Rates of chemical cleavage of DNA and RNA oligomers containing guanine oxidation products.

The nucleobase guanine in DNA (dG) and RNA (rG) has the lowest standard reduction potential of the bases, rendering it a major site of oxidative damage in these polymers. Mapping the sites at which oxidation occurs in an oligomer via chemical reagents utilizes hot piperidine for cleaving oxidized DNA and aniline (pH 4.5) for cleaving oxidized RNA. In the present studies, a series of time-dependent cleavages of DNA and RNA strands containing various guanine lesions were examined to determine the strand scission rate constants. The guanine base lesions 8-oxo-7,8-dihydroguanine (OG), spiroiminodihydantoin (Sp), 5-guanidinohydantoin (Gh), 2,2,4-triamino-2H-oxazol-5-one (Z), and 5-carboxamido-5-formamido-2-iminohydantoin (2Ih) were evaluated in piperidine-treated DNA and aniline-treated RNA. These data identified wide variability in the chemical lability of the lesions studied in both DNA and RNA. Further, the rate constants for cleaving lesions in RNA were generally found to be significantly smaller than for lesions in DNA. The OG nucleotides were poorly cleaved in DNA and RNA; Sp nucleotides were slowly cleaved in DNA and did not cleave significantly in RNA; Gh and Z nucleotides cleaved in both DNA and RNA at intermediate rates; and 2Ih oligonucleotides cleaved relatively quickly in both DNA and RNA. The data are compared and contrasted with respect to future experimental design.

Spirodi(iminohydantoin) products from oxidation of 2'-deoxyguanosine in the presence of NH4Cl in nucleoside and oligodeoxynucleotide contexts.

Upon oxidation of the heterocyclic ring in 2'-deoxyguanosine (dG), the initial electrophilic intermediate displays a wide range of reactivities with nucleophiles leading to many downstream products. In the present study, the product profiles were mapped when aqueous solutions of dG were allowed to react with NH4Cl in the presence of the photooxidants riboflavin and Rose Bengal as well as the diffusible one-electron oxidant Na2IrCl6. Product characterization identified the 2'-deoxyribonucleosides of spiroiminodihydantoin, 5-guanidinohydantoin, and oxazolone resulting from H2O as the nucleophile. When NH3 was the nucleophile, a set of constitutional isomers that are diastereotopic were also observed, giving characteristic masses of dG + 31. ESI(+)-MS/MS of these NH3 adducts identified them to be spirocycles with substitution of either the C5 or C8 carbonyl with an amine. The NH3 adducts exhibit acid-catalyzed hydrolysis to spiroiminodihydantoin. Quantification of the NH3 and H2O adducts resulting from oxidation of dG in the nucleoside, single-stranded, and duplex oligodeoxynucleotide contexts were monitored allowing mechanisms for product formation to be proposed. These data also provide a cautionary note to those who purify their oligonucleotide samples with ammonium salts before oxidation because this will lead to unwanted side reactions in which ammonia participates in product formation.

Endonuclease and Exonuclease Activities on Oligodeoxynucleotides Containing Spiroiminodihydantoin Depend on the Sequence Context and the Lesion Stereochemistry.

8-Oxo-7,8-dihydro-2'-deoxyguanosine (dOG), a well-studied oxidation product of 2'-deoxyguanosine (dG), is prone to facile further oxidation forming spiroiminodihydantoin 2'-deoxyribonucleoside (dSp) in the nucleotide pool and in single-stranded oligodeoxynucleotides (ODNs). Many methods for quantification of damaged lesions in the genome rely on digestion of DNA with exonucleases or endonucleases and dephosphorylation followed by LC-MS analysis of the resulting nucleosides. In this study, enzymatic hydrolysis of dSp-containing ODNs was investigated with snake venom phosphodiesterase (SVPD), spleen phosphodiesterase (SPD) and nuclease P1. SVPD led to formation of a dinucleotide, 5'-d(Np[Sp])-3' (N = any nucleotide) that included the undamaged nucleotide on the 5' side of dSp as the final product. This dinucleotide was a substrate for both SPD and nuclease P1. A kinetic study of the activity of SPD and nuclease P1 showed a sequence dependence on the nucleotide 5' to the lesion with rates in the order dG>dA>dT>dC. In addition, the two diastereomers of dSp underwent digestion at significantly different rates with dSp1>dSp2; nuclease P1 hydrolyzed the 5'-d(Np[Sp1])-3' dinucleotide two- to six-fold faster than the corresponding 5'-d(Np[Sp2])-3', while for SPD the difference was two-fold. These rates are chemically reasoned based on dSp diastereomer differences in the syn vs. anti glycosidic bond orientation. A method for the complete digestion of dSp-containing ODNs is also outlined based on treatment with nuclease P1 and SVPD. These findings have significant impact on the development of methods to detect dSp levels in cellular DNA.

Structural context effects in the oxidation of 8-oxo-7,8-dihydro-2'-deoxyguanosine to hydantoin products: electrostatics, base stacking, and base pairing.

8-Oxo-7,8-dihydroguanine (OG) is the most common base damage found in cells, where it resides in many structural contexts, including the nucleotide pool, single-stranded DNA at transcription forks and replication bubbles, and duplex DNA base-paired with either adenine (A) or cytosine (C). OG is prone to further oxidation to the highly mutagenic hydantoin products spiroiminodihydantoin (Sp) and 5-guanidinohydantoin (Gh) in a sharply pH-dependent fashion within nucleosides. In the present work, studies were conducted to determine how the structural context affects OG oxidation to the hydantoins. These studies revealed a trend in which the Sp yield was greatest in unencumbered contexts, such as nucleosides, while the Gh yield increased in oligodeoxynucleotide (ODN) contexts or at reduced pH. Oxidation of oligomers containing hydrogen-bond modulators (2,6-diaminopurine, N(4)-ethylcytidine) or alteration of the reaction conditions (pH, temperature, and salt) identify base stacking, electrostatics, and base pairing as the drivers of the key intermediate 5-hydroxy-8-oxo-7,8-dihydroguanine (5-HO-OG) partitioning along the two hydantoin pathways, allowing us to propose a mechanism for the observed base-pairing effects. Moreover, these structural effects cause an increase in the effective pK(a) of 5-HO-OG, following an increasing trend from 5.7 in nucleosides to 7.7 in a duplex bearing an OG·C base pair, which supports the context-dependent product yields. The high yield of Gh in ODNs underscores the importance of further study on this lesion. The structural context of OG also determined its relative reactivity toward oxidation, for which the OG·A base pair is ~2.5-fold more reactive than an OG·C base pair, and with the weak one-electron oxidant ferricyanide, the OG nucleoside reactivity is >6000-fold greater than that of OG·C in a duplex, leading to the conclusion that OG in the nucleoside pool should act as a protective agent for OG in the genome.

Fate of selected estrogens in two laboratory scale sequencing batch reactors fed with different organic carbon sources under varying solids retention times.

This study compared the performances of two laboratory-scale sequencing batch reactors to remove 17ß-estradiol and 17α-ethinyl estradiol. Both SBRs were operated to achieve organic carbon oxidation and nitrification. However, the overall bacterial population in both SBRs was targeted to be different by feeding the SBRs with peptone and glucose. Furthermore, the reactors were also run at different solid retention times (SRTs) to evaluate the effect of SRT on estrogen removal. The more diverse heterotrophic and ammonia oxidizing bacterial community in the peptone fed SBR1 had superior estrogen removal than the glucose fed SBR 2 which enriched less diverse community, particularly for 17α-ethinyl estradiol. Under a solids retention time (SRT) of 40days, the total 17ß-estradiol mass was 30% of the amount under the SRT of 20days, and the total 17α-ethinyl estradiol mass was likewise 40% of the amount under the shorter SRT.

Copper/H2O2-mediated oxidation of 2'-deoxyguanosine in the presence of 2-naphthol leads to the formation of two distinct isomeric adducts.

Exposure of cells to phenolic compounds through exogenous and endogenous sources can lead to deleterious effects via nucleobase modifications of DNA occurring under oxidative conditions. 2'-Deoxyguanosine (dG) is the most electron rich of the four canonical bases and includes many nucleophilic sites; it is also susceptible to oxidation with numerous reactive oxygen species. In these studies, dG was allowed to react with 2-naphthol in the presence of copper or iron salts yielding two principal isomeric products. Spectroscopic analysis and reactions with alkylated nucleosides support the assignment of compound 1a/1b as a pair of atropisomer N(2) adducts and compound 2a/2b as a diastereomeric mixture of tricyclic [4.3.3.0] adducts. Both products are the result of an overall four-electron oxidation process and consequently have the same masses, though drastically different structures, providing mechanistic insight into their formation. Thus, dG alkylation by 2-naphthol under oxidative conditions yields products whose structural properties are altered, leading to potentially mutagenic effects in genomic DNA.

Comparison of Transition Metal-Mediated Oxidation Reactions of Guanine in Nucleoside and Single-Stranded Oligodeoxynucleotide Contexts.

As the most readily oxidized of DNA's four natural bases, guanine is a prime target for attack by reactive oxygen species (ROS) and transition metal-mediated oxidants. The oxidation products of a modified guanosine nucleoside and of a single-stranded oligodeoxynucleotide, 5'-d(TTTTTTTGTTTTTTT)-3' have been studied using oxidants that include Co(II), Ni(II), and Ir(IV) compounds as well as photochemically generated oxidants such as sulphate radical, electron-transfer agents (riboflavin) and singlet oxygen. The oxidized lesions formed include spiroiminodihydantoin (Sp), guanidinohydantoin (Gh), imidazolone (Iz), oxazolone (Z) and 5-carboxamido-5-formamido-2-iminohydantion (2-Ih) nucleosides with a high degree of dependence on the exact oxidation system employed. Interestingly, a nickel(II) macrocyclic complex in conjunction with KHSO(5) leads to the recently reported 2-Ih heterocycle as the major product in both the nucleoside and oligonucleotide contexts.

Characterization of 2'-deoxyguanosine oxidation products observed in the Fenton-like system Cu(II)/H2O2/reductant in nucleoside and oligodeoxynucleotide contexts.

Reactive oxygen species attack both base and sugar moieties in DNA with a preference among the bases for reaction at guanine. In the present study, 2'-deoxyguanosine (dG) was oxidized by a copper-mediated Fenton reaction with the reductants ascorbate or N-acetyl-cysteine, yielding oxidation on both the base and the sugar. The primary oxidized lesions observed in these studies include the 2'-deoxyribonucleosides of 8-oxo-7,8-dihydroguanosine (dOG), spiroiminodihydantoin (dSp), guanidinohydantoin (dGh), oxazolone (dZ), and 5-carboxamido-5-formamido-2-iminohydantoin (d2Ih), as well as the free base guanine. d2Ih was the major product observed in the nucleoside, single- and double-stranded oligodeoxynucleotide contexts and is proposed to arise from oxidation at C5 of guanine. Product distribution studies provide insight into the role of the reductant in partitioning of dG base oxidation along the C5 and C8 pathways.

Mechanistic aspects of the formation of guanidinohydantoin from spiroiminodihydantoin under acidic conditions.

Experimentally, it was observed that the oxidized guanine lesion spiroiminodihydantoin (Sp) contained in highly purified oligodeoxynucleotides slowly converts to guanidinohydantoin (Gh). The reaction is accelerated in the presence of acid. The possible mechanisms of this transformation have been analyzed computationally. Specifically, the potential energy surface for formation of Gh from Sp has been mapped using B3LYP density functional theory, the aug-cc-pVTZ and 6-31+G(d,p) basis sets, and the integral equation formalism for the polarizable continuum model (IEF-PCM) solvation model. The results favor a mechanism in which proton-assisted hydration of the C6 carbonyl group forming a gem-diol leads to ring opening of the iminohydantoin ring. The resulting species resembles a beta-ketoacid in its ability to decarboxylate; tautomerization of the resulting enol forms Gh. The results of these studies indicate that incubation of nucleosides or oligonucleotides containing Sp should be avoided in acidic media when high purity or an accurate assessment of the amounts of hydantoin lesions is desired.

Electronic structure of DNA--unique properties of 8-oxoguanosine.

8-Oxo-7,8-dihydroguanosine (8-oxoG) is among the most common forms of oxidative DNA damage found in human cells. The question of damage recognition by the repair machinery is a long standing one, and it is intriguing to suggest that the mechanism of efficiently locating damage within the entire genome might be related to modulations in the electronic properties of lesions compared to regular bases. Using laser-based methods combined with organizing various oligomers self-assembled monolayers on gold substrates, we show that indeed 8-oxoG has special electronic properties. By using oligomers containing 8-oxoG and guanine bases which were inserted in an all thymine sequences, we were able to determine the energy of the HOMO and LUMO states and the relative density of electronic states below the vacuum level. Specifically, it was found that when 8-oxoG is placed in the oligomer, the HOMO state is at higher energy than in the other oligomers studied. In contrast, the weakly mutagenic 8-oxo-7,8-dihydroadenosine (8-oxoA) has little or no effect on the electronic properties of DNA.

Formation of tricyclic [4.3.3.0] adducts between 8-oxoguanosine and tyrosine under conditions of oxidative DNA-protein cross-linking.

8-Oxo-7,8-dihydro-2'-deoxyguanosine (OG), a prevalent product of oxidative stress on cellular DNA, is readily further oxidized forming adducts with nucleophiles. In the presence of tyrosine or p-cresol, an unusual tricyclo[4.3.3.0] adduct has been characterized in both nucleoside and oligodeoxynucleotide studies. The adduct is more stable in oligomers than nucleosides and undergoes slow reversion and hydration to spiroiminodihydantoin.

DNA-protein cross-links between guanine and lysine depend on the mechanism of oxidation for formation of C5 vs C8 guanosine adducts.

The reaction between N(alpha)-acetyllysine methyl ester (Lys) and 2'-deoxyguanosine (dGuo) was used to study structural aspects of DNA-protein cross-link (DPC) formation. The precise structure of DPCs depended on the nature of the oxidant and cross-linking reactions in which a series of different oxidation conditions generated a distribution of adducts, principally spirodiiminodihydantoins with lysine appended at the purine position of C5 (5-Lys-Sp), C8 (8-Lys-Sp), or both C5 and C8 (5,8-diLys-Sp). Singlet oxygen oxidation of dGuo produced 5-Lys-Sp exclusively when Rose Bengal or methylene blue was used to photochemically generate (1)O2 in the presence of Lys, whereas riboflavin or benzophenone-mediated photochemistry generated lysine radicals and led to C8 adduct formation, yielding 8-Lys-Sp and 5,8-diLys-Sp. Notably, the yield of dGuo modifications from riboflavin photooxidation increased dramatically in the presence of lysine. Oxidation of deoxyguanosine/lysine mixtures with Na2IrCl6 or sulfate radicals produced both 5-Lys-Sp and 8-Lys-Sp. The same adducts were formed in single and double-stranded oligodeoxynucleotides, and these could be analyzed after nuclease digestion. Adduct formation in duplex DNA was somewhat dependent on the accessibility of lysine to C5 vs C8 of the purine. No adduct formation was detected between lysine and the other nucleobases T, C, or A. Overall, the precise location of adduct formation at C5 vs C8 of guanine appears to be diagnostic of the oxidation pathway.

Unusual structural features of hydantoin lesions translate into efficient recognition by Escherichia coli Fpg.

Oxidation of guanine (G) and 8-oxoguanine (OG) with a wide variety of oxidants yields the hydantoin lesions, guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp). These two lesions have garnered much recent attention due to their unusual structures and high mutagenic potential. We have previously shown that duplexes containing Gh and Sp are substrates for the base excision repair glycosylase Escherichia coli Fpg (EcFpg). To evaluate the recognition features of these unusual lesions, binding and footprinting experiments were performed using a glycosylase inactive variant, E3Q EcFpg, and 30 bp duplexes containing the embedded lesions. Surprisingly, E3Q EcFpg was found to bind significantly more tightly ( approximately 1000-fold) to duplexes containing Gh or Sp over the corresponding duplexes containing OG. This may be a consequence of the helix-destabilizing nature of the hydantoin lesions that facilitates their recognition within duplex DNA. Though DNA binding affinities of E3Q EcFpg with Gh- and Sp-containing duplexes were found to be similar to each other, hydroxyl radical footprinting using methidium-propyl-EDTA (MPE)-Fe(II) revealed subtle differences between binding of E3Q EcFpg to the two lesions. Most notably, in the presence of E3Q EcFpg, the Sp nucleotide (nt) is hyperreactive toward cleavage by MPE-Fe(II)-generated hydroxyl radicals, suggestive of the formation of an intercalation site for the MPE-Fe(II) reagent at the Sp nt. Interestingly, increasing the duplex length from 18 to 30 bp enhanced the excision efficiency of Gh and Sp paired with C, G, or T by EcFpg such that these substrates are processed as efficiently as the signature substrate lesion, OG. Moreover, the base removal activity with these two lesions was more efficient than removal of OG when in a base pairing context opposite A. The high affinity and efficient activity of EcFpg toward the hydantoin lesions suggest that EcFpg mediates repair of the lesions in vivo. Notably, the facile activity of EcFpg toward Gh and Sp in base pairing contexts with G and A, which are likely to be present after DNA replication, would be detrimental and enhance mutagenesis.

In vitro ligation of oligodeoxynucleotides containing C8-oxidized purine lesions using bacteriophage T4 DNA ligase.

Ligases conduct the final stage of repair of DNA damage by sealing a single-stranded nick after excision of damaged nucleotides and reinsertion of correct nucleotides. Depending upon the circumstances and the success of the repair process, lesions may remain at the ligation site, either in the template or at the oligomer termini to be joined. Ligation experiments using bacteriophage T4 DNA ligase were carried out with purine lesions in four positions surrounding the nick site in a total of 96 different duplexes. The oxidized lesion 8-oxo-7,8-dihydroguanosine (OG) showed, as expected, that the enzyme is most sensitive to lesions on the 3' end of the nick compared to the 5' end and to lesions located in the intact template strand. In general, substrates containing the OG.A mismatch were more readily ligated than those with the OG.C mismatch. Ligations of duplexes containing the OA.T base pair (OA = 8-oxo-7,8-dihydroadenosine) that could adopt an anti-anti conformation proceeded with high efficiencies. An OI.A mismatch-containing duplex (OI = 8-oxo-7,8-dihydroinosine) behaved like OG.A. Due to its low reduction potential, OG is readily oxidized to secondary oxidation products, such as the guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp) nucleosides; these lesions also contain an oxo group at the original C8 position of the purine. Ligation of oligomers containing Gh and Sp occurred when opposite A and G, although the overall ligation efficiencies were much lower than those of most OG base pairs. Steady-state kinetic studies were carried out for representative examples of lesions in the template. Km increased by 90-100-fold for OG.C-, OI.C-, OI.A-, and OA.T-containing duplexes compared to that of a G.C-containing duplex. Substrates containing Gh.A, Gh.G, Sp.A, and Sp.G base pairs exhibited Km values 20-70-fold higher than that of the substrate containing a G.C base pair, while the Km value for OG.A was 5 times lower than that for G.C.

Synthesis and characterization of the oxidized dGTP lesions spiroiminodihydantoin-2'-deoxynucleoside-5'- triphosphate and guanidinohydantoin-2'-deoxynucleoside-5'- triphosphate.

Two convenient synthetic routes to the oxidized guanosine triphosphate lesions spiroiminodihydantoin-2'-deoxynucleoside-5'-triphosphate (dSpTP) and guanidinohydantoin-2'-deoxynucleoside-5'-triphosphate (dGhTP) are reported. Both two-electron oxidation of 2'-deoxy-7,8-dihydro-8-oxoguanosine-5'-triphosphate (dOGTP) using SO4*- generated photolytically from K2S2O8 or four-electron oxidation of 2'-deoxyguanosine-5'-triphosphate (dGTP) from singlet oxygen provide either dSpTP or dGhTP at pH 8.0 or 4.4, respectively. Highly purified triphosphates are obtained by ion pair reversed-phase HPLC.

Oxidatively induced DNA-protein cross-linking between single-stranded binding protein and oligodeoxynucleotides containing 8-oxo-7,8-dihydro-2'-deoxyguanosine.

The formation of covalent cross-links between amino acid side chains and DNA bases in DNA-protein complexes is a significant pathway in oxidative damage to the genome, yet much remains to be learned about their chemical structures and mechanisms of formation. In the present study, DNA-protein cross-links (DPCs) were formed between synthetic oligodeoxynucleotides containing an 8-oxo-7,8-dihydro-2'deoxyguanosine (OG) or an 8-oxo-7,8-dihydro-2'-deoxyadenosine (OA) nucleotide and Escherichia coli singled-stranded binding protein (SSB) under oxidative conditions. Studies with various sequences indicated that DNA homopolymers and those lacking 8-oxopurines were less reactive toward DPC formation. DPCs were formed in the presence of HOCl, peroxynitrite, and the one-electron oxidants Na(2)IrCl(6), Na(2)IrBr(6), and Na(3)Fe(CN)(6). Protein-protein cross-linking was also observed, particularly for oxidants of high reduction potential such as Na(2)IrCl(6). The adducted oligodeoxynucleotides were sensitive to hot piperidine treatment leading to strand scission at the site of cross-linking. In addition, the covalent cross-links were somewhat heat and acid labile, which may be related to the difficulties encountered in obtaining complete characterization of trypsin digests of the DPCs. However, model reactions involving the single amino acids lysine, arginine, and tyrosine, residues known to be involved in base contacts in the DNA:SSB complex, could be studied, and the adduct formed between N(alpha)-acetyllysine methyl ester and an 18-mer containing OG was tentatively characterized by electrospray ionization mass spectrometry as analogues of spiroiminodihydantoin and guanidinohydantoin. A mechanism involving nucleophilic attack of an amino acid side chain (e.g. the epsilon-amino group of lysine) at C5 of a 2-electron oxidized form of OG is proposed.

Spermine participates in oxidative damage of guanosine and 8-oxoguanosine leading to deoxyribosylurea formation.

Oxidation of single- or double-stranded DNA containing a 7,8-dihydro-8-oxoguanosine lesion with the one-electron oxidant Na2IrCl6 in the presence of spermine led to formation of a covalent adduct that was analyzed by gel electrophoresis, HPLC, ESI-MS, and UV-vis. The adduct was labile to heat, exhibiting a t1/2 of 12 h at 37 degrees C, and the ultimate hydrolysis product was characterized as a deoxyribosylurea lesion. Data from model studies with 1,3-diaminopropane vs 1,4-diaminobutane are consistent with initial formation of a C5 spermine adduct from a dehydro-8-oxoguanosine intermediate, followed by rearrangement to a spiroaminal subject to slow hydrolysis at C4 of the purine. Spermine adducts could also be formed from oxidation of the analogous G-containing oligomer from reaction with singlet oxygen, albeit in lower yield. These results are surprising in light of the traditional view that spermine is radioprotective against DNA oxidation.

Mechanism of two-electron oxidation of deoxyguanosine 5'-monophosphate by a platinum(IV) complex.

Many transition metal complexes mediate DNA oxidation in the presence of oxidizing radiation, photosensitizers, or oxidants. The final DNA oxidation products vary depending on the nature of metal complexes and the structure of DNA. Here we propose a mechanism of oxidation of a nucleotide, deoxyguanosine 5'-monophosphate (dGMP) by trans-d,l-1,2-diaminocyclohexanetetrachloroplatinum (trans-Pt(d,l)(1,2-(NH(2))(2)C(6)H(10))Cl(4), [Pt(IV)Cl(4)(dach)]; dach = diaminocyclohexane) to produce 7,8-dihydro-8-oxo-2'-deoxyguanosine 5'-monophosphate (8-oxo-dGMP) stoichiometrically. The reaction was studied by high-performance liquid chromatography (HPLC), (1)H and (31)P nuclear magnetic resonance (NMR), and electrospray ionization mass spectrometry (ESI-MS). The proposed mechanism involves Pt(IV) binding to N7 of dGMP followed by cyclization via nucleophilic attack of a phosphate oxygen at C8 of dGMP. The next step is an inner-sphere, two-electron transfer to produce a cyclic phosphodiester intermediate, 8-hydroxyguanosine cyclic 5',8-(hydrogen phosphate). This intermediate slowly converts to 8-oxo-dGMP by reacting with solvent H(2)O.

Formation of 13C-, 15N-, and 18O-labeled guanidinohydantoin from guanosine oxidation with singlet oxygen. Implications for structure and mechanism.

Guanosine labeled with 15N at N1, amino, and N7 and 13C at either C2 or C8 was oxidized by Rose Bengal photosensitization (singlet oxygen) in buffered aqueous solution. At pH > 7, spiroiminodihydantoin was the major product, while at pH < 7, guanidinohydantoin (Gh) was the principal product. 15N and 13C NMR studies confirmed that Gh was formed as a mixture of slowly equilibrating diastereomers. Experiments conducted in H218O indicated that Gh and Sp each contained one oxygen atom derived from O2 and one from H2O. Tandem mass spectrometry was used to identify the C4 carbonyl of Gh as the one labeled with 18O, supporting a mechanism involving attack of water at C5 of a dehydro-8-oxoguanosine intermediate.

Recognition and removal of oxidized guanines in duplex DNA by the base excision repair enzymes hOGG1, yOGG1, and yOGG2.

8-Oxo-7,8-dihydroguanine (OG) is susceptible to further oxidation in vitro to form two secondary oxidation products, guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp). Previous work from this laboratory has shown that OG, Gh, and Sp are recognized and excised from duplex DNA substrates by the Escherichia coli DNA repair enzyme Fpg. In this report, we extend these studies to the functionally related eukaryotic OG glycosylases (OGG) from yeast and humans: yOGG1, yOGG2, and hOGG1. The hOGG1 enzyme was active only toward the removal of 8-oxoguanine, exhibiting a 1000-fold faster rate of removal of 8-oxoguanine from OG.C-containing duplexes relative to their OG.A counterparts. Duplexes containing Gh or Sp opposite any of the four natural bases were not substrates for the hOGG1 enzyme. In contrast, both yOGG1 and yOGG2 enzymes removed Gh and Sp in a relatively efficient manner from an 18 bp duplex. No significant difference was observed in the rate of reaction of Gh- and Sp-containing duplexes with yOGG1. However, yOGG2 removed Sp at a faster rate than Gh. Both yOGG enzymes exhibit a negligible dependence on the base opposite the lesion, suggesting that the activity of these enzymes may be promutagenic. Surprisingly, in the 18 bp sequence context, both yOGG enzymes did not exhibit OG removal activity. However, both removed OG in a 30 bp duplex with a different sequence surrounding the OG. The wide range of repair efficiencies observed by these enzymes with different substrates in vitro suggests that this could greatly affect the mutagenicity of these lesions in vivo. Indeed, the greater efficiency of the yOGG proteins for removal of the further oxidized products, Gh and Sp, over their 8-oxoguanine parent, suggests that these lesions may be the preferred substrates in vivo.