Effects of human serum albumin on post-mortem changes of malathion.

Malathion, diethyl 2-[(dimethoxyphosphinothioyl)thio]butanedioate, is one of most widely used organophosphoryl pesticide, and it has been detected in several clinical cases of accidental exposure and suicide. It is reported that the observed malathion concentration in blood of persons who suffer from malathion poisoning is smaller than the expected concentration. Because malathion is bound to human serum albumin (HSA), recovery of malathion in the free form is insufficient. We detected malathion adducts in HSA by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q/TOF-MS). The mass spectra showed that malathion was preferably bound to the lysine (K) and cysteinylproline (CP) residues of HSA. The K- and CP-adducts of malathion were increased in vitro with a dose-dependent fashion when its concentration was smaller than the lethal dose. Further, the K-adduct was also detected in post-mortem blood of an autopsied subject suffering from intentional malathion ingestion. These results suggest that the K-adduct seems to be available to use a biomarker of malathion poisoning, and the determination of the K-adduct could make possible to estimate the amount of malathion ingestion.

Malathion dermal permeability in relation to dermal load: Assessment by physiologically based pharmacokinetic modeling of in vivo human data.

Estimates of dermal permeability (Kp), obtained by fitting an updated human PBPK model for malathion to previously reported data on excreted urinary metabolites after 29 volunteers were dermally exposed to measured values of [14C]malathion dermal load (L), were used to examine the empirical relationship between Kp and L. The PBPK model was adapted from previously reported human biokinetic and PBPK models for malathion, fit to previously reported urinary excretion data after oral [14C]malathion intake by volunteers, and then augmented to incorporate a standard Kp approach to modeling dermal-uptake kinetics. Good to excellent PBPK-model fits were obtained to all of 29 sets of cumulative urinary metabolite-excretion data (ave. [±1 SD] R2 = 0.953 [±0.064]). Contrary to the assumption that Kp and L are independent typically applied for dermally administered liquids or solutions, the 29 PBPK-based estimates of Kp obtained for malathion exhibit a strong positive association with the 2/3rds power of L (log-log Pearson correlation = 0.925, p = ∼0). Possible explanations of this observation involving physico-chemical characteristics and/or in vivo cutaneous effects of malathion are discussed. The PBPK model presented, and our observation that Kp estimates obtained by fitting this model to human experimental urinary-excretion data correlate well with L2/3, allow more realistic assessments of absorbed and metabolized dose during or after a variety of scenarios involving actual or potential dermal or multi-route malathion exposures, including for pesticide workers or farmers who apply malathion to crops.

Exposure to environmentally relevant concentrations of malathion induces significant cellular, biochemical and histological alterations in Labeo rohita.

The extensive use of malathion, an organophosphate pesticide, raises the possibility of its undesirable toxicity to non-target organisms. Agricultural run-off and vector control sprays are the major sources of exposure to this pesticide for aquatic organisms. Some earlier studies have reported the presence of malathion at concentrations ranging from 18.12µg/L to 105.2µg/L in various water samples. In this study, we have tested the hypothesis that these sub-lethal yet environmentally significant concentrations of malathion has serious toxicological implications on the fingerlings of Labeo rohita. Exposure to increasing concentration of malathion (10, 50 and 100µg/L) was reflected in the serum concentration of the pesticide and also in the inhibition of acetylcholinesterase activity in fish brains. Increased abnormalities in liver function test coupled with a rise in the oxidative stress response were observed in gills, liver and kidney. However, the increase in antioxidant enzyme activities like superoxide dismutase and glutathione-S-transferase by malathion exposure suggested a hormetic response. Tissue injury due to malathion was evident from the morphological and nuclear anomalies in the H-E stained sections of gill, liver and kidney. Cell cycle analysis of these organs further fortified the histopathological findings. This study elucidates the sub-lethal toxicity of environmentally relevant malathion concentrations on Labeo rohita which indicates the potential health hazard posed to human beings consuming this fish. This calls for careful application of malathion in areas adjoining to inland fisheries.

Nutritional aspects of detoxification in clinical practice.

Detoxification is a vital cellular task that, if lacking, can lead to early morbidity and mortality. The process of detoxification involves the mobilization, biotransformation, and elimination of toxicants of exogenous and endogenous origin. This article discusses the phase I and phase II detoxification and biotransformation pathways and promotes using food to support these highly complex processes. The author identifies the comprehensive elimination diet as a useful therapeutic tool for clinicians and patients to use to achieve detoxification. Using this diet, the patient removes the most common allergenic foods and beverages from the diet and replaces them with nonallergenic choices for a period of 4 wk, gradually adding back the eliminated foods and observing their effects. Another effective clinical tool that the author discusses is the detox-focused core food plan, which identifies the variety of foods required to supply key nutrients that can maximize the effectiveness of detoxification. Finally, the author provides a case study in which these tools were used to help a patient suffering from major, debilitating illnesses that resulted from exposure to malathion, including severe vomiting and diarrhea, headaches, night sweats, severe arthralgias and myalgias, episcleritis, and shortness of breath. The article details the interventions used and the clinical results (ie, successful resolution of most issues after 3 mo).

Contribution of carboxylesterase and cytochrome P450 to the bioactivation and detoxification of isocarbophos and its enantiomers in human liver microsomes.

Organophosphorus pesticides are the most widely used pesticides in modern agricultural systems to ensure good harvests. Isocarbophos (ICP), with a potent acetylcholinesterase inhibitory effect is widely utilized to control a variety of leaf-eating and soil insects. However, the characteristics of the bioactivation and detoxification of ICP in humans remain unclear. In this study, the oxidative metabolism, esterase hydrolysis, and chiral inversion of ICP in human liver microsomes (HLMs) were investigated with the aid of a stereoselective LC/MS/MS method. The depletion of ICP in HLMs was faster in the absence of carboxylesterase inhibitor (BNPP) than in the presence of NADPH and BNPP, with t1/2 of 5.2 and 90 min, respectively. Carboxylesterase was found to be responsible for the hydrolysis of ICP, the major metabolic pathway. CYP3A4, CYP1A2, CYP2D6, CYP2C9, and CYP2C19 were all involved in the secondary metabolism pathway of desulfuration of ICP. Flavin-containing monooxygenase (FMO) did not contribute to the clearance of ICP. The hydrolysis and desulfuration of (±)ICP, (+)ICP, and (-)ICP in HLMs follow Michaelis-Menten kinetics. Individual enantiomers of ICP and its oxidative desulfuration metabolite isocarbophos oxon (ICPO) were found to be inhibitors of acetylcholinesterases at different extents. For example, (±)ICPO is more potent than ICP (IC50 0.031µM vs. 192µM), whereas (+)ICPO is more potent than (-)ICPO (IC50 0.017µM vs. 1.55µM). Given the finding of rapid hydrolysis of ICP and low abundance of oxidative metabolites presence in human liver, the current study highlights that human liver has a greater capacity for detoxification of ICP.

Evaluation of in vitro absorption, decontamination and desorption of organophosphorous compounds from skin and synthetic membranes.

Chemical warfare agents, such as soman, and pesticides, such as chlorpyrifos, dichlorvos or malathion, are toxic organophosphorous compounds (OPCs) that are readily absorbed by the skin. Decontamination using solvents or surfactants may modify the cornified layer - the skin's main barrier against xenobiotic penetration. Thus, effective skin decontamination with fewer side effects is desired. We determined the membrane absorption, decontamination and desorption of toxic OPCs using human skin and synthetic membrane (cuprophane, cellulose acetate, methyl ethyl cellulose, acetophane and nylon) models, and estimated the efficacy of adsorptive powders (bentonite and magnesium trisilicate) at inhibiting this transfer. Using validated flow-through and static diffusion cell and HPLC methods, we found that the transfer of OPCs depends on their membrane affinity. The chlorpyrifos transfer decreased with a decrease in the membrane hydrophilicity, and that of malathion across hydrophilic membranes was less than half of that across hydrophobic membranes. We reliably modeled the toxicant transfer through the skin and synthetic membranes as first-order kinetic and/or square root law transfer processes, suggesting a potential application of synthetic membranes for predicting percutaneous absorption of OPCs. All tested adsorptive powders, applied either alone or as mixtures, significantly reduced the toxicant amount transferred across all membrane models, suggesting a potential therapeutic application with fewer later undesired effects on intact skin.

Absorption and excretion of organophosphorous insecticide biomarkers of malathion in the rat: implications for overestimation bias and exposure misclassification from environmental biomonitoring.

Malathion is an organophosphorous (OP) insecticide widely used for crop protection. Its degradates, malathiondiacid (MDA), malathion monoacid (MMA), dimethylphosphate (DMP), dimethylthiophosphate (DMTP) and dimethyldithiophosphate (DMDTP) are formed in strawberries and other produce. These same chemical biomarkers are measured in urine in human studies as quantitative measures of exposure. The excretion of malathion and its common biomarkers including MDA, MMA, DMP, DMTP and DMDTP at equal molar doses (73 µmol/kg b.w.) was studied following oral dosing of female Holtzmann rats (240-300 g). Following MDA administration, 36.3±5.4% was recovered as MDA, 0.05±0.02% as DMP, 5.5±0.3% as DMTP, 3.8±2.9% as DMDTP (mole percent), and totally 45.6±7.0% of administered dose in urine after 120 h (over 94% in the first 24h). Following DMTP administration, 8.3±7.7% was recovered as DMP, 46.6±16.5% as DMTP, and totally 55.0±10.3% of administered dose in urine after 120 h (over 92% in the first 24h). Similar results were obtained with other malathion biomarkers. Preformed biomarkers of malathion and other OP insecticides when ingested in produce are readily absorbed and excreted. Low level human dietary and non-occupational urine biomonitoring studies will be confounded by preformed pesticide biomarkers used to infer potential human pesticide exposure. This has profound implications for epidemiology studies where subject's biomarker excretion is used as a surrogate for OP exposures that cannot be related to a particular insecticide residue.

Transcriptional regulation of xenobiotic detoxification in Drosophila.

Living organisms, from bacteria to humans, display a coordinated transcriptional response to xenobiotic exposure, inducing enzymes and transporters that facilitate detoxification. Several transcription factors have been identified in vertebrates that contribute to this regulatory response. In contrast, little is known about this pathway in insects. Here we show that the Drosophila Nrf2 (NF-E2-related factor 2) ortholog CncC (cap 'n' collar isoform-C) is a central regulator of xenobiotic detoxification responses. A binding site for CncC and its heterodimer partner Maf (muscle aponeurosis fibromatosis) is sufficient and necessary for robust transcriptional responses to three xenobiotic compounds: phenobarbital (PB), chlorpromazine, and caffeine. Genetic manipulations that alter the levels of CncC or its negative regulator, Keap1 (Kelch-like ECH-associated protein 1), lead to predictable changes in xenobiotic-inducible gene expression. Transcriptional profiling studies reveal that more than half of the genes regulated by PB are also controlled by CncC. Consistent with these effects on detoxification gene expression, activation of the CncC/Keap1 pathway in Drosophila is sufficient to confer resistance to the lethal effects of the pesticide malathion. These studies establish a molecular mechanism for the regulation of xenobiotic detoxification in Drosophila and have implications for controlling insect populations and the spread of insect-borne human diseases.

Acute toxicity and tissue distributions of malathion in Ambystoma tigrinum.

The kinetics of the bioaccumulation of malathion (O,O-dimethyl phosphorodithioate of diethyl mercaptosuccinate) and the biological impact of exposure for tiger salamanders, Ambystoma tigrinum, were assessed through exposure to soil surface contaminated with 50 microg/cm(2) or 100 microg/cm(2 )malathion and ingestion of an earthworm exposed to soil contaminated with 200 microg/cm(2) malathion. Malathion and malaoxon burdens in salamanders sampled at different times after exposure(s) were measured by gas chromatography in four tissue/organ subgroups: liver, epaxial muscle, pooled viscera (except the liver and brain), and pooled avisceral carcass (muscle, skin, and bone). The total tiger salamander xenobiotic burdens were calculated from these data. The malathion/malaoxon burden 1 day after exposure was greatest in the avisceral carcass and 2 days after exposure was greatest in the viscera. Bioconcentration and bioaccumulation factors remained less than unity throughout the experiment and did not support the hypothesis of bioaccumulation of malathion in the tiger salamander. Biological impact was assessed with a colorimetric brain cholinesterase microassay. Brain cholinesterase activities in salamanders exposed to malathion-contaminated soil (50 microg/cm(2) or 100 microg/cm(2 )malathion) were suppressed approximately 50-65% and 90%, respectively, compared to unexposed controls. The exposed animals did not exhibit overt clinical signs of malathion toxicosis.

Assessment of the effect of varying soil organic matter content on the bioavailability of malathion to the common nightcrawler, Lumbricus terrestris L.

This study investigated the effect of soil organic matter content on the bioavailability of malathion to the common nightcrawler, Lumbricus terrestris. Earthworms were exposed for 72 h to malathion on two soil types, 8% organic matter and 55% organic matter. Two different measures of bioavailability, malathion body burdens and tissue cholinesterase activities, were then measured in the malathion exposed animals. There were no significant differences in body burden or cholinesterase levels in L. terrestris exposed to malathion on soils with differing organic matter content. This suggests that absorption into organic matter is not a limiting factor of malathion bioavailability to earthworm species.

Determination of malathion levels and the effect of malathion on the growth of Chrysomya megacephala (Fibricius) in malathion-exposed rat carcass.

This study was conducted to examine the effect of malathion on the development of Chrysomya megacephala. A total of 12 adult Sprague-Dawley rats was divided into 4 groups. Each animal in the 4 groups was given orally 0 (control), 10, 25 and 50ml/kg body weight of malathion, respectively. Chrysomya megacephala larvae were then allowed to grow on the liver of carcass. Larvae development was estimated by means of weight and length, time of adult emergence and survival rate. Results indicated that for the first 6 to 30 hours, larvae from control group developed more rapidly than larvae feeding on tissue containing malathion. However, the 3 doses of malathion did not exhibit significant impact on larvae length and weight. The time required for adult emergence was significantly greater for malathion-treated colony which was 10 days compared to 7 days in control colony. Control larvae of C. megacephala had higher survival rate compared to larvae exposed to the three different doses of malathion. Analysis of the tissues indicated that all rats and fly samples were positive for malathion. Malathion concentration was highest in liver. It was concluded that the presence of malathion altered the development rate of C. megacephala and thus disrupted normal postmortem interval estimation.

[Changes in the toxicity and immunotoxity of tetrachloromethane and carbophos under the action of 2,4,6-triphenyl-4H-selenopyran and their connection with cytochrome P-450 dependent monooxygenase system].

It is established in experiments on noninbred rats that 2,4,6-triphenyl-4H-selenopyrane (peroral administration in a dose of 0.8 mg/kg during 3 days) induces cytochrome P450, thus increasing the toxicity and immunotoxicity of carbon tetrachloride (metabolized via "lethal synthesis"), and reduces the analogous effects of carbophos, the biotransformation of which proceeds via the formation of low-toxicity and nontoxic metabolites.

Análisis morfométrico de los túbulos testiculares del ratón después de la administración de malation y maca / Morphometric analysis of mice testicular tubules after administration of malathion and maca

Los agropesticidas organofosforados (OF) son ampliamente usados para incrementar la producción alimentaría. Sin embargo, se ha demostrado que inducen alteraciones a nivel testicular, relacionadas con la disminución de la fertilidad tanto en humanos como en animales. Por otra parte, diferentes estudios han sido llevados a cabo para desarrollar compuestos químicos o naturales que puedan inducir un efecto antagónico sobre los OF. En estudios anteriores, un extracto de una planta de los Andes peruanos (Maca) ha sido reconocido por su acción estimulante sobre la espermatogénesis. En el presente trabajo se estudiaron los efectos de ambos agentes externos sobre el testículo. Una población de 52 ratones machos adultos de la cepa CF1 fue dividida al azar en 4 grupos (Control, Malation, Maca, Malation-Maca), con intervalos de sacrificio de 1, 7, 14 y 21 días. Para analizar el grado de alteración inducida por estos agentes, se utilizaron técnicas de histomorfometría con ayuda del programa Image tools 3.1, en secciones testiculares. Nuestros resultados muestran que la altura del epitelio aumentó al día 1 en el grupo tratado con malation, cayendo al día 7 y llegando a valores similares al control, al día 14. Sin embargo, el grupo Malation-Maca no mostró cambios significativos. El lumen tubular, disminuyó al día 7 y 14, para normalizarse al día 21 en el grupo Malation. Sin embargo, en el grupo Malation-Maca el lumen tubular sólo bajó al día 14. El diámetro tubular disminuyó a los días 7 (p < 0.01), 14 y 21 en el grupo Malation, con respecto al control (p< 0.05). Sin embargo, el grupo Malation-Maca mostró valores normales. En conclusión, es posible establecer que el daño inducido por Malation es revertido al día 21 post administración de maca.

Organophosphoric (OP) agropesticidas are amply used to increase food production. However, it has been verified that they induce alterations at testicular level related to the diminution of fertility in humans as in animals. On the other hand, different studies have been made to develop chemical or natural compounds that can induce an antagonistic effect to OP. In previous studies an extract from a plant, from the Peruvian Andes (Maca) has been recognized by its stimulating action on spermatogenesis. In the present study the effects of both external agents were evaluated on testicular sections of testis of adult male mice on a population of 52 mice CF1, divided at random in 4 groups (Control, Malathion, Maca, Malathion-Maca), with sacrifice intervals of 1,7, 14 and 21 days. By means of morphometric technique and using the "Image Tools 3,1" software, the histology of testicular sections was evaluated, to analyze the degree of alteration induced by these agents. The epithelial height mainly has a rise in day 1, for Malathion group and then fall to day 7 to be normal by day 14. However, Malathion-Maca groups show no changes. The tubular lumen decreases at day 7 and day 14 to be normal by day 21 in Malathion group. However, in Malathion - Maca group, the tubular lumen decreases only at 14 days. The tubular diameter, at day 7 (p<0.01), 14 and 21 in Malathion group decreases respect to control (p<0.05), However, Malathion-Maca groups show normal values. In conclusion, it is possible to establish that the damage induced by Malathion is reverted by 21 days post-administration of Maca.

In vitro dermal absorption of methyl salicylate, ethyl parathion, and malathion: first responder safety.

In vitro tests with fresh dermatomed (0.3 to 0.4 mm thick) female breast skin and one leg skin specimen were conducted in Bronaugh flow-through Teflon diffusion cells with three chemicals used to simulate chemical warfare agents: 14C-radiolabeled methyl salicylate (MES), ethyl parathion (PT), and malathion (MT), at three dose levels (2, 20, and 200 mM). Tests were conducted at a skin temperature of 29 degrees C using a brief 30-min exposure to the chemical and a 6.5-h receivor collection period. Rapid absorption of all three chemicals was observed, with MES absorbed about 10-fold faster than PT and MT. For MES, PT, and MT, respectively, there was 32%, 7%, and 12% absorption into the receivor solution (Hank's HEPES buffered saline with 4% bovine serum albumin [BSA], pH 7.4) at the low dose (2 mM), 17%, 2%, and 3% at the medium dose (20 mM), and 11%, 1%, and 1% at the high dose (200 mM) levels. Including the skin depot for MES, PT, and MT, respectively, there was 40%, 41%, and 21% (low dose), 26%, 16%, and 8% (medium dose), and 13%, 19%, and 10% (high does) absorption. Efficacy of skin soap washing conducted at the 30 min exposure time ranged from 31% to 86%, varying by chemical and dose level. Skin depot levels were highest for the relatively lipophilic PT. "Pseudo" skin permeability coefficient (K(p)) data declined with dose level, suggesting skin saturation had occurred. An in-depth comparison with literature data was conducted and risk assessment of first responder exposure was briefly considered.

Comparison of the human skin grafted onto nude mouse model with in vivo and in vitro models in the prediction of percutaneous penetration of three lipophilic pesticides.

The evaluation of the degree of percutaneous penetration of agrochemicals is a key part of risk assessment for operators. The availability of suitable and predictive experimental models is crucial, in particular in the case of lipophilic compounds which persist in the stratum corneum (SC). Regulatory models (rat in vivo, human and rat in vitro) and the innovative human skin grafted onto nude mice (HuSki) model were compared for their ability to predict the human skin absorption. Radiolabelled malathion, lindane and cypermethrin (4microg/cm(2)) were topically applied to each model. The % of applied dose absorbed and that present in skin and SC were evaluated at 24h. Additionally, the absorption profile of cypermethrin was evaluated in the in vivo rat and HuSki models for up to 11 days. We found that the human in vitro and HuSki models closely predicted the human skin absorption at 24h, while rat models overestimated the human skin absorption. Furthermore, our experiments with cypermethrin indicated that evaluation of % percutaneous absorption over extended periods of time was feasible with the HuSki model. In our studies the HuSki model overcame the limitations of the regulatory models and is promising to realistically refine the dermal absorption assessment of topically applied chemicals.

[Determination of insecticide-resistance and resistance mechanisms of Blattella germanica (Dictyoptera: Blattellidae)]. / Determinación de la resistencia a insecticidas y mecanismos de resistencia en cepas de Blattella germanica (Dictyoptera: Blattellidae).

In this paper, the level of resistance to four insecticides of 3 Blatella germanica strains collected from various places in the City of Havana province was evaluated. These strains were resistant to two pyrethroids (cypermethrin and lambda-cyalothrine) and to organophosphorate malathion but susceptible to carbamate propoxur. The values of alpha and beta esterases, acetylcholinesterase and gluthatione-S-transferase were estimated in three strains involved in the study. The results of the study showed high esterase activity in all the strains, mainly beta esterases and two of the three strains presented with high gluthation-S-transferase enzyme. No changes in acetylcholinesterase were demonstrated in relation to the reference strain. The association of levels of resistance to insecticides, the possible resistance mechanisms in each strain and the results of the enzymatic activity were also analyzed.

Potential uses of biomonitoring data: a case study using the organophosphorus pesticides chlorpyrifos and malathion.

BACKGROUND: Organophosphorus pesticides such as chlorpyrifos and malathion are widely used insecticides. They do not bioaccumulate appreciably in humans and are rapidly metabolized and excreted in the urine. In nonoccupational settings, exposures to these pesticides are typically sporadic and short-lived because the pesticides tend to degrade in the environment over time; however, dietary exposures may be more chronic. Biologic monitoring has been widely used to assess exposures, susceptibility, and effects of chlorpyrifos and malathion; thus, the information base on these compounds is data rich. For biomonitoring of exposure, chlorpyrifos and malathion have been measured in blood, but most typically their urinary metabolites have been measured. For assessing early effects and susceptibility, cholinesterase and microsomal esterase activities, respectively, have been measured. OBJECTIVES: Although many biologic monitoring data have been generated and published on these chemicals, their interpretation is not straightforward. For example, exposure to environmental degradates of chlorpyrifos and malathion may potentially increase f urinary metabolite levels, thus leading to overestimation of exposure. Also, the temporal nature of the exposures makes the evaluation of both exposure and effects difficult. We present an overview of the current biomonitoring and other relevant data available on exposure to chlorpyrifos and malathion and the use of these data in various environmental public health applications.

Decreasing malathion application time for lice treatment reduces transdermal absorption.

OBJECTIVE: Head lice are the most common parasitic infestation in the United States requiring topical treatment with pediculicides. Ovide, the 0.5% malathion formulation used in treatment of head lice requires placement on dry hair for 8-12 h. Malathion, however, is effective at killing lice and nits in 10 min. Our concern of over exposing children to malathion has led us to examine whether significantly more malathion will penetrate transdermally when applied for the recommended 8 h than for a shorter but apparently equally effective period. METHODS: In vitro absorption studies were performed across haired rat skin and human abdominal skin to determine whether reducing malathion application time decreased skin absorption. RESULTS: A 0.5 h exposure caused 0.36+/-0.14% of the donor malathion to penetrate through human skin after 24 h and 2.1+/-0.6% remained in the skin after washing with shampoo. After 8 h of topical applications penetration was approximately three-fold greater (1.02+/-0.41) and 3.4+/-0.5% remained in the skin (p<0.05 versus 0.5 h). The relationship between absorption and exposure time also occurred for haired rat skin (p<0.05). This differential continued for 72 h even after removal of the source. CONCLUSIONS: Significantly less malathion penetrated from Ovide after 0.5 h versus the suggested 8 h application, without decreasing the product's efficacy. Further clinical studies in children are warranted to confirm the efficacy of this shortened application time.

Malathion detoxification by human hepatic carboxylesterases and its inhibition by isomalathion and other pesticides.

The organophosphorothioate (OPT) pesticide malathion (MAL) in mammals is readily hydrolyzed by mammalian carboxylesterases (CE). The reaction competes with the CYP-catalyzed formation of malaoxon (MOX), the toxic metabolite. Alterations or individual variations in CE activity may result in increased MOX formation, enhancing MAL toxicity. We have characterized the human hepatic CE activity in a panel of 18 human liver microsomes as well as the inhibitory effect of IsoMAL, a major impurity of MAL commercial formulations, parathion (PAR), chlorpyrifos (CPF), and chlorpyrifos-oxon (CPFO). CE activity showed a low level of variation among individuals (4-fold). The reaction consists of two different phases, differing in their affinity for mal (k(m1)=0.25-0.69 microm; K(m2)=10.3-26.8 microM). The relatively low K(m1) values confirmed that human CE efficiently detoxify MAL. IsoMAL was shown to be a potent noncompetitive inhibitor of MAL detoxification (K(i)=0.6 microM), with a higher inhibitory potency than CPF and PAR (K(i)=7.5 microM and 50 microM, respectively). These two latter compounds very likely act as mixed inhibitors. CPFO showed the highest inhibitory potency toward CE-mediated detoxification, being characterized by a K(i)=22 nM. The present results provide useful information for a better understanding of possible interactions between different OPTs and for assessing the potential cumulative risk for exposure to OPT mixtures.