Lactoperoxidase-Catalyzed Activation of Carcinogenic Aromatic and

Chem. Res. Toxicol. 2004, 17, 1659-1666

1659

Lactoperoxidase-Catalyzed Activation of Carcinogenic Aromatic and Heterocyclic Amines Katarzyna M. Gorlewska-Roberts, Candee H. Teitel, Jackson O. Lay, Jr., Dean W. Roberts, and Fred F. Kadlubar* National Center for Toxicological Research, HFT 100, 3900 NCTR Road, Jefferson, Arkansas 72079 Received August 4, 2004

Lactoperoxidase, an enzyme secreted from the human mammary gland, plays a host defensive role through antimicrobial activity. It has been implicated in mutagenic and carcinogenic activation in the human mammary gland. The potential role of heterocyclic and aromatic amines in the etiology of breast cancer led us to examination of the lactoperoxidase-catalyzed activation of the most commonly studied arylamine carcinogens: 2-amino-1-methyl-6-phenylimidazo[4,5-b]-pyridine (PhIP), benzidine, 4-aminobiphenyl (ABP), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx). In vitro activation was performed with lactoperoxidase (partially purified from bovine milk or human milk) in the presence of hydrogen peroxide and calf thymus DNA. Products formed during enzymatic activation were monitored by HPLC with ultraviolet and radiometric detection. Two of these products were characterized as hydrazo and azo derivatives by means of mass spectrometry. The DNA binding level of 3H- and 14C-radiolabeled amines after peroxidase-catalyzed activation was dependent on the hydrogen peroxide concentration, and the highest levels of carcinogen binding to DNA were observed at 100 µM H2O2. Carcinogen activation and the level of binding to DNA were in the order of benzidine > ABP > IQ > MeIQx > PhIP. One of the ABP adducts was identified, and the level at which it is formed was estimated to be six adducts/105 nucleotides. The susceptibility of aromatic and heterocyclic amines for lactoperoxidase-catalyzed activation and the binding levels of activated products to DNA suggest a potential role of lactoperoxidase-catalyzed activation of carcinogens in the etiology of breast cancer.

Introduction Breast cancer is one of the major causes of cancer mortality in women. Risk factors for this multifactorial disease include genetic predisposition, exposure to estrogen, estrogenic chemicals or radiation, and age at menarche and menopause. However, studies of the migrant populations strongly implicate lifestyle factors as the major reason for the development of this disease. Exposure to environmental carcinogens and dietary causes may be associated with an increased risk of breast cancer (1, 2). This is supported by the fact that ionizing radiation induces breast cancer (3), which indicates that mammary epithelial cells are susceptible to DNA-damaging agents. Aromatic and heterocyclic amines are synthetic and naturally occurring environmental and dietary carcinogens that have been implicated in human breast carcinogenesis (4-7). Aromatic amines are present in dyes and tobacco smoke (8), whereas heterocyclic amines are formed during the cooking of proteinaceous foods such as meat and fish at high temperatures or for a long period of time. The most abundant of the heterocyclic amines found in a typical nonvegetarian diet is 2-amino-1methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) (9). It has been postulated that aromatic and heterocyclic amines due to their lipophilic nature can bioaccumulate, especially in fatty tissues such as adipose tissue of human * To whom correspondence should be addressed. E-mail: fkadlubar@ nctr.fda.gov.

10.1021/tx049787n

breast (10). It has been shown by DeBruin et al. (11) that breast milk from healthy lactating women contains PhIP, which is absorbed from food, mainly animal meat, and distributed to the mammary gland. No PhIP was detected in the milk of the vegetarian donors studied. This indicates that ductal mammary epithelial cells are directly exposed to carcinogens, which may lead to DNA damage and mutation and eventually to breast carcinomas (12, 13). Aromatic and heterocyclic amines require metabolic activation to chemically reactive mutagenic species that bind to DNA, forming DNA adducts. There are significant variations between individuals in the ability of human mammary ductal epithelial cells to mediate this activation (14). It is likely that hepatic enzymes metabolize potential breast carcinogens, but it is also probable that locally expressed enzymes in breast tissue play an important role in the metabolism of these compounds and in modulating levels of DNA reactive species. Peroxidase enzymes such as lactoperoxidase (LPO) and myeloperoxidase are involved in mutagenic and carcinogenic activation in the human mammary gland (7, 15). They catalyze one- or two-electron oxidations of organic substrates to produce reactive radical cations or oxidized intermediates such as quinone imines, dimines, or nitrenium/carbenium ions. LPO is an enzyme present in human milk (16), secreted by human mammary ductal epithelial cells into the breast ducts (17). Peroxidases play a host defensive role through antimicrobial activity and removal of toxic hydrogen peroxide (18). These

This article not subject to U.S. Copyright. Published 2004 by the American Chemical Society Published on Web 11/09/2004

1660

Chem. Res. Toxicol., Vol. 17, No. 12, 2004

Gorlewska-Roberts et al.

Figure 1. Chemical synthesis of the bis(phosphate) standard for the C8-dG adduct of ABP.

enzymes, by utilization of aromatic and heterocyclic amines as oxidizable cosubstrates in the reduction of peroxides, can mediate in their oxidation to electrophilic metabolites capable of binding to DNA (19). Hydrogen peroxide, required for peroxidase-dependent oxidations, comes from respiratory burst of neutrophils and is also supplied by xanthine oxidase. The potential role of heterocyclic and aromatic amines in the etiology of breast cancer led us to the investigation of the LPO-catalyzed activation of the most commonly studied arylamine carcinogens: PhIP, benzidine (BZ), 4-aminobiphenyl (ABP), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) (Figure 1). In this study, we examined the oxidation of these compounds by bovine and human LPO in the presence of hydrogen peroxide, and we estimated the levels of DNA binding of activated metabolites.

Experimental Procedures Caution: The following chemicals are hazardous and should be handled carefully: PhIP, ABP, BZ, IQ, MeIQx, and adenosine 5′-[(-32P]triphosphate. Chemicals. PhIP was purchased from Toronto Research Chemicals (Toronto, Ontario, Canada); ABP was purchased from Aldrich (Milwaukee, WI); and BZ, IQ, and MeIQx were purchased from Eagle-Picher ChemSyn (Lenexa, KS). Affinitypurified IgGs from sheep directed against bovine LPO B were obtained from Fitzgerald Industries International (Concord, MA). Adenosine 5′-[(-32P]triphosphate, with an original specific activity of ∼7000 Ci/mmol, was obtained from ICN Biomedicals (Irvine, CA), and T4 polynucleotide kinase (PNK) was obtained from USB (Cleveland, OH). Unless stated otherwise, all other materials were obtained from Sigma Chemical Co. (St. Louis, MO). Bovine LPO-Catalyzed Activation of Aromatic and Heterocyclic Amines. Reactions of PhIP, IQ, MeIQx, ABP, and BZ (50 µM) in the presence of bovine LPO (150 µg) and H2O2 (100 µM) were conducted in 10 mM potassium phosphate buffer (pH 6.8) for 1, 5, 15, and 30 min and 1 h, respectively. After the

reaction was completed, the products were extracted with ethyl acetate, evaporated to dryness, dissolved in MeOH:water (50: 50), and analyzed by HPLC (Waters, Milford, MA) with ultraviolet (model 996 photodiode array detector) and radiometric (Radiomatics model A-500 FLO-ONE radioactivity detector) detection on a µBondapack C18 analytical column. The eluting solvents were 10 mM ammonium acetate (pH 7) and methanol, with a flow rate of 1 mL/min. The solvent program was as follows: 30-100% methanol from 0 to 30 min and 100% methanol from 30 to 35 min (linear gradient). Samples were also analyzed by positive ion electrospray mass spectrometry using a Finnigan TSQ 7000 LC/MS (San Jose, CA). Reactions for MS experiments were carried out for 1 min and 1 h for BZ, ABP, and IQ and for 15 min and 1 h for MeIQx and PhIP. The products were extracted with ethyl acetate, evaporated, dissolved in MeOH:water (50:50), and analyzed using a 2 mm × 250 mm Phenomenex ODS-3 Prodigy column with a flow rate of 200 µL/min. The capillary temperature was set to 300 °C, and the spray voltage was set to 3.5 keV. The solvent system consisted of two components, where solvent A was 5% acetonitrile (in water) with 0.1% formic acid and solvent B was 95% acetonitrile (5% water) with 0.1% formic acid. The HPLC program was 100% A for 4 min followed by a linear gradient to 10% A at 30 min. This solvent composition was held for 3 min. Full scans were used from 100 to 900 Da, scanning quadrupole 3. Data were acquired in the centroid mode at about 1 s per scan. The sheath gas was on, whereas no auxiliary gas was used. Injection was made using an HP1100 autosampler with a draw speed of 200 µL/min and an injection volume of 5-25 µL depending upon the concentration of the analyte; source current, 22.85 µA; capillary voltage, 50 V; and tube lens, 158 V. DNA Binding of Carcinogens Activated by Bovine and Human LPO. To estimate the optimal concentration of H2O2 for DNA binding, 14C-IQ (13 mCi/mmol) was reacted with DNA (5 mg/mL) in 2 mL of 10 mM potassium phosphate (pH 6.8) in the presence of bovine LPO (150 µg) and the following concentrations of hydrogen peroxide: 20, 50, 75, 100, 150, and 200 µM. The control incubation did not contain hydrogen peroxide. The reactions were started by the addition of H2O2 and incubated at 37 °C for 2 h. Carcinogen binding to DNA was estimated as described below. 3H-BZ (37 mCi/mmol), 3H-ABP (45 mCi/mmol), 3H-PhIP (95 mCi/mmol), 14C-IQ (13 mCi/mmol), and 14C-MeIQx (10 mCi/

LPO-Catalyzed Activation of AAs and HAAs

Chem. Res. Toxicol., Vol. 17, No. 12, 2004 1661

Table 1. Purification of Human LPO

step whey partially purified product

total specific volume protein activity activity recovery of (mL) (mg) (munits) (munits/mg) activity (%) 150 10

1560 0.18

2250 5

1.4 27.7

100 0.22

mmol) were each reacted with DNA (5 mg/mL) in 2 mL of 10 mM potassium phosphate (pH 6.8) in the presence of bovine LPO (150 µg) and 100 µM H2O2. The reaction was started by the addition of hydrogen peroxide and continued at 37 °C for 2 h. The reaction was stopped by the addition of cold ethanol:phenol (99:1). The DNA was precipitated, dissolved in 2 mL of water, and extracted twice with an equal vol of water-saturated n-butanol followed by extraction with 2 vol of phenol. NaCl (5 M) was added to give a final concentration of 0.5 M, and the DNA was precipitated with 5 vol of cold ethanol. The modified DNA was dissolved in water. The DNA concentration was determined by the diphenylamine method (20), and radioactivity was measured in 10 mL of scintillation fluid. All experiments were repeated three times. Human milk samples were collected at 2-4 months of lactation and kept frozen at -20 °C. The fat was removed by centrifugation (38000g for 45 min at 4 °C). Casein was precipitated by adjusting the pH to 4.6, followed by cooling for 1 h at 0 °C and centrifugation at 38000g for 45 min. The pH of the resulting whey was adjusted to 7. The DNA binding experiments, described above, were conducted using 1 and 2 mg of total protein content. LPO from human milk was partially purified by immunoaffinity adsorption. Affinity-purified IgGs from sheep (directed against bovine LPO B) were immobilized on Poros Protein G affinity matrix in 200 mM triethanolamine-HCl, pH 8.5. The immobilized antibodies were covalently coupled to the Protein G matrix by reaction with 15 mM dimethyl pimelimidate hydrochloride (DMP) for 30 min at room temperature. Excess DMP was quenched by washing with 200 mM ethanolamine, pH 8. Human breast milk with fat and casein removed was added to the antibody affinity matrix and kept at 4 °C overnight with gentle rocking. Human LPO was eluted from the column with 0.1 M glycine, pH 2.8, and each collected fraction (1 mL) was immediately neutralized. Peroxidase activity was measured at 470 nm by the guaiacol method (21) (Table 1). Synthesis of DNA Adduct of ABP. 1. 3′,5′-dG-Bis(phosphate) (pdGp) Synthesis. N2-Isobutyryl dG was dissolved in a minimal volume of pyridine. Five equivalents of bis(pyridinium)cyanoethyl phosphate and 10 equivalents of dicyclohexylcarbodiimide were added and stirred at room temperature overnight. The reaction was quenched with water and filtered. The filtrate was evaporated, and 2 M ammonium hydroxide at 55 °C was used to deblock the N2 and PO4 groups. Ammonium hydroxide was evaporated, and the final product was purified on a 1 mL C18 precolumn. 2. pdGp-ABP Standard Adduct Synthesis. N-Trifluoroacetyl-N-acetoxy-ABP (TFAA-ABP) was prepared as a reactant by dissolving 25 mg of N-hydroxy-ABP in 12 mL of argonsaturated anhydrous ethyl acetate and cooling on ice. Trifluoroacetic anhydride (27 µL) was added, and the mixture was left at room temperature for 45 min. The ethyl acetate was evaporated, and the residue was redissolved in 2.5 mL of argonsaturated anhydrous ethyl acetate and cooled on ice. Anhydrous pyridine (50 µL) and acetic anhydride (25 µL) were added, and the mixture was set at room temperature for 30 min. The TFAAABP solution was quickly evaporated, and the residue was redissolved in 1 mL of absolute ethanol. The adduct standard was prepared by reacting TFAA-ABP with pdGp (Figure 1). pdGp (25 mM) was dissolved in 4 mM potassium citrate, pH 7, cooled on ice, and flushed with argon for 2 min. Freshly prepared TFAA-ABP in ethanol (1 mL) was added, and the reaction was flushed with argon and placed at

Figure 2. Chemical structures of carcinogens discussed in the text. 37 °C with continuous mixing for 10 min and then mixing every 10 min for up to 1 h. Twenty milligrams of ascorbic acid was added, and ethanol was removed by flushing with argon. The reaction was extracted twice with water-saturated ether and once with water-saturated ethyl acetate. The ethyl acetate was removed by flushing with argon, and the final product was purified by semipreparative HPLC and kept frozen. A mass spectrum of the synthesized standard was obtained to confirm purity and identity. 3. Synthesis of the DNA Adduct of ABP Formed by Activation with Bovine LPO. The activation of ABP (100 µM) in the presence of bovine LPO (300 µg), H2O2 (100 µM), and DNA (2 mg/mL) was conducted in 10 mM potassium phosphate buffer (pH 6.8). After a 2 h incubation, DNA was isolated as described above. DNA (30 µg) was hydrolyzed by adding a solution of 0.2 U/µL micrococcal nuclease (MN) and 3.6 mU/µL spleen phosphodiesterase (SPD) (30 µL) and incubating at 37 °C overnight. The MN/SPD had been dialyzed ∼10 h against water twice. The enrichment of the adducted nucleotides was performed on 1 mL HLB Oasis Sep-Paks (Waters) in digestion buffer. Normal nucleotides were removed by washing with water, and adducted nucleotides were eluted in 3 column vol of methanol. The sample was evaporated to dryness and [5′-32P]phosphorylated using PNK (2 U) and [5′-32P]ATP (100 µCi, ∼2 µM) in 50 mM TrisHCl buffer (pH 7.6) containing 10 mM MgCl2 and 10 mM 2-mercaptoethanol in a total vol of 20 µL and incubated at 37 °C for 45 min. HPLC analysis of 32P-postlabeled DNA adducts was performed by injecting the total 32P-postlabeled mixture containing pdGp-ABP standard as an UV marker into the HPLC. The adduct was identified by ultraviolet and chromatographic comparison with the synthesized standard.

Results Enzymatic Activation of Carcinogens by LPO. PhIP, IQ, MeIQx (carcinogens from food), ABP (carcinogen from cigarette smoke), and BZ (industrial dye intermediate) (Figure 2) were enzymatically activated by bovine LPO in the presence of hydrogen peroxide. The products were separated on HPLC with ultraviolet and radiometric detection. ABP formed one product after 1 min of reaction, and after 15 min, the formation of four main products was observed. For IQ, the formation of two

1662





Figure 3. Electrospray full scan mass spectra of the oxidation products of (A) IQ (protonated molecules [M + H]+ at m/z 395 hydrazo derivative; at m/z 393 azo derivative), (B) MeIQx (protonated molecule [M + H]+ at m/z 425 hydrazo derivative), (C) ABP (protonated molecule [M + H]+ at m/z 378 adduct of hydrazo derivative with acetonitrile), and (D) PhIP (protonated molecule [M + H]+ at m/z 447 hydrazo derivative). Reactions of IQ, MeIQx, ABP, and PhIP (50 µM) were conducted in 10 mM potassium phosphate buffer (pH 6.8) in the presence of bovine LPO (150 µg) and H2O2 (100 µM). After indicated times of the reaction (see the Experimental Procedures), the products were extracted with ethyl acetate, evaporated, redissolved in MeOH:water (50:50), and analyzed by LCMS.

1664


products was observed after the first minute of the reaction and total of three products after 15 min. For both carcinogens, complete loss of substrate was observed after 15 min of reaction. One product of PhIP oxidation was observed at 15 min and three others at 30 min. For MeIQx, the formation of two main products was observed at 15 min. After 1 h of reaction, there were still significant amounts of PhIP and MeIQx in the reaction mixtures. For BZ, a complete loss of substrate was observed within the first 5 min of the reaction. Only one minor product was detected on HPLC. It is likely that reactive metabolites of BZ, which is most susceptible to activation, bind to enzymatic protein or form polymeric compounds that are not seen on HPLC. Kinetic studies revealed that susceptibility of the examined compounds to enzymatic activation was in the order of BZ > ABP > IQ > MeIQx > PhIP as the turnover numbers of amines were BZ (10.2), ABP (7.1), IQ (5.2), MeIQx (1.2), and PhIP (0.98). LC-MS experiments were performed to identify the products formed during the activation reactions. Mass spectra are shown in Figure 3. Two main products of IQ, formed after 1 min of reaction, were identified as hydrazo and azo derivatives. The protonated molecules [M + H]+ were observed at m/z 395 (hydrazo) and at m/z 393 (azo derivatives) (Figure 3A); the main products of MeIQx, ABP, and PhIP were hydrazo derivatives and the protonated molecules [M + H]+ were observed at m/z 425 (Figure 3B) (after 15 min of reaction); m/z 378 (acetonitrile adduct) (Figure 3C) (after 1 min of reaction); and m/z 447 (Figure 3D) (after 15 min of reaction), respectively. Horseradish peroxidase-catalyzed reaction of ABP has been studied previously, and the formation of an azo derivative was observed (22). The mass spectrum of azoxy derivative of IQ formed as an oxidation product was also shown earlier by Snyderwine et al. (23). The formation of the hydrazo and azo derivatives in case of IQ and the hydrazo derivatives in the case of ABP, PhIP, and MeIQx with LPO strongly suggests that the nitrenium ion is formed as a reactive intermediate as a consequence of two-electron oxidation. The nitrenium ion is known to bind to deoxyguanosine at position 8 to form adducts with DNA, which are promutagenic lesions. The scheme of activation is shown in Figure 4. Binding of Carcinogens to DNA. The DNA binding assays were conducted to determine the relative metabolic activation potential of the amines. 3H-BZ, 3H-ABP, 3 H-PhIP, 14C-IQ, and 14C-MeIQx were all activated to DNA binding derivatives by bovine LPO in the presence of hydrogen peroxide and calf thymus DNA. The DNA binding level of carcinogens was dependent on the hydrogen peroxide concentration, and the highest level of carcinogen binding to DNA was observed at 100 µM H2O2 concentration, as presented for IQ in Figure 5. The highest level of binding to DNA and highest susceptibility to oxidation by LPO was observed for BZ, and the lowest level was for PhIP (Figure 6). Human breast milk obtained from three healthy mothers was pooled and processed in order to remove fat and casein, and the whey was used for the activation of BZ in the presence of calf thymus DNA. The level of DNA binding of 3H-BZ was 0.279 ( 0.007 nmol BZ bound/mg DNA/mg protein for 1 mg of total protein content in milk and 0.347 ( 0.018 for 2 mg of total protein content. We also isolated human LPO from breast milk by immunoaffinity adsorption using purified IgGs from sheep


Figure 4. Scheme of heterocyclic and aromatic amines activation by LPO.

Figure 5. DNA binding of IQ activated by bovine LPO (nmol IQ bound/mg DNA/mg protein) dependent on H2O2 (µM) concentration. 14C-IQ (13 mCi/mmol) was reacted with DNA (5 mg/ mL) in 2 mL of 10 mM potassium phosphate (pH 6.8) in the presence of bovine LPO (150 µg) and following concentrations of H2O2: 0 (control incubation), 20, 50, 75, 100, 150, and 200 mM. Each reaction was started by the addition of H2O2 and was incubated at 37 °C for 2 h. DNA was precipitated and extracted as described in the Experimental Procedures, and the level of carcinogen binding to DNA was determined.

directed against bovine LPO B. The DNA binding level of BZ activated by this enzyme was estimated as 1.84 ( 0.21 nmol BZ bound/mg DNA/mg protein. The DNA adduct with ABP formed after activation with bovine LPO was characterized. ABP was incubated with bovine LPO/H2O2 in the presence of DNA. DNA was then isolated, hydrolyzed, and 32P-postlabeled. The ABPDNA adduct was found to cochromatograph by HPLC with the known adduct standard, N-(deoxyguanosin-8yl)-4-aminobiphenyl-3′,5′-bis(phosphate) (pdGp-C8-ABP), and its level was estimated as six adducts per 105 nucleotides (Figure 7).

Discussion In this study, we investigated the ability of the most commonly studied carcinogenic arylamines to undergo


Figure 6. Levels of DNA binding (nmol substrate bound/mg DNA/mg protein) for tested carcinogens. 3H-BZ (37 mCi/mmol), 3H-ABP (45 mCi/mmol), 3H-PhIP (95 mCi/mmol), 14C-IQ (13 mCi/mmol), and 14C-MeIQx were each reacted with DNA (5 mg/ mL) in 10 mM potassium phosphate (pH 6.8) in the presence of bovine LPO (150 µg) and 100 µM H2O2. Control incubation did not contain H2O2. The reactions were started by the addition of H2O2 and incubated for 2 h. DNA was precipitated and extracted as described in the Experimental Procedures, and the level of carcinogen binding to DNA was determined.

metabolic activation by bovine and human LPO, an enzyme present in breast milk. We identified the activation products and found that all studied carcinogens formed DNA reactive species as a result of this activation. Levels of cytochrome P450 enzymes are approximately 500 times lower in the breast than in the liver (24). It has been suggested that peroxidases such as LPO and myeloperoxidase, present in breast, may be responsible for a greater proportion of the total amount of carcinogen


activation in the mammary gland than in the liver (15). Aromatic and heterocyclic amines, like other lipophilic compounds, can possibly bioaccumulate in fatty tissues, such as breast, where they may be oxidized by the enzymes present in breast ducts. Human mammary epithelial cells, the most common site of origin of breast carcinomas, are lining the mammary ducts that are suspended in adipose tissue. These epithelial cells are exposed to carcinogens accumulating in adipose tissue but also present in breast milk or in circulating blood. The levels of locally activated mammary carcinogens depend on the levels of activating enzymes in the epithelial cells, myeloperoxidase activity in neutrophils and polymorphonuclear leukocytes, LPO levels in the breast milk/fluid, and diffusion of carcinogen from the site of activation to the ductal cells of the epithelium. The enzymatic activation of aromatic and heterocyclic amines by bovine LPO studied here leads to the formation of products as monitored by HPLC with ultraviolet, MS, and radiometric detection. Hydrazo and azo derivatives formed during this activation indicate that the reactive intermediate formed in two-electron oxidation process is a nitrenium ion derivative, which is a DNA binding agent. All of the carcinogens studied bound covalently to calf thymus DNA after activation by bovine LPO, which has similar properties to human LPO in terms of absorption spectrum, recognition by anti-bLPO antibodies, molecular mass, glycosylation, and sensitivity to inhibitors (25). The highest level of binding was observed for BZ, which also showed the highest susceptibility to enzymatic oxidation. PhIP was least susceptible to enzymatic oxidation and

Figure 7. HPLC/32P-radioactivity chromatogram (frame A) and UV spectral detection (frame B) of DNA adduct detected in calf thymus DNA after activation of ABP in the presence of the LPO/hydrogen peroxide system (marked a), coinjected with pdGp-C8ABP standard (marked A).

1666


had the lowest level of binding to DNA. ABP activated by bovine LPO in the presence of DNA formed C8-dG adduct at a level of 6 adducts/105 nucleotides in the assay. Peroxidase activity is present in milk of all mammalian species tested so far. Human milk contains LPO at a very low concentration that is highest in colostrum but declines rapidly during the first days of lactation (26). LPO (isolated from human breast milk by immunoaffinity adsorption) catalyzed the oxidation of BZ to reactive species able to form covalent DNA adducts. This suggests that not only bovine but also human LPO catalyzes the oxidative transformations of carcinogens present in breast, leading to DNA adduct formation. In summary, the present study shows that carcinogens, like aromatic and heterocyclic amines, are oxidatively metabolized by the enzyme LPO, present in breast milk, to DNA-binding intermediates. The ease of LPO-catalyzed activation and the high binding level of products of this activation to DNA suggest a significant role of LPO-catalyzed activation of carcinogens in human breast. These findings are supported by a study performed by Gorlewska et al (27) on qualitative and quantitative analysis of the DNA-carcinogen adducts present in DNA isolated from epithelial cells from human breast milk. For three main groups of carcinogens, namely, heterocyclic amines (PhIP), aromatic amines (ABP), and polycyclic aromatic hydrocarbons (benzo[a]pyrene), evidence for the presence of DNA adducts was found by 32Ppostlabeling/HPLC. These data indicate that women are exposed to several classes of dietary and environmental carcinogens that are activated, likely by locally expressed enzymes such as LPO, react with DNA, and form DNA adducts in breast ductal epithelial cells, the cells from which most breast cancers arise.

Acknowledgment. We thank all of the mothers who donated their time and breast milk for this study and Manal Fares and Gail Runnells who organized and collected the milk specimens. This research was supported in part by an appointment of K.G. to the Postgraduate Research Participation Program at the National Center for Toxicological Research administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Food and Drug Administration.

References (1) Harris, J. R., Lippman, M. W., Veronesi, U., and Willet, W. (1992) Breast cancer (1). N. Engl. J. Med. 327, 319-328. (2) Henderson, I. C. (1993) Risk factors for breast cancer development. Cancer 71, 2127-2140. (3) Tokunaga, M., Land, C. E., Yamamoto, T., Asano, M., Tokuoka, S., and Nishimori, I. (1987) Incidence of female breast cancer among atomic bomb survivors, Hiroshima and Nagasaki. Radiat. Res. 112, 243-272. (4) Ezaki, H., Ambrosone, C. B., Freudenheim, J. L., Graham, S., Marshall, J. R., Vena, J. E., Brasure, J. R., and Michalek, A. M. (1996) Cigarette smoking, N-acetyltransferase 2 genetic polymorphisms and breast cancer risk. J. Am. Med. Assoc. 276, 14941501. (5) DeBruin, L. S., Pawliszyn, J. B., and Josephy, P. D. (1999) Detection of monocyclic andaromatic amines, possible mammary carcinogens, in human milk. Chem. Res. Toxicol. 12, 78-82.

Gorlewska-Roberts et al. (6) Zheng, W., Gustafson, D. R., Sinha, R., Cerhan, J. R., Moore, D., Hong, C. P., and Anderson, K. E. (1998) Well-done meat intake and the risk of breast cancer. J. Natl. Cancer Inst. 90, 17241729. (7) Williams, J. A., and Phillips D. H. (2000) Mammary expression of xenobiotic metabolizing enzymes and their potential role in breast cancer. Cancer Res. 60, 4667-4677. (8) Vineis, P., and Pirastu, R. (1997) Aromatic amines and cancer. Cancer Causes Control 8, 346-355. (9) Ito, N., Hasegawa, R., Sano, M., and Tamano, S. (1991) A new colon and mammary carcinogen in cooked food 2-amino-1-methyl6-phenylimidazo[4,5-b]pyridine (PhIP). Carcinogenesis 12, 15031506. (10) Beer, A. E., and Billingham, R. E. (1978) Adipose tissue, a neglected factor in aetiology of breast cancer. Lancet 2 (8084), 296. (11) DeBruin, L. S., Martos, P. A., and Josephy, P. D. (2001) Detection of PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) in the milk of healthy women. Chem. Res. Toxicol. 14 (11), 1523-1528. (12) Martin, F. L., and Carmichael, P. L. (1996) Genotoxicity of human mammary lipid. Cancer Res. 56, 5342-5346. (13) Martin, F. L., and Venitt, S. (1997) DNA damage in breast epithelium: Detection by the single-cell gel (comet) assay and induction by human mammary lipid. Carcinogenesis 18, 22992305. (14) Stone, E. M., and Williams, J. A. (1998) Interindividual variations in the metabolicactivation of heterocyclic amines and their N-hydroxy derivatives in primary cultures of human mammary epithelial cells. Carcinogenesis 19, 873-879. (15) Josephy, P. D. (1996) The role of peroxidase-catalyzed activation of aromatic amines in breast cancer. Mutagenesis 11, 3-7. (16) Ueda, T., and Sakamaki, K. (1997) Molecular cloning and characterization of thechromosomal gene for human lactoperoxidase. Eur. J. Biochem. 243, 32-41. (17) Anderson, W. A. (1979) Mammalian endogenous peroxidases as cellular markersand as biosynthetic endpoints of hormone mediated activity: Viewpoint from cytochemistry. Prog. Histochem. Cytochem. 11, 1-27. (18) Tenovuo, J. O. (1985) The peroxidase system in human secretions. In Lactoperoxidase System, Chemistry and Biological Significance (Pruitt, K. M., and Tenovuo, J. O., Eds.) pp 101-122, Marcel Dekker, New York. (19) Kadlubar, F. F., Frederick, C. B., Weis, C. C., and Zenser, T. V. (1982) Prostaglandin endoperoxide synthetase-mediated metabolism of carcinogenic aromatic amines and their binding to DNA and protein. Biochem. Biophys. Res. Commun. 108, 253-258. (20) Burton, K. (1968) Determination of DNA concentration with diphenylamine. Methods Enzymol. 12, 163-169. (21) George, P. (1952) Intermediate compound formation with peroxidase and strongoxidizing agents. J. Biol. Chem. 201, 413-426. (22) Hughes, M. F., Smith, B. J., and Eling, T. E. (1992) The oxidation of 4-aminobiphenyl by horseradish peroxidase. Chem. Res. Toxicol. 5, 340-345. (23) Snyderwine, E. G., Roller, P. P., Wirth, P. J., Adamson, R. H., Sato, S., and Thorgeirsson, S. S. (1987) Synthesis, purification and mutagenicity of 2-hydroxyamino-3-methylimidazolo [4,5-f] quinoline. Carcinogenesis 8, 1017-1020. (24) Hellmold, H., Rylander, T., Magnusson, M., Reihner, E., Warner, M., and Gustafsson, J. A. (1998) Characterization of cytochrome P450 enzymes in human breast tissue from reduction mammoplasties. J. Clin. Endocrinol. Metab. 83, 886-895. (25) Shin, K., Tomita, M., and Lonnerdal, B. (2000) Identification of lactoperoxidase in mature human milk. J. Nutr. Biochem. 11, 94102. (26) Gothefors, L., and Marklund, S. (1975) Lactoperoxidase activity in human milk and saliva of newborn infants. Infect. Immun. 11, 1210-1215. (27) Gorlewska-Roberts, K. M., Green, B., Fares, M., Ambrosone, C. B., and Kadlubar, F. F. (2002) Carcinogen-DNA adducts in human breast epithelial cells. Environ. Mol. Mutagen. 39, 184-192.

TX049787N

Lactoperoxidase-Catalyzed Activation of Carcinogenic Aromatic and

Recommend Documents