i. ml^-1 for chlorantraniliprole, phosalone, and spinosad, respectively. The effect of a low lethal concentration (LC₃₀) of the compounds was evaluated on various adult biological traits, such as longevity, fecundity, emergence rate and other life table parameters. All compounds caused detrimental effects on all the estimated demographical indexes. Chlorantraniliprole affected the net reproductive rate, mean generation time and doubling time in comparison to the control; while, phosalone and spinosad adversely affected all assessed parameters. Phosalone and spinosad significantly reduced gross reproductive rate, net reproductive rate, intrinsic rate of increase, finite rate of increase, mean generation time and doubling time and reduced longevity, fecundity, emergence rate related to other biological parameters in comparison with control. The results suggest that all compounds are not fully compatible with the activity of T. brassicae, and that the inclusion of chlorantraniprole, spinosad and phosalone into Integrated Pest Management (IPM) involving this parasitoid has to be avoided. Nevertheless, further studies in open field conditions and on a multiple generation scale are necessary for providing a more definitive conclusion on the IPM suitability of the three tested insectcides.

Elucidating the potential neurotoxicity of chiral phenthoate: Molecular insight from experimental and computational studies

Chiral organophosphorus pollutants are existed ubiquitously in the ecological environment, but the enantioselective toxicities of these nerve agents to humans and their molecular bases have not been fully elucidated. Using experimental and computational approaches, this story was to explore the neurotoxic response process of the target acetylcholinesterase (AChE) to chiral phenthoate and further decipher the microscopic mechanism of such toxicological effect at the enantiomeric level. The results showed that the toxic reaction of AChE with chiral phenthoate exhibited significant enantioselectivity, and (R)-phenthoate (K＝1.486 × 10⁵ M^-1) has a bioaffinity for the nerve enzyme nearly three times that of (S)-phenthoate (K＝4.503 × 10⁴ M^-1). Dynamic research outcomes interpreted the wet experiments, and the inherent conformational flexibility of the target enzyme has a great influence on the enantioselective neurotoxicological action processes, especially reflected in the conformational changes of the three key loop regions (i.e. residues His-447, Gly-448, and Tyr-449; residues Gly-122, Phe-123, and Tyr-124; and residues Thr-75, Leu-76, and Tyr-77) around the reaction patch. This was supported by the quantitative results of conformational studies derived from circular dichroism spectroscopy (α-helix: 34.7%→30.2%/31.6%; β-sheet: 23.6%→19.5%/20.7%; turn: 19.2%→22.4%/21.9%; and random coil: 22.5%→27.9%/25.8%). Meanwhile, via analyzing the modes of toxic action and free energies, we can find that (R)-phenthoate has a strong inhibitory effect on the enzymatic activity of AChE, as compared with (S)-phenthoate, and electrostatic energy (-23.79/－17.77 kJ mol^-1) played a critical role in toxicological reactions. These points were the underlying causes of chiral phenthoate displaying different degrees of enantioselective neurotoxicity.

Effect of Agrochemical Exposure on Schistosoma mansoni Cercariae Survival and Activity

Singular use of activity assays or staining dyes to assess pathogen agrochemical tolerance can underestimate tolerance if pesticides cause sublethal effects. We exposed Schistosoma mansoni cercariae, the aquatic life stage of this trematode that infects humans, to 4 insecticides at 5 concentrations using a 24-h time-to-death assay. We used Trypan blue dye, which stains dead tissue, and activity assays simultaneously to discriminate dead from live but paralyzed individuals. Whereas cypermethrin, deltamethrin, and dimethoate exposure did not affect cercariae at any ecologically relevant concentrations, methamidophos exposure increased survival of cercariae compared with those in the controls. This was because methamidophos-induced paralysis reduced cercarial activity and thus energy expenditures, extending the lifespan of this short-lived parasite that causes human schistosomiasis. These findings highlight that sublethal effects should be considered when pesticide effects on disease are under investigation. Environ Toxicol Chem 2020;39:1421-1428. © 2020 SETAC.

Structural and kinetic evidence of aging after organophosphate inhibition of human Cathepsin A

Human Cathepsin A (CatA) is a lysosomal serine carboxypeptidase of the renin-angiotensin system (RAS) and is structurally similar to acetylcholinesterase (AChE). CatA can remove the C-terminal amino acids of endothelin I, angiotensin I, Substance P, oxytocin, and bradykinin, and can deamidate neurokinin A. Proteomic studies identified CatA and its homologue, SCPEP1, as potential targets of organophosphates (OP). CatA could be stably inhibited by low µM to high nM concentrations of racemic sarin (GB), soman (GD), cyclosarin (GF), VX, and VR within minutes to hours at pH 7. Cyclosarin was the most potent with a kinetically measured dissociation constant (K_I) of 2 µM followed by VR (K_I = 2.8 µM). Bimolecular rate constants for inhibition by cyclosarin and VR were 1.3 × 10³ M^-1sec^-1 and 1.2 × 10³ M^-1sec^-1, respectively, and were approximately 3-orders of magnitude lower than those of human AChE indicating slower reactivity. Notably, both AChE and CatA bound diisopropylfluorophosphate (DFP) comparably and had K_I^DFP = 13 µM and 11 µM, respectively. At low pH, greater than 85% of the enzyme spontaneously reactivated after OP inhibition, conditions under which OP-adducts of cholinesterases irreversibly age. At pH 6.5 CatA remained stably inhibited by GB and GF and <10% of the enzyme spontaneously reactivated after 200 h. A crystal structure of DFP-inhibited CatA was determined and contained an aged adduct. Similar to AChE, CatA appears to have a "backdoor" for product release. CatA has not been shown previously to age. These results may have implications for: OP-associated inflammation; cardiovascular effects; and the dysregulation of RAS enzymes by OP.

Comparative examination on synergistic toxicities of chlorpyrifos, acephate, or tetraconazole mixed with pyrethroid insecticides to honey bees (Apis mellifera L.)

Potential synergistic toxicity of pesticide mixtures has increasingly become a concern to the health of crop pollinators. The toxicities of individual and mixture of chlorpyrifos (CHL), acephate (ACE), or tetraconazole (TET) with nine pyrethroid insecticides to honey bees (Apis mellifera L.) were evaluated to reveal any aggregated interaction between pesticides. Results from feeding toxicity tests of individual pesticides indicated that organophosphate insecticides CHL and ACE had higher toxicities to honey bees compared to nine pyrethroids. Moreover, different pyrethroids exhibited considerable variation in toxicity with LC₅₀ values ranging from 10.05 (8.60-11.69) to 1125 (922.4-1442) mg a.i. L^-1 after exposure for 7 days. Among the 12 examined pesticides, a relatively low toxicity to A. mellifera was detected from the fungicide TET. All the binary mixtures of ACE or TET in combination with pyrethroids exhibited synergistic effects. However, TET in combination with pyrethroids showed greater synergistic toxicity to A. mellifera than ACE in combination with pyrethroids. Approximately 50% binary mixtures of CHL in combination with pyrethroids also showed synergistic responses in honey bees. In particular, CHL, ACE, or TET in combination with either lambda-cyhalothrin (LCY) or bifenthrin (BIF) showed the strongest synergy in A. mellifera, followed by CHL, ACE, or TET in combination with either zeta-cypermethrin (ZCY) or cypermethrin (CYP). The findings indicated that the co-exposure of various pesticides in natural settings might lead to severe injury to crop pollinators. Therefore, pesticide mixtures should be applied carefully in order to minimize negative effects on honey bees while maintaining effective management against crop pests.

Impairment of testicular development in rats exposed to acephate during maternal gestation and lactation

Acephate is an organophosphate insecticide that disrupts the endocrine system and impairs the male reproductive system. Thus, the aim of the present study was to evaluate whether exposure to acephate during maternal gestation and lactation histologically damages the testes of male Wistar rats in adulthood. For this study, adult Wistar rats were divided into the following groups: ACE-mother, (2.5 mg/kg/bw, gestational day (GD) 7 to postnatal day (PND) 21) and oil-mother (corn oil (control), GD 7 to PND 21). The male offspring (PND 90) were euthanized, and the prostates and testes were collected and weighed. The testes were utilized for histopathological analyses and to determine the sperm count. A spermatogenesis kinetic analysis revealed an increased number of seminiferous tubules in stages I-VI in the ACE-mother group. Additionally, we observed a decrease in the epithelium and the diameters of the evaluated seminiferous tubules and in the number of Sertoli cells in the group exposed to acephate. The sperm count analysis showed no difference between the groups. We conclude that maternal exposure to the pesticide acephate did not affect testicular function, but led to the impairment of testicular development and morphology of the tissue in adulthood.

Effects of phoxim exposure on gut microbial composition in the silkworm, Bombyx mori

Organophosphate pesticides are widely applied worldwide for agricultural purposes, and their exposures often result in adverse effects on Bombyx mori. The insect gut is a complicated ecosystem inhabited by a large number of microbes that play important roles in insect physiology and behavior. Recent studies have reported that alteration of their microbiota due to stressful conditions or environmental changes has been linked to a compromised health status and a susceptibility to diseases. In the present study, we aimed to assess the effects of phoxim exposure on intestinal microbes in silkworms. The results showed that phoxim exposure increased the bacterial community evenness and altered the structure of gut microbiota in silkworm larvae. The abundances of several genera, such as Methylobacterium and Aurantimonadaceae, in phoxim-treated larval guts were significantly reduced compared with the H₂O-treated group, whereas the abundances of non-dominant bacteria, such as Staphylococcus, were significantly increased. Moreover, phoxim inhibited the expressions of antimicrobial peptides (AMPs) at the mRNA level and enhanced the pathogenesis of Enterobacter cloacae (E. cloacae) against silkworm larvae, suggesting that the immune system was inhibited after phoxim exposure. Therefore, the gut microbial community shifts were apparent after phoxim exposure. The compositional and structural changes of intestinal microbes caused by phoxim exposure might affect the normal function of the intestinal tract of silkworm. These results highlighted the importance of the gut bacterial community when investigating the mechanisms of midgut injury after pesticide exposure in Bombyx mori.

Evaluation of high-affinity phenyltetrahydroisoquinoline aldoximes, linked through anti-triazoles, as reactivators of phosphylated cholinesterases

Acetylcholinesterase (AChE) is a pivotal enzyme in neurotransmission. Its inhibition leads to cholinergic crises and could ultimately result in death. A related enzyme, butyrylcholinesterase (BChE), may act in the CNS as a co-regulator in terminating nerve impulses and is a natural plasma scavenger upon exposure to organophosphate (OP) nerve agents that irreversibly inhibit both enzymes. With the aim of improving reactivation of cholinesterases phosphylated by nerve agents sarin, VX, cyclosarin, and tabun, ten phenyltetrahydroisoquinoline (PIQ) aldoximes were synthesized by Huisgen 1,3 dipolar cycloaddition between alkyne- and azide-building blocks. The PIQ moiety may serve as a peripheral site anchor positioning the aldoxime moiety at the AChE active site. In terms of evaluated dissociation inhibition constants, the aldoximes could be characterized as high-affinity ligands. Nevertheless, high binding affinity of these oximes to AChE or its phosphylated conjugates did not assure rapid and selective AChE reactivation. Rather, potential reactivators of phosphylated BChE, with its enlarged acyl pocket, were identified, especially in case of cyclosarin, where the reactivation rates of the lead reactivator was 100- and 6-times that of 2-PAM and HI-6, respectively. Nevertheless, the return of the enzyme activity was affected by the nerve agent conjugated to catalytic serine, which highlights the lack of the universality of reactivators with respect to both the target enzyme and OP structure.

Does the exposure of parental female adults of the invasive Trogoderma granarium Everts to pirimiphos-methyl on concrete affect the morphology of their adult progeny? A geometric morphometric approach

Insecticidal applications may result to morphological deformations upon exposed insects or their offspring production. In the present study, we tested whether pirimiphos-methyl can induce deformities to wings of progeny production of the invasive khapra beetle, Trogoderma granarium (Coleoptera: Dermestidae) when its parental female adults have been treated with this organophosphorus active ingredient. For that purpose, we analysed both elytra and hindwings of both sexes of T. granarium progeny production by using the geometric morphometrics method. Our results showed that the wings of progeny of the pirimiphos-methyl-treated T. granarium parental female adult individuals suffered certain changes in their usual shape depending on size. Deformations occurred on both pairs of wings, but changes were more noticeable on the hindwings. A longer than 5-h exposure of parental female adults to pirimiphos-methyl, resulted in progeny with more deformed wings than in those individuals emerged after the exposure of their parental female adults in shorter periods on the toxicant. Generally, wings of both sexes were sensitive to pirimiphos-methyl, distinguishing the control group from the insecticidal treatments. The existence of deformed adults could be a useful indicator of earlier insecticidal applications as surface treatments and/or grain protectants in the storage facilities.

Role of biomarkers in the evaluation of cadmium and ethoprophos combination in male mice

This work aimed to study the risk assessment procedures of a combination of single and repeated dose of the widely used pesticide, ethoprophos (Etho) and heavy metal cadmium (Cd), on the hematological, biochemical, reproduction and cytogenetic parameters in male mice. The results revealed that the sub-lethal dose (1/50 LD₅₀) of the tested toxic substances (Etho and/or Cd) reduced the body and organ weights, the most hematological profile and the activity of acetylcholine esterase (AChE). The tested pollutants significantly increased the parameters of liver function, including aspartate aminotransferase (AST) and alanine aminotransferase (ALT), as well as renal function tests, including creatinine and urea. In addition, they have deleterious effects on reproductive function tests by stimulating the number of sperm abnormalities (SA) and cytogenetic assays by increasing the frequency of chromosomal abnormalities (CA) and the mitotic index (MI). The overall results of this exploratory study suggest that the co-existence of the two tested compounds (Etho and Cd) had the propensity to cause a more pronounced effect than that of each compound alone on all the battery measured biomarkers, especially in the repeated treatment (14 doses) than that in the single one. Also, the combination of a range of simple and sensitive assays as endpoints gives a comprehensive picture and provides better insights to evaluate the potential effects of other commonly encountered environmental pollutants.

Combination of chemical and toxicological methods to assess bioavailability of Tolclofos-methyl by earthworms

Tolclofos-methyl (TM) is an organophosphorus fungicide and widely utilized to control soil-borne diseases. However, toxic effects of TM on terrestrial invertebrates are still unknown. Here we measured the bioaccumulation of TM in earthworms (Eisenia fetida) to assess its environmental bioavailability. Mortality, weight change, and oxidative damage of earthworms were determined to investigate the toxicological bioavailability of TM. ROS, SOD and MDA in highest concentration treatment group significantly increased compared to the control group, suggesting that hazardous effects of TM to earthworms were caused by the oxidative stress. To further examine its toxicological bioavailability, cytotoxicity test was carried out by using extracted earthworm coelomocytes. The biomarkers, e.g., intracellular ROS, extracellular LDH, and cell viability showed correlation with TM in the culture media, demonstrating that cytotoxicity test could be employed to reflect the toxicological bioavailability of pollutants to earthworms or other organisms.

Apigenin attenuates edifenphos-induced toxicity by modulating ROS-mediated oxidative stress, mitochondrial dysfunction and caspase signal pathway in rat liver and kidney

Edifenphos (EDF) (O-ethyl-S, S-diphenyldithiophosphate) is an organophosphate pesticide that is extensively used as a fungicide in agricultural rice fields. However, EDF accumulated in various agricultural products and caused potential health hazards to human and other living organisms. Therefore, the present study was investigated to evaluate the ameliorative role of apigenin (APG); a natural antioxidant against EDF-induced hepato-renal toxicity in rats. Six groups with five male Wistar rats each, were used for this purpose; these groups included the control group (A) that received corn oil; (B) 10 mg/kg APG; (C) 10 mg/kg EDF; (D) 25 mg/kg EDF; (E) 10 mg/kg APG pretreatment for 1 h then 10 mg/kg EDF; (F) 10 mg/kg APG pretreatment for 1 h then 25 mg/kg EDF for 14 consecutive days. Oral administration of EDF led to disruption of the intracellular antioxidant machinery which cause the generation of intracellular reactive oxygen species (ROS). However, EDF promotes deleterious effects like oxidative stress, DNA damage, reduced mitochondrial membrane potential, generation of ROS production, activation of caspase 3/9 activities and causing hepato-renal histopathological changes. However, the pretreatment of APG ameliorated the EDF-induced oxidative damage and apoptosis, through their antioxidant activity or by directly scavenging free radical property. Overall, these results suggest that EDF exerts oxidative stress, and APG could be a potent dietary anti-oxidant regimen against EDF-induced toxicity.

Metabolic mechanisms and acetylcholinesterase sensitivity involved in tolerance to chlorpyrifos-ethyl in the earwig Forficula auricularia

Apple orchards are highly treated crops, in which organophosphorus (OP) are among the most heavily sprayed insecticides. These pesticides are toxic to non-target arthropods and their repeated use increases the risk of resistance. We studied mechanisms involved in tolerance and resistance to OP insecticides in the earwig Forficula auricularia, an effective generalist predator in pomefruit orchards. Adult earwigs were sampled in three apple orchards managed under contrasting strategies: conventional, Integrated Pest Management, and organic. The threshold activities of enzyme families involved in pesticides tolerance: Glutathione-S-transferases (GSTs) and Carboxylesterases (CbEs) were measured in earwig extracts. Acetylcholinesterase (AChE) was monitored as a toxicological endpoint. Variations in these activities were assessed prior to and after exposure to chlorpyrifos-ethyl at the normal application rate. We observed that the mortality of earwigs exposed to chlorpyrifos-ethyl depended on the management strategy of orchards. Significantly lower mortality was seen in individuals sampled from conventional orchard. The basal activities of CbEs and GSTs of collected organisms were higher in conventional orchard. After in vivo exposure, AChE activity appeared to be inhibited in surviving males with no difference between orchards. However an in vitro inhibition trial with chlorpyrifos-oxon showed that AChE from earwigs collected in organic and IPM orchards were more sensitive than from conventional ones. These observations support the hypothesis of a molecular target modification in AChE and highlight the possible role of CbEs in effective protection of AChE. Our findings suggest that the earwigs with a high historic level of insecticide exposure could acquire resistance to chlorpyrifos-ethyl.

Novel centrally active oxime reactivators of acetylcholinesterase inhibited by surrogates of sarin and VX

A novel oxime platform, the substituted phenoxyalkyl pyridinium oximes (US patent 9,227,937), was invented at Mississippi State University with an objective of discovering a brain-penetrating antidote to highly potent organophosphate anticholinesterases, such as the nerve agents. The goal was reactivation of inhibited brain acetylcholinesterase to attenuate the organophosphate-induced hypercholinergic activity that results in glutamate-mediated excitotoxicity and neuropathology. The currently approved oxime antidote in the US, 2-PAM, cannot do this. Using highly relevant surrogates of sarin and VX that leave acetylcholinesterase phosphylated with the same chemical moiety as their respective nerve agents, in vitro screens and in vivo tests in rats were conducted to identify the most efficacious members of this platform. The most promising novel oximes provided 24-h survival of lethal level surrogate exposure better than 2-PAM in almost all cases, and two of the oximes shortened the time to cessation of seizure-like behavior while 2-PAM did not. The most promising novel oximes attenuated neuropathology as indicated by immunohistochemical stains for both glia and neurons, while 2-PAM did not protect either glia or neurons. These results strongly suggest that these novel oximes can function within the brain to protect it, and therefore show great promise as potential future nerve agent antidotes.

Methamidophos Influences Midgut Proteinase Activity and Subcellular Structures in the Wolf Spider Pardosa pseudoamulata (Araneae: Lycosidae)

A piezoelectric quartz crystal impedance (PQCI) sensor was used to investigate influences of the insecticide methamidophos on proteinase activity in midguts of the wolf spider, Pardosa pseudoamulata (Araneae: Lycosidae). Results from PQCI indicated that low-concentration dose methamidophos (0.008%) can activate the proteinase but high-concentration dose methamidophos (0.016-0.032%) can inhibit the enzyme activity. The changes in subcellular structure of spider midgut cells were also observed. Electron micrographs of spider midgut epithelial cells showed that the low-dose methamidophos did not visibly impact the structure of these cells. Conversely, high-concentration dose methamidophos led to severe changes in the cell structure, including the karyotheca dissolved, the nucleolus, and the endoplasmic reticulum disappeared. These may contribute to changes in proteinase activity of spider. This work documents a feasible method for rapid and reliable detection of proteinase activity.

Comparative evaluation of toxicological effects and recovery patterns in zebrafish (Danio rerio) after exposure to phosalone-based and cypermethrin-based pesticides

This study evaluated the toxic effects and recovery patterns in zebrafish (Danio rerio) after exposure to phosalone-based (PBP) and cypermethrin-based (CBP) pesticides. Initially, the 96 h LC₅₀ values of the pesticides were calculated as being 5.35 µg of active ingredient (AI) L^-1 for CBP and 217 µg AI L^-1 for PBP based on measured concentrations. Accordingly, experimental groups were exposed to three sublethal concentrations of pesticides for 96 h, separately, and then zebrafish were transferred to pesticide-free conditions for 10 and 20 days recovery periods. Biochemical markers were assessed including carboxylesterase (CaE), acetylcholinesterase (AChE), glutathione S-transferase (GST), lactate dehydrogenase, glutathione peroxidase, catalase, alanine and aspartate aminotransferase (ALT, AST) activities after the exposure and recovery periods. Also, the pesticide concentrations in test water were quantified by high-performance liquid chromatography (HPLC) analysis. Our results showed that AChE and CaE activities were significantly inhibited and GST was induced by both pesticides after 96 h exposure. For PBP exposure, the decreases for GST induction and CaE inhibition showed a partial recovery in pesticide-free conditions. However, the decreases in AChE activity for CBP exposure and insufficient increases in same enzyme activity for PBP exposure after 20 days in pesticide-free conditions indicated that the projected recovery period was not enough to the recovery of AChE activities and for the improvement of fish health.

Vitamin E alleviates phoxim-induced toxic effects on intestinal oxidative stress, barrier function, and morphological changes in rats

Phoxim is an organic phosphorus pesticide that remains easily in the environment, such as human food and animal feed. The objective of this study was to explore the effect of vitamin E on phoxim-induced oxidative stress in the intestinal tissues of Sprague-Dawley (SD) rats. Forty-eight Sprague-Dawley rats were randomly assigned to a control group and three treatment groups: treatment group 1 (phoxim: 20 mg/kg·BW), treatment group 2 (phoxim: 180 mg/kg·BW), and treatment 3 (vitamin E + phoxim: 200 mg/kg·BW + 180 mg/kg·BW). Phoxim was given by gavage administration once a day for 28 days. The results showed that phoxim significantly reduced jejunum villus height in rats (P < 0.05), and decreased the mRNA expression of junction protein genes of rats, including Occlidin and Claudin-4 (P < 0.05). Phoxim reduced GSH content and T-AOC level in the intestinal mucosa (P < 0.05). The mRNA expression levels of oxidative stress-related genes (Nrf2 and GPx2) were decreased. The mRNA expression of SOD was significantly increased. In addition, phoxim increased the level of interleukin-6 (IL-6) in jejunum mucosa and significantly reduced the level of IL-8 in ileum mucosas, while significantly increased TNF-α secretion. The mRNA expression levels of IL-1β, IL-6, and IL-8 were significantly decreased, and mRNA expression of TNF-α was significantly increased (P < 0.05). Phoxim also increased the DNA expression of total cecal bacteria and Escherichia coli, inhibited the DNA expression of Lactobacillus and destroyed the intestinal barrier. Two hundred milligrams per kilogram BW vitamin E reduced the effect of phoxim on intestinal structure, alleviated the oxidative stress in intestinal tissue, and decreased the level of TNF-α. The mRNA expressions of antioxidative stress genes (SOD and GPx2) were significantly increased. The DNA expression level of Lactobacillus was significantly increased. In conclusion, vitamin E helped reduce the toxicity of organophosphate pesticides, such as phoxim on rat intestinal tissue.

The inhibition, reactivation and mechanism of VX-, sarin-, fluoro-VX and fluoro-sarin surrogates following their interaction with HuAChE and HuBuChE

In this study, the mechanisms of HuAChE and HuBChE inhibition by Me-P(O) (OPNP) (OR) [PNP = p-nitrophenyl; R = CH2CH3, CH2CH2F, OCH(CH3)2, OCH(CH3) (CH2F)] representing surrogates and fluoro-surrogates of VX and sarin were studied by in vitro kinetics and mass spectrometry. The in vitro measures showed that the VX- and fluoro-VX surrogates were relatively strong inhibitors of HuAChE and HuBChE (ki ∼ 105-106 M-1min-1) and underwent spontaneous and 2-PAM-mediated reactivation within 30 min. The sarin surrogates were weaker inhibitors of HuAChE and HuBChE (ki ∼ 104-105 M-1min-1), and in general did not undergo spontaneous reactivation, although HuAChE adducts were partially reactivatable at 18 h using 2-PAM. The mechanism of HuAChE and HuBChE inhibition by the surrogates was determined by Q-TOF and MALDI-TOF mass spectral analyses. The surrogate-adducted proteins were trypsin digested and the active site-containing peptide bearing the OP-modified serine identified by Q-TOF as triply- and quadruply-charged ions representing the respective increase in mass of the attached OP moiety. Correspondingly, monoisotopic ions of the tryptic peptides representing the mass increase of the OP-adducted peptide was identified by MALDI-TOF. The mass spectrometry analyses validated the identity of the OP moiety attached to HuAChE or HuBChE as MeP(O) (OR)-O-serine peptides (loss of the PNP leaving group) via mechanisms consistent with those found with chemical warfare agents. MALDI-TOF MS analyses of the VX-modified peptides versus time showed a steady reduction in adduct versus parent peptide (reactivation), whereas the sarin-surrogate-modified peptides remained largely intact over the course of the experiment (24 h). Overall, the presence of a fluorine atom on the surrogate modestly altered the rate constants of inhibition and reactivation, however, the mechanism of inhibition (ejection of PNP group) did not change.

Influences of acephate and mixtures with other commonly used pesticides on honey bee (Apis mellifera) survival and detoxification enzyme activities

Acephate (organophosphate) is frequently used to control piercing/sucking insects in field crops in southern United States, which may pose a risk to honey bees. In this study, toxicity of acephate (formulation Bracket^®97) was examined in honey bees through feeding treatments with sublethal (pollen residue level: 0.168 mg/L) and median-lethal (LC₅₀: 6.97 mg/L) concentrations. Results indicated that adult bees treated with acephate at residue concentration did not show significant increase in mortality, but esterase activity was significantly suppressed. Similarly, bees treated with binary mixtures of acephate with six formulated pesticides (all at residue dose) consistently showed lower esterase activity and body weight. Clothianidin, λ-cyhalothrin, oxamyl, tetraconazole, and chlorpyrifos may interact with acephate significantly to reduce body weight in treated bees. The dose response data (LC50: 6.97 mg/L) revealed a relatively higher tolerance to acephate in Stoneville bee population (USA) than populations elsewhere, although in general the population is still very sensitive to the organophosphate. In addition to killing 50% of the treated bees acephate (6.97 mg/L) inhibited 79.9%, 20.4%, and 29.4% of esterase, Glutathione S-transferase (GST), and acetylcholinesterase (AChE) activities, respectively, in survivors after feeding treatment for 48 h. However, P450 activity was elevated 20% in bees exposed to acephate for 48 h. Even though feeding on sublethal acephate did not kill honey bees directly, chronic toxicity to honey bee was noticeable in body weight loss and esterase suppression, and its potential risk of synergistic interactions with other formulated pesticides should not be ignored.

Evaluation of the joint toxicity of chlorpyrifos ethyl and fenobucarb on climbing perch (Anabas testudineus) from rice fields in the Mekong Delta, Vietnam

Organophosphates (e.g. chlorpyrifos ethyl) and carbamates (e.g. fenobucarb) are commonly used to control a wide range of pests in rice fields of the Mekong Delta in Vietnam. This study assesses the combined effect of chlorpyrifos ethyl (CPF) and fenobucarb (F), applied at concentrations used by rice farmers, on the brain acetylcholinesterase (AChE) activity in climbing perch fingerlings from rice fields in the Mekong Delta. It also investigates if Pyridine-2-aldoxime methiodide (2-PAM) can be used to reactivate cholinesterase that has been blocked by CPF. Three days after spraying, the water concentrations of both insecticides decreased quickly below the detection levels. However, the brain AChE activity in fish was inhibited for more than 7 days. The results indicate a quicker but less prolonged inhibition of the brain AChE activity by the mixture than by only CPF. The inhibition levels were above 70 % only during the first 48 h, which could explain why all fish survived. 2-PAM resulted in a significant reactivation of the cholinesterase blocked by a combination of CPF and F, and it is proposed that the reactivation by 2-PAM could provide a way to assess the AChE inhibition levels in organisms, when no unexposed individuals are available as controls. The results indicate that the current use of CPF and F in rice farming in the Mekong Delta is likely to cause negative effects on non-target organisms. Many of these effects may be sub-lethal, and there is a need to develop biomarkers that are relevant, inexpensive and easy to apply. The results show that brain AChE in climbing perch fingerling is a relevant biomarker for monitoring of exposure to, and sub-lethal impacts from organophosphates and carbamates under tropical conditions in developing countries.

Developmental toxicity and neurotoxicity of synthetic organic insecticides in zebrafish (Danio rerio): A comparative study of deltamethrin, acephate, and thiamethoxam

Synthetic organic insecticides, including pyrethroids, organophosphates, neonicotinoids and other types, have the potential to alter the ecosystems and many are harmful to humans. This study examines the developmental toxicity and neurotoxicity of three synthetic organic insecticides, including deltamethrin (DM), acephate (AP), and thiamethoxam (TM), using embryo-larval stages of zebrafish (Danio rerio). Results showed that DM exposure led to embryo development delay and a significant increase in embryo mortality at 24 and 48 h post-fertilization (hpf). DM and AP decreased embryo chorion surface tension at 24 hpf, along with the increase in hatching rate at 72 hpf. Moreover, DM caused ntl, shh, and krox20 misexpression in a dose-dependent manner with morphological deformities of shorter body length, smaller eyes, and larger head-body angles at 10 μg/L. TM did not show significant developmental toxicity. Furthermore, results of larval rest/wake assay indicated that DM (>0.1 μg/L) and AP (0.1 mg/L) increased activity behavior with different patterns. Interestingly, as an insect-specific pesticide, TM still could alter locomotor activity in zebrafish larvae at concentrations as low as 0.1 mg/L. Our results indicate that different types of synthetic organic insecticides could create different toxicity outcomes in zebrafish embryos and larvae.

Age-dependent behaviors, seizure severity and neuronal damage in response to nerve agents or the organophosphate DFP in immature and adult rats

Exposure to nerve agents (NAs) and other organophosphates (OPs) can initiate seizures that rapidly progress to status epilepticus (SE). While the electrographic and neuropathological sequelae of SE evoked by NAs and OPs have been characterized in adult rodents, they have not been adequately investigated in immature animals. In this study postnatal day (PND) 14, 21 and 28 rat pups, along with PND70 animals as adult controls, were exposed to NAs (sarin, VX) or another OP (diisopropylfluorophosphate, DFP). We then evaluated behavioral and electrographic (EEG) correlates of seizure activity, and performed neuropathology using Fluoro-Jade B. Although all immature rats exhibited behaviors that are often characterized as seizures, the incidence, duration, and severity of the electrographic seizure activity were age-dependent. No (sarin and VX) or brief (DFP) EEG seizure activity was evoked in PND14 rats, while SE progressively increased in severity as a function of age in PND21, 28 and 70 animals. Fluoro-Jade B staining was observed in multiple brain regions of animals that exhibited prolonged seizure activity. Neuronal injury in PND14 animals treated with DFP was lower than in older animals and absent in rats exposed to sarin or VX. In conclusion, we found that NAs and an OP provoked robust SE and neuronal injury similar to adults in PND21 and PND28, but not in PND14, rat pups. Convulsive behaviors were often present independent of EEG seizures and were unaccompanied by neuronal damage. These differential responses should be considered when investigating medical countermeasures for NA and OP exposure in pediatric populations.

Understanding the fate of human serum albumin upon interaction with edifenphos: Biophysical and biochemical approaches

Edifenphos (EDF), an important organophosphate fungicide used in agriculture, is a great threat to human health and environment. To assess the toxicity of EDF at the level of protein molecule, the effect of EDF on human serum albumin (HSA) was investigated by biophysical and biochemical approaches. EDF-HSA complex is formed as a result of static quenching as revealed by the intrinsic fluorescence analysis. Thermodynamic analysis of the binding data suggests involvement of hydrophobic interactions in EDF-HSA complex formation, which is in line with molecular docking results. Moreover, thermodynamic parameters of binding between EDF and HSA suggest entropy-driven spontaneous interaction, presumably dominated by hydrophobic forces. Further, binding site of EDF seems to have been located within sub-domain IIA of HSA. EDF binding to HSA decreases its alpha helical content as analyzed by CD spectra. Marked micro-environmental changes around tryptophan/tyrosine residues in HSA upon EDF binding were recorded via three-dimensional fluorescence spectroscopy. Substantial release of protein carbonyl from HSA as a result of EDF treatment suggested involvement of ROS in EDF induced protein damage. This work is expected to provide some leads toward EDF induced toxicity in humans and would be helpful in reinforcing the check on food safety.

Methamidophos, an Organophosphorus Insecticide, Induces Pro-aggressive Behaviour in Mice

Although evidence indicates that exposure to organophosphorus (OP) pesticides induces neurobehavioral disorders, little is known about the effects of OP on aggressive behaviour. Our study investigated the effects of repeated exposure to an OP pesticide, methamidophos, on the isolation-induced aggressive behaviour in mice. Forty seven male mice were individually housed for a month. Socially isolated animals were then confronted with a standard non-isolated opponent for 15 min (pre-treatment trial), and the latency and frequency of aggressive and general exploratory behaviours were recorded. Based on the presence of attack behaviour in the pre-treatment trial, mice were classified as isolation-induced aggressive and non-aggressive. All mice were then treated for 7 days with methamidophos (3.5 mg/kg/day, n = 22, intraperitoneal (i.p.)) or saline (1 mL/kg/day, control group, n = 25, i.p.), and a second trial was performed. Repeated exposure to methamidophos induced attack behaviour in non-aggressive mice. The treatment with methamidophos also decreased plasma butyrylcholinesterase and brain acetylcholinesterase activity. These results suggest that methamidophos has a pro-aggressive effect on socially isolated mice.

Apoptosis of rat renal cells by organophosphate pesticide, quinalphos: Ultrastructural study

Quinalphos or Ekalux, an organophosphate pesticide, is used in controlling the pests of a variety of crops. Quinalphos was studied on male Sprague-Dawley albino rats. The acute po LD50 of technical Ekalux was 19.95 mg/kg in males. Ekalux, produced several pathological changes in the kidney. A glomerulus demonstrated capillary lumina occluded by degenerated cellular debris. Basement membrane showed irregular wrinkling and branching. The proximal tubular cells showed damage such as dilation of endoplasmic reticulum, accumulation of glycogen granules, and pyknotic nucleus. The changes also included swelling of the mitochondria and reduction of the cristae up to total destruction. The distal tubular changes included electron lucency and vacuolation of cytoplasm. The distal convoluted tubule wall showed edematous epithelial cells, formation of blebs, and microvilli loss. These results suggest that subchronic exposure of rats to Ekalux causes ultrastructural changes in renal corpuscle and marked ultrastructural changes in proximal and distal tubules.

Multiple resistance to pirimiphos-methyl and bifenthrin in Tribolium castaneum involves the activity of lipases, esterases, and laccase2

Several recent studies have elucidated the molecular mechanisms that confer insecticide resistance on insect pests. However, little is known about multiple resistance in red flour beetle (Tribolium castaneum) at molecular level. The multiple resistance is characterized as resistance to different classes of insecticides that have different target sites, and is mediated by several enzymatic systems. In this study, we investigated the biochemical and molecular mechanisms involved in multiple resistance of T. castaneum to bifenthrin (pyrethroid [Pyr]) and pirimiphos-methyl (organophosphate [Org]). We used artificial selection, biochemical and in silico approaches including structural computational biology. After five generations of artificial selection in the presence of bifenthrin (F5Pyr) or pirimiphos-methyl (F5Org), we found high levels of multiple resistance. The hierarchical enzymatic cluster revealed a pool of esterases (E), lipases (LIPs) and laccase2 (LAC2) potentially contributing to the resistance in different ways throughout development, after one or more generations in the presence of insecticides. The enzyme-insecticide interaction network indicated that E2, E3, LIP3, and LAC2 are enzymes potentially required for multiple resistance phenotype. Kinetic analysis of esterases from F5Pyr and F5Org showed that pirimiphos-methyl and specially bifenthrin promote enzyme inhibition, indicating that esterases mediate resistance by sequestering bifenthrin and pirimiphos-methyl. Our computational data were in accordance with kinetic results, indicating that bifenthrin has higher affinity at the active site of esterase than pirimiphos-methyl. We also report the capability of these insecticides to modify the development in T. castaneum. Our study provide insights into the biochemical mechanisms employed by T. castaneum to acquire multiple resistance.

Antioxidant enzymes and their role in phoxim and carbaryl stress in Caenorhabditis elegans

Pesticide exposure can induce oxidative stress and cause changes to antioxidant enzymes in living organisms. In the present study, the effects of phoxim (an organophosphorus insecticide) and carbaryl (a carbamate insecticide) on antioxidant enzyme activity and gene expression were investigated in the model organism Caenorhabditis elegans. The results show that phoxim exposure can induce superoxide dismutase (SOD) and catalase (CAT) activities and decrease glutathione peroxidase (GPx) activity at lower concentrations. The expression levels of sod-3, sod-5, ctl-1, gpx-6, and gpx-8 were up-regulated after treatment with phoxim. The mRNA expression levels of sod-5, ctl-1 and gpx-6 were increased approximately 70-, 170- and 130-fold, respectively, in the 0.25mM treatment group compared to the control group. Carbaryl exposure decreased SOD activity and induced CAT and GPx activities. The addition of carbaryl up-regulated the expression of sod-5, ctl-1, ctl-3 and gpx-8. Specifically, ctl-1 expression increased approximately 10-fold, and gpx-8 expression increased <30-fold in the 0.5mM treatment group relative to the control group. The transcript level of sod-5 increased >20-fold, and ctl-3 increased approximately 10-fold in the 1mM treatment group. The functions of the antioxidant enzymes during oxidative stress caused by the two insecticides were investigated using deletion mutants. The LC₅₀ values phoxim for the of sod-3 (tm760), sod-5 (tm1146), ctl-1 (ok1242), ctl-3 (ok2042) and gpx-8 (tm2108) mutant strains were lower than those observed for the N2 strain. The LC₅₀ values of carbaryl for the ctl-1 (ok1242), ctl-3 (ok2042) and gpx-6 (tm2535) deletion mutant strains decreased in comparison to the N2 strain. The results suggest that these two insecticides caused oxidative stress and changed altered the antioxidant enzyme activities and their gene expressions in C. elegans. The sod-3, sod-5, ctl-1, ctl-3, gpx-6, and gpx-8 encoding enzymes may play roles in defending cells from oxidative stress caused by these two insecticides.

Terbufos-sulfone exacerbates cardiac lesions in diabetic rats: a sub-acute toxicity study

Organophosphorus compounds (OPCs) have a wide range of applications, from agriculture to warfare. Exposure to these brings forward a varied kind of health issues globally. Terbufos is one of the leading OPCs used worldwide. The present study investigates the cardiac effect of no observable dose of a metabolite of terbufos, terbufos-sulfone (TS), under non-diabetic and streptozotocin-induced diabetic condition. One hundred nanomoles per rat (1/20 of LD50) was administered intraperitoneally to adult male Wister rats daily for fifteen days. The left ventricle was collected for ultrastructural changes by transmission electron microscopy. The blood samples were collected for biochemical tests including RBC acetylcholinesterase, creatinine kinase (CK), lactate dehydrogenase (LDH), cholesterol, high density lipoprotein (HDL), low density lipoprotein (LDL), triglycerides, ALT, AST, and GGT. The study revealed about 10 % inhibition of RBC-AChE in two weeks of TS treatment in non-diabetic rats whereas RBC-AChE activity was significantly decreased in diabetic TS treated rats. CK, LDH, and triglycerides were significantly higher in diabetic TS treated rats. Electron microscopy of the heart showed derangement and lesions of the mitochondria of cardiomyocytes in the TS treated groups. The present study concludes that a non-lethal dose of TS causes cardiac lesions which exacerbate under diabetic condition. Biochemical tests confirmed the ultrastructural changes. It is concluded that a non-lethal dose of TS may be a risk factor for a cardiovascular disease, which may be fatal under diabetic condition.

Sublethal dose of phoxim and Bombyx mori nucleopolyhedrovirus interact to elevate silkworm mortality

BACKGROUND

Silkworm (Bombyx mori) is an economically important insect. It is relatively less resistant to certain chemicals and environment exposures such as pesticides and pathogens. After pesticide exposures, the silkworms are more susceptible to microbial infections. The mechanism underlying the susceptibility might be related to immune response and oxidative stress.

RESULTS

A sublethal dose of phoxim combined with Bombyx mori nucleopolyhedrovirus (BmNPV) elevated the silkworm mortality at 96 h. We found a higher content of H₂ O₂ and increased levels of genes related to oxidative stress and immune response after treatment with a sublethal dose of phoxim for 24 h or 48 h. However, such response decreased with longer pesticide treatment. Mortality increased by 44% when B. mori was exposed to combined treatment with BmNPV and phoxim rather than BmNPV alone. The level of examined immune-related and oxidative-stress-related genes significantly decreased in the combined treatment group compared with the BmNPV group. Our results indicated that, with long-term exposure to pesticides such as OPs, even at sublethal dose, the oxidative stress response and immune responses in silkworm were inhibited, which may lead to further immune impairment and accumulation of oxidative stress, resulting in susceptibility to the virus and harm to the silkworm.

CONCLUSION

Our study provided insights for understanding the susceptibility to pathogen after pesticide exposures, which may promote the development of better pesticide controls to avoid significant economic losses. © 2016 Society of Chemical Industry.

Single and joint toxic effects of five selected pesticides on the early life stages of zebrafish (Denio rerio)

Instead of individual ones, pesticides are usually detected in water environment as mixtures of contaminants. Laboratory tests were conducted in order to investigate the effects of individual and joint pesticides (phoxim, atrazine, chlorpyrifos, butachlor and λ-cyhalothrin) on zebrafish (Denio rerio). Results from 96-h semi-static toxicity test indicated that λ-cyhalothrin had the greatest toxicity to the three life stages (embryonic, larval and juvenile stages) of D. rerio with LC₅₀ values ranging from 0.0031 (0.0017-0.0042) to 0.38 (0.21-0.53) mg a.i. L^-1, followed by butachlor and chlorpyrifos with LC₅₀ values ranging from 0.45 (0.31-0.59) to 1.93 (1.37-3.55) and from 0.28 (0.13-0.38) to 13.03 (7.54-19.71) mg a.i. L^-1, respectively. In contrast, atrazine showed the least toxicity with LC₅₀ values ranging from 6.09 (3.34-8.35) to 34.19 (24.42-51.9) mg a.i. L^-1. The larval stage of D. rerio was a vulnerable period to most of the selected pesticides in the multiple life stages tested. Pesticide mixtures containing phoxim and λ-cyhalothrin exerted synergistic effects on the larvae of D. rerio. Moreover, the binary mixture of phoxim-atrazine also displayed synergistic response to zebrafish. It has been assumed that most chemicals are additive in toxicity. Therefore, it is crucial to clarify the synergistic interaction for pesticide regulators and environment managers. In the present study, our data provided a clear picture on ecological risk of these pesticide mixtures to aquatic organisms. Moreover, joint effects play a more important role than individual ones, which require more attention when defining standard for water environment quality and risk assessment protocols.

Combined toxicity of imidacloprid and three insecticides to the earthworm, Eisenia fetida (Annelida, Oligochaeta)

Although the earthworm Eisenia fetida has been used in many ecotoxicological studies in recent years, most of these studies have only focused on assessing the effects of individual insecticides. In the present study, we aimed to compare the individual and combined toxic effects of imidacloprid and three insecticides (phoxim, chlorpyrifos, and lambda-cyhalothrin) on E. fetida. We showed that imidacloprid had the highest intrinsic toxicity to the worms in filter paper contact test, followed by phoxim and lambda-cyhalothrin, while the least toxicity was found from chlorpyrifos. Moreover, 14-day soil toxicity test revealed that the highest toxicity was still detected for imidacloprid with an LC₅₀ value of 2.82 (2.61∼3.17) mg a.i. kg^-1 dry weight (DW), followed by chlorpyrifos with an LC₅₀ value of 384.9 (353.5∼440.3) mg a.i. kg^-1 DW. Meanwhile, a relatively less toxicity was found for lambda-cyhalothrin with an LC₅₀ value of 560.3 (475.9∼718.5) mg a.i. kg^-1 DW, while the lowest toxicity to E. fetida was observed for phoxim with an LC₅₀ value of 901.5 (821.3∼1017) mg a.i. kg^-1 DW. In addition, significant synergistic responses were found from the ternary mixture of imidacloprid-phoxim-lambda-cyhalothrin and quaternary mixture of imidacloprid-phoxim-chlorpyrifos-lambda-cyhalothrin in both bioassay systems. Therefore, our findings highlighted that the simultaneous presence of several insecticides in the soil environment might lead to increased toxicity, resulting in serious damage to the nontarget organisms compared with individual insecticides.

RSDL decontamination of human skin contaminated with the nerve agent VX

Dermal exposure to low volatile organophosphorus compounds (OPC) may lead to penetration through the skin and uptake in the blood circulation. Skin decontamination of toxic OPCs, such as pesticides and chemical warfare nerve agents, might therefore be crucial for mitigating the systemic toxicity following dermal exposure. Reactive skin decontamination lotion (RSDL) has been shown to reduce toxic effects in animals dermally exposed to the nerve agent VX. In the present study, an in vitro flow-through diffusion cell was utilized to evaluate the efficacy of RSDL for decontamination of VX exposed to human epidermis. In particular, the impact of timing in the initiation of decontamination and agent dilution in water was studied. The impact of the lipophilic properties of VX in the RSDL decontamination was additionally addressed by comparing chemical degradation in RSDL and decontamination efficacy between the VX and the hydrophilic OPC triethyl phosphonoacetate (TEPA). The epidermal membrane was exposed to 20, 75 or 90% OPC diluted in deionized water and the decontamination was initiated 5, 10, 30, 60 or 120min post-exposure. Early decontamination of VX with RSDL, initiated 5-10min after skin exposure, was very effective. Delayed decontamination initiated 30-60min post-exposure was less effective but still the amount of penetrated agent was significantly reduced, while further delayed start of decontamination to 120min resulted in very low efficacy. Comparing RSDL decontamination of VX with that of TEPA showed that the decontamination efficacy at high agent concentrations was higher for VX. The degradation mechanism of VX and TEPA during decontamination was dissected by ³¹P NMR spectroscopy of the OPCs following reactions with RSDL and its three nucleophile components. The degradation rate was clearly associated with the high pH of the specific solution investigated; i.e. increased pH resulted in a more rapid degradation. In addition, the solubility of the OPC in RSDL also influenced the degradation rate since the degradation of VX was significantly faster when the NMR analysis was performed in the organic solvent acetonitrile compared to water. In conclusion, we have applied the in vitro flow-through diffusion cell for evaluation of skin decontamination procedures of human epidermis exposed to OPCs. It was demonstrated that early decontamination is crucial for efficient mitigation of epidermal penetration of VX and that almost complete removal of the nerve agent from the skin surface is possible. Our data also indicate that the pH of RSDL together with the solubility of OPC in RSDL are of primary importance for the decontamination efficacy.

Exploring hazards of acute exposure of Acephate in Drosophila melanogaster and search for l-ascorbic acid mediated defense in it

This study reveals protective role of l-ascorbic acid (25, 50 and 100μg/mL) against toxic impacts of acute sub-lethal exposure of Acephate (5μg/mL) in a non-target organism Drosophila melanogaster. Organismal effect was evident from increased impairment in climbing activities (9 folds) of treated individuals who also manifested altered ocular architecture. These anomalies were reduced with l-ascorbic acid (l-AA) supplementation. Acephate induced apoptotic lesions in eye imaginal discs and gut confirmed tissue damage that also reduced with l-AA co-treatment. Reduction in viability of fat body cells (∼41%), neural cells (∼42%) and hemocytes (3 folds) indicates cytotoxic and immunotoxic potential of Acephate, which were significantly mitigated with l-AA co-administration. The sub-cellular toxic impacts of Acephate treatment became obvious from enhancement in activities of antioxidant enzymes (CAT by ∼1.63 folds, SOD by ∼1.32 folds), detoxifying enzymes (Cyp450 by ∼1.99 folds and GST by ∼1.34 folds), 2.1 times boost in HSP 70 expression, and inhibition of cholinesterase activity (by ∼0.66 folds). DNA breaks evident through comet assay confirmed Acephate triggered genotoxicity which could also be prevented through co-administration of. L-AA Furthermore, the study proposes the use of Drosophila as a model to screen chemicals for their protective potential against pesticide toxicity.

Methamidophos induces cytotoxicity and oxidative stress in human peripheral blood mononuclear cells

Previous studies have shown that organophosphate pesticide (OP) exposure is associated with oxidative stress. Methamidophos (MET) is an OP widely used in agriculture, which is regarded as a highly toxic pesticide and it is a potent inhibitor of acetylcholinesterase. The aim of this study was to evaluate whether MET can induce oxidative stress at low concentrations in primary cultures of human peripheral blood mononuclear cells (PBMCs). PBMCs from healthy individuals were exposed to MET (0-80 mg/L) for 0-72 h. We performed the MTT and neutral-red assays to assess the cytotoxicity. As indicators of oxidative stress, the levels of reactive oxygen species (ROS) were assessed using flow cytometry, and the malondialdehyde (MDA) and reduced glutathione (GSH) levels were determined. MET decreased the viability of PBMCs in a dose-dependent manner. At concentrations of 3, 10, or 20 mg/L for 24 h, MET increased the ROS production significantly compared with the vehicle control. Similarly, MET increased the levels of MDA at the same concentrations that increased ROS (10 and 20 mg/L); however, no changes in GSH levels were observed. These results suggest that MET increased the generation of oxidative stress in PBMCs. © 2015 Wiley Periodicals, Inc. Environ Toxicol 32: 147-155, 2017.