Mass spectrometric detection and measurement of N2-(2'-deoxyguanosin-8-yl)PhIP adducts in DNA

Ethanol potentiates the genotoxicity of the food-derived mammary carcinogen PhIP in human estrogen receptor-positive mammary cells: mechanistic support for lifestyle factors (cooked red meat and ethanol) associated with mammary cancer

Consumption of cooked/processed meat and ethanol are lifestyle risk factors in the aetiology of breast cancer. Cooking meat generates heterocyclic amines such as 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Epidemiology, mechanistic and animal studies indicate that PhIP is a mammary carcinogen that could be causally linked to breast cancer incidence; PhIP is DNA damaging, mutagenic and oestrogenic. PhIP toxicity involves cytochrome P450 (CYP1 family)-mediated metabolic activation to DNA-damaging species, and transcriptional responses through Aryl hydrocarbon receptor (AhR) and estrogen-receptor-α (ER-α). Ethanol consumption is a modifiable lifestyle factor strongly associated with breast cancer risk. Ethanol toxicity involves alcohol dehydrogenase metabolism to reactive acetaldehyde, and is also a substrate for CYP2E1, which when uncoupled generates reactive oxygen species (ROS) and DNA damage. Here, using human mammary cells that differ in estrogen-receptor status, we explore genotoxicity of PhIP and ethanol and mechanisms behind this toxicity. Treatment with PhIP (10^-7-10^-4 M) significantly induced genotoxicity (micronuclei formation) preferentially in ER-α positive human mammary cell lines (MCF-7, ER-α+) compared to MDA-MB-231 (ER-α-) cells. PhIP-induced CYP1A2 in both cell lines but CYP1B1 was selectively induced in ER-α(+) cells. ER-α inhibition in MCF-7 cells attenuated PhIP-mediated micronuclei formation and CYP1B1 induction. PhIP-induced CYP2E1 and ROS via ER-α-STAT-3 pathway, but only in ER-α (+) MCF-7 cells. Importantly, simultaneous treatments of physiological concentrations ethanol (10^-3-10^-1 M) with PhIP (10^-7-10^-4 M) increased oxidative stress and genotoxicity in MCF-7 cells, compared to the individual chemicals. Collectively, these data offer a mechanistic basis for the increased risk of breast cancer associated with dietary cooked meat and ethanol lifestyle choices.

Acid-labile protein-adducted heterocyclic aromatic amines in human blood are not viable biomarkers of dietary exposure: A systematic study

Heterocyclic aromatic amines (HCA) are carcinogenic mutagens formed during cooking of protein-rich foods. HCA residues adducted to blood proteins have been postulated as biomarkers of HCA exposure. However, the viability of quantifying HCAs following hydrolytic release from adducts in vivo and correlation with dietary intake are unproven. To definitively assess the potential of labile HCA-protein adducts as biomarkers, a highly sensitive UPLC-MS/MS method was validated for four major HCAs: 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx) and 2-amino-3,7,8-trimethylimidazo[4,5-f]quinoxaline (7,8-DiMeIQx). Limits of detection were 1-5 pg/ml plasma and recoveries 91-115%. Efficacy of hydrolysis was demonstrated by HCA-protein adducts synthesised in vitro. Plasma and 7-day food diaries were collected from 122 fasting adults consuming their habitual diets. Estimated HCA intakes ranged from 0 to 2.5 mg/day. An extensive range of hydrolysis conditions was examined for release of adducted HCAs in plasma. HCA was detected in only one sample (PhIP, 9.7 pg/ml), demonstrating conclusively for the first time that acid-labile HCA adducts do not reflect dietary HCA intake and are present at such low concentrations that they are not feasible biomarkers of exposure. Identification of biomarkers remains important. The search should concentrate on stabilised HCA-peptide markers and use of untargeted proteomic and metabolomic approaches.

The cooked meat-derived mammary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) elicits estrogenic-like microRNA responses in breast cancer cells

The cooking of meat results in the generation of heterocyclic amines (HCA), the most abundant of which is 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Data from epidemiological, mechanistic, and animal studies indicate that PhIP could be causally linked to breast cancer incidence. Besides the established DNA damaging and mutagenic activities of PhIP, the chemical is reported to have oestrogenic activity that could contribute to its tissue specific carcinogenicity. In this study we investigated the effect of treatment with PhIP and 17-β-estradiol (E2) on global microRNA (miRNA) expression of the oestrogen responsive MCF-7 human breast adenocarcinoma cell line. PhIP and E2 caused widespread and largely over-lapping effects on miRNA expression, with many of the commonly affected miRNA reported to be regulated by oestrogen and have been implicated in the initiation and progression of breast cancer. The regulatory activity of the miRNAs we show here to be responsive to PhIP treatment, are also predicted to mediate cellular phenotypes that are associated with PhIP exposure. Consequently, this study offers further support to the ability of PhIP to induce widespread effects via activation of oestrogen receptor alpha (ERα). Moreover, this study indicates that deregulation of miRNA by PhIP could potentially be an important non-DNA-damaging carcinogenic mechanism in breast cancer.

Metabolism and biomarkers of heterocyclic aromatic amines in molecular epidemiology studies: lessons learned from aromatic amines

Aromatic amines and heterocyclic aromatic amines (HAAs) are structurally related classes of carcinogens that are formed during the combustion of tobacco or during the high-temperature cooking of meats. Both classes of procarcinogens undergo metabolic activation by N-hydroxylation of the exocyclic amine group to produce a common proposed intermediate, the arylnitrenium ion, which is the critical metabolite implicated in toxicity and DNA damage. However, the biochemistry and chemical properties of these compounds are distinct, and different biomarkers of aromatic amines and HAAs have been developed for human biomonitoring studies. Hemoglobin adducts have been extensively used as biomarkers to monitor occupational and environmental exposures to a number of aromatic amines; however, HAAs do not form hemoglobin adducts at appreciable levels, and other biomarkers have been sought. A number of epidemiologic studies that have investigated dietary consumption of well-done meat in relation to various tumor sites reported a positive association between cancer risk and well-done meat consumption, although some studies have shown no associations between well-done meat and cancer risk. A major limiting factor in most epidemiological studies is the uncertainty in quantitative estimates of chronic exposure to HAAs, and thus, the association of HAAs formed in cooked meat and cancer risk has been difficult to establish. There is a critical need to establish long-term biomarkers of HAAs that can be implemented in molecular epidemioIogy studies. In this review, we highlight and contrast the biochemistry of several prototypical carcinogenic aromatic amines and HAAs to which humans are chronically exposed. The biochemical properties and the impact of polymorphisms of the major xenobiotic-metabolizing enzymes on the biological effects of these chemicals are examined. Lastly, the analytical approaches that have been successfully employed to biomonitor aromatic amines and HAAs, and emerging biomarkers of HAAs that may be implemented in molecular epidemiology studies are discussed.

Detection and quantitation of N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine adducts in DNA using online column-switching liquid chromatography tandem mass spectrometry

The heterocyclic aromatic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is formed by the grilled cooking of certain foods such as meats, poultry and fish. PhIP has been shown to induce tumours in the colon, prostate and mammary glands of rats and is regarded as a potential human dietary carcinogen. PhIP is metabolically activated via cytochrome P450 mediated oxidation to an N-hydroxylamino-PhIP intermediate that is subsequently converted to an ester by N-acetyltransferases or sulfotransferases and undergoes heterolytic cleavage to produce a PhIP-nitrenium ion, which reacts with DNA to form the N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP-C8-dG) adduct. Thus far, the detection and quantification of PhIP-DNA adducts has relied to a large extent on (32)P-postlabelling methodologies. In order to expand the array of available techniques for the detection and improved quantification of PhIP-C8-dG adducts in DNA we have developed an online column-switching liquid chromatography (LC)-electrospray ionization (ESI)-tandem mass spectrometry (MS/MS) selected reaction monitoring (SRM) method incorporating an isotopically [(13)C(10)]-labelled PhIP-C8-dG internal standard for the analysis of DNA enzymatically hydrolysed to 2'-deoxynucleosides. A dose-dependent increase was observed for PhIP-C8-dG adducts when salmon testis DNA was reacted with N-acetoxy-PhIP. Analysis of DNA samples isolated from colon tissue of mice treated by oral gavage daily for 5 days with 50 mg/kg body weight of PhIP resulted in the detection of an average level of 14.8+/-3.7 PhIP-C8-dG adducts per 10(6) 2'-deoxynucleosides. The method required 50 microg of hydrolysed animal DNA on column and the limit of detection for PhIP-C8-dG was 2.5 fmol (1.5 PhIP-C8-dG adducts per 10(8) 2'-deoxynucleosides). In summary, the LC-ESI-MS/MS SRM method provides for the rapid automation of the sample clean up and a reduction in matrix components that would otherwise interfere with the mass spectrometric analysis, with sufficient sensitivity and precision to analyse DNA adducts in animals exposed to PhIP.

METHODS FOR AROMATIC AND HETEROCYCLIC AMINE CARCINOGEN-DNA ADDUCT ANALYSIS BY LIQUID CHROMATOGRAPHY-TANDEM MASS SPECTROMETRY

Xenobiotic-DNA adducts are used as biomarkers to assess the genotoxic effects of carcinogens. Rats were dosed with 4-aminobiphenyl (4-ABP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). DNA was isolated from the colons of vehicle and carcinogen-treated rats and digested using different nucleases and alkaline phosphatase. Deoxyribonucleoside adducts were quantified by capillary liquid chromatography-tandem mass spectrometry (LC-MS/MS) using isotope dilution methods with deuterated internal standards. Major adducts were those bound to the C8 position of deoxyguanosine. 3'- and 5'-Exonucleases were the most efficient nucleases at isolating dG-C8-ABP adducts. However, bulky adducts such as dG-C8-MeIQx and dG-C8-PhIP were better isolated using nuclease P1 rather than a combination of micrococcal nuclease and spleen phosphodiesterase. The use of DNase I enhanced the detection of all three adducts. We describe LC-MS/MS methods for DNA adduct detection and support the testing of different nucleases that increase DNA digestion efficiency and make available more DNA adducts for detection.

Electrochemical biosensor featuring a two-enzyme pathway and DNA for screening toxic reactive metabolites of arylamines

We demonstrate for the first time a biosensor featuring a sequential two-enzyme pathway suitable to screen potentially toxic reactive metabolites generated during metabolism.

New insights in the formation of deoxynucleoside adducts with the heterocyclic aromatic amines PhIP and IQ by means of ion trap MSn and accurate mass measurement of fragment ions

The formation of adducts by reaction of active metabolites of two heterocyclic aromatic amines (NHOH-PhIP and NHOH-IQ) at nucleophilic sites of deoxynucleosides has been studied by LC-MS(n) analyses of the obtained reaction mixtures. Sequential MS(3) experiments were carried out on an ion trap mass spectrometer to gain extensive structural information on each adduct detected in the first MS step. Attribution of ions was supported by accurate mass measurements performed on an Orbitrap mass analyzer. Particular attention was given to ions diagnostic of the linking between the heterocyclic aromatic amine (HAA) and the deoxynucleoside. By this way, the structures of five adducts have been characterized in this study, among which two are new compounds: dG-N7-IQ and dA-N(6)-IQ. No depurinating adduct was found in the reactions investigated therein. As expected, the C8 and N(2) atoms of dG were found as the most reactive sites of deoxynucleosides, resulting in the formation of two different adducts with IQ and one adduct with PhIP. An unusual non-depurinating dG-N7-IQ adduct has been characterized and a mechanism is proposed for its formation on the basis of the reactivity of arylamines. A dA-N(6)-IQ adduct has been identified for the first time in this work, showing that HAAs can generate DNA adducts with bases other than dG.

2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine-induced DNA adducts and genotoxicity in chinese hamster ovary (CHO) cells expressing human CYP1A2 and rapid or slow acetylator N-acetyltransferase 2

Heterocyclic amine carcinogens such as 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) are present in diet and cigarette smoke. Bioactivation in humans includes N-hydroxylation catalyzed by cytochrome P4501A2 possibly followed by O-acetylation catalyzed by N-acetyltransferase 2 (NAT2). Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells were stably transfected with human CYP1A2 and either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles. CYP1A2 and NAT2 catalytic activities were undetectable in untransfected CHO cell lines. CYP1A2 catalytic activity levels did not differ significantly (P > 0.05) among the CYP1A2-transfected cell lines. Cells transfected with NAT2*4 had significantly higher levels of N-acetyltransferase (P = 0.0001) and N-hydroxy-PhIP O-acetyltransferase (P = 0.0170) catalytic activity than cells transfected with NAT2*5B. PhIP caused dose-dependent decreases in cell survival and significant (P < 0.001) increases in mutagenesis measured at the hypoxanthine phosphoribosyl transferase (hprt) locus in all the CYP1A2-transfected cell lines. Transfection with NAT2*4 or NAT2*5B did not further increase hprt mutagenesis. PhIP-induced hprt mutant cDNAs were sequenced, and 80% of the mutations were single base substitutions at G:C base pairs. dG-C8-PhIP DNA adduct levels were dose-dependent in the order: untransfected < transfected with CYP1A2 < transfected with CYP1A2 and NAT2*5B < transfected with CYP1A2 and NAT2*4. Following incubation with 1.2 microM PhIP, DNA adduct levels were significantly (P < 0.05) higher in CHO cells transfected with CYP1A2/NAT2*4 versus CYP1A2/NAT2*5B. These results strongly support an activation role for CYP1A2 in PhIP-induced mutagenesis and DNA damage and suggest a modest effect of human NAT2 and its genetic polymorphism on PhIP DNA adduct levels.

Novel LC-ESI/MS/MS(n) method for the characterization and quantification of 2'-deoxyguanosine adducts of the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by 2-D linear quadrupole ion trap mass spectrometry

An accurate and sensitive liquid chromatography-electrospray ionization/multi-stage mass spectrometry (LC-ESI/MS/MS(n)) technique has been developed for the characterization and quantification of 2'-deoxyguanosine (dG) adducts of the dietary mutagen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). PhIP is an animal and potential human carcinogen that occurs in grilled meats. Following enzymatic digestion and adduct enrichment by solid-phase extraction (SPE), PhIP-DNA adducts were analyzed by MS/MS and MS(n) scan modes on a 2-D linear quadrupole ion trap mass spectrometer (QIT/MS). The major DNA adduct, N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (dG-C8-PhIP), was detected in calf thymus (CT) DNA modified in vitro with a bioactivated form of PhIP and in the colon and liver of rats given PhIP as part of the diet. The lower limit of detection (LOD) was 1 adduct per 10(8) DNA bases, and the limit of quantification (LOQ) was 3 adducts per 10(8) DNA bases in both MS/MS and MS(3) scan modes, using 27 microg of DNA for analysis. Measurements were based on isotope dilution with the internal standard, N-(deoxyguanosin-8-yl)-2-amino-1-(trideutero)methyl-6-phenylimidazo[4,5-b]pyridine (dG-C8-[2H3C]-PhIP). The selected reaction monitoring (SRM) scan mode in MS/MS was employed to monitor the loss of deoxyribose (dR) from the protonated molecules of the adducts ([M + H - 116]+). The consecutive reaction monitoring (CRM) scan modes in MS(3) and MS(4) were used to measure and further characterize product ions of the aglycone ion (BH2+) (Guanyl-PhIP). The MS(3) scan mode was effective in eliminating isobaric interferences observed in the MS/MS scan mode and resulted in an improved signal-to-noise (S/N) ratio. Moreover, the product ion spectra obtained by the MS(n) scan modes provided rich structural information about the adduct and were used to corroborate the identity of dG-C8-PhIP. In addition, an isomeric dG-PhIP adduct was detected in vivo. This LC-ESI/MS/MS(n) method is the first reported application on the use of the MS(3) scan mode for the analysis of DNA adducts in vivo.

Liquid chromatography-electrospray ionization-mass spectrometry: the future of DNA adduct detection

Over the past 40 years considerable emphasis has been placed on the development of accurate and sensitive methods for the detection and quantitation of DNA adducts. The formation of DNA adducts resulting from the covalent interaction of genotoxic carcinogens with DNA, derived from exogenous and endogenous sources, either directly or following metabolic activation, can if not repaired lead to mutations in critical genes such as those involved in the regulation of cellular growth and subsequent development of cancer. The major analytical challenge has been to detect levels of DNA adducts at the level of 0.1-1 adducts per 10(8) unmodified DNA bases using only low microgram amounts of DNA, and with high specificity and accuracy, in humans exposed to genotoxic carcinogens derived from occupational, environmental, dietary and life-style sources. In this review we will highlight the merits as well as discuss the progress made by liquid chromatography coupled to electrospray ionization mass spectrometry as a method for DNA adduct detection.

Formation and analysis of heterocyclic aromatic amine–DNA adducts in vitro and in vivo

The detection and quantification of heterocyclic aromatic amine (HAA)-DNA adducts, critical biomarkers in interspecies extrapolation of toxicity data for human risk assessment, remains a challenging analytical problem. The two main analytical methods currently in use to screen for HAA-DNA adducts are the 32P-postlabeling assay and mass spectrometry, using either accelerated mass spectrometry (AMS) or liquid chromatography and electrospray ionization mass spectrometry (LC-ESI-MS). In this review, the principal methods to synthesize and characterize DNA adducts, and the methods applied to measure HAA-DNA adduct in vitro and vivo are discussed.

Genetic polymorphisms and modulation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-DNA adducts in human lymphocytes

2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the most abundant heterocyclic amine derived from food, possibly involved in human carcinogenesis. We evaluated the formation of PhIP-DNA adducts in lymphocytes from 76 incident colorectal cancer patients likely to be exposed to dietary PhIP. To address the role of the metabolic polymorphisms relevant to PhIP-DNA adduct formation, the patients were genotyped for common polymorphisms in the N-acetyltransferase (NAT1 and NAT2), sulfotransferase (SULT1A1) and glutathione S-transferase (GSTM1 and GSTA1) genes. PhIP released from adducted DNA after hydrolysis was quantitated by liquid chromatography-tandem mass spectrometry. Overall, adducts were 3.24 +/- 3.58/10(8) nucleotides (mean +/- SD); they were not related to sex, smoking habits or age, though levels were not significantly higher in smokers, young subjects and high meat consumers. High vegetable intake significantly reduced PhIP-DNA adducts (Mann-Whitney U, p = 0.044). Individuals with the GSTM1 null genotype showed colon cancer onset at earlier age (58.8 +/- 1.8 vs. 63.5 +/- 1.6 years; Mann-Whitney U, p = 0.047). None of the genetic polymorphisms studied significantly affected PhIP-DNA adducts. However, individuals carrying 2 mutated GSTA1 alleles and younger than the median age had higher adduct levels than homozygous wild-type and heterozygous ones (Kruskal-Wallis p = 0.0008). In conclusion, these preliminary data indicate that PhIP-DNA adducts are formed in people likely to be exposed to this carcinogen through the diet, suggesting this biomarker may be useful to detect human exposure and DNA damage. Overall, the genetic polymorphisms considered had limited effect on PhIP-DNA levels, but young people with lower detoxification capacity may form a subgroup particularly susceptible to dietary carcinogen.

Methodologies for bulky DNA adduct analysis and biomonitoring of environmental and occupational exposures

It is undisputed that DNA adduct formation is one of the key processes in early carcinogenesis. Therefore, analysis of DNA adduct levels may be one of the best tools available to characterize exposure to complex mixtures of genotoxic chemicals as occurring in different environmental and occupational exposure settings. However, from an analytical point of view the detection and quantification of DNA adducts is a challenging enterprise as extremely high sensitivity and selectivity are required. The entire spectrum of chromatographic techniques, including thin-layer chromatography (TLC), gas and liquid chromatography as well as capillary electrophoresis has been used in combination with different detection systems, all with their own specific characteristics. Among the various combinations of techniques, the TLC-(32)P-postlabeling combination appears to meet best with criteria of sensitivity and requirements of minimal amounts of material. Recent developments in the application of capillary electrophoresis in combination with either immunochemical or mass spectrometric detection techniques may offer new and promising approaches, with higher selectivity as compared to TLC-(32)P postlabeling. The applicability of these new techniques in biomonitoring studies aiming at the exposure and risk assessment of low and chronic exposures remains to be determined. In this paper we compare and discuss the advantages and limitations of different techniques used in DNA adduct analysis, with specific emphasis on those adducts formed by the polycyclic aromatic hydrocarbons and heterocyclic aromatic amines.

Heterocyclic aromatic amine metabolism, DNA adduct formation, mutagenesis, and carcinogenesis

Heterocyclic aromatic amines (HAAs) are carcinogenic compounds formed in meats, fish, and poultry prepared under common household cooking practices. Some HAAs are also formed in tobacco smoke condensate. Because of the widespread occurrence of HAAs in these daily staples, health concerns have been raised regarding the potential role of HAAs in the etiology of some human cancers associated with frequent consumption of these products. In this review, the metabolism of HAAs to biologically active metabolites that bind to DNA and provoke mutations and cancer in various biological systems is discussed. Some of the current analytical and molecular methods that are used to measure biomarkers of HAA exposure and genetic damage in experimental animal models and humans are also presented. These biochemical data combined may help to better assess the role that HAAs may have in the development of some common forms of human cancers.

Analysis and quantification of DNA adducts of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in liver of rats by liquid chromatography/electrospray tandem mass spectrometry

Liquid chromatography with electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) was used to measure DNA adducts of the carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) with a microbore C-18 reversed-phase column. Quantification of the isomeric adducts N-(deoxyguanosin-8-yl)-2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (dG-C8-MeIQx) and 5-(deoxyguanosin-N(2)-yl)-2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (dG-N(2)-MeIQx) was achieved using synthetic, isotopically labeled internal standards. The reaction of the N-acetoxy ester of 2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline (HONH-MeIQx) with calf thymus DNA (ct DNA) resulted in formation of these adducts in a ratio of 5:1 (dG-C8-MeIQx:dG-N(2)-MeIQx). The detection limit by LC/ESI-MS/MS in the selected reaction monitoring (SRM) mode ([MH(+) --> MH - 116](+)) (loss of deoxyribose) approached 500 fg (1 fmol) of adduct standard, and 1 adduct per 10(8) DNA bases using 100 microg of DNA following solid-phase extraction. The SRM analysis of rat liver DNA 24 h after an oral dose of MeIQx (10 and 0.5 mg/kg) revealed the presence of isomeric dG-MeIQx adducts at levels of 3.07 +/- 0.84 and 0.45 +/- 0.27 adducts per 10(7) bases, respectively. LC/ESI-MS/MS product ion spectra were acquired on both adducts from the elevated dose of MeIQx for unambiguous adduct identification. The contribution of dG-N(2)-MeIQx to the total adducts in vivo was significantly more important than that observed in vitro. dG-C8-MeIQx was the principal adduct formed at the 10 mg/kg dose, (dG-C8-MeIQx:dG-N(2)-MeIQx (3:2)); however, dG-N(2)-MeIQx was the major lesion detected at the 0.5 mg/kg dose (dG-C8-MeIQx:dG-N(2)-MeIQx 1:10). The striking differences between the relative amounts of dG-C8-MeIQx and dG-N(2)-MeIQx formed in vivo as a function of dose suggest that reactive esters of HONH-MeIQx other than N-acetoxy-MeIQx may be formed in vivo and react preferentially with the N(2) atom of guanine, or that dG-C8-MeIQx is removed at a significantly more rapid rate than dG-N(2)-MeIQx. The dG-N(2)-MeIQx adduct, previously thought to be a minor adduct, is likely to be an important contributor to the genotoxic damage of MeIQx.