Substitution of p- and o-hydroxyphenyl radicals at the 8 position of purine nucleosides by reaction with mutagenic p- and o-diazoquinones

Utility of 5'-O-2,7-dimethylpixyl for solid-phase synthesis of oligonucleotides containing acid-sensitive 8-aryl-guanine adducts

To study the structural and biological impact of 8-aryl-2'-deoxyguanosine adducts, an efficient protocol is required to incorporate them site-specifically into oligonucleotide substrates. Traditional phosphoramidite chemistry using 5'-O-DMT protection can be limiting because 8-aryl-dG adducts suffer from greater rates of acid-catalyzed depurination than dG and are sensitive to the acidic deblock conditions required to remove the DMT group. Herein we show that the 5'-O-2,7-dimethylpixyl (DMPx) protecting group can be used to limit acid exposure and improve DNA synthesis efficiency for DNA substrates containing 8-aryl-dG adducts. Our studies focus on 8-aryl-dG adducts with 8-substituents consisting of furyl ((Fur)dG), phenyl ((Ph)dG), 4-cyanophenyl ((CNPh)dG), and quinolyl ((Q)dG). These adducts differ in ring size and sensitivity to acid-promoted deglycosylation. A kinetic study for adduct hydrolysis in 0.1 M aqueous HCl determined that (Fur)dG was the most acid-sensitive (55.2-fold > dG), while (Q)dG was the most resistant (5.6-fold > dG). The most acid-sensitive (Fur)dG was chosen for optimization of solid-phase DNA synthesis. Our studies show that the 5'-O-DMPx group can provide a 4-fold increase in yield compared to 5'-O-DMT for incorporation of (Fur)dG into DNA substrates critical for determining adduct impact on DNA synthesis and repair.

C8-linked bulky guanosine DNA adducts: experimental and computational insights into adduct conformational preferences and resulting mutagenicity

Bulky DNA adducts are formed through the covalent attachment of aryl groups to the DNA nucleobases. Many of these adducts are known to possess conformational heterogeneity, which is responsible for the variety of mutagenic outcomes associated with these lesions. The present contribution reviews several conformational and mutagenic themes that are prevalent among the DNA adducts formed at the C8-site of the guanine nucleobase. The most important conclusions obtained (to date) from experiments are summarized including the anti/syn conformational preference of the adducts, their potential to inflict DNA mutations and mismatch stabilization, and their interactions with DNA polymerases and repair enzymes. Additionally, the unique role that computer calculations can play in understanding the structural properties of these adducts are highlighted.

Fluorescent C-linked C8-aryl-guanine probe for distinguishing syn from anti structures in duplex DNA

The synthesis and optical properties of the carbon (C)-linked C(8)-(2"-benzo[b]thienyl)-2'-deoxyguanosine ((Bth)dG), which acts as a fluorescent reporter of syn versus anti glycosidic conformations in duplex DNA, are described. In the syn-conformation, the probe stabilizes a G:G mismatch, emits at ∼385 nm (excitation ∼285 nm), and shows an induced circular dichroism (ICD) signal at ∼320 nm. Molecular dynamics (MD) simulations predict a wedge (W)-conformation for the mismatched duplex with the C(8)-benzo[b]thienyl moiety residing in the minor groove. In contrast, the probe destabilizes the duplex when base paired with its normal pyrimidine partner C. With flanking purine bases, a major groove B-type duplex is favored with (Bth)dG present in the anti-conformation emitting at ∼413 nm (excitation ∼326 nm) and no ICD signal. However, with flanking pyrimidine bases, (Bth)dG adopts the syn-conformation when base paired with C, and MD simulations predict a base-displaced stacked (S)-conformation, with the opposing C flipped out of the helix. The different duplex (B-, S-, and W-) conformers formed upon incorporation of (Bth)dG are known to play a critical role in the biological activity of N-linked C8-dG adducts formed by arylamine carcinogens. Bulky environment-sensitive fluorescent C(8)-dG adducts that mimic the duplex structures formed by carcinogens may be useful in luminescence-based DNA polymerase assays.

Conformational flexibility of C8-phenoxylguanine adducts in deoxydinucleoside monophosphates

M06-2X/6-31G(d,p) is used to calculate the structure of all natural deoxydinucleoside monophosphates with G in the 5' or 3' position, the anti or syn conformation, and each natural (A, C, G, T) base in the corresponding flanking position. When the ortho or para C8-phenoxyl-2'-deoxyguanosine (C8-phenoxyl-dG) adduct replaces G in each model, there is little change in the relative base-base orientation or backbone conformation. However, the orientation of the C8-phenoxyl group can be characterized according to the position (5' versus 3'), conformation (anti versus syn), and isomer (ortho versus para) of damage. Although the degree of coplanarity between the phenoxyl ring and G base in the ortho adduct is highly affected by the sequence since the hydroxyl group can interact with neighboring bases, the para adduct generally does not exhibit discrete interactions with flanking bases. For both adducts, steric clashes between the phenoxyl group and the backbone or flanking base destabilize the anti conformation preferred by the natural nucleotide and thereby result in a clear preference for the syn conformation regardless of the sequence or position. This contrasts the conclusions drawn from smaller (nucleoside, nucleotide) models previously used in the literature, which stresses the importance of using models that address the steric constraints present due to the surrounding environment. Since replication errors for other C8-dG bulky adducts have been linked to a preference for the syn conformation, our findings provide insight into the possible mutagenicity of phenolic adducts.

Fluorescent properties and conformational preferences of C-linked phenolic-DNA adducts

Phenolic toxins and mutagenic diazoquinones generate C-linked adducts at the C8 site of 2'-deoxyguanosine (dG) through the intermediacy of radical species. We have previously reported the site-specific incorporation of these adducts into oligonucleotides using a postsynthetic palladium-catalyzed cross-coupling strategy [Omumi (2011 ) J. Am. Chem. Soc. 133 , 42 - 50 ]. We report here the structural impact of these lesions within two decanucleotide sequences containing either 5'- and 3'-flanking pyrimidines or purines. In the complementary strands, the base opposite (N) the C-linked adduct was varied to determine the possibility of mismatch stabilization by the modified nucleobases. The resulting adducted duplex structures were characterized using UV thermal denaturation studies, circular dichroism, fluorescence spectroscopy, and molecular dynamics (MD) simulations. The experimental data showed the C-linked adducts to destabilize the duplex when base paired with its normal partner C but to increase duplex stability within a G:G mismatch. The stabilization within the G:G mismatch was sequence dependent, with flanking purine bases playing a key role in the stabilizing influence of the adduct. MD simulations showed no large structural changes to the B form double helix, regardless of the (anti/syn) adduct preference. Consideration of H-bonding and stacking interactions derived from the MD simulations together with the thermal melting data and changes in fluorescent emission of the adducts upon hybridization to the complementary strands implied that the C-linked phenolic adducts preferentially adopt the syn-conformation within both duplexes regardless of the opposite base N. Given that biological outcome in terms of mutagenicity appears to be strongly correlated to the conformational preference of the corresponding N-linked C8-dG adducts, the potential biological implications of phenolic C-linked adducts are discussed.

Tautomerization in gas-phase ion chemistry of isomeric C-8 deoxyguanosine adducts from phenol-induced DNA damage

Collision-induced dissociation (CID) of 8-(4''-hydroxyphenyl)-2'-deoxyguanosine and 8-(2''-hydroxyphenyl)-2'-deoxyguanosine was investigated using sequential tandem mass spectrometry. These adducts represent biomarkers of DNA damage linked to phenolic radicals and were investigated to gain insight into the effects of chemical structure of a C-8 modification on fragmentation pathways of modified 2'-deoxyguanosine (dG). CID in MS(2) of the deprotonated molecules of both the isomers generated the same product ion having the same m/z values. CID in MS(3) of the product ion at m/z 242 and CID in MS(4) experiments carried out on the selected product ions at m/z 225 and m/z 218 afford distinct fragmentation patterns. The conformational properties of isomeric product ions from CID showed that the ortho-isomers possess the unique ability to tautomerize through an intramolecular proton transfer between the phenolic OH group and the imine nitrogen (N7). Tautomerization of ortho-isomers to their keto-tautomers led to differences in their system of conjugated double bonds compared with either their enol-tautomer or the para-isomer. The charge redistribution through the N-7 site on the imidazole ring is a critical step in guanosine adduct fragmentation which is disrupted by the formation of the keto-tautomer. For this reason, different reaction pathways are observed for 8-(4''-hydroxyphenyl)-2'-deoxyguanosine and 8-(2''-hydroxyphenyl)-2'-deoxyguanosine. We present herein the dissociation and the gas-phase ion-molecule reactions for highly conjugated ions involved in the CID ion chemistry of the investigated adducts. These will be useful for those using tandem mass spectrometry for structural elucidation of C-8 modified dG adducts. This study demonstrates that the modification at the C-8 site of dG has the potential to significantly alter the reactivity of adducts. We also show the ability of tandem mass spectrometry to completely differentiate between the isomeric dG adducts investigated.

Postsynthetic guanine arylation of DNA by Suzuki-Miyaura cross-coupling

Direct radical addition reactions at the C(8)-site of 2'-deoxyguanosine (dG) can afford C(8)-Ar-dG adducts that are produced by carcinogenic arylhydrazines, polycyclic aromatic hydrocarbons, and certain phenolic toxins. Such modified nucleobases are also highly fluorescent for sensing applications and possess useful electron transfer properties. The site-specific synthesis of oligonucleotides containing the C(8)-Ar-G adduct can be problematic. These lesions are sensitive to acids and oxidants that are commonly used in solid-phase DNA synthesis and are too bulky to be accepted as substrates for enzymatic synthesis by DNA polymerases. Using the Suzuki-Miyaura cross-coupling reaction, we have synthesized a number of C(8)-Ar-G-modified oligonucleotides (dimers, trimers, decamers, and a 15-mer) using a range of arylboronic acids. Good to excellent yields were obtained, and the reaction is insensitive to the nature of the bases flanking the convertible 8-Br-G nucleobase, as both pyrimidines and purines are tolerated. The impact of the C(8)-Ar-G lesion was also characterized by electrospray ionization tandem mass spectrometry, UV melting temperature analysis, circular dichroism, and fluorescence spectroscopy. The C(8)-Ar-G-modified oligonucleotides are expected to be useful substrates for diagnostic applications and understanding the biological impact of the C(8)-Ar-G lesion.

NO-dependent modifications of nucleic acids

This review is devoted to chemical transformations of nucleic acids and their components under the action of nitrogen oxide metabolites. The deamination reaction of bases is discussed in the context of possible competing transformations of its intermediates (nitrosamines, diazonium cations, diazotates, triazenes, and diazoanhydrides) and mechanisms of crosslink formation with proteins and nucleic acids. The oxidation and nitration of bases by NO2 is considered together with the possibility of radical transfer to domains from the base stacks in DNA. Reduction of redox potentials of bases as a result of stacking interactions explains the possibility of their reactions within nucleic acids with the oxidants whose redox potential is insufficient for the effective reactions with mononucleotides. Modifications of nucleic acids with peroxynitrite derivatives are discussed in the context of the effect of the DNA primary structure and the modification products formed on the reactivity of single bases. The possibility of reduction of nitro groups within modified bases to amino derivatives and their subsequent diazotation is considered. The substitution of oxoguanine for nitroguanine residues may result; the reductive diazotation can lead to undamaged guanine. The intermediate modified bases, e.g., 8-aminoguanine and 8-diazoguanine, were shown to participate in noncanonical base pairing, including the formation of more stable bonds with two bases, which is characteristic of the DNA Z-form. A higher sensitivity of RNA in comparison with DNA to NO-dependent modifications (NODMs) is predicted on the basis of the contribution of medium microheterogeneity and the known mechanisms of nitrosylation and nitration. The possible biological consequences of nucleic acids NODMs are briefly considered. It is shown that the NODMs under the action of nitrogen oxide metabolites generated by macrophages and similar cells in inflammations or infections should lead to a sharp increase in the number of mutations in the case of RNA-containing viruses. As a result, the defense mechanisms of the host organism may contribute to the appearance of new, including more dangerous, variants of infecting viruses.

Conformational Properties of a Phototautomerizable Nucleoside Biomarker for Phenolic Carcinogen Exposure

We have characterized the conformational properties of the C8-deoxyguanosine (C8-dG) nucleoside adduct, 8-(2"-hydroxyphenyl)-2'-dG (1), which is a potential biomarker for exposure to phenolic carcinogens. Adduct 1 possesses the unique ability to phototautomerize, through an excited-state intramolecular proton transfer (ESIPT) process, to generate its keto form. This tautomerization depends on the presence of an intramolecular hydrogen (H)-bond between the phenolic OH and the imine nitrogen (N7) and has permitted insight into the equilibrium ground states of adduct 1. The results of our studies demonstrate that adduct 1 undergoes an ESIPT despite preferring a nonplanar "twisted" conformation that is imposed by the deoxyribose (dR) sugar moiety. Interestingly, a planar conformation of adduct 1 is also formed in certain aprotic solvents due to the anchoring effect of the intramolecular H-bond. Our results provide a basis for future studies aimed at determining the conformations of adduct 1 within DNA that will aid in our understanding of phenol-mediated carcinogenesis.

Tumor-promoting activity of p-hydroxybenzenediazonium is accelerated in Mg-deficient rats

Tumor-promoting activity caused by the short-term administration of p-hydroxybenzenediazonium (PDQ) has been assayed in rats fed on a Mg-deficient diet as a reference model versus rats fed on a standard diet as controls. For 5 weeks groups of 20 rats, fed either on the Mg-deficient or standard diet, were treated simultaneously with PDQ. A group of 10 Mg-deficient rats remained untreated. Topical application of PDQ was followed by the appearance of macroscopic alterations in the skin, which were more evident in the Mg-deficient rats. No deaths occurred during the treatment. After 5 weeks' PDQ treatment the rats were killed and histological analyses were made. Tissues from the skin, liver, heart, kidney, lung and thymus were screened by conventional staining methods. Both the PDQ-treated Mg-deficient and PDQ-treated control rats presented tissue lesions, although to a different extent. The untreated Mg-deficient rats showed no such lesions. Mg-deficient rats treated with PDQ developed significant incipient fibrosarcomas (p<0.05) and extended hyperplasia (p<0.001), particularly in the skin, accompanied by a significant increase in the thickness of collagen (mean value: 445.4+/-47.2microm, p<0.05) compared to the control PDQ-treated group (mean values: 258.7+/-36.4microm). The overall results provide objective proof of tumor-promoting activity after 5 weeks' treatment with PDQ. Such a fast response is interpreted as being linked to the increased vulnerability of the membrane caused by Mg deficiency, which would more readily facilitate the toxic activity of p-hydroxybenzenediazonium ions.

Evidence for Covalent DNA Adduction by Ochratoxin A following Chronic Exposure to Rat and Subacute Exposure to Pig

Ochratoxin A (OTA) is a nephrotoxic mycotoxin that is a potent renal carcinogen in male rats and is suspected of being the etiological agent of Balkan endemic nephropathy (BEN) and its associated urinary tract cancers. Conflicting results have been obtained regarding the genotoxicity of OTA and its ability to react directly with DNA upon oxidative bioactivation to yield covalent DNA adducts. To characterize DNA adduction by OTA, the present study utilizes the photooxidative properties of the toxin to generate authentic C8 OTA-3'-monophosphate-deoxyguanosine (3'-dGMP) adducts for use as cochromatographic standards for (32)P-postlabeling detection of OTA-mediated DNA adduction in the kidney of rat and pig. Our results show evidence for the photooxidation of OTA to yield carbon (C)- and oxygen (O)-bonded C8-3'-dGMP adducts (C-C8 and O-C8) that have been isolated and characterized by LC/MS with in-line UV and electrospray negative ionization (ES(-)) detection. A comparison to previously published work on related C8-dG adducts supports C8 attachment by OTA. The C-C8 OTA-3'-dGMP adduct standard is shown by (32)P-postlabeling to comigrate with the major lesion detected in the kidney of rat following chronic exposure to OTA and with one of four adducts detected in the kidney of pig following subacute exposure to the toxin. The O-C8 OTA-3'-dGMP adduct standard is also shown to coelute with a lesion detected in rat kidney. These findings suggest a role for the OTA phenoxyl radical in OTA-mediated DNA adduction in vivo, provide a rationale for the tumorigenesis of OTA, and strengthen the OTA hypothesis in the etiology of BEN and the associated urinary tract tumors.

Synthesis, properties, and NMR studies of a C8-phenylguanine modified oligonucleotide that preferentially adopts the Z DNA conformation

Carcinogenic aryl hydrazines produce C8-arylated purine adducts. The effect of these adducts on DNA conformation and their role in hydrazine carcinogenesis are unknown. Here, we describe a new synthetic route to produce these adducts that is also compatible with the synthesis of the corresponding phosphoramidites needed for oligonucleotide synthesis. Two oligonucleotides were prepared, an unmodified oligonucleotide, d((5)(')CGCGCGCGCG(3)(')), and a C8-phenylguanine modified oligonucleotide, d((5)(')CGCGCGCGCG(3)(')) (G = 8-phenylguanine). These oligonucleotides were compared using thermal denaturation, circular dichroism, NMR, and molecular modeling. The phenyl modification destabilizes the B DNA form and stabilizes the Z DNA form such that the B:Z ratio is near one under physiological conditions. In light of recent studies that show a role for Z DNA in gene expression and cell transformation, Z DNA stabilization by C8-arylguanine formation from aryl hydrazines may be relevant to their role in carcinogenesis.

Oxidative effects induced by dediazoniation of the p-hydroxybenzenediazonium ion in a neutral aqueous medium

The toxicity of arenediazonium ions is believed to result from the appearance of very reactive compounds during the dediazoniation process. In the case of the p-hydroxybenzenediazonium ion (PDQ), radical species generated during dediazoniation could potentially initiate lipid peroxidation. The data obtained in spectrophotometric experiments suggest that an interaction between PDQ and linoleic acid (LA) gives rise to the characteristic absorption of oxidized products deriving from LA, both in the presence and absence of a mixed micellar medium containing the surfactant Tween 20 (Tw20). Spectroscopic evidence also clearly points to the interference of these processes in the dediazoniation of PDQ. Analysis by reverse-phase, high-pressure liquid chromatography (HPLC) confirms that the decomposition of PDQ in a mixed micellar medium induces the peroxidation of both LA and methyl linoleate (MEL), thus causing the appearance of peaks characteristic of dienic conjugated hydroperoxides. The same products are observed after interaction between LA and the water-soluble 2,2'-azobis (2-amidinopropane), a frequently used initiator of lipid peroxidation. The proportion of isomers produced during the peroxidation process agrees well with that reported for reactions mediated by free radicals. A further chromatographic analysis of the decomposition of PDQ in the presence of 2-methylcyclohexa-2,5-diene-1-carboxylic acid (CHD) shows that phenol and quinone are the main products of the reaction. These results are discussed on the understanding that aryl and peroxyl radicals abstract a hydrogen atom from CHD, in accordance with our general scheme for PDQ dediazoniation described in a previous publication.

Efficient one-step Suzuki arylation of unprotected halonucleosides, using water-soluble palladium catalysts

Modification of nucleosides to give pharmaceutically active compounds, mutagenesis models, and oligonucleotide structural probes continues to be of great interest. The aqueous-phase modification of unprotected halonucleosides is reported herein. Using a catalyst derived from tris(3-sulfonatophenyl)phosphine (TPPTS) and palladium acetate, 8-bromo-2'-deoxyguanosine (8-BrdG) is coupled with arylboronic acids to give 8-aryl-2'-deoxyguanosine adducts (8-ArdG) in excellent yield in a 2:1 water:acetonitrile solvent mixture. The TPPTS ligand was found to be superior to water-soluble alkylphosphines for this coupling reaction. The coupling chemistry has been extended to 8-bromo-2'-deoxyadenosine (8-BrdA) and 5-iodo-2'-deoxyuridine (5-IdU), as well as the ribonucleosides 8-bromoguanosine and 8-bromoadenosine. Good to excellent yields of arylated adducts are obtained in all cases. With use of tri(4,6-dimethyl-3-sulfonatophenyl)phosphine (TXPTS), the Suzuki coupling of 8-BrdA and 5-IdU can be accomplished in less than 1 h at room temperature. This methodology represents an efficient and general method for halonucleoside arylation that does not require prior protection of the nucleoside.

Decomposition of N-nitrosamines, and concomitant release of nitric oxide by Fenton reagent under physiological conditions

N-Nitrosodimethylamine (NDMA) in phosphate buffer was rapidly decomposed by Fenton reagent composed of H2O2, and Fe(II) ion. Electron spin resonance (ESR) studies using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) showed that characteristic four line 1:2:2:1 ESR signals due to the DMPO-OH adduct formed on treatment of DMPO with Fenton reagent disappeared in the presence of NDMA, and N-nitrosodiethylamine (NDEA), suggesting the interaction of the N-nitrosamines with Fenton reagent. Treatment of the N-nitrosamines with Fenton reagent generated nitric oxide (NO) as estimated by ESR technique using cysteine-Fe(II), and N-methyl-D-glucaminedithiocarbamate (MGD)-Fe(II) complexes. Characteristic 3, and single line signals due to 2 cysteine-Fe(II)-NO, and 2 cysteine-Fe(II)-2 NO complexes, respectively, and three line signals due to MGD-Fe(II)-NO were observed. Considerable amount of NO were liberated as determined by NO2-, the final oxidation product of NO formed by reaction with dissolved oxygen in the aqueous medium. Spontaneous release of a small amount of NO from the N-nitrosamines was observed only on incubation in neutral buffers. Above results indicate that the N-nitrosamines were decomposed accompanying concomitant release of NO on contact with reactive oxygen species.

Excision of 8-methylguanine site-specifically incorporated into oligonucleotide substrates by the AlkA protein of Escherichia coli

8-Methyl-2'-deoxyguanosine (8-medGuo) has been shown to be a major stable alkylation product of 2'-deoxyguanosine induced by methyl radical attack on DNA. Moreover, by using primer extension assays, the latter DNA modification has recently been reported to be a miscoding lesion by generating G to C and G to T transversions and deletions in vitro. However, no data have been reported up to now, concerning the processing of this C8-alkylated nucleoside by the DNA repair machinery. Therefore, we have investigated the capability of excision of 8-methylguanine (8-meGua) site specifically incorporated into oligonucleotide substrates by several bacterial, yeast and mammalian DNA N-glycosylases. The results show that the 3-methyladenine (3-meAde) DNA glycosylase II (AlkA protein) from Escherichia coli is the only DNA N-glycosylase tested able to remove 8-meGua from double-stranded DNA fragments. Moreover, the activity of AlkA for 8-meGua varied markedly depending on the opposite base in DNA, being the highest with Adenine and Thymine and the lowest with Cytosine and Guanine. The removal of 8-meGua by AlkA protein was compared to that of 7-methylguanine (7-meGua) and hypoxanthine (Hx). The rank of damage as a substrate for AlkA being 7-meGua>8-meGua>Hx. In contrast, the human 3-meAde DNA N-glycosylase (Mpg) is not able to release 8-meGua paired with any of the four DNA bases. We also show that, DNA N-glycosylases involved in the removal of oxidative damage, such as Fpg or Nth proteins from E. coli, Ntg1, Ntg2 or Ogg1 proteins of Saccharomyces cerevisiae, or human Ogg1 do not release 8-meGua placed opposite any of the four DNA bases. Furthermore, HeLa and Chinese hamster ovary (CHO) cell free protein extracts do not show any cleavage activity at 8-meGua paired with adenine or cytosine, which suggests the absence of base excision repair (BER) of this lesion in mammalian cells.

Activation of AP-1 through the MAP kinase pathway: a potential mechanism of the carcinogenic effect of arenediazonium ions

Arenediazonium ions such as those found in the common mushroom Agaricus bisporus have been convincingly demonstrated to be tumorigenic. The specific mechanism of their tumorigenicity remains unclear. It has been shown that arenediazonium ions can be metabolized to aryl radicals, and that reaction of these aryl radicals with DNA produces aryl adducts. These metabolic processes also produce the reactive oxygen species superoxide and hydroxyl radicals which have been implicated in AP-1 activation. To further investigate the mechanism of tumorigenesis by arenediazonium ions, we studied the effect of a representative arenediazonium ion on AP-1 activation and phosphorylation of the signal transduction proteins ERK1, ERK2, JNK, and p38 kinase, both in vitro and in vivo. We also identified the specific radicals produced by spin trapping and ESR analysis. Here, it was found that p-methylbenzenediazonium ion (2a) induced a 16-fold increase in the extent of AP-1 activation at micromolar concentrations, and that this increase coincided with phosphorylation of the signaling kinases ERK1 and -2 and p38 kinase, but not JNK, in JB6 mouse epithelial cells. In vivo studies using AP-1 luciferase reporter-bearing transgenic mice supported the increase in the extent of AP-1 activation in 2a-treated mice over controls, and showed that this effect was different in different tissue types. The antioxidant N-acetylcysteine (NAC), a general antioxidant, showed an inhibitory effect on 2a-mediated AP-1 induction, while aspirin, a hydroxyl radical scavenger, had no effect. Spin trapping studies showed that while NAC suppressed radical formation from 2a, aspirin did not alter radical production from 2a. It appears that 3a, a carbon-centered radical formed from 2a, is responsible for AP-1-induced activation, and therefore, radical species that are not oxygen-centered are also capable of inducing AP-1. These results represent a step toward understanding the mechanism of tumorigenicity of arenediazonium ions.

In vivo metabolism of tert-butyl hydroperoxide to methyl radicals. EPR spin-trapping and DNA methylation studies

Metabolic activation of peroxides and hydroperoxides to free radicals is associated with the tumor promoting activity of these compounds. tert-Butyl hydroperoxide (t-BOOH) metabolism has been extensively studied as a model of peroxide biotransformation. In vivo studies are limited, and the hemoglobin-thiyl radical was the only species thus far identified in the blood of treated rats. Here we further examine t-BOOH metabolism in vivo with regard to free radical and DNA adduct production. Spin-trapping experiments with phenyl-N-tert-butylnitrone (PBN) led to the detection of EPR signals in the blood, bile, and organic extracts of the liver and stomach of rats treated with t-BOOH. Analysis of these signals demonstrated that t-BOOH metabolism in vivo produces alkyl radicals, detected in the bile and organic extracts of liver and stomach, in addition to the previously identified hemoglobin-thiyl radical. To characterize the produced alkyl radicals, experiments were performed with (13)C-labeled t-BOOH and two spin traps, PBN and alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). The latter was used because the EPR signals obtained with PBN were too weak to be unambiguous. Nevertheless, the EPR signals present in the bile of animals treated with (13)C-labeled t-BOOH and PBN or POBN were consistent with adducts of (13)C-labeled methyl radical and an unidentified alkyl radical. The latter is probably derived from lipids oxidized by the metabolically produced primary radicals, methyl and its precursor, tert-butoxyl. The presence of 8-methylguanine and 7-methylguanine in hydrolysates of DNA from liver and stomach of rats treated with t-BOOH was also examined. 8-Methylguanine, a typical product of methyl radical attack on DNA, was detectable in both the liver and stomach of treated rats. The results may be relevant to the understanding of the genotoxic properties of other peroxides, particularly of cumene hydroperoxide.

Dediazoniation of p-hydroxybenzenediazonium ion in a neutral aqueous medium

The dediazoniation of p-hydroxybenzenediazonium ion (PDQ) in a neutral aqueous medium has been studied under controlled experimental conditions to prevent photochemical and/or heterolytic side-reactions. Oxygen increased the dediazoniation rate of PDQ and caused the appearance of quinone and hydroquinone. An accumulation of quinone deriving from the reduction of PDQ by hydroquinone was also observed. In ESR analyses with different spin traps, the most stable signal was identified as belonging to the adduct of the p-hydroxyphenyl radical, even in the presence of dimethylsulfoxide or ethanol. A general scheme is proposed including three pathways for the homolytic dediazoniation of PDQ. Pathway 1 represents dediazoniation induced by a hydroxyl ion, a slow process at neutral pH and an even slower one with deaerated samples; a favored quinoid structure is put forward to explain these results. In pathway 2, the formation of a semiquinone radical via the reaction of an aryl radical with oxygen is put forward to justify the increase in the dediazoniation rate in the presence of oxygen. In pathway 3, hydroquinone, produced by semiquinone dismutation, may act as a reducing agent.

DNA methylation by tert-butyl hydroperoxide-iron (II): a role for the transition metal ion in the production of DNA base adducts

Metabolic degradation of both endogenous and exogenous peroxides is associated with the etiology of several diseases including cancer. Tert-butyl hydroperoxide (TBHP) has been widely employed as a model compound to study the cytotoxicity and promoting effects of organic peroxides. Recently, we reported that incubations of TBHP with iron (II) and calf thymus DNA led to generation of high yields of methyl radicals and to DNA methylation. Interestingly, DNA was methylated to products expected from both free radical and ionic mechanisms such as 8-methylguanine (C8-MeGua) and 7-methylguanine (N7-MeGua), respectively. To elucidate the mechanisms by which methyl radicals can produce different types of DNA adducts, we examined the effects of transition metal ions (iron (II), iron (III) and copper (I)) and metal ion chelators (ethylenediamine-N,N,N",N"-tetraacetate (EDTA) and desferal) on the nature and the yields of the DNA adducts produced during TBHP decomposition. The results led us to propose that a direct methyl radical attack on DNA guanine residues produces C8-MeGua whereas N7-MeGua and 3-methyladenine (N3-MeAde) are likely to be produced by attack of nucleophilic DNA centers on methyl radical generated in situ by the assistance of transition metal ions bound to DNA.

Aryl radical formation during the metabolism of arylhydrazines by microsomes

Many arylhydrazines are genotoxins, although the mechanism of their genotoxicity is unknown. Previous studies have shown that arylhydrazines are metabolized to arenediazonium ions, which produce C8-arylguanine adducts in DNA suggesting the intermediacy of an aryl radical. Here we have looked for the formation of aryl radicals from arylhydrazines and microsomes by ESR spin trapping. Only hydroxyl radicals are trapped upon incubation of p-methylphenylhydrazine with rat liver microsomes and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). However, hydroxyl and aryl radicals were trapped upon incubation of p-(methoxymethyl)phenylhydrazine with rat liver microsomes. Evidence for hydroperoxyl radical formation was also obtained. In contrast, when either of these substrates was incubated with microsomes from C5O cells, aryl and hydroxyl radicals were trapped. The ESR signal intensity of the spin-trapped aryl radicals parallels the extent of C8-arylguanine formation in DNA, and therefore, the aryl radical is likely the intermediate responsible for C8-arylguanine adduct formation. Aryl radicals and C8-arylguanine adducts may be related to the genotoxicity of arylhydrazines and related compounds that are oxidatively metabolized to arenediazonium ions, the precursor to aryl radicals, including arylalkyl nitrosamines, arylazo compounds, and triazenes.

8-Arylguanine adducts from arenediazonium ions and DNA

Arenediazonium ions (ArN2+) are genotoxic though the source of their genotoxicity is unknown. The present studies were undertaken to determine if reductive decomposition of ArN2+ to aryl radicals (Ar) in the presence of calf thymus DNA (ctDNA) or in cells results in the formation of DNA adducts. We found that when arenediazonium ions of the general structure p-X-ArN2+ (X = CH3, CH2OCH3, CH2OH) are allowed to react with ctDNA or incubated with cells under conditions that produce p-X-Ar, DNA adducts are formed with guanine. The structure of the adduct is the C8-substitution product derived from guanine and p-X-Ar. Formation of p-X-Ar was determined by ESR spin-trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The extent of C8-arylguanine adduction was measured by high performance liquid chromatography (HPLC) analysis of the DNA hydrosylate and comparison with authentic synthetic standards. The C8-arylguanine adducts observed to form may be important in regard to the genotoxicity of ArN2+, though other DNA adducts such as the N6-triazene of adenine or C8-aryladenine adducts can form. Finally, though the formation of C8-arylguanine adducts from arenediazonium ions has been proposed, this is the first report demonstrating their formation in DNA.

DNA strand cleavage by tumor-inhibiting antibiotic 6-diazo-5-oxo-L-norleucine

A tumor-inhibiting antibiotic, 6-diazo-5-oxo-L-norleucine (DON), caused DNA single-strand breaks. Thus, supercoiled plasmid DNA was transformed into an open circular relaxed form DNA by incubation with DON at pH 7.4. DNA strand cleavage by DON was not inhibited by superoxide dismutase, but inhibited by catalase. The inhibition by catalase may not be due to the destruction of hydrogen peroxide, but to the masking DON by the interaction with the heme moiety of the enzyme. DNA strand cleavage by DON was inhibited by azide, mannitol, ethanol, cysteine and 2-mercaptoethanol, suggesting the involvement of radical species in the cleavage. The cleavage, however, was not suppressed by removal of dissolved oxygen from the reaction mixture, indicating that no oxygen-derived radicals participated in the cleavage. Electron spin resonance spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) elucidated the generation of a carbon-centered radical from DON. Hence, the carbon-centered radical may participate in DNA strand cleavage by DON.

DNA strand breaking by the carbon-centered radical generated from 4-(hydroxymethyl) benzenediazonium salt, a carcinogen in mushroom Agaricus bisporus

4-(Hydroxymethyl)benzenediazonium salt (HMBD), a carcinogen in mushroom Agaricus bisporus, was found to generate a carbon-centered radical, 4-(hydroxymethyl)phenyl radical, during incubation at pH 7.4 and 37 degrees C, when estimated by Electron Spin Resonance (ESR) spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO), N-tert-butylphenyl-alpha-nitrone (PBN) and 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS). Formation of a substantial amount of benzyl alcohol during incubation of HMBD in the presence of a hydrogen donor, ethanol, supported the generation of the carbon-centered radical. When plasmid supercoiled DNA was incubated with HMBD at pH 7.4 and 37 degrees C for 30 min, the supercoiled DNA was converted into a nicked circular relaxed form and subsequently into a linear form. Sequence analysis indicated that the compound cleaved the plasmid DNA strand non-specifically. The intracellular double stranded DNA of Escherichia coli was fragmented by the compound, which may be responsible for its cytotoxicity. The compound induced mouse micronucleated peripheral reticulocytes. The compound was active in breaking DNA strands in the absence of molecular oxygen and in the presence of superoxide dismutase and catalase, indicating that no oxygen-derived radicals participated in the breaking. DNA breaking was inhibited by hydrogen donors butyl hydroxyanisole and ethanol, thiol compounds L-cysteine and 2-mercaptoethanol, and spin-trapping agents DMPO and PBN, indicating the direct contribution of the carbon-centered radical to the breaking.

Mechanisms of the DNA breaking activity of mutagenic 5-diazouracil

5-Diazouracil in monohydrated form showed mutagenicity and cytotoxicity on Salmonella typhimurium TA98 and TA100 strains without metabolic activation, and induced mouse micronucleated peripheral reticulocytes. Incubation of a plasmid supercoiled DNA with the compound caused DNA single-strand breaking: the supercoiled form was transformed into an open circular relaxed form and then into a linear form. The breaking was similarly caused in the absence of molecular oxygen. The breaking was not inhibited by superoxide dismutase and catalase, but inhibited by ethanol, butyl hydroxyanisole and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), suggesting the involvement of radical species other than oxygen-derived radical species. Sequencing analysis of the singly 5'-end-labeled DNA fragment showed that the phosphodiester breaking was not site-specific. When Escherichia coli cells were incubated with the compound, the intracellular double-strand DNA was fragmented. The fragmentation was inhibited by ethanol, DMPO, N-tert.-butyl-alpha-phenylnitrone (PBN) and thiol compounds. Generation of the carbon-centered radical was confirmed by the electron spin resonance spin-trapping technique using DMPO and PBN. The mutagenicity and the DNA breaking activity of 5-diazouracil can be ascribed to the carbon-centered radical.

Interaction of trichloromethyl free radicals with thymine in a model system: a mass spectrometric study

In previous studies from our laboratory we found that the CCl4 reactive metabolites produced during enzymatic in vitro or in vivo CCl4 biotransformation covalently bind to DNA. Further, chemically produced.CCl3 produce many adducts of unknown structure with the four DNA bases when the reaction proceeds in model systems. In the present work, we describe our attempt to elucidate by GLC/MS the structures of the adducts resulting when chemically generated.CCl3 interact with thymine. The following reaction products were identified: (i) 5-hydroxymethyl uracil; (ii) thymineglycol; (iii) 5-trichloroethyl uracil (tentative) and (iv) two isomeric 5,6-monochloro monohydroxy adducts of thymine (tentative). Reaction products found do not involve thymine positions directly participating in base-pairing processes. However, alterations in thymine structure reported if they occurred in DNA from livers of CCl4 poisoned animals, might potentially have biological significance that remains to be established.

Alkylation and cleavage of DNA by carbon-centered radical metabolites

Although carbon-centered radicals are formed during the metabolism of several genotoxic compounds, they have received little attention as DNA damaging agents. Carbon-centered radicals, however, can both cleave the DNA backbone and alkylate DNA bases, as has been demonstrated to occur in chemical and biochemical systems. Also, in vivo DNA alkylation by methyl radicals has been evidenced by isolation of C8-methylguanine in hydrolysates of DNA from rats administered 1,2-dimethylhydrazine. While most of the studies related to DNA damage by free radicals have been focused on oxyradicals, further studies on DNA alterations promoted by carbon-centered radicals may be necessary to elucidate the mechanisms of action of chemical mutagens and carcinogens.

DNA strand breakage by hydroxyphenyl radicals generated from mutagenic diazoquinone compounds

The mutagenic diazoquinone compounds p-diazoquinone (p-DQ), o-diazoquinone (o-DQ) and 3-diazo-N-nitrosobamethan (D-BM) cleaved the phosphodiester bond of lambda DNA, phi X174 RFI DNA and M13mp8ss DNA. p-DQ also cleaved the phosphodiester bond of bis(p-nitrophenyl)phosphate. The breakage of the phosphodiester bond was inhibited by the antioxidant butyl hydroxyanisole (BHA), ethanol, the spin trapping agent DMPO, cysteine and 2-mercaptoethanol. While incubation of p-DQ and o-DQ alone gave p-hydroquinone and catechol, respectively, incubation of these compounds in the presence of BHA and ethanol gave phenol in large yields. Incubation of p-DQ and o-DQ with the spin trapping agents DMPO and PBN gave spin adducts assignable as p- and o-hydroxyphenyl adducts, respectively. The breakage of the phosphodiester bond of DNA by the diazoquinone compounds is suggested to be due to the hydroxyphenyl radicals generated during incubation.