Identification of three protein targets for reactive metabolites of bromobenzene in rat liver cytosol

Assessment of hepatoprotective and nephroprotective potential of withaferin A on bromobenzene-induced injury in Swiss albino mice: possible involvement of mitochondrial dysfunction and inflammation

Bromobenzene is a well-known environmental toxin which causes liver and kidney damage through CYP450-mediated bio-activation to generate reactive metabolites and, consequently, oxidative stress. The present study aimed to evaluate the possible protective role of withaferin A against bromobenzene-induced liver and kidney damage in mice. Withaferin A (10 mg/kg) was administered orally to the mice for 8 days before intragastric intubation of bromobenzene (10 mmol/kg). As results of this experiment, the levels of liver and kidney functional markers, lipid peroxidation, and cytokines (TNF-α and IL-1β) presented an increase and there was a decrease in anti-oxidant activity in the bromobenzene-treated group of mice. Pre-treatment with withaferin A not only significantly decreased the levels of liver and kidney functional markers and cytokines but also reduced oxidative stress, as evidenced by improved anti-oxidant status. In addition, the mitochondrial dysfunction shown through the decrease in the activities of mitochondrial enzymes and imbalance in the Bax/Bcl-2 expression in the livers and kidneys of bromobenzene-treated mice was effectively prevented by pre-administration of withaferin A. These results validated our conviction that bromobenzene caused liver and kidney damage via mitochondrial pathway and withaferin A provided significant protection against it. Thus, withaferin A may have possible usage in clinical liver and kidney diseases in which oxidative stress and mitochondrial dysfunction may be existent.

Bioinformatic analysis of 302 reactive metabolite target proteins. Which ones are important for cell death?

Many low molecular weight compounds undergo biotransformation to chemically reactive metabolites (CRMs) that covalently modify cellular proteins. However, the mechanisms by which this covalent binding leads to cytotoxicity are not understood. Prior analyses of lists of target proteins sorted by functional categories or hit frequency have not proven informative. In an attempt to move beyond covalent binding, we hypothesized that xenobiotic posttranslational modification of proteins might disrupt important protein-protein interactions (PPIs) and thereby direct cells from homeostasis into cell death pathways. To test this hypothesis, we analyzed a list of 302 proteins (66% rat, 26% mouse, 5% human) known to be targeted by 41 different cytotoxic CRMs. Human orthologs of rodent proteins were found by blast sequence alignment, and their interacting partners were found using the Human Protein Reference Database. The combined set of target orthologs and partners was sorted into KEGG pathways and Gene Ontology categories. Those most highly ranked based on sorting statistics and toxicological relevance were heavily involved with intracellular signaling pathways, protein folding, unfolded protein response, and regulation of apoptosis. Detailed examination revealed that many of the categories were flagged primarily by partner proteins rather than target proteins and that a majority of these partners interacted with just a small number of proteins in the CRM target set. A similar analysis performed without the partner proteins flagged very few categories as significant. These results support the hypothesis that disruption of important PPIs may be a major mechanism contributing to CRM-induced acute cytotoxicity.

Protein targets of thioacetamide metabolites in rat hepatocytes

Thioacetamide (TA) has long been known as a hepatotoxicant whose bioactivation requires S-oxidation to thioacetamide S-oxide (TASO) and then to the very reactive S,S-dioxide (TASO2). The latter can tautomerize to form acylating species capable of covalently modifying cellular nucleophiles including phosphatidylethanolamine (PE) lipids and protein lysine side chains. Isolated hepatocytes efficiently oxidize TA to TASO but experience little covalent binding or cytotoxicity because TA is a very potent inhibitor of the oxidation of TASO to TASO2. However, hepatocytes treated with TASO show extensive covalent binding to both lipids and proteins accompanied by extensive cytotoxicity. In this work, we treated rat hepatocytes with [(14)C]-TASO and submitted the mitochondrial, microsomal, and cytosolic fractions to 2DGE, which revealed a total of 321 radioactive protein spots. To facilitate the identification of target proteins and adducted peptides, we also treated cells with a mixture of TASO/[(13)C2D3]-TASO. Using a combination of 1DGE- and 2DGE-based proteomic approaches, we identified 187 modified peptides (174 acetylated, 50 acetimidoylated, and 37 in both forms) from a total of 88 nonredundant target proteins. Among the latter, 57 are also known targets of at least one other hepatotoxin. The formation of both amide- and amidine-type adducts to protein lysine side chains is in contrast to the exclusive formation of amidine-type adducts with PE phospholipids. Thiobenzamide (TB) undergoes the same two-step oxidative bioactivation as TA, and it also gives rise to both amide and amidine adducts on protein lysine side chains but only amidine adducts to PE lipids. Despite their similarity in functional group chemical reactivity, only 38 of 62 known TB target proteins are found among the 88 known targets of TASO. The potential roles of protein modification by TASO in triggering cytotoxicity are discussed in terms of enzyme inhibition, protein folding, and chaperone function, and the emerging role of protein acetylation in intracellular signaling and the regulation of biochemical pathways.

Liver protein targets of hepatotoxic 4-bromophenol metabolites

The hepatotoxicity of bromobenzene (BB) is directly related to the covalent binding of both initially formed epoxide and secondary quinone metabolites to at least 45 different liver proteins. 4-Bromophenol (4BP) is a significant BB metabolite and a precursor to reactive quinone metabolites; yet, when administered exogenously, it has negligible hepatotoxicity as compared to BB. The protein adducts of 4BP were thus labeled as nontoxic [Monks, T. J., Hinson, J. A., and Gillette, J. R. (1982) Life Sci. 30, 841-848]. To help identify which BB-derived adducts might be related to its cytotoxicity, we sought to identify the supposedly nontoxic adducts of 4BP and eliminate them from the BB target protein list. Administration of [(14)C]-4BP to phenobarbital-induced rats resulted in covalent binding of 0.25, 0.33, and 0.42 nmol equiv 4BP/mg protein in the mitochondrial, microsomal, and cytosolic fractions, respectively. These values may be compared to published values of 3-6 nmol/mg protein from a comparable dose of [(14)C]-BB. After subcellular fractionation and 2D electrophoresis, 47 radioactive spots on 2D gels of the mitochondrial, microsomal, and cytosolic fractions were excised, digested, and analyzed by LC-MS/MS. Twenty-nine of these spots contained apparently single proteins, of which 14 were nonredundant. Nine of the 14 are known BB targets. Incubating freshly isolated rat hepatocytes with 4BP (0.1-0.5 mM) produced time- and concentration-dependent increases in lactate dehydrogenase release and changes in cellular morphology. LC-MS/MS analysis of the cell culture medium revealed rapid and extensive sulfation and glucuronidation of 4BP as well as formation of a quinone-derived glutathione conjugate. Studies with 7-hydroxycoumarin, (-)-borneol, or D-(+)-galactosamine showed that inhibiting the glucuronidation/sulfation of 4BP increased the formation of a GSH-bromoquinone adduct, increased covalent binding of 4BP to hepatocyte proteins, and potentiated its cytotoxicity. Taken together, our data demonstrate that protein adduction by 4BP metabolites can be toxicologically consequential and provide a mechanistic explanation for the failure of exogenously administered 4BP to cause hepatotoxicity. Thus, the probable reason for the low toxicity of 4BP in vivo is that rapid conjugation limits its oxidation and covalent binding and thus its toxicity.

Mechanisms of the hepatoprotective effects of tamoxifen against drug-induced and chemical-induced acute liver injuries

Although estrogen receptor (ER)α agonists, such as estradiol and ethinylestradiol (EE2), cause cholestasis in mice, they also reduce the degree of liver injury caused by hepatotoxicants as well as ischemia-reperfusion. The functional mechanisms of ERα have yet to be elucidated in drug-induced or chemical-induced liver injury. The present study investigated the effects of an ERα agonist, selective ER modulators (SERMs) and an ER antagonist on drug-induced and chemical-induced liver injuries caused by acetaminophen, bromobenzene, diclofenac, and thioacetamide (TA). We observed hepatoprotective effects of EE2, tamoxifen (TAM) and raloxifene pretreatment in female mice that were exposed to a variety of hepatotoxic compounds. In contrast, the ER antagonist did not show any hepatoprotective effects. DNA microarray analyses suggested that monocyte to macrophage differentiation-associated 2 (Mmd2) protein, which has an unknown function, is commonly increased by TAM and RAL pretreatment, but not by pretreatment with the ER antagonist. In ERα-knockout mice, the hepatoprotective effects of TAM and the increased expression of Mmd2 mRNA were not observed in TA-induced liver injury. To investigate the function of Mmd2, the expression level of Mmd2 mRNA was significantly knocked down to approximately 30% in mice by injection of siRNA for Mmd2 (siMmd2). Mmd2 knockdown resulted in a reduction of the protective effects of TAM on TA-induced liver injury in mice. This is the first report of the involvement of ERα in drug-induced or chemical-induced liver injury. Upregulation of Mmd2 protein in the liver was suggested as the mechanism of the hepatoprotective effects of EE2 and SERMs.

Analysis of naphthalene adduct binding sites in model proteins by tandem mass spectrometry

The electrophilic metabolites of the polyaromatic hydrocarbon naphthalene have been shown to bind covalently to proteins and covalent adduct formation correlates with the cytotoxic effects of the chemical in the respiratory system. Although 1,2-naphthalene epoxide, naphthalene diol epoxide, 1,2-naphthoquinone, and 1,4-napthoquinone have been identified as reactive metabolites of interest, the role of each metabolite in total covalent protein adduction and subsequent cytotoxicity remains to be established. To better understand the target residues associated with the reaction of these metabolites with proteins, mass spectrometry was used to identify adducted residues following (1) incubation of metabolites with actin and protein disulfide isomerase (PDI), and (2) activation of naphthalene in microsomal incubations containing supplemental actin or PDI. All four reactive metabolites bound to Cys, Lys or His residues in actin and PDI. Cys₁₇ of actin was the only residue adducted by all metabolites; there was substantial metabolite selectivity for the majority of adducted residues. Modifications of actin and PDI, following microsomal incubations containing ¹⁴C-naphthalene, were detected readily by 2D gel electrophoresis and phosphor imaging. However, target modifications on tryptic peptides from these isolated proteins could not be readily detected by MALDI/TOF/TOF and only three modified peptides were detected using high resolution-selective ion monitoring (HR-SIM). All the reactive metabolites investigated have the potential to modify several residues in a single protein, but even in tissues with very high rates of naphthalene activation, the extent of modification was too low to allow unambiguous identification of a significant number of modified residues in the isolated proteins.

Identification of protein targets of reactive metabolites of tienilic acid in human hepatocytes

Tienilic acid (TA) is a uricosuric diuretic that was withdrawn from the market only months after its introduction because of reports of serious incidents of drug-induced liver injury including some fatalities. Its hepatotoxicity is considered to be primarily immunoallergic in nature. Like other thiophene compounds, TA undergoes biotransformation to a S-oxide metabolite which then reacts covalently with cellular proteins. To identify protein targets of TA metabolites, we incubated [(14)C]-TA with human hepatocytes, separated cellular proteins by 2D gel electrophoresis, and analyzed proteins in 36 radioactive spots by tryptic digestion followed by LC-MS/MS. Thirty-one spots contained at least one identifiable protein. Sixteen spots contained only one of 14 nonredundant proteins which were thus considered to be targets of TA metabolites. Six of the 14 were also found in other radioactive spots that contained from 1 to 3 additional proteins. Eight of the 14 had not been reported to be targets for any reactive metabolite other than TA. The other 15 spots each contained from 2 to 4 identifiable proteins, many of which are known targets of other chemically reactive metabolites, but since adducted peptides were not observed, the identity of the adducted protein(s) in these spots is ambiguous. Interestingly, all the radioactive spots corresponded to proteins of low abundance, while many highly abundant proteins in the mixture showed no radioactivity. Furthermore, of approximately 16 previously reported protein targets of TA in rat liver ( Methogo, R., Dansette, P., and Klarskov, K. ( 2007 ) Int. J. Mass Spectrom. , 268 , 284 -295 ), only one (fumarylacetoacetase) is among the 14 targets identified in this work. One reason for this difference may be statistical, given that each study identified a small number of targets from among thousands present in hepatocytes. Another may be the species difference (i.e., rat vs human), and still another may be the method of detection of adducted proteins (i.e., Western blot vs C-14). Knowledge of human target proteins is very limited. Of more than 350 known protein targets of reactive metabolites, only 42 are known from humans, and only 21 of these are known to be targets for more than one chemical. Nevertheless, the demonstration that human target proteins can be identified using isolated hepatocytes in vitro should enable the question of species differences to be addressed more fully in the future.

Filling and mining the reactive metabolite target protein database

The post-translational modification of proteins is a well-known endogenous mechanism for regulating protein function and activity. Cellular proteins are also susceptible to post-translational modification by xenobiotic agents that possess, or whose metabolites possess, significant electrophilic character. Such non-physiological modifications to endogenous proteins are sometimes benign, but in other cases they are strongly associated with, and are presumed to cause, lethal cytotoxic consequences via necrosis and/or apoptosis. The Reactive Metabolite Target Protein Database (TPDB) is a searchable, freely web-accessible (http://tpdb.medchem.ku.edu:8080/protein_database/) resource that attempts to provide a comprehensive, up-to-date listing of known reactive metabolite target proteins. In this report we characterize the TPDB by reviewing briefly how the information it contains came to be known. We also compare its information to that provided by other types of "-omics" studies relevant to toxicology, and we illustrate how bioinformatic analysis of target proteins may help to elucidate mechanisms of cytotoxic responses to reactive metabolites.

Identification of the Protein Targets of the Reactive Metabolite of Teucrin A in Vivo in the Rat

Covalent modification of proteins is associated with the toxicity of many electrophiles, and the identification of relevant in vivo protein targets is a desirable but challenging goal. Here, we describe a strategy for the enrichment of adducted proteins utilizing single-chain fragment variable (ScFv) antibodies selected using phage-display technology. Teucrin A is a furan-containing diterpenoid found in the herb germander that is primarily responsible for the herb's hepatotoxicity in rodents and humans following metabolic activation by cytochrome P450 enzymes. Conjugates of the 1,4-enedial derivative of teucrin A, its presumed toxic metabolite, with lysine- and cysteine-containing peptides were synthesized and used to select ScFvs from a rodent phage-displayed library, which recognized the terpenoid moiety of the teucrin-derived adducts. Immunoaffinity isolation of adducted proteins from rat liver homogenates following administration of a toxic dose of teucrin A afforded a family of proteins that were identified by liquid chromatography/tandem mass spectrometry. Of the 46 proteins identified in this study, most were of mitochondrial and endoplasmic reticulum origin. Several cytosolic proteins were found, as well as four peroxisomal and two secreted proteins. Using Ingenuity Pathway Analysis software, two significant networks involving the target genes were identified that had major functions in gene expression, small molecule biochemistry, and cellular function and maintenance. These included proteins involved in lipid, amino acid, and drug metabolism. This study illustrates the utility of chemically synthesized biological conjugates of reactive intermediates and the potential of the phage display technology for the generation of affinity reagents for the isolation of adducted proteins.

Site-specific arylation of rat glutathione s-transferase A1 and A2 by bromobenzene metabolites in vivo

The hepatotoxicity of bromobenzene (BB) derives from its reactive metabolites (epoxides and quinones), which arylate cellular proteins. Application of proteomic methods to liver proteins from rats treated with a hepatotoxic dose of [14C]-BB has identified more than 40 target proteins, but no adducted peptides have yet been observed. Because such proteins are known to contain bromophenyl- and bromodihydroxyphenyl derivatives of cysteine, histidine, and lysine, the failure to observe modified peptides has been attributed to the low level of total covalent binding and to the "dilution" effect of multiple metabolites reacting at multiple sites on multiple proteins. In this work glutathione S-transferase (GST), a well-known and abundant BB-target protein, was isolated from liver cytosol of rats treated with 14C-BB by use of a glutathione (GSH)-agarose affinity column and further resolved by reverse-phase high-performance liquid chromatography (HPLC) into subunits M1, M2, A1, A2 and A3. The subunits were identified by a combination of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), whole-molecule mass spectrometry, and peptide mass mapping and found to contain radioactivity corresponding to 0.01-0.05 adduct per molecule of protein. Examination of tryptic digests of these subunits by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) and electrospray ionization mass spectrometry (ESI-MS) failed to reveal any apparent adducted peptides despite observed sequence coverages up to 87%. However, use of HPLC-linear ion-trap quadrupole Fourier transform mass spectrometry (LTQ-FTMS) to search for predicted modified tryptic peptides revealed peaks corresponding, with a high degree of mass accuracy, to a bromobenzoquinone adduct of peptide 89-119 in both GSTA1 and A2. The identity of these adducts and their location at Cys-111 was confirmed by tandem mass spectrometry (MS-MS). No evidence for the presence of any putative BB-adducts in GST M1, M2, or A3 was obtained. This work highlights the challenges involved in the unambiguous identification of reactive metabolite adducts formed in vivo.

Ecotoxicological assessment of bromobenzene using a test battery with five model systems

Bromobenzene (BrB) is used as a solvent for crystallization and as an additive to motor oils and may be released into the environment through various waste streams. However, there is limited available information about the toxic hazard of BrB in the aquatic environment. Consequently, the ecotoxicological effects induced by BrB were investigated using five model systems with representants from four trophic levels. The battery included bioluminescence inhibition of the bacterium Vibrio fischeri, growth inhibition of the alga Chlorella vulgaris and immobilization of the cladoceran Daphnia magna. Total protein content, neutral red uptake and MTS metabolization were reduced, while lysosomal function, succinate dehydrogenase activity, G6PDH activity and leakage, metallothionein levels and EROD activity were stimulated in PLHC-1 and RTG-2 fish cell lines. The most sensitive bioindicator was the bioluminiscence of V. fischeri, with an EC(50) of 0.04mM BrB at 15min and a non-observed adverse effect level of 0.02 mM BrB. There is a large difference in sensitivity to BrB among the model systems probably due to the metabolic capacity of the different species. PLHC-1 cells were more sensitive to BrB than RTG-2 cells. The most prominent morphological effects observed were hydropic degeneration, loss of cells and of the perinuclear pattern of distribution of lysosomes. Therefore, BrB should be classified as toxic to aquatic organisms.

The reactive metabolite target protein database (TPDB)--a web-accessible resource

Background

The toxic effects of many simple organic compounds stem from their biotransformation to chemically reactive metabolites which bind covalently to cellular proteins. To understand the mechanisms of cytotoxic responses it may be important to know which proteins become adducted and whether some may be common targets of multiple toxins. The literature of this field is widely scattered but expanding rapidly, suggesting the need for a comprehensive, searchable database of reactive metabolite target proteins.

Description

The Reactive Metabolite Target Protein Database (TPDB) is a comprehensive, curated, searchable, documented compilation of publicly available information on the protein targets of reactive metabolites of 18 well-studied chemicals and drugs of known toxicity. TPDB software enables i) string searches for author names and proteins names/synonyms, ii) more complex searches by selecting chemical compound, animal species, target tissue and protein names/synonyms from pull-down menus, and iii) commonality searches over multiple chemicals. Tabulated search results provide information, references and links to other databases.

Conclusion

The TPDB is a unique on-line compilation of information on the covalent modification of cellular proteins by reactive metabolites of chemicals and drugs. Its comprehensiveness and searchability should facilitate the elucidation of mechanisms of reactive metabolite toxicity. The database is freely available at

Detecting and characterizing reactive metabolites by liquid chromatography/tandem mass spectrometry

Metabolic activation of a drug leading to reactive metabolite(s) that can covalently modify proteins is considered an initial step that may lead to drug-induced organ toxicities. Characterization of reactive metabolites is critical to designing new drug candidates with an improved toxicological profile. High performance liquid chromatography (HPLC) coupled with mass spectrometry (MS) predominates over all analytical tools used for screening and characterization of reactive metabolites. In this review, a brief description of experimental approaches employed for assessing reactive metabolites is followed by a discussion on the reactivity of acyl glucuronides and acyl coenzyme A thioesters. Techniques for high-throughput screening and quantitation of reactive metabolite formation are also described, along with proteomic approaches used to identify protein targets and modification sites by reactive metabolites. Strategies for dealing with reactive metabolites are reviewed. In conclusion, we discuss the challenges and future needs in this field of research.

Identification of proteins adducted by reactive naphthalene metabolitesin vitro

Metabolic activation of inert chemicals to electrophilic intermediates has been correlated with the incidence and severity of cytotoxicity. The current studies have identified several proteins adducted by reactive metabolites of the lung toxicant, naphthalene. Proteins isolated from microsomal incubations of (14)C-naphthalene were separated by 2-DE, proteins were blotted to PVDF membranes and radioactive proteins were localized by storage phosphor analysis. Adducted proteins were isolated from complimentary gels and identified by peptide mass mapping. A total of 18 adducted proteins were identified including: protein disulfide isomerase precursor, ER-60 protease, alpha actin, mouse urinary proteins, and cytochrome b5 reductase. In supernatant fractions, protein disulfide isomerase, heat shock protein 70, and alpha-actin were key proteins to which reactive naphthalene metabolites were bound. All of the proteins adducted, with the exception of cytochrome b5 reductase were sulfhydryl rich. Although several of the proteins found to be adducted in these studies have also been shown to be adducted by other electrophiles, several others have not been reported as common targets of reactive metabolites. These studies provide a basis for both in situ and in vivo work designed to follow the fate and formation of reactive metabolite protein adducts.

Synthesis of Nτ-arylhistidine derivatives via direct N-arylation

N(tau)-Aryl-histidine derivatives were synthesized using a modified one-step Cu-catalyzed coupling of aryl halides and N-acetylhistidine methyl ester. The latter is much less reactive than imidazole toward aryl halides. p-Chloroiodobenzene coupled with iodine displacement only, whereas m- and p-bromoiodobenzene both gave mixtures of bromo- and iodophenyl products.

Bromobenzene-induced hepatotoxicity at the transcriptome level

Rats were exposed to three levels of bromobenzene, sampled at 6, 24, and 48 h, and liver gene expression profiles were determined to identify dose and time-related changes. Expression of many genes changed transiently, and dependent on the dose. Few changes were identified after 6 h, but many genes were differentially expressed after 24 h, while after 48 h, only the high dose elicited large effects. Differentially expressed genes were involved in drug metabolism (upregulated GSTs, mEH, NQO1, Mrps, downregulated CYPs, sulfotransferases), oxidative stress (induced HO-1, peroxiredoxin, ferritin), GSH depletion (induced GCS-l, GSTA, GSTM) the acute phase response, and in processes like cholesterol, fatty acid and protein metabolism, and intracellular signaling. Trancriptional regulation via the electrophile and sterol response elements seemed to mediate part of the response to bromobenzene. Recovery of the liver was suggested in response to BB by the altered expression of genes involved in protein synthesis and cytoskeleton rearrangement. Furthermore, after 48 h, rats in the mid dose group showed no toxicity, and gene expression patterns resembled the normal situation. For certain genes (e.g., CYP4A, metallothioneins), intraday variation in expression levels was found, regardless of the treatment. Selected cDNA microarray measurements were confirmed using the specific and sensitive branched DNA signal amplification assay.

Toxicogenomics of bromobenzene hepatotoxicity: a combined transcriptomics and proteomics approach

Toxicogenomics is a novel approach integrating the expression analysis of thousands of genes (transcriptomics) or proteins (proteomics) with classical methods in toxicology. Effects at the molecular level are related to pathophysiological changes of the organisms, enabling detailed comparison of mechanisms and early detection and prediction of toxicity. This report addresses the value of the combined use of transcriptomics and proteomics technologies in toxicology. Acute hepatotoxicity was induced in rats by bromobenzene administration resulting in depleted glutathione levels and reduced average body weights, 24hr after dosage. These physiological symptoms coincided with many changes of hepatic mRNA and protein content. Gene induction confirmed involvement of glutathione-S-transferase isozymes and epoxide hydrolase in bromobenzene metabolism and identified many genes possibly relevant in bromobenzene toxicity. Observed glutathione depletion coincided with induction of the key enzyme in glutathione biosynthesis, gamma-glutamylcysteine synthetase. Oxidative stress was apparent from strong upregulation of heme oxygenase, peroxiredoxin 1 and other genes. Bromobenzene-induced protein degradation was suggested from two-dimensional gel electrophoresis, upregulated mRNA levels for proteasome subunits and lysosomal cathepsin L, whereas also genes were upregulated with a role in protein synthesis. Both protein and gene expression profiles from treated rats were clearly distinct from controls as shown by principal component analysis, and several proteins found to significantly change upon bromobenzene treatment were identified by mass spectrometry. A modest overlap in results from proteomics and transcriptomics was found. This work indicates that transcriptomics and proteomics technologies are complementary to each other and provide new possibilities in molecular toxicology.