Measurement of MeIQx and DiMeIQx in fried beef by capillary column gas chromatography electron capture negative ion chemical ionisation mass spectrometry

A gas chromatographic—mass spectrometric assay has beendeveloped for the simultaneous measurement of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) and 2-amino-3,4,8-dimethylimidazo[4,5-f]quinoxaline(DiMeIQx) in fried beef. The method employs capillary columngas chromatography, electron capture negative ion chemical ionisationmass spectrometry and a stable isotope labelled analogue ofMeIQx (the synthesis of which is described) as common internalstandard. Two patties of lean minced beef which had been cookedseparately were analysed and found to contain both compounds(patty 1–2.4 ng MeIQx/g meat, 1.2 ng DiMeIQx/g meat; patty2–1.3 ng MeIQx/g meat, 0.5 ng DiMeIQx/g meat). Neithercompound was present in the meat prior to cooking.     

Effect of cooking temperature on the formation of heterocyclic amines in fried meat products and pan residues

Frequent consumers of meat have an increased risk of colorectalcancer and possibly also of breast, stomach, pancreas and urinarybladder cancer. Bacon, ‘Falusausage’, ground beef,meatballs, pork belly, pork chops and sliced beef account formore than one-third of the intake of fried meat of the populationof Stockholm of age 50–75. These dishes were fried atfour temperatures (150, 175, 200 and 225 °C) representingnormal household cooking practices in Stockholm. Heterocyclicamines in these dishes were analysed using solid-phase extractionand HPLC. The heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline(IQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx),2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and2-amino-1-methyl-6-phenylimidazo[4,5-b]-pyridine (PhIP) wererecovered. The formation of IQ was favoured by moderate cookingtemperatures; IQ was detected in one meat sample cooked at 150°Cand in some pan residues. The yield of MeIQx, DiMeIQx and PhIPincreased with the temperature. For several of the meat dishes,the content of heterocyclic amines in the pan residue was aslarge or larger than for corresponding piece of meat. The highestlevels of MeIQx were 23.7 ng/g in the meat and 233 ng/g in thepan residue. Corresponding data for DiMeIQx were 2.7 and 4.1ng/g and for PhIP 12.7 and 82.4 ng/g. The study leaves littledoubt that mutagenic heterocyclic amines are ingested by thepopulation of Stockholm, and added to previous epidemiologicalstudies from the same area, the combined data are consistentwith human carcinogenicity of heterocyclic amines. However,analytical epidemiological studies are needed before any statementon causality can be made.     

Simple methods for quantifying mutagenic heterocyclic aromatic amines in food products

Two solid-phase extraction methods were developed for the determinationof mutagenic heterocyclic aromatic amines in heated meat products.The copper phthalocyanine (CPC) tandem extraction was performedon coupled cartridges of diatomaceous earth and CPC-derivatizedSephasorb HP, followed by further clean-up on Sephasorb HP.Parts per billion levels of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) and its homologs as well as 2-amino-3-methylimidazo[4,5-f]quinoline(IQ), 2-amino-3,8-dimethyl-imidazo[4,5-f]quinoline (MeIQ), 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine(PhIP), amino--carboline (AC), 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole(Trp-P-1), 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2),harman (H) and norharman (NH) can then be simultaneously quantifiedby HPLC with UV detection. The propylsulfonyl silica gel (PRS)tandem extraction is a one-step clean-up method on coupled cartridgesof diatomaceous earth and PRS, suitable for the determinationof MeIQx, IQ and their homologs, as well as the glutamic acidpyrolysates 2-amlno-6-methyldipyrido[1,2-a:3',2']imidazole (Glu-P-1)and 2-aminodipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2). 4,7,8-TriMeIQxor 7,8-DiMeIQx were used as internal standards. Four grams ofsample or less are required for analysis. The recovery of theamineswas between 46 and 83% and the detection limit was in the lowp.p.b. range with coefficients of variation ranging between5 and 18%. The major mutagenic contaminant found in meat extractswas MeIQx (from <1 to 44 p.p.b.), followed by 4,8-DiMeIQx(1.3–5 p.p.b.) whereas the major contaminant in friedmeat was PhIP (23–48 p.p.b.), followed by MeIQx (5.1–8.3p.p.b.), AC (3.2–8.9 p.p.b.) and 4,8-DiMeIQx (1.3–2p.p.b.).The co-mutagens NH and H were found in fried meat atlevels of 8.7–19 p.p.b. and 3–4.8 p.p.b. respectively.     

Metabolism of the food carcinogen 2-amino-3, 8-dimethylimidazo[4, 5-f]quinoxaline in isolated rat liver cells

2-Amino-3, 8-dimethylimidazo[4, 5-f]quinoxaline (MelQx) wastransformed to at least 10 metabolites in suspensions of hepatocytesisolated from Aroclor 1254 treated rats. Combining biochemicaldata such as effects on MeIQx metabolism of metabolic modulatorsand incorporation of radioisotopic sulfur with UV, mass and¹H-NMR spectroscopy, we elucidated the structures of six metabolites.About 40% of the MeIQx was transformed to 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxalin-4(or5)-yl sulfate. Other oxygenated metaboliteswere 2-amino-8-hydroxymethy1–3-methylimidazo[4, 5-f)quinoxalin-4(or5)-yl sulfate and 2-amino-4(or5)-ß-D-glucuronopyranosyloxy-3,8-dimethylimidazo[4, 5-f]quinoxaline. Evidence was obtainedthat a glutathione conjugate was formed. This metabolite, andthe other oxygenated metabolites were probably formed in P-450catalyzed reactions. Two metabolites, 2-ß-D-glucurono-pyranosylamino-3,8-dimethylimidazo[4, 5-f)quinoxaline and the N(3, 8-dimethylimidazo[4,5-f]quinoxaline-2-yl)sulfamate, were direct conjugates of MeIQx.     

Synergistic enhancement of hepatic foci development by combined treatment of rats with 10 heterocyclic amines at low doses

Potential synergism between 10 carcinogenic heterocyclic amines[3-amino-l,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), 3-amino-1-methyl-5H-pyrido[4,3-b]indole(Trp-P-2), 2-amino-6 methyldipyrido[l,2-a:3',2'-d]imidazole(Glu-P-1), 2-ammo-dipyrido[l,2-a:3',2'-d]imidazoIe (Glu-P-2),2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethyl-imidazo[4,5-f]quinoline(MeIQ), 2-amino-3,8-dimethyl-imidazo[4,5-f]quinoxaline (MeIQx),2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeA     

Influence of amino acids on the formation of mutagenic/carcinogenic heterocydic amines in a model system

Mixtures of creatinine, glucose and various single amino acidswere heated at 180°C for 10 min in an aqueous model system.The heated mixtures all showed mutagenic activity, ranging from80 to 2400 TA98 revertant colonies/µmol creatinine withmetabolic activation. Testing of HPLC fractions for mutagenicactivity showed each mixture to contain several mutagenic components,some of which corresponded to known heterocyclic amines andothers to unknown compounds. The presence of 2-amino-3-methyl-imidazo[4,5-f]quinoxaline,2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline and 2-amino-3,7,8-trimethylimidazo[4,5-f]quinoxalinein most of the samples was established using HPLC with photodiodearray detection and liquid chromatography/mass spectrometrywith electrospray interface and single ion monitoring. In addition,2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine,3-amino-1,4-di-methyl-5H-pyrido[4,3-b]indole and 3-amino-1-methyl-5H-pyrido[4,3-b]indoleand the co-mutagenic compounds 9H-pyrido[3,4-bindole and 1-methyl-9H-pyrido[3,4-b]indolewere detected in some samples.     

Heterocyclic aromatic amine formation in grilled bacon, beef and fish and in grill scrapings

The heterocyclic aromatic amines (HAAs) 2-amino-l-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3, 8-di-methylimidazo[4,5-f)quinoxaline(MelQx), 2-amino-3, 4, 8-tri-methylimidazo[4, 5-f)quinoxaline(4, 8-DiMeIQx) and 2-amino     

The heterocyclic amines IQ and MeIQx show no promotive effect in a short-term in vivo liver carcinogenesis assay

The food-derived heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) are carcinogenic in rodents as well as in non-humanprimates (IQ). Using a shortterm liver carcinogenesis modelthe compounds were found to be only weak initiators. It is alsopossible that these amines are involved in other phases of thecarcinogenesis process. In an attempt to study the promotiveeffects of IQ and MeIQx, these heterocyclic amines were repeatedlygiven by gavage, over a period of 10 days, to Wistar rats previouslyinitiated with diethylnitrosamine (DEN) and subjected to a partialhepatectomy. The rats were killed 1 month after DEN administrationand their livers examined for glutathione-S-transferase positive(GST-P) foci. The positive control, 2-acetylaminofluorene, significantlyincreased the development of GST-P foci in the DEN-treated ratscompared to the negative control, saline. However, neither theIQ- nor the MeIQx-treated groups differed from the controlsand thus these amines do not promote the growth of DEN-initiatedGST-P liver foci in this model.     

Major routes of metabolism of the food-borne carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in the rat

A metabolite of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) was incorrectly characterized in this paper. The metabolitewas thought to be an acetyl conjugate of the 5-hydroxyl-atedderivative of MeIQx. This assignment is incorrect. The correctassignment is a sulfate conjugate of 5-hydroxy-MeIQx, 2-amino-3,8-dimethylirnidazo[4,5-f]quinoxaUn-5-yl-sulfate.This conclusion is based upon repurified sample analyzed by¹H NMR and ¹³C NMR, enzyme hydrolysis assays, IR spectro-scopyand FAB/MS (accurate mass measurement).     

Major routes of metabolism of the food-borne carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in the rat

The metabolic fate of 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline(MeIQx), a carcinogen formed in cooked meat and fish, has beeninvestigated in male Sprague-Dawley rats. Five metabolites wererecovered from bile of animals given an intragastric dose of{2-¹⁴C]MeIQx. These accounted for nearly all of the radioactivityin bile. The chemical structures of these metabolites were elucidatedby proton NMR, UV and mass spectroscopy. Three structures maybe assigned unambiguously: two sulfamates, N-(3,8.dimethylimidazo[4,5f]quinoxalin-2-yl)sulfamic acid and N-(8-hydroxymethyl-3-methylimidazo[4,5f]quinoxalin-2-yl)sulfamic acid, and N-(8-one glucuronide, N²(ß-1-glucosiduronyl)-2-amino-3,8-dimelhyliinidazo[4,5f]quinoxaline In addition, an acetyl and a glucosiduronylconjugate of 5-hydroxy-MeIQx were observed. The spectral evidencedid not allow an unambiguous assignment of the site of conjugation.The two glucuronides were excreted in urine and the sulfamateof MeIQx was found in feces as well as urine. All five metaboliteswere found to be non-mutagenic to Salmonella typhimurium TA98with or without metabolic activation. The glucuronide conjugateswere found also to be non-mutagenic when ß- glucuronidasewas incorporated with S-9 mixture in the mutation assay, andthus all appear to be detoxification products. The previouslyreported metabolite, 2-amino-8-hydroxymethyl-3-methylimidazo[4,5f]quinoxalinewhich is mutagenic to Salmonella typhimurium TA98 with metabolicactivation, was identified as a minor component in both urineand feces.     

32P-Post-labelling analysis of DNA adducts formed by food-derived heterocyclic amines: evidence for incomplete hydrolysis and a procedure for adduct pattern simplification

Food-derived aminoimidazoazarenes have been shown to be mutagenicand carcinogenic and to form covalent DNA adducts. ³²P-Post-labellinganalysis of DNA modified with these heterocyclic amines (HA),including 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ), 2-amino-1-methyl-6-phenylimid-azo[4,5-b]pyridine(PhIP), 2-amino-3,4-dimethylimidazo [4,5-fquinoline (MeIQ),2-amino-3,4,8-trimethylimidazo [4,5-f1 quinoxaline (4,8-DiMeIQx),2-amino-3,7,8-trimethylimidazo[4,5-f]quinoxaline (7,8-DiMeIQx)and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) hasresulted in considerable interlaboratory variation in the characteristicpatterns of DNA adduct spots, with up to six being detectedfor each compound. Similar complex patterns were observed whenazido-derivatives of HA were photoreacted with calf thymus DNA.When deoxyguanosine 3'-monophosphate was modified with the azidoderivatives and analysed using the ³²P-post-labelling procedure,one major spot was observed for IQ, 4,8-DilMeIQx, 7,8-DiMeIQxor PhIP and two major spots for MeIQ or MeIQx. In each case,these adducts were chromatographically indistinguishable fromthe major adducts formed with DNA. No major adduct spots wereobserved when 3'-phosphate derivatives of deoxyadenosine, deoxycytidineor thymidine were reacted with the azido-derivatives of HA.In an attempt to identify the additional spots, azido derivativesof PhIP or IQ were reacted with the synthetic homopolymer poly(dG)·poly(dC),the alternating copolymer poly(dC-dG) or a synthetic oligonucleotide(TTT-GTTTTTTCTTTCCCT): in each case a reduced number of adductspots were detected. The introduction of an additional nucleaseP1 hydrolysis step following the labelling reaction furtherreduced the number of adduct spots to only one or two majorspots. Reversed-phase HPLC analysis showed that the number ofpeaks of radioactivity was also reduced to one or two, presumablycorresponding to the [³²P]-5'-monophosphate deoxyguanosine adducts.We suggest that many of the additional spots commonly observedin conventional ³²P-post-labelling analysis of HA-modified DNAare adducted oligonucleotides that are partly resistant to hydrolysisby micrococcal nuclease and spleen phosphodiesterase but aresusceptible to hydrolysis by nuclease P1.     

Incorporation of carbon atoms from glucose into the food mutagens MeIQx and 4,8-DiMeIQx using 14C-labelled glucose in a model system

Mixtures of creatinine, glucose and threonine with the additionof a small amount, 250 µCi, of [U-¹⁴C]glucose, [1-¹⁴C]glucoseor [6-¹⁴C]glucose were heated at 180°C for 30 min in anaqueous model system. The mixtures were purified and analysedusing HPLC, scintillation and Ames tests. 2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline(4,8-DiMeIQx) were detected as the main radioactive mutagens.The amount of MeIQx and 4,8-DiMeIQx produced from threoninewas estimated at 18 and 60 nmol/mmol glucose respectively. Radioactivecarbon atoms originating from glucose were also shown to beincorporated into 2-amino-3-methylimidazo[4,5-f]quinoxaline(IQx). The specific activity was calculated to be 0.6, 0.3 and0.1–0.3 mCi/mmol for MeIQx, 4,8-DiMeIQx and IQx respectivelyfor all three labelled forms of glucose. By the incorporationof carbon atoms originating from glucose into the imidazoquinoxalinemutagens it was clearly demonstrated that glucose is a precursorin the formation of these food mutagens.     

SHORT COMMUNICATION: Metabolic activation and DNA binding of food mutagens and other environmental carcinogens in human mammary epithelial cells

Cultures of human mammary epithelial cells were treated withone of seven heterocyclic amine food mutagens [2-amino-3-methylimidazo[4,5-f)quinoline (IQ), 2-amino-3, 4-dimethylimidazo[4, 5-f)quinoline(MelQ), 2-amino-3, 8-di-methylimidazo[4, 5-f]quinoxaline (MeIQx),2-amino-3, 4, 8-trimethylimidazo[4, 5-f]quinoxaline (4, 8-DiMelQx)2-amino-3, 7, 8-trimethylimidazo[4, 5-f]quinoline (7, 8-DiMelQx),2-amino-3, 4, 7, 8-tetramethylimidazo[4, 5-f]quinoxaline (4,7, 8-TriMelQx) or 2-amino-1-methy1–6-phenylimidazo[4,5-b] pyridine (PhlP)], four nitropyrenes (1-nitropyrene (1-NP),1, 3-dinitropyrene (1, 3-DNP), 1, 6-dinitropyrene (1, 6-DNP)or 1, 8-dinitropyrene (1, 8-DNP) or the Polycyclic aromatichydrocarbon dibenzo[a, l]pyrene (DB[a, l]P). DNA isolated fromthe cultures was analysed by ³²P-post-labelling and in eachcase the presence of carcinogen-DNA adducts was detected. Thepatterns and numbers of adducts obtained when human mammarycell DNA digests were separated on polyethyleneimine-celluloseTLC were found to closely resemble those previously demonstratedto be present in the DNA of tissues from rodents and other primatestreated with the same agents. Up to six DNA adducts were detectedin human breast cells treated with IQ and MelQ. Fewer adducts(1–3) were detected following treatment with MelQx orits methylated derivatives, whilst PhIP gave rise to at leastfour distinct adduct spots. Five adduct spots were detectedin breast cells treated with DB[a, l]P or with 1-NP, but feweradduct spots were formed by 1, 3-, 1, 6- and 1, 8-DNP. Thesedata demonstrate the ability of human breast epithelial cellsto activate to DNA binding species a range of carcinogenic compoundsknown to be present in the human diet or to which humans areknown to be exposed environmentally.     

32P-Postlabeling analysis of IQ, MelQx and PhIP adducts formed in vitro in DNA and polynucleotides and found in vivo in hepatic DNA from IQ-, MelQx- and PhIP-treated monkeys

The ³²P-postlabeling method was used to examine the adductsin DNA, polynucleotides, and mononucleotides reacted in vitrowith the N-hydroxy and N-acetoxy derivatives of 2-amino-3-methylimidazo[4,5-f]quinoline(IQ), 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MelQx)or 2-amino-1-methy1-6-phenylimidazo[4,5-b]pyridine (PhIP). Adductprofiles were compared to those found in vivo in liver of cynomolgusmonkeys fed IQ, MeIQx or PhIP. The N-acetoxy derivatives ofIQ, MelQx and PhIP (generated in situ from the correspondingN-hydroxylamine in the presence of acetic anhydride) each formedthree principal adducts in DNA. Adduct 1 of IQ, MeIQx and PhlPwas chromatographically identical to the ³²P-labeled bis(phosphate)derivative of N-(deoxyguanosin-8-yI)-IQ, N-(deoxyguanosin-8-yI)-MeIQx,and N-(deoxyguanosin-8-yl)-PhIP respectively, and this adductcomprised     

Lower levels of urinary 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline (MeIQx) in humans with higher CYP1A2 activity

Heterocyclic aromatic amines (HAAs), such as 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx), are metabolically activated by cytochrome P4501A2 (CYP1A2)and N-acetyltransferase (NAT2). We examined the relationshipbetween CYP1A2 and NAT2 activity and the excretion of totalunconjugated MeIQx in 66 healthy subjects. The subjects atea controlled diet for 7 days containing lean ground beef cookedat low temperature. On day 8, they were tested for CYP1A2 andNAT2 activity by caffeine phenotyping. On the evening of day8, subjects consumed lean ground beef cooked at high temperaturecontaining 9.0 ng of MeIQx/g of meat. The subjects ate 3.1–4.0g meat/ kg body wt. Twelve-hour urine samples were collectedand MeIQx was measured by gas chromatography-mass spectrometry.Using linear regression analyses, we found that higher CYP1A2activity was associated with lower levels of total unconjugatedMeIQx in the urine (P = 0.008) when adjusted for amount of meateaten, while NAT2 activity showed no relationship with the latter.This suggests that a greater percentage of MeIQx is convertedto metabolites such as the N-hydroxy derivative when CYP1A2activity is higher. This finding supports the concept that inter-individualvariation in CYP1A2 activity may be relevant for cancers associatedwith exposure to HAAs.     

Effect of heterocyclic amines and beef extract on chromosome aberrations and sister chromatid exchanges in cultured human lymphocytes

Two of the major bacterial mutagens formed in heated meat products,2-amino-3-methylimidazo[4, 5-f]quinoline and 2-amino-3, 8-dimethylimidazo[4,5-f quinoxaline or the basic fraction of beef extract induceda low frequency of sister chromatid exchanges in human lymphocytecultures in the presence of metabolic activation. Structuralchromosome aberrations were not induced at comparable high concentrationsin human lymphocytes with intact repair system, suggesting thatrepair or induction of point mutations are involved in the DNA-damagingeffect of heterocyclic amines rather than structural chromosomeaberrations. Accordingly it may be concluded that mammaliancells with both intact repair and enzyme systems are more relevantthan bacterial systems for evaluating the carcinogenic potentialof heterocyclic amines.     

Mutagenic activation of IQ, PhIP, and MeIQx by hepatic microsomes from rat, monkey and man: low mutagenic activation of MeIQx in cynomolgus monkeys in vitro reflects low DNA adduct levels in vivo

Cooked meat, poultry and fish contain a number of mutagenicand carcinogenic heterocyclic amines, including 2-amino-3-methylimidazo[4,5-f]quinoline(IQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP).In the present study we examined the capacity of hepatic microsomesfrom Fischer 344 rats, cynomolgus monkeys and humans to metabolicallyactivate IQ, MeIQx and PhIP in vitro using the Ames Salmonellamutagenicity assay. The mutagenic activation of IQ was similaramong the three species; however, there were significant differencesamong the species in the activation of PhIP and MeIQx. Livermicrosomes from humans showed the greatest capacity to activatePhIP and MeIQx, followed by rats, and then monkeys. The largestdifferences between the species were observed when MeIQx wasused as the mutagen. MeIQx–DNA adducts formed in vivowere then compared among rats and monkeys given MeIQx by gavage(20 mg/kg/day, 10 doses). ³²P-Postlabeling analysis, carriedout under intensification conditions, was used to examine MeIQx–DNAadducts in the liver, kidney, heart, colon and white blood cells.MeIQx–DNA adducts were highest in all tissues examinedfrom male rats, followed by female rats, and much lower in monkeys.In the liver, the total MeIQx–DNA adduct levels of monkeyswere {small tilde}19 and {small tilde}10 times lower than inmale and female rats respectively. In extrahepatic tissues,the differences in MeIQx–DNA adduct levels between monkeysand rats were even greater. The results suggest that the lowlevel of MeIQx–DNA adducts found in vivo in cynomolgusmonkeys reflects a low capacity to activate MeIQx via the hepaticcytochrome P450 monooxygenase system.     

Identification of N2-(deoxyguanosin-8-yl)-2-amino-3,8-dimethylimidazo[4,5-f)quinoxaline 3',5'-diphosphate, a major DNA adduct, detected by nuclease P1 modification of the 32P-postlabeling method, in the liver of rats fed MeIQx

The carcinogenic heterocyclic amine 2-amino-3,8-dimethyl-imidazo[4,5-f]quinoxaline(MeIQx) is widely distributed in cooked foods. The nucleaseP1 method increased the sensitivity of the standard ³²P-postlabelinganalysis about 1000-fold for detection of MeIQx-DNA adducts.The recovery of MeIQx-DNA adducts by the nuclease P1 methodwas determined to be about 50% using liver DNA of a rat treatedwith [¹⁴C]MeIQx intragastrically. By the nuclease P1 methodfive adducts were detected in the liver DNA of rats fed MeIQxand two of them, including the most abundant one, were identifiedas MeIQx-deoxyguanosine adducts by comparison with the adductsformed in in vitro reactions of N-acetoxy-2-amino-3,8-dimethylimidazo[4,5-f)quinoxalinewith the four 2'-deoxyribonucleotides. The most abundant adductin vivo was identified as N²-(deoxyguanosin-8-yl)-MeIQx 3',5'-diphosphate(3',5'-pdGp-C8-MeIQx). MeIQx-DNA adduct levels in human tissuescould be determined by the nuclease P1 modification of the ³²P-postlabelingmethod in combination with HPLC, and thus provide informationon the roles of MeIQx in human carcinogenesis.     

Detection and measurement of MeIQx in human urine after ingestion of a cooked meat meal

A gas chromatographic - mass spectrometric assay has been developedfor the measurement of 2-ainino-3,8-dimethylimidazo [4,5-fquinoxaline(MeIQx)in urine. The method employs capillary column gas chromatography,electron capture negative-ion chemical ionization mass spectrometry,a stable isotope-labelled analogue of MeIQx as internal standardand has a limit of detection of 5 pg MeIQx/ml urine. Six subjectsconsumed a fried beef meal and urine was collected before andafter this event. While no MeIQx could be detected in urinecollections made prior to meat consumption, the 12-h urine collectionsof all six subjects made after the meal contained the amine.When the amounts of MeIQx measured in the urine collectionswere compared to the quantities of amine ingested in the friedbeef, it was found that 1.8–4.9% of the oral dose wasexcreted unchanged in urine. Metabolic studies in animals leadus to believe that MeIQx in the diet is efficiently absorbedand extensively biotransformed.     

Antibodies to the food mutagens, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline: useful for immunoassay and immunoaffinity chromatography of biological samples

Monoclonal mouse IgG₁ and IgG₃ antibodies were developed tothe food mutagens, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine(PhIP) and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline(4,8-DiMeIQx) in order to make specific and sensitive detectionand purification systems suitable for biological samples. Theantibodies were developed with the strategy that cross-reactionwith analogues modified in the N²-position was desirable. Competitiveenzyme-linked immunosorbent assays (ELISA) with 50% inhibitionby 0.4–6 pmol food mutagen were developed. The epitopesrecognized by the antibodies have been characterized by ELISAusing 52 synthetic analogues and metabolites of PhIP, 4,8-DiMeIQx,and other food mutagens. One of the anti-PhIP antibodies onlyrecognizes PhIP and those PhIP-analogues which have minor modificationsin the N²-amino group, whereas the other, 7B7-1, is less stringentand also recognizes several other modified metabolites, includingbulky adducts at the N²-amino group e.g. the major guanine anddeoxyguanosine adducts isolated from PhIP-modified DNA. Theantibodies to DiMeIQx also recognize the food mutagens 2-amino-3,4-dimethylimidazo[4,5-f]quinoxaline(4-MeIQx), 2-amino-3, 8-dimethylimidazo[4, 5-f]quinoxaline (8-MeIQx),and the corresponding quinolines (4-MeIQ and 8-MeIQ). Two ofthese antibodies only bind analogues with minor modificationsin the free amino group, whereas analogues with major modificationsin this position, including a deoxyguanosine adduct, react withthe third antibody. Urine samples and faecal extracts from ³H-PhIPor 2-¹⁴C-DiMeIQx dosed rats were analysed by these ELISA assays,and high correlations between radioactivity and response inthe ELISA assays were observed. Urine samples and faecal extractsfrom ³H-PhIP-dosed rats were purified on an affinity columncontaining the less stringent anti-PhIP antibody, 7B7-1. Theaffinity column was found by high performance liquid chromatography(HPLC) analysis to concentrate exclusively labelled material.This affinity column also bound PhIP-related materials fromdilute samples of acid hydrolysed PhIP-DNA with high efficiency.Only