The role of phosphotriesterases in the detoxication of organophosphorus compounds

Cu2+-dependent hydrolysis of O-hexyl 2,5-dichlorophenyl phosphoramidate by reptile sera

This study shows the EDTA-resistant, Ca2+ and Cu2+-dependent hydrolysis of O-hexyl 2,5-dichlorophenyl phosphoramidate (HDCP) compound in reptiles sera determined by spectrophotometry UV/Vis and chiral chromatography. Samples of ten reptile species were incubated with aliquot of 100 or 400 μM HDCP in presence of 100 or 300 μM Cu2+, or 2.5 mM Ca2+ or 5 mM EDTA at 37 °C for 30-60 min. The results shown an activator effect of Cu2+ on HDCP hydrolysis in freshwater turtles sera (Trachemys scripta, Chelydra serpentina and Macrochelys temminckii) because the levels of 2,5-dichlorophenol (DCP; product hydrolysis) were similar (∼37 μM DCP) to chicken serum (positive control group). The marine turtles (Chelonia mydas and Eretmochelys imbricata) and crocodiles (Crocodylusacutus and Crocodylus moreletii) showed ∼50% less HDCPase activity (13-17 μM DCP) compared to the HDCPase activity of the freshwater turtle species. Terrestrial reptile species (snakes and lizards) showed around 25% of activity (7-13 μM DCP) with both copper concentrations. These Cu2+-dependent hydrolysis were stereospecific to R(+)-HDCP (p˂0.05) in the three freshwater turtle species that showed similar hydrolysis to the chicken serum. However, the Ca2+ did not show a significant activating effect on the HDCPase activity (1-8 μM DCP) in any reptile serum. Their hydrolysis levels were very similar to those of EDTA-resistant activity. The present study demonstrates a Cu2+-dependent A-esterase (HDCPase) activity in turtles and points serum albumin as the cuproprotein responsible for this activity, reinforcing its N-terminal sequence (DAEH) as a catalytic center.

Computational study to find the goat serum albumin (GSA) binding site as A-esterase

A-esterases are a classical term applied to enzymatic activity of the proteins by a mechanism not involving intermediate covalent phosphorylation, but requiring a divalent cation cofactor. Recently, a copper-dependent A-esterase activity has been identified in goat serum albumin (GSA) on the organophosphorus insecticide trichloronate. This hydrolysis was identified ex vivo with spectrophotometry and chromatography techniques. Albumin mechanism of action and catalytic site as Cu²⁺-dependent A-esterase are still unknown. Therefore, to know the copper bind to albumin is relevant. N-terminal sequence has been reported as the high affinity site for this cation, due to the histidine in position 3. The aim of this work In Silico is to explore how occurs this metallic binding and active the esterase catalytic function. The GSA crystallized structure (PDB: 5ORI) was chosen for molecular docking and dynamics. A site-directed docking, for N-terminal site and a blind docking was done with trichloronate as ligand. Root-mean-square deviation and frequency plot was calculated to find the most frequent predicted structure and visualize the amino acids involved in binding site. The affinity energy in the blind docking (-5.804 kcal/mol) is almost twice lower than site-directed docking (-3.816 kcal/mol) and N-terminal amino acids do not appear in the most repeated structure binding site, suggesting that the protein has a site with higher affinity to the trichloronate ligand. His145 could be involved in the binding site as has been reported in previous studies.

Predicting the bioremediation potential of earthworms of different ecotypes through a multi-biomarker approach

Earthworms are attracting the attention of bioremediation research because of their short-term impact on pollutant fate. However, earthworm-assisted bioremediation largely depends on the earthworm sensitivity to target pollutants and its metabolic capacity to break down contaminants. The most studied species in soil bioremediation has been Eisenia fetida, which inhabits the soil surface feeding on decomposing organic residues. Therefore, its bioremediation potential may be limited to organic matter-rich topsoil. We compared the detoxification potential against organophosphate (OP) pesticides of three earthworm species representative of the main ecotypes: epigeic, anecic, and endogeic. Selected biomarkers of pesticide detoxification (esterases, cytochrome P450-dependent monooxygenase, and glutathione S-transferase) and oxidative homeostasis (total antioxidant capacity, glutathione levels, and glutathione reductase [GR] and catalase activities) were measured in the muscle wall and gastrointestinal tract of E. fetida (epigeic), Lumbricus terrestris (anecic) and Aporrectodea caliginosa (endogeic). Our results show that L. terrestris was the most suitable species to bioremediate OP-contaminated soil for the following reasons: 1) Gut carboxylesterase (CbE) activity of L. terrestris was higher than that of E. fetida, whereas muscle CbE activity was more sensitivity to OP inhibition than that of E. fetida, which means a high capacity to inactivate the toxic oxon metabolites of OPs. 2) Muscle and gut phosphotriesterase activities were significantly higher in L. terrestris than in the other species. 3) Enzymatic (catalase and GR) and molecular mechanisms of free radical inactivation (glutathione) were 3- to 4-fold higher in L. terrestris concerning E. fetida and A. caliginosa, which reveals a higher potential to keep the cellular oxidative homeostasis against reactive metabolites formed during OP metabolism. Together with biological and ecological traits, these toxicological traits suggest L. terrestris a better candidate for soil bioremediation than epigeic earthworms.

Acute toxicity and metabolism of pesticides in birds

The median lethal dose of pesticide in acute oral toxicity, used as a conservative index in avian risk assessment, varies by the species with differences of less than one order of magnitude, depending on body size, feeding habit, and metabolic enzyme activity. The profiles of pesticide metabolism in birds with characteristic conjugations are basically common to those in mammals, but less information is available on their relevant enzymes. The higher toxicity of some pesticides in birds than in mammals is due to the lower activity of avian metabolic enzymes. The bioaccumulation in birds is limited for very hydrophobic pesticides resistant to metabolic degradation. Several in silico approaches using the descriptors of a pesticide molecule have recently been employed to estimate the profiles of acute oral toxicity and bioaccumulation.

Phosphorus Deficiency Promoted Hydrolysis of Organophosphate Esters in Plants: Mechanisms and Transformation Pathways

The biotransformation of organophosphate esters (OPEs) in white lupin (Lupinus albus) and wheat (Triticum aestivum L.) was investigated in hydroponic experiments with different phosphorus (P)-containing conditions. The hydrolysis rates of OPEs followed the order of triphenyl phosphate (TPHP) > tri-n-butyl phosphate (TnBP) > tris(1,3-dichloro-2-propyl) phosphate (TDCPP). Hydrolysis of OPEs was accelerated at P-deficient conditions, and faster hydrolysis took place in white lupin than in wheat. Coincidingly, the production of acid phosphatase (ACP) in both plants was promoted, and much higher intracellular and extracellular ACPs were observed in white lupin under P-deficient conditions. In vitro experiments revealed that ACP was a key enzyme to hydrolyze OPEs. The hydrolysis rates of OPEs were significantly correlated with the Hirshfeld charges, calculated by density functional theory, of the oxygen atom in the single P-O bond. Using ultra-high-performance liquid chromatography coupled with Orbitrap Fusion mass spectrometer, 30 metabolites were successfully identified. Some of these metabolites, such as sulfate-conjugated products, hydration of cysteine-conjugated products of TPHP, and reductively dechlorinated metabolites of TDCPP, were observed for the first time in plants. It is noteworthy that OPEs may transform into many hydroxylated metabolites, and special attention should be paid to their potential environmental effects.

Cu2+-dependent stereoselective hydrolysis of a chiral organophosphonothioate insecticide for domestic mammals' sera and its albumins

Acute toxicity of organophosphate (OPs) pesticides is a public health problem. The adverse effects are associated with the inhibition and aging of nervous system B-esterases such as acetyl cholinesterase (AChE) and neuropathic target esterase (NTE). Treatment based on A-esterases such as mammal serum paraoxonase-1 has been suggested. This ex vivo study shows the Cu²⁺-dependent hydrolysis of trichloronate (TCN), a racemic organophosphonothioate insecticide, in human and domestic mammal serum (dog, goat, pig, sheep and cow). Ca²⁺-dependent (2.5 mM) or EDTA-resistant (5 mM) activity (1-6%) was not significant (p>0.05) in all samples, except goat serum and its albumin, which showed higher levels of TCN hydrolysis (38-58%) than other mammals with 100 and 300 μM copper sulfate at physiological conditions for 60 min. Goat serum albumin (GSA) showed significant (p˂0.05) stereoselective hydrolysis (+)-TCN ˃ (-)-TCN (45% versus 33%). This suggests that GSA is the protein responsible for Cu²⁺-dependent TCNase activity in goat serum. This is the first report on Cu²⁺-dependent A-esterase activity in mammalian tissues. This goat serum cuproprotein could be considered as an alternative in future biotechnological applications including enantiomeric synthesis, bioremediation and antidotal treatment of organophosphonothioate pesticide poisoning.

Characterization of carbaryl-degrading strain Bacillus licheniformis B-1 and its hydrolase identification

Pesticides introduced inadvertently or deliberately into environment by anthropogenic activity have caused growing global public concern, therefore the search of approaches for elimination of such xenobiotics should be encouraged. A cypermethrin-degrading bacterial strain Bacillus licheniformis B-1 was found to efficiently degrade carbaryl in LB medium at concentrations of 50-300 mg L^-1 within 48 h, during which temperature and pH played important roles as reflected by increase in pollutant depletion. A stimulatory effect of Fe³⁺ and Mn²⁺ on microbial growth was observed, whereas Cu²⁺ caused inhibition of degradation. Results showed that 1-naphthol was a major transformation product of carbaryl which was further metabolised. An approximately 29 kDa carbaryl-degrading enzyme was purified from B-1 with 15.93-fold purification and an overall yield of 6.02% was achieved using ammonium sulphate precipitation, DEAE-Sepharose CL-6B anion-exchange chromatography and Sephadex G-100 gel filtration. The enzyme was identified through nano reversed-phase liquid chromatography coupled with hybrid triple quadrupole time-of-flight mass spectrometry as a phosphodiesterase (PDE). This is the first report on the characterization of carbaryl-degrading by Bacillus spp. and the role of a PDE in carbaryl-detoxifying. Also, strain B-1 showed versatile in carbosulfan, isoprocarb and chlorpyrifos degradation, demonstrating as ideal candidate for environment bioremediation.

Paraoxonase-1 genetic polymorphisms in organophosphate metabolism

Organophosphates (OPs) are a class of chemicals commonly used in agriculture as pesticides, that can often lead to severe toxicity in humans. Paraoxonase-1 (PON1) belongs to a family of A-esterases and hydrolyses several OPs while also serving other biological roles. Two main genetic polymorphisms have been shown to affect enzymatic ability; an A > G transition in the 192nd position (192 Q/R, rs662), and an A > T at codon 55 (55 M/L, rs854560). In this review, we searched PubMed for relevant articles published from its inception till June 2018 and included publications from 1996 to 2018. We aimed to address the distribution of the polymorphisms in various populations, the way they affect enzymatic activity and the possible use of PON1 as a biomarker. The polymorphisms present great heterogeneity between populations, with the data being clearer over 192 Q/R, and this heterogeneity is related to the phylogenetic origins of each population. Concerning enzymatic activity, the different genotypes react better or worse to different OP substrates, with studies presenting a variety of findings. Detecting the "paraoxonase status" of an individual -referring to PON1 function- seems to be important in predicting OP toxicity, as studies have shown that some specific-genotype individuals present symptoms of toxicity in higher rates than others. We are strongly convinced that in order for the scientific community to reach a consensus over which polymorphisms confer susceptibility to toxicity and whether PON1 can eventually be used as a biomarker, more studies need to be carried out, since the data thus far does not seem to reach a universal conclusion.

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.

Comparative intestinal esterases amongst passerine species: Assessing vulnerability to toxic chemicals in a phylogenetically explicit context

Inhibition of blood esterase activities by organophosphate (OP) pesticides has been used as a sensitive biomarker in birds. Furthermore, compared to mammalian vertebrates, less is known about the role of these enzyme activities in the digestive tracts of non-mammalian vertebrates, as well as the environmental and biological stressors that contribute to their natural variation. To fill this gap, we examined butyrylcholinesterase (BChE) and carboxylesterases (CbE) in the digestive tracts of sixteen passerine species from central Chile. Whole intestine enzyme activities were positively and significantly correlated with body mass. After correcting for body mass and phylogenetic effect, we found only a marginal effect of dietary category on BChE activity, but a positive and significant association between the percentage of dietary nitrogen and the mass-corrected lipase activity. Our results suggest that observed differences may be due to the dietary composition in the case of lipases and BChE, and also we predict that all model species belonging to the same order will probably respond differently to pesticide exposure, in light of differences in the activity levels of esterase activities.

Enzyme therapeutics for systemic detoxification

Life relies on numerous biochemical processes working synergistically and correctly. Certain substances disrupt these processes, inducing living organism into an abnormal state termed intoxication. Managing intoxication usually requires interventions, which is referred as detoxification. Decades of development on detoxification reveals the potential of enzymes as ideal therapeutics and antidotes, because their high substrate specificity and catalytic efficiency are essential for clearing intoxicating substances without adverse effects. However, intrinsic shortcomings of enzymes including low stability and high immunogenicity are major hurdles, which could be overcome by delivering enzymes with specially designed nanocarriers. Extensive investigations on protein delivery indicate three types of enzyme-nanocarrier architectures that show more promise than others for systemic detoxification, including liposome-wrapped enzymes, polymer-enzyme conjugates, and polymer-encapsulated enzymes. This review highlights recent advances in these nano-architectures and discusses their applications in systemic detoxifications. Therapeutic potential of various enzymes as well as associated challenges in achieving effective delivery of therapeutic enzymes will also be discussed.

Use of OpdA, an organophosphorus (OP) hydrolase, prevents lethality in an African green monkey model of acute OP poisoning

Organophosphorus (OP) pesticides are a diverse class of acetylcholinesterase (AChE) inhibitors that are responsible for tremendous morbidity and mortality worldwide, killing approximately 300,000 people annually. Enzymatic hydrolysis of OPs is a potential therapy for acute poisoning. OpdA, an OP hydrolase isolated from Agrobacterium radiobacter, has been shown to decrease lethality in rodent models of OP poisoning. This study investigated the effects of OpdA on AChE activity, plasma concentrations of OP, and signs of toxicity after administration of dichlorvos to nonhuman primates. A dose of 75 mg/kg dichlorvos given orally caused apnea within 10 min with a progressive decrease in heart rate. Blood AChE activity decreased to zero within 10 min. Respirations and AChE activity did not recover. The mean dichlorvos concentration rose to a peak of 0.66 μg/ml. Treated monkeys received 1.2mg/kg OpdA iv immediately after poisoning with dichlorvos. In Opda-treated animals, heart and respiratory rates were unchanged from baseline over a 240-minute observation period. AChE activity slowly declined, but remained above 25% of baseline for the entire duration. Dichlorvos concentrations reached a mean peak of 0.19 μg/ml at 40 min after poisoning and decreased to a mean of 0.05 μg/ml at 240 min. These results show that OpdA hydrolyzes dichlorvos in an African green monkey model of lethal poisoning, delays AChE inhibition, and prevents lethality.

Drug development for the management of organophosphorus poisoning

INTRODUCTION

The continuous application of organophosphate pesticides in developing countries, in addition to the remaining stock piles of chemical warfare nerve agents and their possible use is a significant threat to the public. Yet, today's options for a treatment of organophosphorus poisonings are still inadequate.

AREAS COVERED

This article provides a concise overview of current and future research trying to improve both prophylaxis and treatment of organophosphorus intoxications. The authors provide a summary of current oxime therapy and highlight several new concepts to overcome existing gaps. This overview of therapeutic options is accompanied by two sections on cyclodextrins, related compounds and bioscavengers, which may be used for either prophylaxis or treatment. For both groups, the authors review current drug design and screening approaches, the resulting developments and future challenges.

EXPERT OPINION

While the search for one multipotent oxime has been a fruitless endeavor, combination of multiple oximes with complemental and systemic reactivity appears as a valuable concept. Development of potential scavengers, be it cyclodextrins or bioscavengers, is still hampered by insufficient efficacy of these compounds. Future strategies will aim at improving their catalytic efficacy while minimizing immunogenicity.

Acetylcholinesterase inhibitors: pharmacology and toxicology

Acetylcholinesterase is involved in the termination of impulse transmission by rapid hydrolysis of the neurotransmitter acetylcholine in numerous cholinergic pathways in the central and peripheral nervous systems. The enzyme inactivation, induced by various inhibitors, leads to acetylcholine accumulation, hyperstimulation of nicotinic and muscarinic receptors, and disrupted neurotransmission. Hence, acetylcholinesterase inhibitors, interacting with the enzyme as their primary target, are applied as relevant drugs and toxins. This review presents an overview of toxicology and pharmacology of reversible and irreversible acetylcholinesterase inactivating compounds. In the case of reversible inhibitors being commonly applied in neurodegenerative disorders treatment, special attention is paid to currently approved drugs (donepezil, rivastigmine and galantamine) in the pharmacotherapy of Alzheimer's disease, and toxic carbamates used as pesticides. Subsequently, mechanism of irreversible acetylcholinesterase inhibition induced by organophosphorus compounds (insecticides and nerve agents), and their specific and nonspecific toxic effects are described, as well as irreversible inhibitors having pharmacological implementation. In addition, the pharmacological treatment of intoxication caused by organophosphates is presented, with emphasis on oxime reactivators of the inhibited enzyme activity administering as causal drugs after the poisoning. Besides, organophosphorus and carbamate insecticides can be detoxified in mammals through enzymatic hydrolysis before they reach targets in the nervous system. Carboxylesterases most effectively decompose carbamates, whereas the most successful route of organophosphates detoxification is their degradation by corresponding phosphotriesterases.

Optimization of methyl parathion biodegradation and detoxification by cells in suspension or immobilized on tezontle expressing the opd gene

The goal of this study was to optimize methyl parathion (O,O-dimethyl-O-4-p-nitrophenyl phosphorothioate) degradation using a strain of Escherichia coli DH5α expressing the opd gene. Our results indicate that this strain had lower enzymatic activity compared to the Flavobacterium sp. ATCC 27551 strain from which the opd gene was derived. Both strains were assessed for their ability to degrade methyl parathion (MP) in a mineral salt medium with or without the addition of glucose either as suspended cells or immobilized on tezontle, a volcanic rock. MP was degraded by both strains with similar efficiencies, but immobilized cells degraded MP more efficiently than cells in suspension. However, the viability of E. coli cells was much higher than that of the Flavobacterium sp. We confirmed the decrease in toxicity from the treated effluents through acetylcholinesterase activity tests, indicating the potential of this method for the treatment of solutions containing MP.

Chromatographic analysis of toxic phosphylated oximes (POX): a brief overview

Poisoning with organophosphorus compounds (OP), e.g. pesticides and nerve agents, causes inhibition of acetylcholinesterase (AChE) by phosphylation of the active site serine residue. Consequently, accumulation of stimulating acetylcholine in the synaptic cleft induces cholinergic crisis which ultimately may lead to death. For standard causal therapy, enzyme reactivators are administered representing oxime derivatives of quarternary pyridinium compounds, e.g. pralidoxime (2-PAM), obidoxime and HI 6. The mechanism of action includes removal of the phosphyl moiety by a nucleophilic attack of the oximate molecule substituting the enzyme and forming a phosphylated oxime (POX). POX is produced in stoichiometric amounts of reactivated enzyme and exhibits a significantly enhanced toxicity (inhibition rate constant) when compared to the parent OP. However, stability of POX under physiological conditions appears to be highly limited. Nevertheless, the presence of POX reveals a potential critical issue for both therapeutic efficacy in vivo and pharmacokinetic and pharmacodynamic (PK-PD) modelling based on cholinesterase activity data. Detailed characterization represents an important need for elaboration of the entire oxime pharmacology.Nevertheless, reports on POX toxicity and analysis are quite rare and may therefore be indicative of the challenge of POX analysis. This review provides a concise overview of chromatographic approaches applied to POX separation. Chromatography represents the key technology for POX purification and quantification in kinetic in vitro studies using buffers and biological fluids. Applications based on reversed-phase chromatography (RPC), ion pair chromatography (IPC) and an affinity approach as well as thin layer chromatography (TLC) are discussed and novel applications and data are presented.

Phosphotriesterase variants with high methylphosphonatase activity and strong negative trade-off against phosphotriesters

The most lethal organophosphorus nerve agents (NA), like sarin, soman, agent-VX and Russian-VX, share a methylphosphonate moiety. Pseudomonas diminuta phosphotriesterase (PTE) catalyses the hydrolysis of methylphosphonate NA analogues with a catalytic efficiency orders of magnitude lower than that towards the pesticide paraoxon. With a view to obtaining PTE variants that more readily accept methylphosphonate NA, ~75,000 PTE variants of the substrate-binding residues Gly-60, Ile-106, Leu-303 and Ser-308 were screened with fluorogenic analogues of the NA Russian-VX and cyclosarin. Seven new PTE variants were isolated, purified and their k(cat)/K(M) determined against five phosphotriesters and five methylphosphonate analogues of sarin, cyclosarin, soman, agent-VX and Russian-VX. The novel PTE variants exhibited as much as a 10-fold increase in activity towards the methylphosphonate compounds--many reaching a k(cat)/K(M) of 10⁶ M⁻¹ s⁻¹--and as much as a 29,000-fold decrease in their phosphotriesterase activity. The mutations found in two of the variants, SS0.5 (G60V/I106L/S308G) and SS4.5 (G60V/I106A/S308G), were modelled into a high-resolution structure of PTE-wild type and docked with analogues of cyclosarin and Russian-VX using Autodock 4.2. The kinetic data and docking simulations suggest that the increase in activity towards the methylphosphonates and the loss of function against the phosphotriesters were due to an alteration of the shape and hydrophobicity of the binding pocket that hinders the productive binding of non-chiral racemic phosphotriesters, yet allows the binding of the highly asymmetric methylphosphonates.

Kinetics and efficacy of an organophosphorus hydrolase in a rodent model of methyl-parathion poisoning

OBJECTIVES

Organophosphorus (OP) pesticides exert a tremendous health burden, particularly in the developing world. Limited resources, the severity of intentional OP ingestions, and a paucity of beneficial therapies all contribute to the morbidity and mortality of this broad class of chemicals. A novel theoretical treatment for OP poisoning is the use of an enzyme to degrade the parent OP in the circulation after poisoning. The aims of this study were to determine the pharmacokinetics and efficacy of an OP hydrolase (OpdA) in a rodent model of severe methyl-parathion poisoning.

METHODS

Two animal models were used. First, Wistar rats were administered two different doses of the hydrolase (0.15 and 1.5 mg/kg), and the ex vivo hydrolytic activity of plasma was determined by a fluorometric method. Second, an oral methyl-parathion animal poisoning model was developed to mimic severe human poisoning, and the efficacy of postpoisoning OpdA (as measured by survival to 4 and 24 hours) was determined.

RESULTS

The half-life of OpdA in the Wistar rat was dependent on the dose administered and ranged between 45.0 and 57.9 minutes. The poisoning model of three times the lethal dose to 50% of the population (3 x LD(50)) of methyl-parathion resulted in 88% lethality at 4 and 24 hours. Using a single dose of 0.15 mg/kg OpdA 10 minutes after poisoning resulted in 100% survival at 4 hours (p = 0.001 vs. placebo), but 0% at 24 hours postpoisoning (p = NS vs. placebo).

CONCLUSIONS

The OP hydrolase OpdA exhibits pharmacokinetics suitable for repeated dosing and increases short-term survival after severe methyl-parathion poisoning.

Pharmacokinetics of OpdA, an organophosphorus hydrolase, in the African green monkey

Organophosphorus (OP) pesticides are a broad class of acetylcholinesterase inhibitors that are responsible for tremendous morbidity and mortality worldwide, contributing to an estimated 300,000 deaths annually. Current pharmacotherapy for acute OP poisoning includes the use of atropine, an oxime, and benzodiazepines. However, even with such therapy, the mortality from these agents are as high as 40%. Enzymatic hydrolysis of OPs is an attractive new potential therapy for acute OP poisoning. A number of bacterial OP hydrolases have been isolated. A promising OP hydrolase is an enzyme isolated from Agrobacterium radiobacter, named OpdA. OpdA has been shown to decrease lethality in rodent models of parathion and dichlorvos poisoning. However, pharmacokinetic data have not been obtained. In this study, we examined the pharmacokinetics of OpdA in an African Green Monkey model. At a dose of 1.2mg/kg the half-life of OpdA was approximately 40 min, with a mean residence time of 57 min. As expected, the half-life did not change with the dose of OpdA given: at doses of 0.15 and 0.45 mg/kg, the half-life of OpdA was 43.1 and 38.9 min, respectively. In animals subjected to 5 daily doses of OpdA, the residual activity that was measured 24h after each OpdA dose increased 5-fold for the 0.45 mg/kg dose and 11-fold for the 1.2mg/kg dose. OpdA exhibits pharmacokinetics favorable for the further development as a therapy for acute OP poisoning, particularly for hydrophilic OP pesticides. Future work to increase the half-life of OpdA may be beneficial.

Inhibition, recovery and oxime-induced reactivation of muscle esterases following chlorpyrifos exposure in the earthworm Lumbricus terrestris

Assessment of wildlife exposure to organophosphorus (OP) pesticides generally involves the measurement of cholinesterase (ChE) inhibition, and complementary biomarkers (or related endpoints) are rarely included. Herein, we investigated the time course inhibition and recovery of ChE and carboxylesterase (CE) activities in the earthworm Lumbricus terrestris exposed to chlorpyrifos, and the ability of oximes to reactivate the phosphorylated ChE activity. Results indicated that these esterase activities are a suitable multibiomarker scheme for monitoring OP exposure due to their high sensitivity to OP inhibition and slow recovery to full activity levels following pesticide exposure. Moreover, oximes reactivated the inhibited ChE activity of the earthworms exposed to 12 and 48 mg kg(-1) chlorpyrifos during the first week following pesticide exposure. This methodology is useful for providing evidence for OP-mediated ChE inhibition in individuals with a short history of OP exposure (< or = 1 week); resulting a valuable approach for assessing multiple OP exposure episodes in the field.

Serum albumins and detoxication of anti-cholinesterase agents

Serum albumin displays an esterase activity that is capable of hydrolysing the anti-cholinesterase compounds carbaryl, paraoxon, chlorpyrifos-oxon, diazoxon and O-hexyl, O-2,5-dichlorphenyl phosphoramidate. The detoxication of all these anti-cholinesterase compounds takes place at significant rates with substrate concentrations in the same order of magnitude as expected during in vivo exposures, even when these substrate concentrations are between 15 and 1300 times lower than the recorded K(m) constants. Our data suggest that the efficacy of this detoxication system is based on the high concentration of albumin in plasma (and in the rest of the body), and not on the catalytic efficacy itself, which is low for albumin. We conclude the need for a structure-activity relationship study into the albumin-associated esterase activities because this protein is universally present in vertebrates and could compensate for reduced levels of other esterases, i.e., lipoprotein paraoxonase, in some species. It is also remarkable that the biotransformation of xenobiotics can be reliably studied in vitro, although conditions as similar as possible to in vivo situations are necessary.

Toxic effects of diazinon and its photodegradation products

The toxic effects of diazinon and its irradiated solutions were investigated using cultivated human blood cells (lymphocytes and erythrocytes) and skin fibroblasts. Ultra Performance Liquid Chromatography (UPLC)-UV/VIS system was used to monitor the disappearance of starting diazinon during 115-min photodegradation and formation of its by-products (diazoxon and 2-isopropyl-6-methyl-4-pyrimidinol (IMP)) as a function of time. Dose-dependent AChE and Na(+)/K(+)-ATPase inhibition by diazinon was obtained for all investigated cells. Calculated IC(50) (72 h) values, in M, were: 7.5x10(-6)/3.4x10(-5), 8.7x10(-5)/6.6x10(-5), and 3.0x10(-5)/4.6x10(-5) for fibroblast, erythrocyte and lymphocyte AChE/Na(+)/K(+)-ATPase, respectively. Results obtained for reference commercially purified target enzymes indicate similar sensitivity of AChE towards diazinon (IC(50) (20 min)-7.8x10(-5)M), while diazinon concentrations below 10mM did not noticeably affect Na(+)/K(+)-ATPase activity. Besides, diazinon and IMP induced increasing incidence of micronuclei (via clastogenic mode of action) in a dose-dependent manner up to 2x10(-6)M and significant inhibition of cell proliferation and increased level of malondialdehyde at all investigated concentrations. Although after 15-min diazinon irradiation formed products do not affect purified commercial enzymes activities, inhibitory effect of irradiated solutions on cell enzymes increased as a function of time exposure to UV light and resulted in significant reduction of AChE (up to 28-45%) and Na(+)/K(+)-ATPase (up to 35-40%) at the end of irradiation period. Moreover, photodegradation treatment strengthened prooxidative properties of diazinon as well as its potency to induce cytogenetic damage.

Tissue distribution, isozyme abundance and sensitivity to chlorpyrifos-oxon of carboxylesterases in the earthworm Lumbricus terrestris

A laboratory-based study was conducted to determine the basal carboxylesterase (CbE) activity in different tissues of the earthworm Lumbricus terrestris, and its sensitivity to the organophosphate (OP) pesticide chlorpyrifos-oxon (CPx). Carboxylesterase activity was found in the pharynx, crop, gizzard, anterior intestine, wall muscle and reproductive tissues of L. terrestris, and multiple tissue-specific isozymes were observed by native gel electrophoresis. Esterase activity and sensitivity to CPx inhibition varied on a tissue- and substrate-specific basis, suggesting isoforms-specific selectivity to OP-mediated inhibition. Three practical issues are recommended for the use of earthworm CbE activity as a biomarker of pesticide exposure: (i) CbE should be measured using several routine substrates, (ii) it should be determined in selected tissues instead of whole organism homogenate, and (iii) earthworm CbE activity should be used in conjuncture with other common biomarkers (e.g., ChE) within a multibiomarker approach to assess field exposure of OPs, and potentially other agrochemicals.

Serum albumin is as efficient as paraxonase in the detoxication of paraoxon at toxicologically relevant concentrations

Human serum albumin was able to hydrolyze the organophosphorus compounds paraoxon, chlorpyrifos-oxon, and diazoxon at toxicologically relevant concentrations. Human serum displayed two paraoxon hydrolyzing activities: the so-called paraoxonase, which is associated with the lipoprotein fraction and is calcium dependent and EDTA sensitive, and the activity associated with albumin, which is EDTA resistant and sensitive to fatty acids. Human serum albumin hydrolyzed these compounds with the same relative efficacy as lipoproteins (chlorpyrifos-oxon > diazoxon > paraoxon). The capability of detoxication of activity associated with human serum albumin was similar or even higher than paraoxonase associated with lipoproteins in the case of paraoxon at concentrations as low as those noted in an acute in vivo intoxication. However, paraoxonase activity associated with lipoprotein was more effective than paraoxonase activity associated with albumin at toxicologically relevant chlorpyrifos-oxon concentrations. These results explain why mice deficient in paraoxonase associated with lipoprotein are not more sensitive to paraoxon than wild animals.

Stereospecific hydrolysis of a phosphoramidate as a model to understand the role of biotransformation in the neurotoxicity of chiral organophosphorus compounds

Calcium-dependent and EDTA-resistant hydrolyses of R and S isomers of O-hexyl O-2,5-dicholorophenyl phosphoramidate (HDCP) were observed in serum and subcellular fractions of liver, kidney and brain from hen, rat and rabbit. In serum, the Ca(2+)-dependent hydrolysis was much higher in rabbit than in other species. Liver showed a higher activity than kidney and brain. The S-HDCP isomer was hydrolysed to a higher extent than the other isomer. The fact that this stereospecificity favours the S-isomer is more clearly observed in rabbit serum, and in rat and rabbit liver particulate fractions. In such tissues and species, the EDTA-resistant hydrolysis was not stereospecific. Soluble fractions of rat brain and of hen liver, kidney and brain, showed a lower total activity but with a higher proportion of EDTA-resistant activity and a higher hydrolysis of the R-HDCP isomer. The Ca(2+)-dependent stereoselective biodegradation of S-HDCP is dominant in the most active tissues in rabbit and rat. It can therefore be concluded that S-HDCP would be biodegraded faster than R-HDCP. Furthermore, R-HDCP is the isomer that will remain at a higher proportion to be available for interaction with the target of neurotoxicity.

Comparative hydrolysis of O-hexyl O-2,5-dichlorophenyl phosphoramidate and paraoxon in different tissues of vertebrates

The Ca(2+)-dependent and EDTA-resistant hydrolysis of O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) and paraoxon was studied in serum and subcellular fractions of liver, kidney and brain of hen, rat and rabbit. HDCP was the best substrate among all the tissues studied, except that of rabbit serum which showed the highest Ca(2+)-dependent paraoxon hydrolysing activity (paraoxonase). High HDCP hydrolysing activity (HDCPase) was detected in the brain tissue of the three species studied, whereas low or no paraoxonase was found. The HDCPase/paraoxonase ratio of Ca(2+)-dependent hydrolysing activities ranged from 0.5 to 83 for tissues of the same species. EDTA-resistant HDCPase activity was more than 50% of the total activities in hen tissues, with an almost undetectable Ca(2+)-dependent paraoxonase activity in most organs. The same response was observed in rat tissues, except for serum where the Ca(2+)-dependent HDCPase and paraoxonase activities were higher (70 and 25% of total activities, respectively). EDTA-resistant HDCPase and paraoxonase activities represented less than 25% of all activities in rabbit tissues. Paraoxon has traditionally been the substrate for measuring organophosphorus hydrolysing activities. However, HDCP could be a good substrate in addition to paraoxon for monitoring other phosphotriesterases in biological tissues.

The stability of organophosphorus insecticides in fresh blood

We investigated the stability of 14 organophosphorus insecticides: dichlorvos, fenitrothion, cyanophos, malathion, phenthoate, methidathion, dimethoate, thiometon, isoxathion, diazinon, trichlorfon, EPN, acephate and sulprofos, in fresh blood. The organophosphorus compounds, except for sulprofos, decomposed over time at 37 degrees C, with varying decomposition speed for each compound. Methyl phosphate types (dichlorvos) decomposed most rapidly, followed by methyl thiophosphate types (fenitrothion and cyanophos) and methyl dithiophosphate types (methidathion, dimethoate and thiometon). Methyl thiophosphate types decomposed faster than ethyl thiophosphate types (isoxathion and diazinon). Of the five methyl dithiophosphate type insecticides (malathion, phenthoate, methidathion, dimethoate and thiometon), the compounds with a carboxylic ester bond (malathion and phenthoate) decomposed faster than the others. Compounds left standing at 37 degrees C decomposed faster than those left standing at 4 degrees C. Temperature has a great effect on the decomposition of organophosphorus insecticides in blood. However, the order of the decomposition speeds of each compound was approximately the same at different temperatures. In cases of suspected organophosphate poisoning, it should be considered that the blood concentration of the compound might decrease during the postmortem interval.

Methyl-parathion and organophosphorous pesticide metabolites modify the activation status and interleukin-2 secretion of human peripheral blood mononuclear cells

Organophosphorous (OP) compounds are the most commonly used pesticides. There are reports on susceptibility to the toxic effects of OP pesticides, but no information exists regarding the toxicity of their metabolites. To determine the metabolites' contribution to the OP pesticide immunotoxic effects, human peripheral blood mononuclear cells (PBMCs) were treated with the parent compound methyl-parathion (MP) and the following OP pesticide alkyl-phosphorous metabolites: diethylphosphate (DEP), diethylthiophosphate (DETP), diethyldithiophosphate (DEDTP), dimethylphosphate (DMP), and dimethyldithiophosphate (DMDTP). Activation and function of the PBMCs were examined by assessment of phytohemagglutinin (PHA)-induced proliferative response, interleukin-2 (IL-2) secretion, and CD25 and CD69 expression. Treatments with DMP, DEP, DETP and DEDTP for 48h produced significant toxicity in human PBMCs, but did not affect their proliferative response to PHA. Only MP reduced cell proliferation by 30%. DEDTP decreased the proportion of PBMCs expressing CD25. This effect was associated with a reduction of IL-2 secretion, which was also reduced by MP and DMP treatments. In contrast, DETP and DEDTP treatments increased the expression of CD69. DMP, DETP and DEDTP were more consistently involved in modulating the PBMC response to PHA.

Expression, purification, and characterization of a novel methyl parathion hydrolase

The mpd gene coding for a novel methyl parathion hydrolase (MPH) was previously reported and its putative open reading frame was also identified. To further confirm its coding region, the intact region encoding MPH was obtained by PCR and expressed in Escherichia coli as a hexa-His C-terminal fusion protein. The fusion protein was purified to homogeneity by metal-affinity chromatography. The enzyme activity and zymogram assay showed that the fusion protein was functional in degrading methyl parathion. The amino terminal sequencing of the purified recombinant MPH indicated that a signal peptide of the first 35 amino acids was cleaved from its precursor to form active MPH. A rat polyclonal antiserum was raised against the purified mature fusion protein. The results of Western blot and zymogram demonstrated that mature MPH in native Plesiomonas sp. strain M6 was also processed from its precursor by cleavage of a putative signal peptide at the amino terminus. The production of active MPH in E. coli was greatly improved after the coding region for the signal peptide was deleted. HPLC gel filtration of the purified mature recombinant MPH revealed that the MPH was a monomer.

Future applications of phosphotriesterases in the prophylaxis and treatment of organophosporus insecticide and nerve agent poisonings

Organophosphorus compounds (OPs) are being used as insecticides and warfare agents. OP insecticides represent an important problem of public health, causing around 200,000 deaths annually. The World Health Organization has pointed to the necessity to introduce new medical practices that improve the results of classical treatments. Many studies have shown that the administration of phosphotriesterases (enzymes that detoxify OPs through hydrolysis) is a promising treatment of persons poisoned with OPs. Such an enzyme-based treatment might introduce important improvements in the treatment of patients having ingested large amounts of OPs. Phosphotriesterases might also be suitable for prophylactic treatment of persons at risk to be severely exposed. The new experimental treatments do not exhibit the intrinsic neurotoxicity of the classical prophylaxis based on carbamates and antimuscarinic drugs. Experimental data suggest that might be time to initiate clinical trials in order to study the efficacy of phosphotriesterases in the therapy and prophylaxis of OP intoxication.

Utilization of monocrotophos as phosphorus source by Pseudomonas aeruginosa F10B and Clavibacter michiganense subsp. insidiosum SBL 11

Monocrotophos (dimethyl (E)-1-methyl-2-(methylcarbamoyl) vinyl phosphate, or MCP), an organophosphorus insecticide, was used as a sole phosphorus source by the microorganisms isolated from the soil. None of the isolates could utilize MCP as a sole source of carbon. Two of the potential microbial isolates, Pseudomonas aeruginosa F10B and Clavibacter michiganense subsp. insidiosum SBL 11, could utilize MCP as a sole source of phosphorus. Pseudomonas aeruginosa F10B showed a lag phase of 4 h, while in the case of C. michiganense subsp. insidiosum SBL 11, it was 8 h when cultured in the presence of MCP. The generation time for both strains was increased in the medium containing MCP. It was 2.15 h for P. aeruginosa F10B in MCP medium as compared with 1.29 h in basal medium, while in case of C. michiganense subsp. insidiosum SBL 11 it was increased to 3.4 h in MCP medium as compared with 1.28 h in basal medium. These two strains were able to degrade technical MCP in shake-flask culture up to 98.9 and 86.9%, respectively, and pure MCP up to 79 and 80%, respectively, within 24 h at 37 degrees C. The optimal concentration of MCP required for the normal growth was 500 ppm. In the substrate preference study, Tris-p-nitrophenyl phosphate was the most preferred substrate followed by paraoxon. The enzyme responsible for the break down of MCP was phosphotriesterase, which was localized on the membrane-bound fraction of the disrupted cells. The gene responsible for the production of phosphotriesterase (opd) in P. aeruginosa F10B was plasmid-borne.

Enzymes involved in the detoxification of organophosphorus, carbamate and pyrethroid insecticides through hydrolysis

The most employed insecticides for indoor and agriculture purposes belong to carbamates, pyrethroid or organophosphates. The chemical structures of these three groups correspond to carbamic, carboxylic and triphosphoric esters. Technical monographs suggest that the hydrolysis of ester bonds of carbamates and pyrethroids plays an important role in the detoxification of these compounds. However, detailed studies about enzymes hydrolysing carbamates and pyrethroids in vertebrates are not available. Certain carbamate hydrolysing activities are associated to serum albumin. Phosphotriesterases, being of an unknown physiological role, hydrolyse (in some cases stereospecifically) organophosphorus insecticides (OP). Phosphotriesterases have been found in a multitude of species, from mammals to bacteria. A phosphotriesterase activity, EDTA-resistant, has been detected in serum albumin. Phosphotriesterases in serum of mammals display polymorphisms. Phosphotriesterases offer applications in therapy of organophosphorus poisonings, in biodegradation and bioremedation of organophosphates. Similar studies should be developed with enzymes hydrolysing pyrethroids and carbamate insecticides. Such studies will improve the knowledge of the detoxification routes in non-target species and will help to design specific and safer carbamate and pyrethroid insecticides.