Cytochrome P450 isozymes responsible for the metabolism of toluene and styrene in human liver microsomes

Risk-Based Prioritization of Organic Chemicals and Locations of Ecological Concern in Sediment From Great Lakes Tributaries

With improved analytical techniques, environmental monitoring studies are increasingly able to report the occurrence of tens or hundreds of chemicals per site, making it difficult to identify the most relevant chemicals from a biological standpoint. For the present study, organic chemical occurrence was examined, individually and as mixtures, in the context of potential biological effects. Sediment was collected at 71 Great Lakes (USA/Canada) tributary sites and analyzed for 87 chemicals. Multiple risk-based lines of evidence were used to prioritize chemicals and locations, including comparing sediment concentrations and estimated porewater concentrations with established whole-organism benchmarks (i.e., sediment and water quality criteria and screening values) and with high-throughput toxicity screening data from the US Environmental Protection Agency's ToxCast database, estimating additive effects of chemical mixtures on common ToxCast endpoints, and estimating toxic equivalencies for mixtures of alkylphenols and polycyclic aromatic hydrocarbons (PAHs). This multiple-lines-of-evidence approach enabled the screening of more chemicals, mitigated the uncertainties of individual approaches, and strengthened common conclusions. Collectively, at least one benchmark/screening value was exceeded for 54 of the 87 chemicals, with exceedances observed at all 71 of the monitoring sites. Chemicals with the greatest potential for biological effects, both individually and as mixture components, were bisphenol A, 4-nonylphenol, indole, carbazole, and several PAHs. Potential adverse outcomes based on ToxCast gene targets and putative adverse outcome pathways relevant to individual chemicals and chemical mixtures included tumors, skewed sex ratios, reproductive dysfunction, hepatic steatosis, and early mortality, among others. The results provide a screening-level prioritization of chemicals with the greatest potential for adverse biological effects and an indication of sites where they are most likely to occur. Environ Toxicol Chem 2022;41:1016-1041. Published 2022. This article is a U.S. Government work and is in the public domain in the USA. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Profiles of Volatile Biomarkers Detect Tuberculosis from Skin

Tuberculosis (TB) is an infectious disease that threatens >10 million people annually. Despite advances in TB diagnostics, patients continue to receive an insufficient diagnosis as TB symptoms are not specific. Many existing biodiagnostic tests are slow, have low clinical performance, and can be unsuitable for resource-limited settings. According to the World Health Organization (WHO), a rapid, sputum-free, and cost-effective triage test for real-time detection of TB is urgently needed. This article reports on a new diagnostic pathway enabling a noninvasive, fast, and highly accurate way of detecting TB. The approach relies on TB-specific volatile organic compounds (VOCs) that are detected and quantified from the skin headspace. A specifically designed nanomaterial-based sensors array translates these findings into a point-of-care diagnosis by discriminating between active pulmonary TB patients and controls with sensitivity above 90%. This fulfills the WHO's triage test requirements and poses the potential to become a TB triage test.

Increased toxicity and retention of perflourooctane sulfonate (PFOS) in humanized CYP2B6-Transgenic mice compared to Cyp2b-null mice is relieved by a high-fat diet (HFD)

PFOS is a persistent, fluorosurfactant used in multiple products. Murine Cyp2b's are induced by PFOS and high-fat diets (HFD) and therefore we hypothesized that human CYP2B6 may alleviate PFOS-induced steatosis. Cyp2b-null and hCYP2B6-Tg mice were treated with 0, 1, or 10 mg/kg/day PFOS by oral gavage for 21-days while provided a chow diet (ND) or HFD. Similar to murine Cyp2b10, CYP2B6 is inducible by PFOS. Furthermore, three ND-fed hCYP2B6-Tg females treated with 10 mg/kg/day PFOS died during the exposure period; neither Cyp2b-null nor HFD-fed mice died. hCYP2B6-Tg mice retained more PFOS in serum and liver than Cyp2b-null mice presumably causing the observed toxicity. In contrast, serum PFOS retention was reduced in the HFD-fed hCYP2B6-Tg mice; the opposite trend observed in HFD-fed Cyp2b-null mice. Hepatotoxicity biomarkers, ALT and ALP, were higher in PFOS-treated mice and repressed by a HFD. However, PFOS combined with a HFD exacerbated steatosis in all mice, especially in the hCYP2B6-Tg mice with significant disruption of key lipid metabolism genes such as Srebp1, Pparg, and Hmgcr. In conclusion, CYP2B6 is induced by PFOS but does not alleviate PFOS toxicity presumably due to increased retention. CYP2B6 protects from PFOS-mediated steatosis in ND-fed mice, but increases steatosis when co-treated with a HFD.

Targeted GC-MS analysis of firefighters' exhaled breath: Exploring biomarker response at the individual level

Biomarker measurements can provide unambiguous evidence of environmental exposures as well as the resultant biological responses. Firefighters have a high rate of occupational cancer incidence, which has been proposed to be linked in part to their increased environmental exposure to byproducts of combustion and contaminants produced during fire responses. In this article, the uptake and elimination of targeted volatile organic compounds were investigated by collecting the exhaled breath of firefighters on sorbent tubes before and after controlled structure burns and analyzing samples using automated thermal desorption-gas chromatography (ATD-GC/MS). Volatile organic compounds exposure was assessed by grouping the data according to firefighting job positions as well as visualizing the data at the level of the individual firefighter to determine which individuals had expected exposure responses. When data were assessed at the group level, benzene concentrations were found to be elevated post-exposure in both fire attack, victim search, and outside ventilation firefighting positions. However, the results of the data analysis at the individual level indicate that certain firefighters may be more susceptible to post-exposure volatile organic compounds increases than others, and this should be considered when assessing the effectiveness of firefighting protective gear. Although this work focuses on firefighting activity, the results can be translated to potential human health and ecological effects from building and forest fires.

CYP1A1 activity in rainbow trout is inhibited by the environmental pollutant p-cresol

Naturally- and anthropogenically-produced cresols could pose serious risks to fish health. In this study, three piscine CYP isoforms were investigated for their abilities to interact with p-cresol. Therefore, the activity of 7-ethoxyresorufin-O-deethylase (EROD), 7-benzyloxy-4-trifluoromethylcoumarin O-debenzylase (BFCOD), and p-nitrophenol hydroxylase (PNPH) were evaluated in the hepatic microsomes of juvenile rainbow trout. Results showed that EROD activity was inhibited in a competitive manner, BFCOD activity was inhibited in presence the highest tested p-cresol concentration and PNPH activity was not affected. These results indicate that p-cresol might affect the ability of fish to metabolize numerous aromatic hydrocarbons and dioxin compounds, which are present in the aquatic environment.

Inhalation toxicity of indoor air pollutants in Drosophila melanogaster using integrated transcriptomics and computational behavior analyses

We conducted an inhalation toxicity test on the alternative animal model, Drosophila melanogaster, to investigate potential hazards of indoor air pollution. The inhalation toxicity of toluene and formaldehyde was investigated using comprehensive transcriptomics and computational behavior analyses. The ingenuity pathway analysis (IPA) based on microarray data suggests the involvement of pathways related to immune response, stress response, and metabolism in formaldehyde and toluene exposure based on hub molecules. We conducted a toxicity test using mutants of the representative genes in these pathways to explore the toxicological consequences of alterations of these pathways. Furthermore, extensive computational behavior analysis showed that exposure to either toluene or formaldehyde reduced most of the behavioral parameters of both wild-type and mutants. Interestingly, behavioral alteration caused by toluene or formaldehyde exposure was most severe in the p38b mutant, suggesting that the defects in the p38 pathway underlie behavioral alteration. Overall, the results indicate that exposure to toluene and formaldehyde via inhalation causes severe toxicity in Drosophila, by inducing significant alterations in gene expression and behavior, suggesting that Drosophila can be used as a potential alternative model in inhalation toxicity screening.

Subcellular localization of rat CYP2E1 impacts metabolic efficiency toward common substrates

Cytochrome P450 2E1 (CYP2E1) detoxifies or bioactivates many low molecular-weight compounds. Most knowledge about CYP2E1 activity relies on studies of the enzyme localized to endoplasmic reticulum (erCYP2E1); however, CYP2E1 undergoes transport to mitochondria (mtCYP2E1) and becomes metabolically active. We report the first comparison of in vitro steady-state kinetic profiles for erCYP2E1 and mtCYP2E1 oxidation of probe substrate 4-nitrophenol and pollutants styrene and aniline using subcellular fractions from rat liver. For all substrates, metabolic efficiency changed with substrate concentration for erCYP2E1 reflected in non-hyperbolic kinetic profiles but not for mtCYP2E1. Hyperbolic kinetic profiles for the mitochondrial enzyme were consistent with Michaelis-Menten mechanism in which metabolic efficiency was constant. By contrast, erCYP2E1 metabolism of 4-nitrophenol led to a loss of enzyme efficiency at high substrate concentrations when substrate inhibited the reaction. Similarly, aniline metabolism by erCYP2E1 demonstrated negative cooperativity as metabolic efficiency decreased with increasing substrate concentration. The opposite was observed for erCYP2E1 oxidation of styrene; the sigmoidal kinetic profile indicated increased efficiency at higher substrate concentrations. These mechanisms and CYP2E1 levels in mitochondria and endoplasmic reticulum were used to estimate the impact of CYP2E1 subcellular localization on metabolic flux of pollutants. Those models showed that erCYP2E1 mainly carries out aniline metabolism at all aniline concentrations. Conversely, mtCYP2E1 dominates styrene oxidation at low styrene concentrations and erCYP2E1 at higher concentrations. Taken together, subcellular localization of CYP2E1 results in distinctly different enzyme activities that could impact overall metabolic clearance and/or activation of substrates and thus impact the interpretation and prediction of toxicological outcomes.

Interspecies uncertainty in molecular responses and toxicity of mixtures

Most of the experimental toxicity testing data for chemicals are generated through the use of laboratory animals, namely, rodents such as rats and mice or other species. Interspecies extrapolation is needed to nullify the differences between species so as to use such data for human health/risk assessment. Thus, understanding of interspecies differences is important in extrapolating the laboratory results to humans and conducting human risk assessments based on current credible scientific knowledge. Major causes of interspecies differences in anatomy and physiology, toxicokinetics, injury repair, molecular receptors, and signal transduction pathways responsible for variations in responses to toxic chemicals are outlined. In the risk assessment process, uncertainty associated with data gaps in our knowledge is reflected by application of uncertainty factors for interspecies differences. Refinement of the risk assessment methods is the ultimate goal as we strive to realistically evaluate the impact of toxic chemicals on human populations. Using specific examples from current risk assessment practice, this chapter illustrates the integration of interspecies differences in evaluation of individual chemicals and chemical mixtures.

Identifying potential protein targets for toluene using a molecular similarity search, in silico docking and in vitro validation

The toxicity of chemicals greatly depends on their interaction with macromolecular targets.

The G⁵¹⁶T CYP2B6 germline polymorphism affects the risk of acute myeloid leukemia and is associated with specific chromosomal abnormalities

The etiology of acute myeloid leukemia (AML) underlies the influence of genetic variants in candidate genes. The CYP2B6 enzyme detoxifies many genotoxic xenobiotics, protecting cells from oxidative damage. The CYP2B6 gene is subjected to a single-nucleotide polymorphism (G⁵¹⁶T) with heterozygotes (GT) and homozygotes (TT) presenting decreased enzymatic activity. This case-control study aimed to investigate the association of CYP2B6 G⁵¹⁶T polymorphism with the susceptibility of AML and its cytogenetic and clinical characteristics. Genotyping was performed on 619 AML patients and 430 healthy individuals using RCR-RFLP and a novel LightSNip assay. The major finding was a statistically higher frequency of the variant genotypes (GT and TT) in patients compared to the controls (GT:38.8% vs 29.8% and TT:9.3% vs 5.3% respectively) (p<0.001). More specifically, a significantly higher frequency of GT+TT genotypes in de novo AML patients (46.6%) and an immensely high frequency of TT in secondary AML (s-AML) (20.5%) were observed. The statistical analysis showed that the variant T allele was approximately 1.5-fold and 2.4-fold higher in de novo and s-AML respectively than controls. Concerning FAB subtypes, the T allele presented an almost 2-fold increased in AML-M2. Interestingly, a higher incidence of the TT genotype was observed in patients with abnormal karyotypes. In particular, positive correlations of the mutant allele were found in patients carrying specific chromosomal aberrations [-7/del(7q), -5/del(5q), +8, +21 or t(8;21)], complex or monosomal karyotypes. Finally, a strikingly higher frequency of TT genotype was also observed in patients stratified to the poor risk group. In conclusion, our results provide evidence for the involvement of the CYP2B6 polymorphism in AML susceptibility and suggest a possible role of the CYP2B6 genetic background on the development of specific chromosomal aberrations.

Subtoxic and toxic concentrations of benzene and toluene induce Nrf2-mediated antioxidative stress response and affect the central carbon metabolism in lung epithelial cells A549

Since people in industrialized countries spend most of their time indoors, the effects of indoor contaminants such as volatile organic compounds become more and more relevant. Benzene and toluene are among the most abundant compounds in the highly heterogeneous group of indoor volatile organic compounds. In order to understand their effects on lung epithelial cells (A549) representing lung's first line of defense, we chose a global proteome and a targeted metabolome approach in order to detect adverse outcome pathways caused by exposure to benzene and toluene. Using a DIGE approach, 93 of 469 detected protein spots were found to be differentially expressed after exposure to benzene, and 79 of these spots were identified by MS. Pathway analysis revealed an enrichment of proteins involved in Nrf2-mediated and oxidative stress response glycolysis/gluconeogenesis. The occurrence of oxidative stress at nonacute toxic concentrations of benzene and toluene was confirmed by the upregulation of the stress related proteins NQO1 and SOD1. The changes in metabolism were validated by ion chromatography MS/MS analysis revealing significant changes of glucose-6-phosphate, fructose-6-phosphate, 3-phosphoglycerate, and NADPH. The molecular alterations identified as a result of benzene and toluene exposure demonstrate the detrimental effect of nonacute toxic concentrations on lung epithelial cells. The data provided here will allow for a targeted validation in in vivo models.

Importance of genetic polymorphisms of drug-metabolizing enzymes for the interpretation of biomarkers of exposure to styrene

The objective of this study was to test the infiuence of genetic polymorphisms for metabolic enzymes (CYP2E1, mEH, GSTM1 and GSTT1) implicated in the biotransformation of styrene in humans on the interpretation of urinary biomarkers of exposure. Thirty workers from a fibreglass-reinforced plastics factory took part in the study. Ambient styrene concentration was determined during the whole workshift by passive sampling. Urine was collected at the end of the shift for the determination of mandelic acid (MA) and phenylglyoxylic acid (PGA) (major biotransformation pathway), N-acetyl-S-(1-phenyl-2-hydroxy)ethyl-L-cysteine (M1) and N-acetyl-S-(2-phenyl-2-hydroxy)ethyl-L-cysteine (M2) (minor metabolic pathway) and creatinine. The average airborne styrene concentration of 18.2 ppm (range: 0.9-68.9 ppm) was very close to the current threshold limit value (TLV-TWA) recently adjusted by ACGIH from 50 to 20 ppm. There was a better correlation between external and internal exposure as estimated by urinary MA + PGA (r=0.92; p&lt;0.0001) compared with urinary M1 + M2 (r=0.74; p&lt;0.0001). To investigate to what extent genetic polymorphisms in metabolic enzymes could explain interindividual variations observed in the concentration of urinary biomarkers related to a given external exposure, two 'metabolic indexes' (derived from the ratio between the sum of urinary metabolites for a specific pathway and ambient styrene concentration) were calculated for each worker and compared for different allelic combinations. Monovariate analyses showed that GSTM1 polymorphism was clearly the most significant parameter infiuencing urinary concentrations of mercapturic acids. Based on GSTM1 allelic status, two different biological exposure indexes (BEIs) for M1 + M2 in post-shift urinary samples corresponding to a 20 ppm styrene concentration are proposed (GSTM1null: 1330 µg g(-1) creatinine, GSTM1+: 2878 µg g(-1) creatinine). Multivariate regression analyses were also performed and revealed that the presence of the rare CYP2E1*1B allele linked to TaqI polymorphism (A1/A2) was associated with increased urinary concentrations of metabolites from both pathways. Two previously described polymorphisms for the EPHX gene were also tested but seemed not really relevant for interpretation of biomarkers. In conclusion, while CYP2E1 genotyping, particularly assessment of the CYP2E1*1B allelic status, is useful for a more accurate interpretation of the concentration of urinary biomarkers, GSTM1 genotyping is absolutely necessary when considering a biological monitoring programme based on determination of urinary mercapturic acids.

CYP2E1 metabolism of styrene involves allostery

We are the first to report allosterism during styrene oxidation by recombinant CYP2E1 and human liver microsomes. At low styrene concentrations, oxidation is inefficient because of weak binding to CYP2E1 (K(s) = 830 μM). A second styrene molecule then binds CYP2E1 with higher affinity (K(ss) = 110 μM) and significantly improves oxidation to achieve a k(cat) of 6.3 nmol · min(-1) · nmol CYP2E1(-1). The transition between these metabolic cycles coincides with reported styrene concentrations in blood from exposed workers; thus, this CYP2E1 mechanism may be relevant in vivo. Scaled modeling of the in vitro-positive allosteric mechanism for styrene metabolism to its in vivo clearance led to significant deviations from the traditional model based on Michaelis-Menten kinetics. Low styrene levels were notably much less toxic than generally assumed. We interrogated the allosteric mechanism using the CYP2E1-specific inhibitor and drug 4-methylpyrazole, which we have shown binds two CYP2E1 sites. From the current studies, styrene was a positive allosteric effector on 4-methylpyrazole binding, based on a 10-fold increase in 4-methylpyrazole binding affinity from K(i) 0.51 to K(si) 0.043 μM. The inhibitor was a negative allosteric effector on styrene oxidation, because k(cat) decreased 6-fold to 0.98 nmol · min(-1) · nmol CYP2E1(-1). Consequently, mixtures of styrene and other molecules can induce allosteric effects on binding and metabolism by CYP2E1 and thus mitigate the efficiency of their metabolism and corresponding effects on human health. Taken together, our elucidation of mechanisms for these allosteric reactions provides a powerful tool for further investigating the complexities of CYP2E1 metabolism of drugs and pollutants.

Breath biomarkers of liver cirrhosis

The diagnosis of asymptomatic cirrhosis in patients with liver disease is of importance to start screening for complications in due time. Liver biopsy is neither sensitive nor practical enough to be used as a frequent follow-up test in patients with chronic liver disease. The volatile organic compounds present in exhaled breath offer the possibility of exploring internal physiologic and pathologic process in a non invasive way. This study examined whether a specific pattern of biomarkers can be found in breath samples of patients with cirrhosis. To this aim samples of alveolar breath from patients with cirrhosis and healthy volunteers were analyzed using gas chromatography-mass spectrometry. When linear discriminant analysis was used to search for a model(s)/pattern of compounds characteristic for liver cirrhosis, 24 models of 8 independent compounds could distinguish between the groups. The sensitivity and specificity (between 82% and 88%, and 96% and 100%, respectively) of the models suggest that a specific pattern of breath biomarkers can be found in patients with cirrhosis, which may allow detecting this complication of chronic liver disease in an early stage.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Pulmonary function and oxidative stress in workers exposed to styrene in plastic factory: occupational hazards in styrene-exposed plastic factory workers

Styrene is a volatile organic compound used in factories for synthesis of plastic products. The pneumotoxicity of styrene in experimental animals is known. The aim of the present study was to study the effect of styrene on lung function and oxidative stress in occupationally exposed workers in plastic factory. Thirty-four male workers, between 18 and 40 years of age, exposed to styrene for atleast 8 hours a day for more than a year were studied, while 30 age- and sex-matched healthy subjects not exposed to styrene served as controls. Assessment of lung functions showed a statistically significant reduction (p < 0.05) in most of the lung volumes, capacities (FVC, FEV(1), VC, ERV, IRV, and IC) and flow rates (PEFR, MEF(75%), and MVV) in the study group (workers) as compared to controls. Malondialdehyde (MDA) was observed to be significantly high (p < 0.05) while ferric-reducing ability of plasma (FRAP) was significantly low (p < 0.05) in styrene-exposed subjects. Reduced glutathione (GSH) level was significantly depleted in exposed subjects as compared to control group. The mean value of serum cytochrome c in styrene-exposed subjects was found to be 1.1 ng/ml (0.89-1.89) while in control its levels were under detection limit (0.05 ng/ml). It shows that styrene inhalation by workers leads to increased level of oxidative stress, which is supposed to be the cause of lung damage.

Functional characterization of human cytochrome P4502E1 allelic variants: in vitro metabolism of benzene and toluene by recombinant enzymes expressed in yeast cells

Benzene and toluene are common organic solvents currently in worldwide industrial usage, which are metabolized mainly by hepatic cytochrome P450 2E1 (CYP2E1) in humans. Genetic polymorphism of CYP2E1 in 5'-flanking and coding regions has been found previously in Caucasian and Chinese populations. In this study, the effects of CYP2E1 alleles causing amino acid substitutions (CYP2E1*2, CYP2E1*3 and CYP2E1*4; wild-type, CYP2E1.1A) on benzene hydroxylation and toluene methylhydroxylation were studied using recombinant CYP2E1 enzymes of wild-type (CYP2E1.1) and variants (CYP2E1.2 having Arg76His, CYP2E1.3 having Val389Ile and CYP2E1.4 having Val179Ile) expressed in yeast cells. The K (m), V (max) and CL (int) values of CYP2E1.1 were 10.1 mM, 9.38 pmol/min/pmol CYP and 0.99 nL/min/pmol CYP for benzene hydroxylation, and 3.97 mM, 19.9 pmol/min/pmol CYP and 5.26 nL/min/pmol CYP for toluene methylhydroxylation, respectively. The K (m), V (max) and CL (int) values for benzene and toluene metabolism of CYP2E1.2, CYP2E1.3 and CYP2E1.4 were comparable to those of wild-type CYP2E1. These findings may mean that the polymorphic alleles of CYP2E1 causing amino acid substitutions are not directly associated with the metabolic activation of benzene and toluene. The information gained in this study should help to identify the variations in the toxicity of environmental pollutants.

Styrene Metabolism, Genotoxicity, and Potential Carcinogenicity

This report reviews styrene biotransformation, including minor metabolic routes, and relates metabolism to the genotoxic effects and possible styrene-related carcinogenicity. Styrene is shown to require metabolic activation in order to become notably genotoxic and styrene 7,8-oxide is shown to contribute quantitatively by far the most (in humans more than 95%) to the genotoxicity of styrene, while minor ring oxidation products are also shown to contribute to local toxicities, especially in the respiratory system. Individual susceptibility depending on metabolism polymorphisms and individual DNA repair capacity as well as the dependence of the nonlinearity of the dose-response relationships in the species in question and the consequences for risk evaluation are analyzd.

Applications of CYP-450 expression for biomonitoring in environmental health

Cytochrome P450s (CYPs) are one of the first steps in the metabolism of xenobiotics, such as polycyclic aromatic hydrocarbons (PAHs), which are bioactivated into carcinogens. As such, changes in CYP expression are potential biomarkers in human biomonitoring applications. For the proper biomonitoring of environmental toxicants, it is important to understand the biological relevance of each biomarker and the associations among the biomarkers for uses as exposure, effects, and susceptibility biomarkers. Here, we have reviewed various aspects of CYPs for biomonitoring environmental health in terms of the CYP substrates, such as PAHs, aromatic amines, benzene/toluene, and tobacco smoking-related nitrosamines. This review also includes association studies between CYP phenotypical alterations and other exposure, susceptibility, and effect biomarkers. The association studies were mainly performed in CYP gene-transfected cells and noninvasive human biospecies, such as urine and peripheral blood. In conclusion, we suggest that phenotypical alterations in CYPs with exposure to environmental toxicants are useful as susceptibility or effect biomarkers, particularly when the phenotype-related genotypes are unknown.

The importance of cytochrome P450 2B6 in the human metabolism of environmental chemicals

Cytochrome P450 (CYP) 2B6 (CYP2B6) is a human CYP isoform found in variable amounts in the liver and other organs. It is known to be inducible and polymorphic and has a wide range of xenobiotic substrates. Studies of CYP2B6 to date have concentrated heavily on clinical drugs. In the present communication, however, we concentrate on its role in the metabolism of environmental xenobiotics. The term environment is used, in its broadest sense, to include natural ecosystems and agroecosystems as well as the industrial and indoor domestic environments. In essence, this excludes only clinical drugs and drugs of abuse. Many of these chemicals, including agrochemicals and industrial chemicals, can serve as substrates, inhibitors and/or inducers of CYP2B6, these activities being often modified by the existence of polymorphic variants. Metabolism-based interactions between environmental chemicals are discussed, as well as the emerging possibility of metabolic interactions between environmental chemicals and clinical drugs.

Intra- and Inter-ethnic Differences in the Allele Frequencies of Cytochrome P450 2B6 Gene in Chinese

This study aimed to investigate the allele frequencies of CYP2B6 gene in 193 Han Chinese and compared with 91 Uygur Chinese. Five single nucleotide polymorphisms of CYP2B6, 64C > T, 516G > T, 777C > A, 785A > G and 1459C > T, were tested using the polymerase chain reaction-restriction fragment length polymorphism method. The allele frequencies for CYP2B6*2, CYP2B6*3, CYP2B6*4, CYP2B6*5, CYP2B6*6, CYP2B6*7 and CYP2B6*9 in Han and Uygur Chinese were 0.034 and 0.027, 0 and 0.011, 0.091 and 0.033, 0.003 and 0.049, 0.184 and 0.214, 0 and 0.022, and 0.018 and 0.044, respectively, with CYP2B6*4, CYP2B6*5, and CYP2B6*7 being significantly different between these two races (P < 0.05). CYP2B6*6 was the most prevalent allele among all detected variants in Han and Uygur Chinese. The most frequent genotypes were CYP2B6*1/CYP2B6*1 (50.8%), CYP2B6*1/CYP2B6*6 (24.4%), and CYP2B6*1/CYP2B6*4 (7.3%) in Han subjects, whereas the most frequent genotypes in Uygur subjects were CYP2B6*1/CYP2B6*1 (36.3%), CYP2B6*1/CYP2B6*6 (25.3%), CYP2B6*1/CYP2B6*5 (5.5%) and CYP2B6*6/CYP2B6*6 (5.5%). The frequencies of 64C > T mutation in Han and Uygur Chinese were significantly lower than that in American Caucasian (P < 0.05). These results indicate that there were marked ethnic differences in the mutant frequencies of CYP2B6 between Chinese and other ethnic groups. Further studies are warranted to explore the clinical impact of such ethnic differences.

Genetic polymorphisms of cytochrome P450 2B6 gene in Han Chinese

Cytochrome P450 (CYP2B6) is an important enzyme that metabolizes more than eight compounds and about 3.0% of therapeutic drugs. The genetic polymorphisms of CYP2B6 have earlier been studied in Caucasian, Japanese and Korean, but the data are lacking for Han Chinese. The aim of this study was to investigate the frequencies of allelic variants of CYP2B6 in healthy Han Chinese and compare with those in other ethnic groups reported in the literature. Polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) method was used to test the five common non-synonymous single nucleotide polymorphisms (SNPs) of CYP2B6 gene, namely, 64C>T, 516G>T, 777C>A, 785A>G and 1459C>T in unrelated healthy Han Chinese (n=193). The study demonstrated that the frequencies of 64C>T, 516G>T, 777C>A, 785A>G and 1459C>T SNPs in Han Chinese were 0.03, 0.21, 0, 0.28 and 0.003, respectively. The frequencies of all five SNPs tested in female were higher than those in male, but the statistical difference was insignificant (P>0.05). Compared to the data reported in the literature, the frequencies of common CYP2B6 allelic variants in Chinese are similar to those of other Asian populations including Japanese and Korean, but markedly different from those in Caucasians. These results indicate the presence of marked ethnic difference in CYP2B6 SNP frequencies between Chinese and Caucasian. Further studies are required to explore the impact of these SNPs of CYP2B6 gene on the clinical response (efficacy and toxicity) to drugs that are substrates for CYP2B6 in patients.

Influence of genetic polymorphisms of styrene-metabolizing enzymes and smoking habits on levels of urinary metabolites after occupational exposure to styrene

Here we evaluate the influence of individual genetic polymorphisms of drug-metabolizing enzymes as well as body mass index (BMI) and lifestyle (smoking, alcohol consumption) on urinary metabolites after occupational exposure to styrene. Seventy-three workers exposed to styrene in a reinforced-plastics workplace were studied. The personal styrene exposure in the air and the urinary styrene metabolites mandelic acid and phenylglyoxylic acid were measured. The subjects' genetic polymorphisms in the genes that encode the styrene-metabolizing enzymes CYP2E1, CYP2B6, EPHX1, GSTM1, GSTT1 and GSTP1 were determined. Neither genotype nor lifestyle significantly affected urinary metabolites. There was, however, an interaction between the CYP2E1 genotype and smoking. Among non-smokers, urinary styrene metabolites were significantly decreased in subjects with c1/c1 alleles of CYP2E1 as compared with those with the c1/c2 genotype. There was no significant difference in urinary metabolites among smokers. When the combined influence of the CYP2B6 genotype and the predicted activity of EPHX1 were examined, urinary metabolites in subjects with low enzyme activity were lower than in those with medium or high activity after high styrene exposure (>or=50 ppm). The results suggest that genetic susceptibility and lifestyle should be considered in biological monitoring of exposure to styrene.

Review of the genotoxicity of styrene in humans

Styrene (CAS No. 100-42-5) is an important industrial chemical for which positive results have been reported in in vitro and in vivo genotoxicity assays. Styrene-exposed workers have been studied extensively over two decades for the induction of various types of genotoxic effects. The outcomes of these studies have been conflicting, and where positive responses have been reported, it has proved difficult to demonstrate clear relationships between levels of damage reported and exposure levels. In this review, we have assessed studies addressing mutagenicity (chromosome aberrations, micronuclei and gene mutations) and other endpoints (sister chromatid exchanges, DNA breaks and DNA adducts) using criteria derived from the IPCS guidelines for the conduct of human biomonitoring studies. Based on the re-evaluated outcomes, the data are not convincing that styrene induces gene mutations. The evidence for induction of clastogenicity in occupationally exposed workers is less clear, with a predominant lack of induction of micronuclei in different studies, but conflicting responses in chromosome aberration assays. The results of numerous studies on sister chromatid exchanges do not provide evidence of a clear positive response, despite these being induced in animals exposed to styrene at high concentrations. However, there is evidence that both DNA adducts and DNA single strand breaks are induced in styrene workers. These types of damage are considered indicative of exposure of the target cells and interaction with cellular DNA but do not necessarily result in heritable changes. There is evidence that the metabolism of styrene in humans is affected by genetic polymorphisms of metabolizing genes and that these polymorphisms affect the outcome of in vitro mutagenicity studies on styrene. Therefore, studies that have addressed the potential of this factor to affect in vivo responses were considered. To date, there are no consistent relationships between genetic polymorphisms and induction of genotoxicity by styrene in humans, but further work is warranted on larger samples. The analyses of individual studies, together with a consideration of dose-response relationships and the lack of a common profile of positive responses for the various endpoints in different studies, provide no clear evidence that styrene exposure in workers results in detectable levels of mutagenic damage. However, evidence of exposure to genotoxic metabolites is demonstrated by the formation of DNA adducts and strand breaks.

Effects of pregnancy, age and sex in the metabolism of styrene in rat liver in relation to the regulation of cytochrome P450 enzymes

To elucidate the effect of maternal styrene exposure, which is due to various postnatal changes in the development and behavior of offspring, we investigated pregnancy-induced changes in the metabolism of styrene in rat liver in relation to the regulation of cytochrome P450 enzymes. We also examined age and sex-induced changes in the metabolism of styrene. Pregnancy appeared to exert a negative effect on cytochrome P450 content at the late stage, whereas microsomal protein content showed little change during pregnancy. Pregnancy significantly decreased the rate of formation of styrene glycol at the late stage. The percentage of remaining activity in microsomes exposed to anti-CYP2E1 was lower than that exposed to anti-CYP2C11/6 in pregnant and non-pregnant female rats and immature male rats, indicating that CYP2E1 contributes to the metabolism of styrene more than CYP2C11/6 in these rats. Although pregnancy seemed to decrease styrene metabolism, the contribution of CYP2E1 seemed to be slightly increasing. In conclusion, pregnancy clearly influences the metabolism of styrene as well as other characteristic factors such as age and sex. It is very important to elucidate the changes in specific P450 isozyme composition related to their characteristic modification and in their affinity for chemicals.

The effects of simultaneous exposure to methyl ethyl ketone and toluene on urinary biomarkers of occupational N,N-dimethylformamide exposure

General regulations and risk assessment regarding toxicants are single-compound oriented even though humans are exposed to multi-chemicals in the general environment. This study investigated the effects of different levels of N,N-dimethylformamide (DMF) and co-exposure levels of methyl ethyl ketone (MEK) and toluene (TOL) on two biomarkers of DMF exposure: non-metabolized urinary (U-)DMF and the DMF metabolite urinary N-methylformamide (NMF). Thirty-five workers were selected from a two-stage field investigation strategy and were classified into four groups based on DMF exposure and co-exposure levels. Breathing-zone air concentrations of DMF, MEK, and TOL as well as dermal DMF exposure were determined. Post-shift U-DMF and U-NMF levels were determined for each individual. U-DMF concentrations were significantly higher in high-DMF groups than in low-DMF groups, but U-NMF concentrations were significantly (P<0.05) lower in the high-DMF-high-co-exposure group than in the high-DMF-low-co-exposure group; there were no significant differences between two low-DMF groups. The ratio of U-NMF to U-DMF showed the biotransformation from DMF to NMF was significantly suppressed at high co-exposure (P<0.001) for high-DMF exposure groups, possibly because of competitive inhibition of CYP2E1, the responsible enzyme involved. Due to the ubiquity of MEK/TOL in DMF-exposed occupational settings, the biological exposure index for occupational DMF exposure should be re-evaluated at high co-exposure levels.

Protective effect of Lycium chinense fruit on carbon tetrachloride-induced hepatotoxicity

In the present study, we investigated the protective effect of Lycium chinense Miller (Solanaceae) fruit (LFE) against CCl(4)-induced hepatotoxicity and the mechanism underlying these protective effects in rats. The pretreatment of LFE has shown to possess a significant protective effect by lowering the serum aspartate and alanine aminotransferase (AST and ALT) and alkaline phosphatase (ALP). This hepatoprotective action was confirmed by histological observation. In addition, pretreatment of LFE prevented the elevation of hepatic malondialdehyde (MDA) formation and the depletion of reduced glutathione (GSH) content and catalase activity in the liver of CCl(4)-injected rats. The LFE also displayed hydroxide radical scavenging activity in a dose-dependent manner (IC(50) = 83.6 microg/ml), as assayed by electron spin resonance (ESR) spin-trapping technique. The expression level of cytochrome P450 2E1 (CYP2E1) mRNA and protein, as measured by reverse transcriptase-polymerase chain reaction (RT-PCR) and western blot analysis, was significantly decreased in the liver of LFE-pretreated rats when compared with that in the liver of control group. Based on these results, it was suggested that the hepatoprotective effects of the LFE might be related to antioxidative activity and expressional regulation of CYP2E1.

Robustness of chlorzoxazone as an in vivo measure of cytochrome P450 2E1 activity

AIMS

Chlorzoxazone is metabolized by cytochrome P450 2E1 (CYP2E1) to a single oxidized metabolite, 6-hydroxychlorzoxazone. The aim of the study was to test the robustness of chlorzoxazone as an in vivo probe of CYP2E1 activity in humans, with emphasis on investigating short-term and long-term intra-individual variabilities and effects of different doses of the drug. In addition, the influences of body build, drug metabolizing enzyme genotype, blood sampling time, and moderate recent ethanol intake were investigated.

METHODS

The 6-hydroxychlorzoxazone:chlorzoxazone (metabolic) ratio in plasma was measured at 2 h in 28 male and nine female volunteers following a single oral dose of 500 mg chlorzoxazone. Similarly, the metabolic ratios at 4 h and 6 h were measured in 20 of the males. The metabolic ratio at 2 h was also determined 1.5 and 2.5 years later in 13 and seven males, respectively, and weekly for 3 weeks in seven males, after a dose of 500 mg, once at higher (750 mg) and lower (250 mg) doses, and once (500 mg) following moderate ethanol intake (0.5 g kg(-1) body weight) the preceding evening. Genotypes were determined for CYP2E1 as well as for N-acetyltransferase 2 and glutathione transferase M1.

RESULTS

Excluding an outlier (ratio = 1.6) the metabolic ratio at 2 h ranged from 0.12 to 0.61 (n = 36). A positive correlation with body weight (r = 0.61, P < 0.001) suggested dose-dependent metabolism of chlorzoxazone. The metabolic ratio decreased with increasing chlorzoxazone dose (P = 0.01), again suggesting dose-dependent metabolism. Long-term (yearly intervals) and short-term (weekly intervals) intra- and interindividual variabilities in metabolic ratio were similar (30% and 63%vs 28% and 54%, respectively). Both inter- and intra-individual variabilities tended to decrease with increasing dose of chlorzoxazone. There was no significant influence of moderate ethanol intake the preceding evening, or of CYP2E1 genotype on the metabolic ratio.

CONCLUSIONS

The relatively low intra-individual variability in the metabolism of chlorzoxazone suggests that a single-sample procedure may suffice to assess CYP2E1 activity in vivo. However, chlorzoxazone metabolism is dose-dependent at commonly used doses and it is therefore advisable to adjust the dose for body weight. Moderate intake of ethanol the preceding evening did not significantly affect the chlorzoxazone metabolic ratio.

Response of human cord blood cells to styrene exposure: evaluation of its effects on apoptosis and gene expression by genomic technology

Styrene is one of the most important monomers produced worldwide, and it finds major use in the production of polystyrene, acrylonitrile-butadiene-styrene resins and unsaturated polystyrene resins. Epidemiological studies on styrene showed that the malignancies observed most frequently in humans after exposure are related to the lymphatic and haemopoietic system. IARC classified styrene a possible carcinogenic to humans (Group 2B). In this study, we evaluated the effect of styrene on gene expression profiles of human cord blood cells, as well as its activity on the apoptosis and bcl-2 related protein expression. Data demonstrated that, after 24 and 48 h of exposure, styrene (800 microM) induced an increase in the necrosis of mononuclear cord blood cells, whereas it did not cause any increase in the apoptotic process. Western blot analysis revealed a modified expression of Bax, BCl-2, c-Jun, c-Fos and Raf-1 proteins in the human cord blood cells after direct exposure to styrene, whereas p53 expression did not change. Furthermore, Macroarray analysis showed that styrene changed cord blood gene expression, inducing up-regulation of monocyte chemotactic protein 1 (MCP-1), and down-regulation of CC chemokine receptor type 1 (CCR-1) and SLP-76 tyrosine-phosphoprotein.

Evaluation of Current Biological Exposure Index for Occupational N, N-dimethylformamide Exposure From Synthetic Leather Workers

The aim of this study was (1) to investigate the correlation between external exposure to N, N-dimethylformamide (DMF) and urinary excretion of DMF and N-methylformamide; (2) to assess whether the correspondence between the current occupational exposure limit setting and recommended urinary biological exposure index is substantial; and (3) to evaluate whether coexposure to toluene, methyl ethyl ketone, and ethyl acetate has an effect on urinary excretion of DMF and N-methylformamide (NMF). Urinary DMF and NMF were significantly correlated (P < 0.01) with one another and also significantly correlated with airborne DMF (P < 0.01) over the range of 1.55 to 152.8 mg/m. Urinary DMF can be considered a complementary marker for short-term exposure. Urinary concentration of NMF and DMF, corresponding to the 8-hour exposure to airborne DMF at 30 mg/m, was estimated to 38.4 mg/L or 39.4 mg/g creatinine for NMF and to 0.92 mg/L or 0.96 mg/g creatinine for DMF.

Occupational exposure to styrene: modulation of cytogenetic damage and levels of urinary metabolites of styrene by polymorphisms in genes CYP2E1, EPHX1, GSTM1, GSTT1 and GSTP1

Styrene is widely used in the production of various plastics, synthetic rubber and resins. The aim of this study was to evaluate if individual polymorphisms in xenobiotic metabolizing enzymes, related with the metabolic fate of styrene, could modify individual susceptibility to the possible genotoxic effects of the styrene exposure. Twenty-eight reinforced plastic workers and 28 control subjects were studied. In the selected population the urinary styrene metabolites mandelic (MA) and phenylglyoxylic (PGA) acids were quantified, sister chromatid exchanges (SCE) and micronuclei (MN) were assessed in peripheral lymphocytes and all the subjects were genotyped for GSTM1, GSTT1 (gene deletions), GSTP1 (codon 105 ile==>val), EPHX1 (codons 113 tyr==>his and 139 his==>arg) and CYP2E1 (DraI polymorphism in intron 6). The results obtained showed a significant difference between the levels of SCE, but not in MN levels, in exposed workers as compared with the control group. The GSTP1 and CYP2E1 individual genotypes modulate the baseline levels of SCE that are lower in non-wild type individuals for both polymorphisms. The GSTM1 null individuals with low levels of exposure have significantly higher urinary levels of MA+PGA. The present data seem to suggest that apart from the methodology usually used for monitoring populations occupationally exposed to styrene (urinary metabolites and biomarkers of early biological effects) the analysis of individual genotypes associated with the metabolic fate of styrene should also be carried out in order to evaluate the individual genetic susceptibility of exposed populations.

Effects of genetic polymorphisms in metabolic enzymes on the relationships between 8-hydroxydeoxyguanosine levels in human leukocytes and urinary 1-hydroxypyrene and 2-naphthol concentrations

This study was designed to investigate the relationship between environmental exposure to polycyclic aromatic hydrocarbons (PAHs) and oxidative stress, and to evaluate the effects of cigarette smoking and the genetic polymorphisms of CYP1A1, CYP2E1, GSTM1, NAT2 and UGT1A6 on the relationship. The subjects of this study were 105 healthy Korean males without occupational exposure to PAHs. The 8-hydroxydeoxyguanosine (8-OHdG) level in leukocytes, and urinary 1-hydroxypyrene (1-OHP) and 2-naphthol concentrations, were measured by high-performance liquid chromatography. Genetic polymorphisms of CYP1A1, CYP2E1, GSTM1, NAT2 and UGT1A6 were identified by PCR and PCR-RFLP methods. The 8-OHdG level showed a significant correlation with the 1-OHP concentration in all subjects (p<.001) and in smokers (p<.01), and with the 2-naphthol level in non-smokers (p<.01). The 8-OHdG level was significantly higher in smoking rapid acetylators than in smoking slow or intermediate acetylators, and in individuals with the UGT1A6 wild-type than in those with the UGT1A6 mutant genotype. Significant positive correlations between 8-OHdG and 1-OHP concentrations were found in subjects with every genotype of the CYP1A1 and CYP2E1 genes, with the GSTM1 null-type, with the NAT2 genotype of a rapid acetylator, and with the UGT1A6 wild-type, respectively. The urinary 2-naphthol level significantly correlated with the 8-OHdG level only in subjects with the GSTM1 null-type. In conclusion, there is a significant correlation between the 8-OHdG level in leukocytes and the urinary 1-OHP concentration in the population not occupationally exposed to PAHs. This relationship is affected by genetic polymorphisms in PAH metabolic enzymes.

A recombinant model for assessing the role of GSTM1 in styrene-7,8-oxide toxicity and mutagenicity

Styrene-7,8-oxide (SO) is a highly reactive epoxide able to undergo reactions with endogenous nucleophiles, such as DNA. SO is inactivated by glutathione-S-transferase M1 (GSTM1). This detoxification enzyme is absent in approximately one-half of Caucasian (49%) populations. A GSTM1 recombinant human lymphoblastoid cell line (FB7) was generated from a GSTM1 negative parental cell line (WIL2NS). GSTM1 status was determined using RT-PCR and immunochemistry. Cells were challenged with a range of SO doses and subsequent toxicity (population growth in flasks) and genotoxicity (mutations at the HPRT locus) were monitored. FB7 (GSTM1 positive) exhibited greater cell survival after SO exposure relative to the GSTM1 negative parental line. The IC50 following a 1 h exposure to SO was 0.5 mM for WIL2NS, compared to greater than 2.5 mM for FB7. The extrapolated IC50 for FB7 was 5.5 mM. Significantly fewer mutant cells were induced by SO for FB7 than for WIL2NS at equivalent doses of SO. These findings suggest that the sensitivity of cells to styrene-7,8-oxide is influenced by GSTM1 status and that a recombinant GSTM1 positive cell line can efficiently detoxify styrene-7,8-oxide.

Analysis of differential substrate selectivities of CYP2B6 and CYP2E1 by site-directed mutagenesis and molecular modeling

Human CYP2B6 and CYP2E1 were used to investigate the extent to which differential substrate selectivities between cytochrome P450 subfamilies reflect differences in active-site residues as opposed to distinct arrangement of the backbone of the enzymes. Reciprocal CYP2B6 and CYP2E1 mutants at active-site positions 103, 209, 294, 363, 367, and 477 (numbering according to CYP2B6) were characterized using the CYP2B6-selective substrate 7-ethoxy-4-trifluoromethylcoumarin, the CYP2E1-selective substrate p-nitrophenol, and the common substrates 7-ethoxycoumarin, 7-butoxycoumarin, and arachidonic acid. This report is the first to study the active site of CYP2E1 by systematic site-directed mutagenesis. One of the most intriguing findings was that substitution of CYP2E1 Phe-477 with valine from CYP2B6 resulted in significant 7-ethoxy-4-trifluoromethylcoumarin deethylation. Use of three-dimensional models of CYP2B6 and CYP2E1 based on the crystal structure of CYP2C5 suggested that deethylation of 7-ethoxy-4-trifluoromethylcoumarin by CYP2E1 is impeded by van der Waals overlaps with the side chain of Phe-477. Interestingly, none of the CYP2B6 mutants acquired enhanced ability to hydroxylate p-nitrophenol. Substitution of residue 363 in CYP2E1 and CYP2B6 resulted in significant alterations of the metabolite profile for the side chain hydroxylation of 7-butoxycoumarin. Probing of CYP2E1 mutants with arachidonic acid indicated that residues Leu-209 and Phe-477 are critical for substrate orientation in the active site. Overall, the study revealed that differences in the side chains of active-site residues are partially responsible for differential substrate selectivities across cytochrome P450 subfamilies. However, the relative importance of active-site residues appears to be dependent on the structural similarity of the compound to other substrates of the enzyme.

Interest of genotyping and phenotyping of drug-metabolizing enzymes for the interpretation of biological monitoring of exposure to styrene

In the field of occupational and/or environmental toxicology, the measurement of specific metabolites in urine may serve to assess exposure to the parent compounds (biological monitoring of exposure). Styrene is one of the chemicals for which biological monitoring programs have been validated and implemented in environmental and occupational medicine. However, inter-individual differences in the urinary excretion exist both for the main end-products (mandelic acid and phenylglyoxylic acid) and for its specific mercapturic acids (phenylhydroxyethylmercapturic acids, PHEMA). This limits to a certain extent the use of these metabolites for an accurate assessment of styrene exposure. In a group of 26 volunteers selected with relevant genotypes, and exposed to styrene vapours (50 mg/m3, 8 h) in an inhalation chamber, we evaluated whether genotyping or phenotyping relevant drug-metabolizing enzymes (CYP2E1, EPHX1, GSTM1, GSTT1 and GSTP1) may help to explain the observed inter-individual variability in the urinary metabolite excretion. Peripheral blood lymphocytes were used for genotyping and as reporter cells for the phenotyping of CYP2E1 and EPHX1. The GSTM1 genotype was clearly the most significant parameter explaining the variance in urinary PHEMA excretion (6-fold lower in GSTM1 null subjects; P < 0.0001) so that systematic GSTM1 genotyping should be recommended routinely for a correct interpretation of PHEMA urinary levels. Variant alleles CYP2E1*6 (7632T>A) and His113EPHX1 were associated with a significant reduction of, respectively, the expression (P = 0.047) and activity (P = 0.022) of the enzyme in peripheral blood lymphocytes. In combination with GSTM1 genotyping, the phenotyping approach also contributed to improve the interpretation of urinary results, as illustrated by the combined effect of CYP2E1 expression and GSTM1 allelic status that explained 77% of the variance in PHEMA excretion and allows the recommendation of mercapturates as specific and reliable biomarkers of exposure to styrene.

Metabolite ratio of toluene-exposed rotogravure printing plant workers reflects individual mutagenic risk by sister chromatid exchanges

The study involved a group of 42 printing plant workers and a control group of 45 blood donors. At the working places, the ambient air-toluene concentration amounted from 141 to 328 mg/m(3). Sister chromatid exchanges (SCEs) were significantly elevated by three units in the exposed group. In this group, the concentration of urinary toluene metabolites was also considerably increased-hippuric acid was four times higher and the o-cresol and p-cresol fractions were twice as high. Results of toluene monitoring of ambient air- or blood-toluene concentrations did not show any relationships with individual SCE. While these SCE values revealed only a weak relationship with the corresponding hippuric acid data, a significant correlation with the cresols, which are known to be more genotoxic than hippuric acid, appeared in highly exposed workers. An attempt was made to consider the individual metabolic balance of toluene excretion products. For that reason individual cresol to hippuric acid ratios were calculated and related to corresponding SCE values. In all investigated subpopulations of the exposed group, this ratio correlated with SCE at a level of high significance. This strong interrelationship is a powerful argument for the genotoxic behavior of toluene. Furthermore, the individual metabolic balance, as a consequence of genetic polymorphism, should be considered in the discussion about genetic risk of toluene.