Effect of CYP2B6*6 and CYP2C19*2 genotype on chlorpyrifos metabolism

Human CYP2C19 Substrate and Inhibitor Characterization of Organophosphate Pesticides

CYP2C19 is an important enzyme for organophosphate pesticide (OPP) metabolism. Because the OPPs can be both substrates and inhibitors of CYP2C19, we screened 45 OPPs for their ability to inhibit the activity of this enzyme and investigated the role of CYP2C19 in the metabolism of 22 of these molecules. We identified several nanomolar inhibitors of CYP2C19 as well as determined that thions, in general, are more potent inhibitors than oxons. We also determined that thions are readily metabolized by CYP2C19, although we saw no relationship between IC50 values and intrinsic clearance rates. This study may have implications for mitigating the risk of OPP poisoning.

Multidisciplinary Insights into the Structure-Function Relationship of the CYP2B6 Active Site

Cytochrome P450 2B6 (CYP2B6) is a highly polymorphic human enzyme involved in the metabolism of many clinically relevant drugs, environmental toxins, and endogenous molecules with disparate structures. Over the last 20-plus years, in silico and in vitro studies of CYP2B6 using various ligands have provided foundational information regarding the substrate specificity and structure-function relationship of this enzyme. Approaches such as homology modeling, X-ray crystallography, molecular docking, and kinetic activity assays coupled with CYP2B6 mutagenesis have done much to characterize this originally neglected monooxygenase. However, a complete understanding of the structural details that make new chemical entities substrates of CYP2B6 is still lacking. Surprisingly little in vitro data has been obtained about the structure-function relationship of amino acids identified to be in the CYP2B6 active site. Since much attention has already been devoted to elucidating the function of CYP2B6 allelic variants, here we review the salient findings of in silico and in vitro studies of the CYP2B6 structure-function relationship with a deliberate focus on the active site. In addition to summarizing these complementary approaches to studying structure-function relationships, we note gaps/challenges in existing data such as the need for more CYP2B6 crystal structures, molecular docking results with various ligands, and data coupling CYP2B6 active site mutagenesis with kinetic parameter measurement under standard expression conditions. Harnessing in silico and in vitro techniques in tandem to understand the CYP2B6 structure-function relationship will likely offer further insights into CYP2B6-mediated metabolism. SIGNIFICANCE STATEMENT: The apparent importance of cytochrome P450 2B6 (CYP2B6) in the metabolism of various xenobiotics and endogenous molecules has grown since its discovery with many in silico and in vitro studies offering a partial description of its structure-function relationship. Determining the structure-function relationship of CYP2B6 is difficult but may be aided by thorough biochemical investigations of the CYP2B6 active site that provide a more complete pharmacological understanding of this important enzyme.

Patterns of cardio-respiratory motor outputs during acute and subacute exposure to chlorpyrifos in an ex-vivo in situ preparation in rats

While a considerable body of literature has characterized the clinical features induced by organophosphate pesticides, the field lacks scrutiny into cardio-respiratory changes in different phases of poisoning. Herein, we evaluated the impact of chlorpyrifos (CPF) and its active metabolite chlorpyrifos-oxon (CPO) on the cardiorespiratory system during acute and subacute phases of poisoning using an in situ experimental rodent model. CPF (30 mg/kg) was injected intraperitoneally to rats beforehand (24 h) whereas CPO (15 mg/kg) was added into the perfusate reservoir to evaluate the effects on the motor outputs throughout the three phases of the respiratory cycle: inspiration, post-inspiration and late expiration. Phrenic, recurrent laryngeal (RLN) and thoracic sympathetic nerve activity (tSNA) were recorded. Heart rate was derived from the electrocardiogram (ECG) and the baro- and chemo-reflexes tested. CPF and CPO led to a time-dependent change in cardiorespiratory motor outputs. In the acute phase, the CPO induced bradypnea, transiently reduced the inspiratory time (TI), and increased the amplitude of phrenic. Post-inspiratory (PI) discharge recorded from the RLN was progressively reduced while tSNA was increased. CPO significantly depressed the chemoreflex but had no effect on baroreflex. During subacute phase, CPF prolongated T_I with no effect on respiratory rate. Both the RLN PI discharge, the chemoreflex and the baroreflex sympathetic gain were reduced. In addition, both CPF and CPO shifted the cardiac sympatho-vagal balance towards sympathetic dominance. Our data show that different phases of poisoning are associated with specific changes in the cardio-respiratory system and might therefore demand distinct approaches by health care providers.

3-Indolepropionic acid upturned male reproductive function by reducing oxido-inflammatory responses and apoptosis along the hypothalamic-pituitary-gonadal axis of adult rats exposed to chlorpyrifos

We examined the effect of 3-Indolepropionic acid (3-IPA), an antioxidant on the organophosphorus pesticide chlorpyrifos (CPF)-induced reproductive toxicity in rats. The five experimental rat cohorts were treated per os for 14 consecutive days as follows: Control (Corn oil 2 mL/kg body weight), CPF alone (5 mg/kg), 3-IPA alone (40 mg/kg) and the co-treated rat cohorts (CPF:5 mg/kg + 3-IPA: 20 or 40 mg/kg). Biomarkers of testicular and epididymal function, oxidative stress, myeloperoxidase (MPO) activity and the levels of nitric oxide (NO), reactive oxygen and nitrogen (RONS) species and lipid peroxidation (LPO) were assessed. Also, tumour necrosis factor-alpha (TNF-α), Bcl-2-associated X (Bax) and B cell lymphoma 2 (Bcl-2) proteins were estimated, and tissue histology was microscopically examined. CPF alone significantly (p < 0.05) increased biomarkers of reproductive toxicities were averted in rats co-treated 3-IPA. Decreases in antioxidants and increases in lipid peroxidation and reactive oxygen and nitrogen species were lessened (p < 0.05) in CPF and 3-IPA co-treated rats. CPF mediated increases in TNF-α, NO, Bax, and MPO activity was reduced (p < 0.05) in the epididymis, testes, and hypothalamus of rats co-treated with 3-IPA. In addition, Bcl-2 expression was increased in rats co-treated with 3-IPA dose-dependently. Histopathological examination revealed severe lesions induced by CPF were prevented in rats co-treated with 3-IPA. Our findings demonstrate that exogenous 3-IPA reduced CPF-induced oxidative stress, inflammation, and apoptosis in the epididymis and testes of male rats.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Nicotine oxidation by genetic variants of CYP2B6 and in human brain microsomes

Common variation in the CYP2B6 gene, encoding the cytochrome P450 2B6 enzyme, is associated with substrate-specific altered clearance of multiple drugs. CYP2B6 is a minor contributor to hepatic nicotine metabolism, but the enzyme has been proposed as relevant to nicotine-related behaviors because of reported CYP2B6 mRNA expression in human brain tissue. Therefore, we hypothesized that CYP2B6 variants would be associated with altered nicotine oxidation, and that nicotine metabolism by CYP2B6 would be detected in human brain microsomes. We generated recombinant enzymes in insect cells corresponding to nine common CYP2B6 haplotypes and demonstrate genetically determined differences in nicotine oxidation to nicotine iminium ion and nornicotine for both (S) and (R)-nicotine. Notably, the CYP2B6.6 and CYP2B6.9 variants demonstrated lower intrinsic clearance relative to the reference enzyme, CYP2B6.1. In the presence of human brain microsomes, along with nicotine-N-oxidation, we also detect nicotine oxidation to nicotine iminium ion. However, unlike N-oxidation, this activity is NADPH independent, does not follow Michaelis-Menten kinetics, and is not inhibited by NADP or carbon monoxide. Furthermore, metabolism of common CYP2B6 probe substrates, methadone and ketamine, is not detected in the presence of brain microsomes. We conclude that CYP2B6 metabolizes nicotine stereoselectively and common CYP2B6 variants differ in nicotine metabolism activity, but did not find evidence of CYP2B6 activity in human brain.

Toxicokinetic and toxicodynamic studies of carbaryl alone or in binary mixtures with azinphos methyl in the freshwater gastropod Planorbarius corneus

Carbamate insecticides such as carbaryl and organophosphates such as azinphos-methyl share the ability to inhibit the activity of B-esterases. This study aimed to (1) assess the inhibitory effects of carbaryl on B-esterase activity in soft tissues and hemolymph of Planorbarius corneus; (2) establish whether binary mixtures of carbaryl and azinphos-methyl depart or not from a model of concentration addition on the inhibition of cholinesterase activity; (3) determine the bioconcentration and elimination of the pesticides. The results showed that exposure of gastropods to increasing concentrations of carbaryl (0.1-5 mg L^-1) for 48 h inhibited cholinesterase activity in a concentration-dependent manner, with an EC₅₀ of 1.4 ± 0.3 mg L^-1 and 1.2 ± 0.1 mg L^-1 for soft tissue and hemolymph, respectively. Carboxylesterase activity, measured with the substrates p-nitrophenyl butyrate and p-nitrophenyl acetate, was between 2.3 and 25 times more sensitive to carbaryl inhibition than cholinesterase activity. Binary mixtures corresponding to 0.5 EC₅₀ carbaryl + 0.5 EC₅₀ azinphos-methyl and 0.75 EC₅₀ carbaryl + 0.75 EC₅₀ azinphos-methyl produced inhibitions of cholinesterase activity similar to those of individual pesticides, following a model of concentration addition. Bioconcentration was analyzed using a one-compartment model. The absorption kinetics (k₁) for both pesticides alone (1.4 mg L^-1 of carbaryl or 1.8 mg L^-1 of azinphos-methyl) or mixed (1.4 mg L^-1 of carbaryl + 1.8 mg L^-1 of azinphos-methyl) were similar. The elimination kinetics ratio (k₂) estimated for the pesticides alone or in the mixtures showed that carbaryl was eliminated 3.5 times faster than azinphos-methyl. These results suggest that exposure of Planorbarius corneus to binary mixtures of carbaryl and azinphos-methyl for 48 h follow a concentration addition model on inhibition of cholinesterase activity and that the pesticide mixtures do not change the toxicokinetic parameters of the parent compounds.

Use of Phenoxyaniline Analogues To Generate Biochemical Insights into the Interactio n of Polybrominated Diphenyl Ether with CYP2B Enzymes

Human hepatic cytochromes P450 (CYP) are integral to xenobiotic metabolism. CYP2B6 is a major catalyst of biotransformation of environmental toxicants, including polybrominated diphenyl ethers (PBDEs). CYP2B substrates tend to contain halogen atoms, but the biochemical basis for this selectivity and for species specific determinants of metabolism has not been identified. Spectral binding titrations and inhibition studies were performed to investigate interactions of rat CYP2B1, rabbit CYP2B4, and CYP2B6 with a series of phenoxyaniline (POA) congeners that are analogues of PBDEs. For most congeners, there was a <3-fold difference between the spectral binding constants (K_S) and IC₅₀ values. In contrast, large discrepancies between these values were observed for POA and 3-chloro-4-phenoxyaniline. CYP2B1 was the enzyme most sensitive to POA congeners, so the Val-363 residue from that enzyme was introduced into CYP2B4 or CYP2B6. This substitution partially altered the protein-ligand interaction profiles to make them more similar to that of CYP2B1. Addition of cytochrome P450 oxidoreductase (POR) to titrations of CYP2B6 with POA or 2'4'5'TCPOA decreased the affinity of both ligands for the enzyme. Addition of cytochrome b₅ to a recombinant enzyme system containing POR and CYP2B6 increased the POA IC₅₀ value and decreased the 2'4'5'TCPOA IC₅₀ value. Overall, the inconsistency between K_S and IC₅₀ values for POA versus 2'4'5'TCPOA is largely due to the effects of redox partner binding. These results provide insight into the biochemical basis of binding of diphenyl ethers to human CYP2B6 and changes in CYP2B6-mediated metabolism that are dependent on POA congener and redox partner identity.

Effect of genetic polymorphism of human CYP2B6 on the metabolic activation of chlorpyrifos

Chlorpyrifos (CPS) is a broad-spectrum organophosphate insecticide that is neurotoxic in humans. Chlorpyrifos oxon (CPO) is a toxic metabolite of CPS that is produced by CYP2B6. In this study, we examined the variability of CPS metabolism resulting from single-nucleotide polymorphisms in CYP2B6. Wild-type CYP2B6 (CYP2B6.1) and two variants each with a single amino acid substitution: CYP2B6.5 (R487C) and CYP2B6.8 (K139E) were co-expressed together with human NADPH-dependent cytochrome P450 reductase in Escherichia coli (E. coli). Both of the CYP2B6 variants were successfully expressed in E. coli. The conversion of CPS to CPO by the CYP2B6 variants was analyzed with high-performance liquid chromatography. K_m and V_max of the reaction by CYP2B6.1 were 18.50±2.94μM and 17.07±1.15mol/min/mol P450, respectively. The CYP2B6 variants produced CPO with the following kinetic parameters: K_m for CYP2B6.5 and CYP2B6.8 were 20.44±6.43 and 44.69±9.97μM, respectively; and V_max were 1.10±0.10 and 1.77±0.26mol/min/mol P450, respectively. These results indicate that the amino acid substitutions in the CYP2B6 variants suppressed the metabolic activation of CPS. CYP2B6 variants have altered capacity to bioactivate CPF and may affect individual susceptibility of CPF.

Determination of chlorpyrifos and its metabolites in cells and culture media by liquid chromatography-electrospray ionization tandem mass spectrometry

A sensitive method to simultaneously quantitate chlorpyrifos, chlorpyrifos oxon and the detoxified product 3,5,6-trichloro-2-pyridinol (TCP) was developed using either liquid-liquid extraction for culture media samples, or protein precipitation for cell samples. Multiple reaction monitoring in positive ion mode was applied for the detection of chlorpyrifos and chlorpyrifos oxon, and selected ion recording in negative mode was applied to detect TCP. The method provided linear ranges from 5 to 500, 0.2-20 and 20-2000ng/mL for media samples and from 0.5-50, 0.02-2 and 2-200ng/million cells for CPF, CPO and TCP, respectively. The method was validated using selectivity, linearity, precision, accuracy, recovery, stability and dilution tests. All relative standard deviations (RSDs) and relative errors (REs) for QC samples were within 15% (except for LLOQ, within 20%). This method has been successfully applied to study the neurotoxicity and metabolism of chlorpyrifos in a human neuronal model.

Internal Concentration and Time Are Important Modifiers of Toxicity: The Case of Chlorpyrifos on Caenorhabditis elegans

The internal concentration of chemicals in exposed organisms changes over time due to absorption, distribution, metabolism, and excretion processes since chemicals are taken up from the environment. Internal concentration and time are very important modifiers of toxicity when biomarkers are used to evaluate the potential hazards and risks of environmental pollutants. In this study, the responses of molecular biomarkers, and the fate of chemicals in the body, were comprehensively investigated to determine cause-and-effect relationships over time. Chlorpyrifos (CP) was selected as a model chemical, and Caenorhabditis elegans was exposed to CP for 4 h using the passive dosing method. Worms were then monitored in fresh medium during a 48-h recovery regime. The mRNA expression of genes related to CYP metabolism (cyp35a2 and cyp35a3) increased during the constant exposure phase. The body residue of CP decreased once it reached a peak level during the early stage of exposure, indicating that the initial uptake of CP rapidly induced biotransformation with the synthesis of new CYP metabolic proteins. The residual chlorpyrifos-oxon concentration, an acetylcholinesterase (AChE) inhibitor, continuously increased even after the recovery regime started. These delayed toxicokinetics seem to be important for the extension of AChE inhibition for up to 9 h after the start of the recovery regime. Comprehensive investigation into the molecular initiation events and changes in the internal concentrations of chemical species provide insight into response causality within the framework of an adverse outcome pathway.

Chiral polychlorinated biphenyls: absorption, metabolism and excretion--a review

Seventy eight out of the 209 possible polychlorinated biphenyl (PCB) congeners are chiral, 19 of which exist under ambient conditions as stable rotational isomers that are non-superimposable mirror images of each other. These congeners (C-PCBs) represent up to 6 % by weight of technical PCB mixtures and undergo considerable atropisomeric enrichment in wildlife, laboratory animals, and humans. The objective of this review is to summarize our current knowledge of the processes involved in the absorption, metabolism, and excretion of C-PCBs and their metabolites in laboratory animals and humans. C-PCBs are absorbed and excreted by passive diffusion, a process that, like other physicochemical processes, is inherently not atropselective. In mammals, metabolism by cytochrome P450 (P450) enzymes represents a major route of elimination for many C-PCBs. In vitro studies demonstrate that C-PCBs with a 2,3,6-trichlorosubstitution pattern in one phenyl ring are readily oxidized to hydroxylated PCB metabolites (HO-PCBs) by P450 enzymes, such as rat CYP2B1, human CYP2B6, and dog CYP2B11. The oxidation of C-PCBs is atropselective, thus resulting in a species- and congener-dependent atropisomeric enrichment of C-PCBs and their metabolites. This atropisomeric enrichment of C-PCBs and their metabolites likely plays a poorly understood role in the atropselective toxicity of C-PCBs and, therefore, warrants further investigation.

Subacute nicotine co-exposure has no effect on 2,2',3,5',6- pentachlorobiphenyl disposition but alters hepatic cytochrome P450 expression in the male rat

Polychlorinated biphenyls (PCBs) are metabolized by cytochrome P450 2B enzymes (CYP2B) and nicotine is reported to alter CYP2B activity in the brain and liver. To test the hypothesis that nicotine influences PCB disposition, 2,2',3,5',6-pentachlorobiphenyl (PCB 95) and its metabolites were quantified in tissues of adult male Wistar rats exposed to PCB 95 (6mg/kg/d, p.o.) in the absence or presence of nicotine (1.0mg/kg/d of the tartrate salt, s.c.) for 7 consecutive days. PCB 95 was enantioselectively metabolized to hydroxylated (OH-) PCB metabolites, resulting in a pronounced enrichment of E1-PCB 95 in all tissues investigated. OH-PCBs were detected in blood and liver tissue, but were below the detection limit in adipose, brain and muscle tissues. Co-exposure to nicotine did not change PCB 95 disposition. CYP2B1 mRNA and CYP2B protein were not detected in brain tissues but were detected in liver. Co-exposure to nicotine and PCB 95 increased hepatic CYP2B1 mRNA but did not change CYP2B protein levels relative to vehicle control animals. However, hepatic CYP2B protein in animals co-exposed to PCB 95 and nicotine were reduced compared to animals that received only nicotine. Quantification of CYP2B3, CYP3A2 and CYP1A2 mRNA identified significant effects of nicotine and PCB 95 co-exposure on hepatic CYP3A2 and hippocampal CYP1A2 transcripts. Our findings suggest that nicotine co-exposure does not significantly influence PCB 95 disposition in the rat. However, these studies suggest a novel influence of PCB 95 and nicotine co-exposure on hepatic cytochrome P450 (P450) expression that may warrant further attention due to the increasing use of e-cigarettes and related products.

Azinphos-methyl and chlorpyrifos, alone or in a binary mixture, produce oxidative stress and lipid peroxidation in the freshwater gastropod Planorbarius corneus

Azinphos-methyl (AZM) and chlorpyrifos (CPF) are broad-spectrum organophosphate insecticides used for pest control on a number of food crops in many parts of the world that have been shown to inhibit cholinesterase activity in the non-target freshwater gastropod Planorbarius corneus. The present study was undertaken to determine: (a) whether AZM and CPF induce oxidative stress in P. corneus, and (b) whether a mixture of both organophosphates that causes a higher neurotoxicity than single pesticides also causes an enhanced oxidative stress. To this end, non-enzymatic and enzymatic parameters were measured in the soft tissues of snails acutely exposed to the insecticides in single-chemical (2.5 mg AZM L(-1) and 7.5 μg CPF L(-1)) and a binary-mixture (1.25 mg AZM L(-1) plus 3.75 μg CPF L(-1)) studies. At 24 h, all pesticide-exposed groups showed significantly decreased glutathione (GSH) and glutathione disulfide (GSSG) levels when compared to control animals. At 48 h, all exposed groups showed an alteration of the redox status (GSH/GSSG ratio) and a significant increase in malondialdehyde levels. The exposure for 48 h to AZM and CPF, alone or in the binary mixture, also resulted in a significant decrease of the antioxidant superoxide dismutase activity. The greatest decrease was observed with CPF exposure (59% of decrease relative to the control group). A significant increase in catalase and glutathione S-transferase activities was observed in CPF group and in CPF and AZM+CPF groups, respectively. The activities of glutathione reductase and glucose 6-phosphate dehydrogenase did not show significant changes with respect to controls in any treatment group. In conclusion, the data shown in the present study provide evidence that AZM, CPF and a mixture of both organophosphates are able to induce oxidative stress and oxidative damage in P. corneus tissues. However, no similarities between the degree of neurotoxicity and the degree of alterations of the measured oxidative stress parameters were found.

CYP2B6 non-coding variation associated with smoking cessation is also associated with differences in allelic expression, splicing, and nicotine metabolism independent of common amino-acid changes

The Cytochrome P450 2B6 (CYP2B6) enzyme makes a small contribution to hepatic nicotine metabolism relative to CYP2A6, but CYP2B6 is the primary enzyme responsible for metabolism of the smoking cessation drug bupropion. Using CYP2A6 genotype as a covariate, we find that a non-coding polymorphism in CYP2B6 previously associated with smoking cessation (rs8109525) is also significantly associated with nicotine metabolism. The association is independent of the well-studied non-synonymous variants rs3211371, rs3745274, and rs2279343 (CYP2B6*5 and *6). Expression studies demonstrate that rs8109525 is also associated with differences in CYP2B6 mRNA expression in liver biopsy samples. Splicing assays demonstrate that specific splice forms of CYP2B6 are associated with haplotypes defined by variants including rs3745274 and rs8109525. These results indicate differences in mRNA expression and splicing as potential molecular mechanisms by which non-coding variation in CYP2B6 may affect enzymatic activity leading to differences in metabolism and smoking cessation.

Cholinesterase and carboxylesterase inhibition in Planorbarius corneus exposed to binary mixtures of azinphos-methyl and chlorpyrifos

Though pesticide mixtures are commonly encountered in fresh water systems, the knowledge of their effects on non-target aquatic species is scarce. The present investigation was undertaken to explore the in vivo inhibition of the freshwater gastropod snail Planorbarius corneus cholinesterase (ChE) and carboxylesterases (CES) activities by the organophosphorus pesticides azinphos-methyl (AZM) and chlorpyrifos (CPF). To this end, snails were exposed for 48 h to different concentrations of AZM and CPF in single-chemical and binary-mixture studies, and ChE and CES activities were measured in the whole soft body tissues and hemolymph. ChE activity was measured with acetylthiocholine as substrate and CES activity was measured with four substrates: p-nitrophenyl acetate, p-nitrophenyl butyrate, 1- and 2-naphthyl acetate. Single-chemical experiments showed that CPF was a more potent inhibitor of ChE activity than AZM (350 and 27 times for the whole soft tissue and hemolymph, respectively). CES were 15 times more sensitive than ChE when the activities were measured in the whole soft tissue of the animals exposed to AZM. Based on a default assumption of concentration addition, P. corneus snails were exposed to mixtures of AZM+CPF designed to yield predicted soft tissue ChE inhibitions of 31% (mixture 1), 50% (mixture 2) and 61% (mixture 3). Results showed that ChE inhibition produced by mixture 1 followed a model of concentration addition. In contrast, the other mixtures showed synergism, both in whole soft tissue and hemolymph. In addition, results obtained when the snails were exposed sequentially to the pesticides showed that the sequence AZM/CPF produced at 48 h a higher ChE inhibition than the sequence CPF/CPF. A range of metabolic pathways and responses associated with bioactivation or detoxification may play important roles in organophosphorus interactions. In particular, the data presented in the present study indicate that CES enzymes would be important factors in determining the effects of the mixtures of OPs on P. corneus ChE activity.

Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation

Cytochromes P450 (CYP) are a major source of variability in drug pharmacokinetics and response. Of 57 putatively functional human CYPs only about a dozen enzymes, belonging to the CYP1, 2, and 3 families, are responsible for the biotransformation of most foreign substances including 70-80% of all drugs in clinical use. The highest expressed forms in liver are CYPs 3A4, 2C9, 2C8, 2E1, and 1A2, while 2A6, 2D6, 2B6, 2C19 and 3A5 are less abundant and CYPs 2J2, 1A1, and 1B1 are mainly expressed extrahepatically. Expression of each CYP is influenced by a unique combination of mechanisms and factors including genetic polymorphisms, induction by xenobiotics, regulation by cytokines, hormones and during disease states, as well as sex, age, and others. Multiallelic genetic polymorphisms, which strongly depend on ethnicity, play a major role for the function of CYPs 2D6, 2C19, 2C9, 2B6, 3A5 and 2A6, and lead to distinct pharmacogenetic phenotypes termed as poor, intermediate, extensive, and ultrarapid metabolizers. For these CYPs, the evidence for clinical significance regarding adverse drug reactions (ADRs), drug efficacy and dose requirement is rapidly growing. Polymorphisms in CYPs 1A1, 1A2, 2C8, 2E1, 2J2, and 3A4 are generally less predictive, but new data on CYP3A4 show that predictive variants exist and that additional variants in regulatory genes or in NADPH:cytochrome P450 oxidoreductase (POR) can have an influence. Here we review the recent progress on drug metabolism activity profiles, interindividual variability and regulation of expression, and the functional and clinical impact of genetic variation in drug metabolizing P450s.

Bioactivation of chlorpyrifos by CYP2B6 variants

Chlorpyrifos (CPF), an organophosphorus (OP) pesticide, is bioactivated by cytochrome P450s (CYPs) to the active metabolite chlorpyrifos oxon (CPF-O). Given that human CYP2B6 has the highest intrinsic clearance (CL(int)) for CPF bioactivation, CYP2B6 polymorphisms may impact human susceptibility to CPF at real world environmental and occupational CPF exposure levels. CYP2B6.4,.5,.7, and .18 were over-expressed in mammalian COS-1 cells to assess the impact of CYP2B6 variants on the K(m) and V(max) for bioactivation of CPF. Cell lysates were incubated with CPF (0-100 μM) and the production of CPF-O was measured via HPLC analysis. CYP2B6 content was determined by western blot. CYP2B6.18 had neither detectable protein nor activity levels. The V(max) value for each remaining variant was significantly higher than wild-type (CYP2B6.1, V(max) 4.13 × 10(4) pmol/min/nmol CYP2B6), with CYP2B6.4,.5, and .7 having V(max) values of 4.52 × 10(5), 1.82 × 10(5), and 9.60 × 10(4) pmol/min/nmol CYP2B6, respectively. The K(m) values for these variants ranged from 0.39 to 1.09 μM and were not significantly different from wild-type. All active variants examined had significantly higher CL(int) than CYP2B6.1. Variants of CYP2B6 have altered capacity to bioactivate CPF and may affect individual susceptibility by altering the V(max) for CPF-O formation.