Concentration-Dependent Kinetics of Acetylcholinesterase Inhibition by the Organophosphate Paraoxon

Human Plasma Butyrylcholinesterase Hydrolyzes Atropine: Kinetic and Molecular Modeling Studies

The participation of butyrylcholinesterase (BChE) in the degradation of atropine has been recurrently addressed for more than 70 years. However, no conclusive answer has been provided for the human enzyme so far. In the present work, a steady-state kinetic analysis performed by spectrophotometry showed that highly purified human plasma BChE tetramer slowly hydrolyzes atropine at pH 7.0 and 25 °C. The affinity of atropine for the enzyme is weak, and the observed kinetic rates versus the atropine concentration was of the first order: the maximum atropine concentration in essays was much less than Km. Thus, the bimolecular rate constant was found to be kcat/Km = 7.7 × 104 M-1 min-1. Rough estimates of catalytic parameters provided slow kcat < 40 min-1 and high Km = 0.3-3.3 mM. Then, using a specific organophosphoryl agent, echothiophate, the time-dependent irreversible inhibition profiles of BChE for hydrolysis of atropine and the standard substrate butyrylthiocholine (BTC) were investigated. This established that both substrates are hydrolyzed at the same site, i.e., S198, as for all substrates of this enzyme. Lastly, molecular docking provided evidence that both atropine isomers bind to the active center of BChE. However, free energy perturbations yielded by the Bennett Acceptance Ratio method suggest that the L-atropine isomer is the most reactive enantiomer. In conclusion, the results provided evidence that plasma BChE slowly hydrolyzes atropine but should have no significant role in its metabolism under current conditions of medical use and even under administration of the highest possible doses of this antimuscarinic drug.

Phytochemical Screening of Rosmarinus officinalis L. as a Potential Anticholinesterase and Antioxidant-Medicinal Plant for Cognitive Decline Disorders

The inhibition of acetylcholinesterase (AChE) by cholinergic agents has been promoted as a potent strategy for treating and managing cognitive decline disorders. A wide range of natural products has long been used as potential sources or formulations of cholinergic inhibitors. Therefore, this study aimed to evaluate different Rosmarinus officinalis L. (R. officinalis) extracts for their AChE inhibitory activity using galanthamine as a standard AChE inhibitor. In this study, the ethyl-acetate extract (at a concentration of 250 µg/mL) exhibited the greatest inhibitory effect against AChE with significant inhibition of 75%, comparable to the inhibitor galanthamine with an inhibition of 88%. Kinetic analysis revealed that the extracts could induce a mixed type of inhibition, as observed in the case of galanthamine, with the highest increased Km and decreased Vmax values in the ethyl acetate extract. The antioxidant potential of the three extracts tested was found to be in the order of ethyl-acetate > ethanol > aqueous, with IC₅₀ values of 272 µg/mL, 387 µg/mL, and 534 µg/mL, respectively. Ethyl-acetate was found to have the highest total phenolic content in all extracts. Further, in silico study showed structural binding characterization of rosmarinic acid and carnosic acid with human AChE enzyme. Rosmarinic acid showed strong binding and formed two hydrogen-bonding interactions with Ser-293 and Arg-296. In light of this, the ethyl-acetate extract of the plant may provide some novel potential pharmacological leads for treating and managing cognitive disorders such as Alzheimer's.

Interspecies comparison of plasma metabolism and sample stabilization for quantitative bioanalyses: Application to (R)-CE3F4 in preclinical development, including metabolite identification by high-resolution mass spectrometry

The CE3F4 is an inhibitor of the type 1 exchange protein directly activated by cAMP (EPAC1), which is involved in numerous signaling pathways. The inhibition of EPAC1 shows promising results in vitro and in vivo in different cardiac pathological situations like hypertrophic signaling, contributing to heart failure, or arrhythmia. An HPLC-UV method with a simple and fast sample treatment allowed the quantification of (R)-CE3F4. Sample treatment consisted of simple protein precipitation with 50 µL of ethanol and 150 µL of acetonitrile for a 50 µL biological sample. Two wavelengths were used according to the origin of plasma (220 or 250 nm for human samples and 250 nm for murine samples). Accuracy profile was evaluated for both wavelengths, and the method was in agreement with the criteria given by the EMA in the guideline for bioanalytical method validation for human and mouse plasma samples. The run time was 12 min allowing the detection of the (R)-CE3F4 and a metabolite. This study further permitted understanding the behavior of CE3F4 in plasma by highlighting an important difference between humans and rodents on plasma metabolism and may impact future in vivo studies related to this molecule and translation of results between animal models and humans. Using paraoxon as a metabolism inhibitor was crucial for the stabilization of (R)-CE3F4 in murine samples. HPLC-UV and HPLC-MS/MS studies were conducted to confirm metabolite structure and consequently, the main metabolic pathway in murine plasma.

Covalent inhibition of hAChE by organophosphates causes homodimer dissociation through long-range allosteric effects

Acetylcholinesterase (EC 3.1.1.7), a key acetylcholine-hydrolyzing enzyme in cholinergic neurotransmission, is present in a variety of states in situ, including monomers, C-terminally disulfide-linked homodimers, homotetramers, and up to three tetramers covalently attached to structural subunits. Could oligomerization that ensures high local concentrations of catalytic sites necessary for efficient neurotransmission be affected by environmental factors? Using small-angle X-ray scattering (SAXS) and cryo-EM, we demonstrate that homodimerization of recombinant monomeric human acetylcholinesterase (hAChE) in solution occurs through a C-terminal four-helix bundle at micromolar concentrations. We show that diethylphosphorylation of the active serine in the catalytic gorge or isopropylmethylphosphonylation by the R_P enantiomer of sarin promotes a 10-fold increase in homodimer dissociation. We also demonstrate the dissociation of organophosphate (OP)-conjugated dimers is reversed by structurally diverse oximes 2PAM, HI6, or RS194B, as demonstrated by SAXS of diethylphosphoryl-hAChE. However, binding of oximes to the native ligand-free hAChE, binding of high-affinity reversible ligands, or formation of an S_P-sarin-hAChE conjugate had no effect on homodimerization. Dissociation monitored by time-resolved SAXS occurs in milliseconds, consistent with rates of hAChE covalent inhibition. OP-induced dissociation was not observed in the SAXS profiles of the double-mutant Y337A/F338A, where the active center gorge volume is larger than in wildtype hAChE. These observations suggest a key role of the tightly packed acyl pocket in allosterically triggered OP-induced dimer dissociation, with the potential for local reduction of acetylcholine-hydrolytic power in situ. Computational models predict allosteric correlated motions extending from the acyl pocket toward the four-helix bundle dimerization interface 25 Å away.

Prediction of dose-dependent in vivo acetylcholinesterase inhibition by profenofos in rats and humans using physiologically based kinetic (PBK) modeling-facilitated reverse dosimetry

Organophosphate pesticides (OPs) are known to inhibit acetylcholine esterase (AChE), a critical effect used to establish health-based guidance values. This study developed a combined in vitro–in silico approach to predict AChE inhibition by the OP profenofos in rats and humans. A physiologically based kinetic (PBK) model was developed for both species. Parameter values for profenofos conversion to 4-bromo-2-chlorophenol (BCP) were derived from in vitro incubations with liver microsomes, liver cytosol, and plasma from rats (catalytic efficiencies of 1.1, 2.8, and 0.19 ml/min/mg protein, respectively) and humans (catalytic efficiencies of 0.17, 0.79, and 0.063 ml/min/mg protein, respectively), whereas other chemical-related parameter values were derived using in silico calculations. The rat PBK model was evaluated against literature data on urinary excretion of conjugated BCP. Concentration-dependent inhibition of rat and human AChE was determined in vitro and these data were translated with the PBK models to predicted dose-dependent AChE inhibition in rats and humans in vivo. Comparing predicted dose-dependent AChE inhibition in rats to literature data on profenofos-induced AChE inhibition revealed an accurate prediction of in vivo effect levels. Comparison of rat predictions (BMDL10 of predicted dose–response data of 0.45 mg/kg bw) and human predictions (BMDL10 of predicted dose–response data of 0.01 mg/kg bw) suggests that humans are more sensitive than rats, being mainly due to differences in kinetics. Altogether, the results demonstrate that in vivo AChE inhibition upon acute exposure to profenofos was closely predicted in rats, indicating the potential of this novel approach method in chemical hazard assessment.

Slow-binding inhibitors of acetylcholinesterase of medical interest

Certain ligands slowly bind to acetylcholinesterase. As a result, there is a slow establishment of enzyme-inhibitor equilibrium characterized by a slow onset of inhibition prior reaching steady state. Three mechanisms account for slow-binding inhibition: a) slow binding rate constant k_on, b) slow ligand induced-fit following a fast binding step, c) slow conformational selection of an enzyme form. The slow equilibrium may be followed by a chemical step. This later that can be irreversible has been observed with certain alkylating agents and substrate transition state analogs. Slow-binding inhibitors present long residence times on target. This results in prolonged pharmacological or toxicological action. Through several well-known molecules (e.g. huperzine) and new examples (tocopherol, trifluoroacetophenone and a 6-methyluracil alkylammonium derivative), we show that slow-binding inhibitors of acetylcholinesterase are promising drugs for treatment of neurological diseases such as Alzheimer disease and myasthenia gravis. Moreover, they may be of interest for neuroprotection (prophylaxis) against organophosphorus poisoning. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Poly(2-oxazoline)s with a 2,2'-Iminodiacetate End Group Inhibit and Stabilize Laccase

Poly(2-oxazoline)s (POxs) with 2,2'-iminodiacetate (IDA) end groups were investigated as inhibitors for laccase. The polymers with the IDA end groups are reversible, competitive inhibitors for this enzyme. The IC50 values were found to be in a range of 1-3 mm. Compared with IDA alone, the activity was increased by a factor of more than 30; thus indicating that attaching a polymer chain to an inhibitor can already improve the activity of the former. The enzyme activity drops to practically zero upon increasing the concentration of the most active telechelic inhibitor, IDA-PEtOx30 -IDA (PEtOx: poly(2-ethyl-2-oxazoline)), from 5 to 8 mm. This unusual behavior was investigated by means of dynamic light scattering, which showed specific aggregation above 5 mm. Furthermore, the laccase could be stabilized in the presence of POx-IDA, upon addition at a concentration of 20 mm and higher. Whereas laccase becomes completely inactive at room temperature after one week, the stabilized laccase is fully active for at least a month in aqueous solution.

Studies Exploring the Interaction of the Organophosphorus Compound Paraoxon with Fullerenes

In vitro experiments previously published demonstrated the ability of fullerenes to decrease the capability of organophosphorus (OP) compounds to inhibit acetylcholinesterase. Experiments described herein demonstrate molecular level affinity interactions between fullerenes and the OP test compound paraoxon with NMR spectroscopy. The calculated binding constant of 19 M^-1 indicates that this binding was not covalent.

The Targeted Pesticides as Acetylcholinesterase Inhibitors: Comprehensive Cross-Organism Molecular Modelling Studies Performed to Anticipate the Pharmacology of Harmfulness to Humans In Vitro

Commercially available pesticides were examined as Mus musculus and Homo sapiens acetylcholinesterase (mAChE and hAChE) inhibitors by means of ligand-based (LB) and structure-based (SB) in silico approaches. Initially, the crystal structures of simazine, monocrotophos, dimethoate, and acetamiprid were reproduced using various force fields. Subsequently, LB alignment rules were assessed and applied to determine the inter synaptic conformations of atrazine, propazine, carbofuran, carbaryl, tebufenozide, imidacloprid, diuron, monuron, and linuron. Afterwards, molecular docking and dynamics SB studies were performed on either mAChE or hAChE, to predict the listed pesticides' binding modes. Calculated energies of global minima (E_{glob_min}) and free energies of binding (∆G_binding) were correlated with the pesticides' acute toxicities (i.e., the LD₅₀ values) against mice, as well to generate the model that could predict the LD₅₀s against humans. Although for most of the pesticides the low E_{glob_min} correlates with the high acute toxicity, it is the ∆G_binding that conditions the LD₅₀ values for all the evaluated pesticides. Derived pLD₅₀ = f(∆G_binding) mAChE model may predict the pLD₅₀ against hAChE, too. The hAChE inhibition by atrazine, propazine, and simazine (the most toxic pesticides) was elucidated by SB quantum mechanics (QM) DFT mechanistic and concentration-dependent kinetic studies, enriching the knowledge for design of less toxic pesticides.

Acetylcholinesterase Inhibition and Antioxidant Activity of N-trans-Caffeoyldopamine and N-trans-Feruloyldopamine

Phenolic acids and their derivatives found in nature are well-known for their potential biological activity. In this study, two amides derived from trans-caffeic/ferulic acid and dopamine were synthesized and characterized by Fourier-transform infrared spectroscopy (FTIR), mass spectrometry, proton and carbon-13 nuclear magnetic resonance spectroscopy. The compounds were tested for the inhibition of acetylcholinesterase (AChE) from Electrophorus electricus and for antioxidant activity by scavenging 2,2-diphenyl-1-pycrylhydrazyl free radical (DPPH^•) and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) radical cation (ABTS^•+), reducing ferric ions, and ferrous ions chelation. N-trans-Feruloyldopamine displayed the highest inhibitory effect on AChE with half-maximal inhibitory concentration (IC₅₀) values of 8.52 μM. In addition, an in silico study was done to determine the most favorable AChE cluster with the synthesized compounds. Further, these clusters were investigated for binding positions at the lowest free binding energy. Both synthesized hydroxycinnamates were found to be better antioxidants than the parent acids in in vitro tests applied. N-trans-Caffeoyldopamine showed the best antioxidant activity in the three tested methods—against non-biological stable free radicals IC₅₀ 5.95 μM for DPPH^•, 0.24 μM for the ABTS^•+ method, and for reducing power (ascorbic acid equivalent (AAE) 822.45 μmol/mmol)—while for chelation activity against Fe²⁺ ions N-trans-feruloyldopamine had slightly better antioxidant activity (IC₅₀ 3.17 mM).

Modeling and simulation of organophosphate-induced neurotoxicity: Prediction and validation by experimental studies

Exposure to organophosphorus (OP) compounds, either pesticides or chemical warfare agents, represents a major health problem. As potent irreversible inhibitors of cholinesterase, OP may induce seizures, as in status epilepticus, and occasionally brain lesions. Although these compounds are extremely toxic agents, the search for novel antidotes remains extremely limited. In silico modeling constitutes a useful tool to identify pharmacological targets and to develop efficient therapeutic strategies. In the present work, we developed a new in silico simulator in order to predict the neurotoxicity of irreversible inhibitors of acetyl- and/or butyrylcholinesterase (ChE) as well as the potential neuroprotection provided by antagonists of cholinergic muscarinic and glutamate N-methyl-d-aspartate (NMDA) receptors. The simulator reproduced firing of CA1 hippocampal neurons triggered by exposure to paraoxon (POX), as found in patch-clamp recordings in in vitro mouse hippocampal slices. In the case of POX intoxication, it predicted a preventing action of the muscarinic receptor antagonist atropine sulfate, as well as a synergistic action with the non-competitive NMDA receptor antagonist memantine. These in silico predictions relative to beneficial effects of atropine sulfate combined with memantine were recapitulated experimentally in an in vivo model of POX in adult male Swiss mice using electroencephalic (EEG) recordings. Thus, our simulator is a new powerful tool to identify protective therapeutic strategies against OP central effects, by screening various combinations of muscarinic and NMDA receptor antagonists.

Optimal detection of cholinesterase activity in biological samples: modifications to the standard Ellman's assay

Ellman's assay is the most commonly used method to measure cholinesterase activity. It is cheap, fast, and reliable, but it has limitations when used for biological samples. The problems arise from 5,5-dithiobis(2-nitrobenzoic acid) (DTNB), which is unstable, interacts with free sulfhydryl groups in the sample, and may affect cholinesterase activity. We report that DTNB is more stable in 0.09 M Hepes with 0.05 M sodium phosphate buffer than in 0.1M sodium phosphate buffer, thereby notably reducing background. Using enzyme-linked immunosorbent assay (ELISA) to enrich tissue homogenates for cholinesterase while depleting the sample of sulfhydryl groups eliminates unwanted interactions with DTNB, making it possible to measure low cholinesterase activity in biological samples. To eliminate possible interference of DTNB with enzyme hydrolysis, we introduce a modification of the standard Ellman's assay. First, thioesters are hydrolyzed by cholinesterase to produce thiocholine in the absence of DTNB. Then, the reaction is stopped by a cholinesterase inhibitor and the produced thiocholine is revealed by DTNB and quantified at 412 nm. Indeed, this modification of Ellman's method increases butyrylcholinesterase activity by 20 to 25%. Moreover, high stability of thiocholine enables separation of the two reactions of the Ellman's method into two successive steps that may be convenient for some applications.

Time-dependent kinetic complexities in cholinesterase-catalyzed reactions

Cholinesterases (ChEs) display a hysteretic behavior with certain substrates and inhibitors. Kinetic cooperativity in hysteresis of ChE-catalyzed reactions is characterized by a lag or burst phase in the approach to steady state. With some substrates damped oscillations are shown to superimpose on hysteretic lags. These time dependent peculiarities are observed for both butyrylcholinesterase and acetylcholinesterase from different sources. Hysteresis in ChE-catalyzed reactions can be interpreted in terms of slow transitions between two enzyme conformers E and E'. Substrate can bind to E and/or E', both Michaelian complexes ES and Ε'S can be catalytically competent, or only one of them can make products. The formal reaction pathway depends on both the chemical structure of the substrate and the type of enzyme. In particular, damped oscillations develop when substrate exists in different, slowly interconvertible, conformational, and/or micellar forms, of which only the minor form is capable of binding and reacting with the enzyme. Biphasic pseudo-first-order progressive inhibition of ChEs by certain carbamates and organophosphates also fits with a slow equilibrium between two reactive enzyme forms. Hysteresis can be modulated by medium parameters (pH, chaotropic and kosmotropic salts, organic solvents, temperature, osmotic pressure, and hydrostatic pressure). These studies showed that water structure plays a role in hysteretic behavior of ChEs. Attempts to provide a molecular mechanism for ChE hysteresis from mutagenesis studies or crystallographic studies failed so far. In fact, several lines of evidence suggest that hysteresis is controlled by the conformation of His438, a key residue in the catalytic triad of cholinesterases. Induction time may depend on the probability of His438 to adopt the operative conformation in the catalytic triad. The functional significance of ChE hysteresis is puzzling. However, the accepted view that proteins are in equilibrium between preexisting functional and non-functional conformers, and that binding of a ligand to the functional form shifts equilibrium towards the functional conformation, suggests that slow equilibrium between two conformational states of these enzymes may have a regulatory function in damping out the response to certain ligands and irreversible inhibitors. This is particularly true for immobilized (membrane bound) enzymes where the local substrate and/or inhibitor concentrations depend on influx in crowded organellar systems, e.g. cholinergic synaptic clefts. Therefore, physiological or toxicological relevance of the hysteretic behavior and damped oscillations in ChE-catalyzed reactions and inhibition cannot be ruled out.

Synthesis and anti-acetylcholinesterase properties of novel β- and γ-substituted alkoxy organophosphonates

Activated organophosphate (OP) insecticides and chemical agents inhibit acetylcholinesterase (AChE) to form OP-AChE adducts. Whereas the structure of the OP correlates with the rate of inhibition, the structure of the OP-AChE adduct influences the rate at which post-inhibitory reactivation or aging phenomena occurs. In this report, we prepared a panel of β-substituted ethoxy and γ-substituted propoxy phosphonoesters of the type p-NO(2)PhO-P(X)(R)[(O(CH(2))(n)Z] (R=Me, Et; X=O, S; n=2, 3; Z=halogen, OTs) and examined the inhibition of three AChEs by select structures in the panel. The β-fluoroethoxy methylphosphonate analog (R=Me, Z=F, n=2) was the most potent anti-AChE compound comparable (ki ∼6 × 10(6)M(-1)min(-1)) to paraoxon against EEAChE. Analogs with Z=Br, I, or OTs were weak inhibitors of the AChEs, and methyl phosphonates (R=Me) were more potent than the corresponding ethyl phosphonates (R=Et). As expected, analogs with a thionate linkage (PS) were poor inhibitors of the AChEs.

Novel selective and irreversible mosquito acetylcholinesterase inhibitors for controlling malaria and other mosquito-borne diseases

We reported previously that insect acetylcholinesterases (AChEs) could be selectively and irreversibly inhibited by methanethiosulfonates presumably through conjugation to an insect-specific cysteine in these enzymes. However, no direct proof for the conjugation has been published to date, and doubts remain about whether such cysteine-targeting inhibitors have desirable kinetic properties for insecticide use. Here we report mass spectrometric proof of the conjugation and new chemicals that irreversibly inhibited African malaria mosquito AChE with bimolecular inhibition rate constants (k_inact/K_I) of 3,604–458,597 M⁻¹sec⁻¹ but spared human AChE. In comparison, the insecticide paraoxon irreversibly inhibited mosquito and human AChEs with k_inact/K_I values of 1,915 and 1,507 M⁻¹sec⁻¹, respectively, under the same assay conditions. These results further support our hypothesis that the insect-specific AChE cysteine is a unique and unexplored target to develop new insecticides with reduced insecticide resistance and low toxicity to mammals, fish, and birds for the control of mosquito-borne diseases.

Determination of acetylcholinesterase activity by the Ellman assay: a versatile tool for in vitro research on medical countermeasures against organophosphate poisoning

Inhibition of acetylcholinesterase (AChE) is the main mechanism of action of organophosphorus compounds (OP), and AChE reactivators (oximes) are at present the only causal therapeutic approach. Being the key target of OP toxicity, AChE may serve as a valuable tool for diagnosis of OP exposure as well as for the investigation of the kinetics of interactions between OP and oximes. At present, the rapid, simple, and cheap spectrophotometric Ellman assay is widely used for diagnosis, therapeutic monitoring and in vitro kinetic investigations. Application of the assay for investigation of the interactions between AChE, inhibitors, and oximes requires the consideration of potential matrix effects (e.g. hemoglobin), side reactions (e.g. oximolysis of substrate) and other determinants (e.g. pH, temperature). By taking these factors into account, the Ellman assay allows the precise and reproducible determination of kinetic constants as a basis for the understanding of toxic OP effects and for the development of improved therapies against poisoning by OP. In addition, advanced applications of the Ellman assay, for example, in a dynamic in vitro model for the real-time activity determination of membrane-bound AChE, enables the proper investigation of relevant tissue, primarily respiratory muscle, and extends the applicability of this method.

Chlorpyrifos-oxon disrupts zebrafish axonal growth and motor behavior

Axonal morphology is a critical determinant of neuronal connectivity, and perturbation of the rate or extent of axonal growth during development has been linked to neurobehavioral deficits in animal models and humans. We previously demonstrated that the organophosphorus pesticide (OP) chlorpyrifos (CPF) inhibits axonal growth in cultured neurons. In this study, we used a zebrafish model to determine whether CPF, its oxon metabolite (CPFO), or the excreted metabolite trichloro-2-pyridinol (TCPy) alter spatiotemporal patterns of axonal growth in vivo. Static waterborne exposure to CPFO, but not CPF or TCPy, at concentrations ≥ 0.03 μM from 24- to 72-h post fertilization significantly inhibited acetylcholinesterase, and high-performance liquid chromatography detected significantly more TCPy in zebrafish exposed to 0.1 μM CPFO versus 1.0 μM CPF. These data suggest that zebrafish lack the metabolic enzymes to activate CPF during these early developmental stages. Consistent with this, CPFO, but not CPF, significantly inhibited axonal growth of sensory neurons, primary motoneurons, and secondary motoneurons at concentrations ≥ 0.1 μM. Secondary motoneurons were the most sensitive to axonal growth inhibition by CPFO, which was observed at concentrations that did not cause mortality, gross developmental defects, or aberrant somatic muscle differentiation. CPFO effects on axonal growth correlated with adverse effects on touch-induced swimming behavior, suggesting the functional relevance of these structural changes. These data suggest that altered patterns of neuronal connectivity contribute to the developmental neurotoxicity of CPF and demonstrate the relevance of zebrafish as a model for studying OP developmental neurotoxicity.

Mixture effects of nickel and chlorpyrifos on Folsomia candida (Collembola) explained from development of toxicity in time

Two reference models are commonly used to predict mixture toxicity, Concentration Addition and Independent Action. For accurately predicting mixture effects, both reference models need a full description of the dose-response curve for all single chemicals present in the mixture. We studied the mixture effect of nickel and chlorpyrifos on survival of the soil-dwelling collembolan Folsomia candida exposed for 7weeks in Lufa 2.2 soil. Especially chlorpyrifos toxicity showed extremely steep dose-response curves, making it impossible to fulfil the assumptions required by both reference models. Nevertheless, we showed that by monitoring the development of toxicity in time for both the single compounds and the mixtures it was possible to determine mixture effects. Even at exposure levels below the No Effect Concentration, chlorpyrifos was found to reduce nickel toxicity. In addition, this study clearly showed that toxicity is a dynamic process and that mixture effects may be dependent on exposure time.

Paradox findings may challenge orthodox reasoning in acute organophosphate poisoning

It is generally accepted that inhibition of acetylcholinesterase (AChE) is the most important acute toxic action of organophosphorus compounds, leading to accumulation of acetylcholine followed by a dysfunction of cholinergic signaling. However, the degree of AChE inhibition is not uniformly correlated with cholinergic dysfunction, probably because the excess of essential AChE varies among tissues. Moreover, the cholinergic system shows remarkable plasticity, allowing modulations to compensate for dysfunctions of the canonical pathway. A prominent example is the living (-/-) AChE knockout mouse. Clinical experience indicates that precipitous inhibition of AChE leads to more severe poisoning than more protracted yet finally complete inhibition. The former situation is seen in parathion, the latter in oxydemeton methyl poisoning. At first glance, this dichotomy is surprising since parathion is a pro-poison and has to be activated to the oxon, while the latter is still the ultimate inhibitor. Also oxime therapy in organophosphorus poisoning apparently gives perplexing results: Oximes are usually able to reactivate diethylphosphorylated AChE, but the efficiency may be occasionally markedly smaller than expected from kinetic data. Dimethylphosphorylated AChE is in general less amenable to oxime therapy, which largely fails in some cases of dimethoate poisoning where aging was much faster than expected from a dimethylphosphorylated enzyme. Similarly, poisoning by profenofos, an O,S-dialkyl phosphate, leads to a rapidly aged enzyme. Most surprisingly, these patients were usually well on admission, yet their erythrocyte AChE was completely inhibited. Analysis of the kinetic constants of the most important reaction pathways, determination of the reactant concentrations in vivo and comparison with computer simulations may reveal unexpected toxic reactions. Pertinent examples will be presented and the potentially underlying phenomena discussed.

An evaluation of the inhibition of human butyrylcholinesterase and acetylcholinesterase by the organophosphate chlorpyrifos oxon

Acetylcholinesterase (EC 3.1.1.7) and butyrylcholinesterase (EC 3.1.1.8) are enzymes that belong to the superfamily of alpha/beta-hydrolase fold proteins. While they share many characteristics, they also possess many important differences. For example, whereas they have about 54% amino acid sequence identity, the active site gorge of acetylcholinesterase is considerably smaller than that of butyrylcholinesterase. Moreover, both have been shown to display simple and complex kinetic mechanisms, depending on the particular substrate examined, the substrate concentration, and incubation conditions. In the current study, incubation of butyrylthiocholine in a concentration range of 0.005-3.0 mM, with 317 pM human butyrylcholinesterase in vitro, resulted in rates of production of thiocholine that were accurately described by simple Michaelis-Menten kinetics, with a K(m) of 0.10 mM. Similarly, the inhibition of butyrylcholinesterase in vitro by the organophosphate chlorpyrifos oxon was described by simple Michaelis-Menten kinetics, with a k(i) of 3048 nM(-1) h(-1), and a K(D) of 2.02 nM. In contrast to inhibition of butyrylcholinesterase, inhibition of human acetylcholinesterase by chlorpyrifos oxon in vitro followed concentration-dependent inhibition kinetics, with the k(i) increasing as the inhibitor concentration decreased. Chlorpyrifos oxon concentrations of 10 and 0.3 nM gave k(i)s of 1.2 and 19.3 nM(-1) h(-1), respectively. Although the mechanism of concentration-dependent inhibition kinetics is not known, the much smaller, more restrictive active site gorge of acetylcholinesterase almost certainly plays a role. Similarly, the much larger active site gorge of butyrylcholinesterase likely contributes to its much greater reactivity towards chlorpyrifos oxon, compared to acetylcholinesterase.

Chlorpyrifos and chlorpyrifos-oxon inhibit axonal growth by interfering with the morphogenic activity of acetylcholinesterase

A primary role of acetylcholinesterase (AChE) is regulation of cholinergic neurotransmission by hydrolysis of synaptic acetylcholine. In the developing nervous system, however, AChE also functions as a morphogenic factor to promote axonal growth. This raises the question of whether organophosphorus pesticides (OPs) that are known to selectively bind to and inactivate the enzymatic function of AChE also interfere with its morphogenic function to perturb axonogenesis. To test this hypothesis, we exposed primary cultures of sensory neurons derived from embryonic rat dorsal root ganglia (DRG) to chlorpyrifos (CPF) or its oxon metabolite (CPFO). Both OPs significantly decreased axonal length at concentrations that had no effect on cell viability, protein synthesis or the enzymatic activity of AChE. Comparative analyses of the effects of CPF and CPFO on axonal growth in DRG neurons cultured from AChE nullizygous (AChE -/-) versus wild type (AChE +/+) mice indicated that while these OPs inhibited axonal growth in AChE+/+ DRG neurons, they had no effect on axonal growth in AChE -/- DRG neurons. However, transfection of AChE -/- DRG neurons with cDNA encoding full-length AChE restored the wild type response to the axon inhibitory effects of OPs. These data indicate that inhibition of axonal growth by OPs requires AChE, but the mechanism involves inhibition of the morphogenic rather than enzymatic activity of AChE. These findings suggest a novel mechanism for explaining not only the functional deficits observed in children and animals following developmental exposure to OPs, but also the increased vulnerability of the developing nervous system to OPs.

Concentration-dependent binding of chlorpyrifos oxon to acetylcholinesterase

The organophosphorus insecticides have been known for many years to cause cholinergic crisis in humans as a result of the inhibition of the critical enzyme acetylcholinesterase. The interactions of the activated, toxic insecticide metabolites (termed oxons) with acetylcholinesterase have been studied extensively for decades. However, more recent studies have suggested that the interactions of certain anticholinesterase organophosphates with acetylcholinesterase are more complex than previously thought since their inhibitory capacity has been noted to change as a function of inhibitor concentration. In the present report, chlorpyrifos oxon (O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphate) was incubated with human recombinant acetylcholinesterase in the presence of p-nitrophenyl acetate in order to better characterize kinetically the interactions of this oxon with enzyme. Determination of the dissociation constant, Kd, and the phophorylation rate constant, k2, for chlorpyrifos oxon with a range of oxon and p-nitrophenyl acetate concentrations revealed that Kd, but not k2, changed as a function of oxon concentration. Changes in p-nitrophenyl acetate concentrations did not alter these same kinetic parameters. The inhibitory capacity of chlorpyrifos oxon, as measured by ki (k2/Kd), was also affected as a result of the concentration-dependent alterations in binding affinity. These results suggest that the concentration-dependent interactions of chlorpyrifos oxon with acetylcholinesterase resulted from a different mechanism than the concentration-dependent interactions of acetylthiocholine. In the latter case, substrate bound to the peripheral anionic site of acetylcholinesterase has been shown to reduce enzyme activity by blocking the release of the product thiocholine from the active site gorge. With chlorpyrifos oxon, the rate of release of 3,5,6-trichloro-2-pyridinol is irrelevant since the active site is not available to interact with other oxon molecules after phosphorylation of Ser-203 has occurred.

Preparation and characterization of dialkylphosphoryl-obidoxime conjugates, potent anticholinesterase derivatives that are quickly hydrolyzed by human paraoxonase (PON1192Q)

The potential of the most active pyridinium-4-aldoximes, such as obidoxime and trimedoxime, to reactivate phosphorylated acetylcholinesterase is not fully exploited because of inevitable formation of phosphoryloximes (POXs) with extremely high anticholinesterase activity. Hence, a topochemical equilibrium is expected at the active site, with the freshly reactivated enzyme being rapidly re-inhibited by POX produced during reactivation. In the present study, dimethylphosphoryl-, diethylphosphoryl-, and diisopropyl-obidoxime conjugates were generated and isolated in substance. Their inhibition rate of acetylcholinesterase from human red cell membranes was by a factor of 2250, 480 and 600 higher than that observed with paraoxon-methyl, paraoxon-ethyl, and diisopropyl phosphorofluoridate, respectively. All three POXs were hydrolyzed by human paraoxonase (PON1), with the alloenzyme PON1192Q being about 50-fold more active than PON1192R. The rate of hydrolysis, yielding obidoxime, was 1:6:0.03 for the three POXs, respectively. The rate of non-enzymic degradation, yielding obidoxime mononitrile, was similar with the three POXs and showed a high dependency on the reaction temperature (activation energy 83 kJ/mol), while enzymic hydrolysis required less energy (16 kJ/mol). To determine POX-hydrolase activity, we preferred a reaction temperature of 20 degrees C to reduce the noise of spontaneous degradation. A plot of POX-hydrolase versus salt-stimulated paraoxonase activity showed a highly discriminating power towards the PON1Q192R alloenzymes, which may be based on repulsive forces of the quaternary nitrogen atoms of the protonated arginine subtype and the bisquaternary POXs. It is concluded that the pharmacogenetic PON1Q192R polymorphism may be another contributor to the large variability of susceptible subjects seen in obidoxime-treated patients.

Characterization of a Novel Cell Wall-anchored Protein with Carboxylesterase Activity Required for Virulence in Mycobacterium tuberculosis

Pooled mutant competition assays have shown that the Mycobacterium tuberculosis MT2282 gene (Rv2224c, annotated as encoding a proteinase) is required for bacterial survival in mice. To understand the mechanism of this requirement, we conducted a genetic and biochemical study of the MT2282 gene and its product. MT2282 encodes a member of the microbial esterase/lipase family with active site consensus sequences of G-X-S-X-G, and we have concluded that the MT2282 protein is, in fact, a cell wall-associated carboxylesterase rather than a proteinase, as initially annotated. The MT2282 gene product preferentially hydrolyzes ester bonds of substrates with intermediate carbon chain length. Purified MT2282 is a monomer with enzymatic catalysis properties that fit in the Michaelis-Menten kinetic model. Esterase activity was inhibited by paraoxon and dichlorvos. Replacement of Ser215, Asp450, and His477 by Ala in the consensus motifs completely abolishes esterase activity, suggesting that Ser215-Asp450-His477 forms a catalytic triad with Ser215 as an active site residue. To evaluate the role of the MT2282 in pathogenesis, the gene was deleted from the M. tuberculosis genome. BALB/c mouse aerosol infections showed reduced colony-forming unit loads in lungs and spleens and less lung pathology for the DeltaMT2282 mutant. High dose intravenous infection of mice with the mutant resulted in a significantly delayed time to death compared with the wild type or complemented mutant. These results indicate that MT2282 encodes a cell wall-associated carboxylesterase, which is required for full virulence of M. tuberculosis. We propose that MT2282 (Rv2224c) and its adjacent paralogous gene MT2281 (Rv2223c) be named caeA and caeB respectively, for carboxylesterase A and B.

Concentration-dependent interactions of the organophosphates chlorpyrifos oxon and methyl paraoxon with human recombinant acetylcholinesterase

For many decades it has been thought that oxygen analogs (oxons) of organophosphorus insecticides phosphorylate the catalytic site of acetylcholinesterase by a mechanism that follows simple Michaelis-Menten kinetics. More recently, the interactions of at least some oxons have been shown to be far more complex and likely involve binding of oxons to a second site on acetylcholinesterase that modulates the inhibitory capacity of other oxon molecules at the catalytic site. The current study has investigated the interactions of chlorpyrifos oxon and methyl paraoxon with human recombinant acetylcholinesterase. Both chlorpyrifos oxon and methyl paraoxon were found to have k(i)'s that change as a function of oxon concentration. Furthermore, 10 nM chlorpyrifos oxon resulted in a transient increase in acetylthiocholine hydrolysis, followed by inhibition. Moreover, in the presence of 100 nM chlorpyrifos oxon, acetylthiocholine was found to influence both the K(d) (binding affinity) and k(2) (phosphorylation constant) of this oxon. Collectively, these results demonstrate that the interactions of chlorpyrifos oxon and methyl paraoxon with acetylcholinesterase cannot be described by simple Michaelis-Menten kinetics but instead support the hypothesis that these oxons bind to a secondary site on acetylcholinesterase, leading to activation/inhibition of the catalytic site, depending on the nature of the substrate and inhibitor. Additionally, these data raise questions regarding the adequacy of estimating risk of low levels of insecticide exposure from direct extrapolation of insecticide dose-response curves since the capacity of individual oxon molecules at low oxon levels could be greater than individual oxon molecules in vivo associated with the dose-response curve.

Testing of antidotes for organophosphorus compounds: experimental procedures and clinical reality

According to current knowledge, inhibition of acetylcholinesterase (AChE) is a very important toxic action of organphosphorus compounds (OP). Hence, it is obvious to follow the AChE activity in order to quantify the degree of inhibition and to assess possible reactivation. Red blood cell (RBC)-AChE provides an easily accessible source to follow the AChE status also in humans. There are many reports underlining the appropriateness of RBC-AChE as a surrogate parameter that mirrors the synaptic enzyme. With this tool at hand, we can study the kinetic parameters of inhibition, spontaneous and oxime-induced reactivation, as well as aging with human RBCs under physiological conditions in vitro. Moreover, we can simulate the influence of inhibitor and reactivator on enzyme activity and can calculate what happens when both components change with time. Finally, we can correlate under controlled conditions the AChE-status in intoxicated patients with the clinical signs and symptoms and determine the time-dependent changes of the oxime and OP concentration. Data of a clinical trial performed in Munich to analyze the value of obidoxime has elucidated that obidoxime worked as expected from in vitro studies. Following a 250mg bolus, obidoxime was administered by continuous infusion at 750mg/24h aimed at maintaining a plasma concentration of 10-20microM obidoxime. This oxime concentration reactivated RBC-AChE>20% of normal in most cases of OP poisoning by diethylphosphoryl compounds within a few hours. The degree of reactivation fitted theoretical calculations very well when the obidoxime and paraoxon concentrations were fed into the model. Only in a few cases reactivation was much lower than expected. The reason for this effect is probably based on the polymorphism of paraoxonase (PON1) in that the (192)arginine phenotype does hardly hydrolyze the arising diethylphosphoryl obidoxime. While this variable may complicate a proper assessment even more, we are confident that the in vitro evaluation of all relevant kinetic data enables the prediction of probable effectiveness in humans. These studies also help to understand therapeutic failures and to define scenarios where oximes are virtually ineffective. These include poisonings with rapidly aging phosphylated AChE, late start with an effective oxime and too early discontinuation of oximes in poisonings with a persistent OP. The experience gathered with the experimental and therapeutic approaches to human poisoning by OP pesticides may be helpful when oximes have to be selected against nerve agents.