8-Oxo-7,8-dihydroguanine and uric acid as efficient predictors of survival in colon cancer patients

The aim of this work was to answer the question whether the broad range of parameters which describe oxidative stress and oxidatively damaged DNA and repair are appropriate prognosis factors of colon cancer (CRC) patients survival? The following parameters were analyzed for 89 CRC patients: concentration of uric acid and vitamins A, E, C in plasma; levels of 8-oxodGuo (8-oxo-7,8-dihydro-2'-deoxyguanosine) in DNA of leukocyte and colon tissues; urinary excretion rates of 8-oxodGuo and 8-oxoGua (8-oxo-7,8-dihydroguanine); the activity and mRNA or protein level of repair enzymes OGG1, APE1, ANPG, TDG and PARP1. All DNA modifications and plasma antioxidants were analyzed using high performance liquid chromatography (HPLC) or HPLC/gas chromatography-mass spectrometry techniques. Expression of repair proteins was analyzed by QPCR, Western or immunohistochemistry methods. Longer survival coincided with low levels of 8-oxodGuo/8oxoGua in urine and 8-oxodGuo in DNA as well as with high concentration of uric acid plasma level. In contrast to expectations, longer survival coincided with lower mRNA level in normal colon tissue of the main 8-oxoGua DNA glycosylase, OGG1, but no association was found for PARP-1 expression. When analyzing simultaneously two parameters the discriminating power increased significantly. Combination of low level of urinary 8-oxoGua together with low level of 8-oxodGuo in leukocyte (both below median value) or high concentration of plasma uric acid (above median value) have the best prediction power. Since prediction value of these parameters seems to be comparable to conventional staging procedure, they could possibly be used as markers to predict clinical success in CRC treatment.

Aberrant repair of etheno-DNA adducts in leukocytes and colon tissue of colon cancer patients

To assess the role of lipid peroxidation-induced DNA damage and repair in colon carcinogenesis, the excision rates and levels of 1,N(6)-etheno-2'-deoxyadenosine (epsilondA), 3,N(4)-etheno-2'-deoxycytidine (epsilondC), and 1,N(2)-etheno-2'-deoxyguanosine (1,N(2)-epsilondG) were analyzed in polymorphic blood leukocytes (PBL) and resected colon tissues of 54 colorectal carcinoma (CRC) patients and PBL of 56 healthy individuals. In PBL the excision rates of 1,N(6)-ethenoadenine (epsilonAde) and 3,N(4)-ethenocytosine (epsilonCyt), measured by the nicking of oligodeoxynucleotide duplexes with single lesions, and unexpectedly also the levels of epsilondA and 1,N(2)-epsilondG, measured by LC/MS/MS, were lower in CRC patients than in controls. In contrast the mRNA levels of repair enzymes, alkylpurine- and thymine-DNA glycosylases and abasic site endonuclease (APE1), were higher in PBL of CRC patients than in those of controls, as measured by QPCR. In the target colon tissues epsilonAde and epsilonCyt excision rates were higher, whereas the epsilondA and epsilondC levels in DNA, measured by (32)P-postlabeling, were lower in tumor than in adjacent colon tissue, although a higher mRNA level was observed only for APE1. This suggests that during the onset of carcinogenesis, etheno adduct repair in the colon seems to be under a complex transcriptional and posttranscriptional control, whereby deregulation may act as a driving force for malignancy.

Oxidative stress and 8-oxoguanine repair are enhanced in colon adenoma and carcinoma patients

Oxidative stress is involved in the pathogenesis of colon cancer. We wanted to elucidate at which stage of the disease this phenomenon occurs. In the examined groups of patients with colorectal cancer (CRC, n = 89), benign adenoma (AD, n = 77) and healthy volunteers (controls, n = 99), we measured: vitamins A, C and E in blood plasma, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-oxo-7,8-dihydroguanine (8-oxoGua) in leukocytes and urine, leukocyte 8-oxoGua excision activity, mRNA levels of APE1, OGG1, 8-oxo-7,8-dihydrodeoxyguanosine 5'-triphosphate pyrophosphohydrolase (MTH1) and OGG1 polymorphism. The vitamin levels decreased gradually in AD and CRC patients. 8-OxodG increased in leukocytes and urine of CRC and AD patients. 8-OxoGua was higher only in the urine of CRC patients. 8-OxoGua excision was higher in CRC patients than in controls, in spite of higher frequency of the OGG1 Cys326Cys genotype, encoding a glycosylase with decreased activity. mRNA levels of OGG1 and APE1 increased in CRC and AD patients, which could explain increased 8-oxoGua excision rate in CRC patients. MTH1 mRNA was also higher in CRC patients. The results suggest that oxidative stress occurs in CRC and AD individuals. This is accompanied by increased transcription of DNA repair genes, and increased 8-oxoGua excision rate in CRC patients, which is, however, insufficient to counteract the increased DNA damage.

Human RECQL5beta stimulates flap endonuclease 1

Human RECQL5 is a member of the RecQ helicase family which is implicated in genome maintenance. Five human members of the family have been identified; three of them, BLM, WRN and RECQL4 are associated with elevated cancer risk. RECQL1 and RECQL5 have not been linked to any human disorder yet; cells devoid of RECQL1 and RECQL5 display increased chromosomal instability. Here, we report the physical and functional interaction of the large isomer of RECQL5, RECQL5β, with the human flap endonuclease 1, FEN1, which plays a critical role in DNA replication, recombination and repair. RECQL5β dramatically stimulates the rate of FEN1 cleavage of flap DNA substrates. Moreover, we show that RECQL5β and FEN1 interact physically and co-localize in the nucleus in response to DNA damage. Our findings, together with the previous literature on WRN, BLM and RECQL4’s stimulation of FEN1, suggests that the ability of RecQ helicases to stimulate FEN1 may be a general feature of this class of enzymes. This could indicate a common role for the RecQ helicases in the processing of oxidative DNA damage.

Direct and indirect roles of RECQL4 in modulating base excision repair capacity

RECQL4 is a human RecQ helicase which is mutated in approximately two-thirds of individuals with Rothmund-Thomson syndrome (RTS), a disease characterized at the cellular level by chromosomal instability. BLM and WRN are also human RecQ helicases, which are mutated in Bloom and Werner's syndrome, respectively, and associated with chromosomal instability as well as premature aging. Here we show that primary RTS and RECQL4 siRNA knockdown human fibroblasts accumulate more H(2)O(2)-induced DNA strand breaks than control cells, suggesting that RECQL4 may stimulate repair of H(2)O(2)-induced DNA damage. RTS primary fibroblasts also accumulate more XRCC1 foci than control cells in response to endogenous or induced oxidative stress and have a high basal level of endogenous formamidopyrimidines. In cells treated with H(2)O(2), RECQL4 co-localizes with APE1, and FEN1, key participants in base excision repair. Biochemical experiments indicate that RECQL4 specifically stimulates the apurinic endonuclease activity of APE1, the DNA strand displacement activity of DNA polymerase beta, and incision of a 1- or 10-nucleotide flap DNA substrate by Flap Endonuclease I. Additionally, RTS cells display an upregulation of BER pathway genes and fail to respond like normal cells to oxidative stress. The data herein support a model in which RECQL4 regulates both directly and indirectly base excision repair capacity.

Bacterial DNA repair genes and their eukaryotic homologues: 1. Mutations in genes involved in base excision repair (BER) and DNA-end processors and their implication in mutagenesis and human disease

Base excision repair (BER) pathway executed by a complex network of proteins is the major system responsible for the removal of damaged DNA bases and repair of DNA single strand breaks (SSBs) generated by environmental agents, such as certain cancer therapies, or arising spontaneously during cellular metabolism. Both modified DNA bases and SSBs with ends other than 3'-OH and 5'-P are repaired either by replacement of a single or of more nucleotides in the processes called short-patch BER (SP-BER) or long-patch BER (LP-BER), respectively. In contrast to Escherichia coli cells, in human ones, the two BER sub-pathways are operated by different sets of proteins. In this review the selection between SP- and LP-BER and mutations in BER and end-processors genes and their contribution to bacterial mutagenesis and human diseases are considered.

Bacterial DNA repair genes and their eukaryotic homologues: 1. Mutations in genes involved in base excision repair (BER) and DNA-end processors and their implication in mutagenesis and human disease

Base excision repair (BER) pathway executed by a complex network of proteins is the major system responsible for the removal of damaged DNA bases and repair of DNA single strand breaks (SSBs) generated by environmental agents, such as certain cancer therapies, or arising spontaneously during cellular metabolism. Both modified DNA bases and SSBs with ends other than 3'-OH and 5'-P are repaired either by replacement of a single or of more nucleotides in the processes called short-patch BER (SP-BER) or long-patch BER (LP-BER), respectively. In contrast to Escherichia coli cells, in human ones, the two BER sub-pathways are operated by different sets of proteins. In this review the selection between SP- and LP-BER and mutations in BER and end-processors genes and their contribution to bacterial mutagenesis and human diseases are considered.

Inhibition of DNA repair glycosylases by base analogs and tryptophan pyrolysate, Trp-P-1

DNA base analogs, 2,4,5,6-substituted pyrimidines and 2,6-substituted purines were tested as potential inhibitors of E. coli Fpg protein (formamidopyrimidine -DNA glycosylase). Three of the seventeen compounds tested revealed inhibitory properties. 2-Thioxanthine was the most efficient, inhibiting 50% of 2,6-diamino-4-hydroxy-5N-methyl-formamidopyrimidine (Fapy-7MeG) excision activity at 17.1 microM concentration. The measured K(i) was 4.44 +/- 0.15 microM. Inhibition was observed only when the Fpg protein was first challenged to its substrate followed by the addition of the base analog, suggesting uncompetitive (catalytic) inhibition. For two other compounds, 2-thio- or 2-oxo-4,5,6-substituted pyrimidines, IC(50) was only 343.3 +/- 58.6 and 350 +/- 24.4 microM, respectively. No change of the Fpg glycosylase activity was detected in the presence of Fapy-7MeG, up to 5 microM. We also investigated the effect of DNA structure modified by tryptophan pyrolysate (Trp-P-1) on the activity of base excision repair enzymes: Escherichia coli and human DNA glycosylases of oxidized (Fpg, Nth) and alkylated bases (TagA, AlkA, and ANPG), and for bacterial AP endonuclease (Xth protein). Trp-P-1, which changes the secondary DNA structure into non-B, non-Z most efficiently inhibited excision of alkylated bases by the AlkA glycosylase (IC(50) = 1 microM). The ANPG, TagA, and Fpg proteins were also inhibited although to a lesser extent (IC(50) = 76.5 microM, 96 microM, and 187.5 microM, respectively). Trp-P-1 also inhibited incision of DNA at abasic sites by the beta-lyase activity of the Fpg and Nth proteins, and to a lesser extent by the Xth AP endonuclease. Thus, DNA conformation is critical for excision of damaged bases and incision of abasic sites by DNA repair enzymes.

Contribution of hMTH1 to the maintenance of 8-oxoguanine levels in lung DNA of non-small-cell lung cancer patients

BACKGROUND

The level of 8-oxoguanine (8-oxoG), a general marker of oxidative DNA damage, in DNA is the result of both an equilibrium between the rates of its formation and removal from DNA by DNA repair enzymes and the removal of 8-oxodGTP from the cellular nucleotide pool by hydrolysis to 8-oxodGMP, preventing its incorporation into DNA. To determine the contribution of each component to the level of 8-oxoG in DNA, we compared 8-oxoG-excising activity (encoded by hOGG1), 8-oxodGTPase activity (encoded by hMTH1), and 8-oxoG levels in DNA from tumors and surrounding normal lung tissues from non-small-cell lung cancer patients.

METHODS

We measured the level of 8-oxoG in DNA of 47 patients by high-performance liquid chromatography/electrochemical detection (HPLC/ECD), hOGG1 activity in tissue extracts of 56 patients by the nicking assay using an oligodeoxynucleotide containing a single 8-oxoG, and hMTH1 activity in tissue extracts of 33 patients by HPLC/UV detection. All statistical tests were two-sided.

RESULTS

The 8-oxoG level was lower in tumor DNA than in DNA from normal lung tissue (geometric mean: 5.81 versus 10.18 8-oxoG/10(6) G, geometric mean of difference = 1.75; P<.001). The hOGG1 activity was also lower in tumor than in normal lung tissue (geometric mean: 8.76 versus 20.91 pmol/h/mg protein, geometric mean of difference = 2.39; P<.001), whereas the hMTH1 activity was higher in tumor than in normal lung tissue (geometric mean: 28.79 versus 8.94 nmol/h/mg protein, geometric mean of difference = 0.31; P<.001). The activity of hMTH1 was three orders of magnitude higher than that of hOGG1 (nanomoles versus picomoles per hour per milligram of protein, respectively).

CONCLUSIONS

Several different components contribute to the maintenance of 8-oxoG levels in human DNA, with the greatest contributor being the removal of 8-oxodGTP from the cellular nucleotide pool by hMTH1.

Inhibition of DNA repair glycosylases by base analogs and tryptophan pyrolysate, Trp-P-1

DNA base analogs, 2,4,5,6-substituted pyrimidines and 2,6-substituted purines were tested as potential inhibitors of E. coli Fpg protein (formamidopyrimidine -DNA glycosylase). Three of the seventeen compounds tested revealed inhibitory properties. 2-Thioxanthine was the most efficient, inhibiting 50% of 2,6-diamino-4-hydroxy-5N-methyl-formamidopyrimidine (Fapy-7MeG) excision activity at 17.1 microM concentration. The measured K(i) was 4.44 +/- 0.15 microM. Inhibition was observed only when the Fpg protein was first challenged to its substrate followed by the addition of the base analog, suggesting uncompetitive (catalytic) inhibition. For two other compounds, 2-thio- or 2-oxo-4,5,6-substituted pyrimidines, IC(50) was only 343.3 +/- 58.6 and 350 +/- 24.4 microM, respectively. No change of the Fpg glycosylase activity was detected in the presence of Fapy-7MeG, up to 5 microM. We also investigated the effect of DNA structure modified by tryptophan pyrolysate (Trp-P-1) on the activity of base excision repair enzymes: Escherichia coli and human DNA glycosylases of oxidized (Fpg, Nth) and alkylated bases (TagA, AlkA, and ANPG), and for bacterial AP endonuclease (Xth protein). Trp-P-1, which changes the secondary DNA structure into non-B, non-Z most efficiently inhibited excision of alkylated bases by the AlkA glycosylase (IC(50) = 1 microM). The ANPG, TagA, and Fpg proteins were also inhibited although to a lesser extent (IC(50) = 76.5 microM, 96 microM, and 187.5 microM, respectively). Trp-P-1 also inhibited incision of DNA at abasic sites by the beta-lyase activity of the Fpg and Nth proteins, and to a lesser extent by the Xth AP endonuclease. Thus, DNA conformation is critical for excision of damaged bases and incision of abasic sites by DNA repair enzymes.

Identification of new genes regulated by the Crt1 transcription factor, an effector of the DNA damage checkpoint pathway in Saccharomyces cerevisiae

The Crt1 (RFX1) protein in Saccharomyces cerevisiae is an effector of the DNA damage checkpoint pathway. It recognizes a 13-bp cis-regulatory element in the 5'-untranslated region (5'-UTR) of the ribonucleotide reductase genes RNR2, RNR3, and RNR4; the HUG1 gene; and itself. We calculated the weight matrix representing the Crt1p binding site motif according to analysis of the 5'-UTR sequences of the genes that are under its regulation. We subsequently searched the 5'-UTR sequences of all the genes in the yeast genome for the occurrence of this motif. The motif was found in regulatory regions of 30 genes. A statistical analysis showed that it is unlikely that a random gene cluster contains the motif conserved as well as the Crt1p binding site. Analysis of microarray data provided supporting evidence for five putative Crt1p targets: FSH3, YLR345W, UBC5, NDE2, and NTH2. We used reverse transcription-PCR to compare the expression levels of these genes in wild-type and crt1Delta strains. Our results indicated that FSH3, YLR345W, and NTH2 are indeed under the regulation of Crt1p. Sequence analysis of the FSH3p indicated that this protein may be involved in folate metabolism either by carrying serine hydrolase activity required for the novel metabolic pathway involving dihydrofolate reductase (DHFR) or by directly interacting with the DHFR enzyme. We postulate that Crt1p may influence deoxyribonucleotide synthesis not only by regulating expression of the RNR genes but also by modulating DHFR activity. FSH3p shares significant sequence similarity with the product of the human tumor suppressor gene OVCA2. YLR345Wp and NTH2p are enzymes involved in the central metabolism under stress conditions.

Chemical rearrangement and repair pathways of 1,N6-ethenoadenine

1,N(6)-Ethenoadenine (epsilonA) is an exocyclic DNA adduct introduced to DNA by vinyl chloride and related compounds as well as in the consequence of oxidative stress and lipid peroxidation (LPO). This highly genotoxic DNA damage is chemically unstable and either depurinates or converts into pyrimidine ring-opened secondary lesions. We have studied the structures of derivatives formed during epsilonA chemical rearrangement and identified enzymes repairing one of the rearrangement products. Rearrangement involves a water molecule addition to the C(2)-N(3) bond of epsilonA, resulting in formation of pyrimidine ring-closed B1 product, which is in equilibrium with pyrimidine ring-opened B2 compound. B2 further deformylates to yield compound C. N-Glycosidic bond of compound C is unstable and C depurinates, yielding compound D. These secondary lesions are not repaired by alkylpurine DNA N-glycosylase, which excises the parental epsilon A. Compound B, when paired with thymine and cytosine is efficiently excised by Escherichia coli formamidopirymidine DNA N-glycosylase (Fpg), and thymine glycol DNA N-glycosylases from E. coli (Nth) and Saccharomyces cerevisiae (Ntg2). B is eliminated from B:G pair only by Nth and Ntg2 glycosylases, however none of the enzymes studied is excising B from B:A pair. This enables finishing of rearrangement, formation of AP sites and subsequently DNA strand breaks. During in vitro translesion synthesis, C is much easier bypassed by DNA polymerases, than compound B, and also than the parental epsilonA as well as than the AP site. This bypass beyond C proceeds mainly by misinsertion of adenine and guanine, or by insertion of thymine, the latter restoring the parental A:T pair. Alternatively, looping out of adducted nucleotide alone or with adjacent one generates one- or two-nucleotide deletions. This may explain the previously reported 20-fold higher mutagenic potency of product C in comparison to epsilon A in E. coli [Biochemistry 32 (1993) 12793].

Products of oxidative DNA damage and repair as possible biomarkers of susceptibility to lung cancer

The broad spectrum of oxidative DNA damage biomarkers [urinary excretion of 8-hydroxy-2'-deoxyguanosine (8-OH-dGuo) and 8-hydroxyguanine (8-OH-Gua)] and the level of oxidative DNA damage and repair in leukocytes DNA were analyzed in three groups of subjects: (a) lung cancer patients [all smokers (n = 51)]; (b) healthy smokers with comparable smoking status (n = 26); and (c) healthy nonsmokers (n = 38). The mean level of 8-OH-Gua in urine samples of 38 healthy nonsmokers reached a value of 1.783 +/- 0.785 nmol/day/kg. This level was significantly lower than that in the urine of the two smoker groups (cancer patients and healthy smokers), in whom the levels reached values of 2.319 +/- 1.271 and 2.824 +/- 0.892 nmol/day/kg, respectively. Urinary excretion of 8-OH-dGuo was similar in all groups of subjects. The level of 8-OH-dGuo in DNA isolated from leukocytes of cancer patients was significantly higher than that in DNA isolated from the group of healthy smokers and nonsmokers (9.44 +/- 4.77 versus 7.20 +/- 2.83 and 5.88 +/- 2.47 molecules/10(6) deoxyguanosine, respectively). Repair activity of 8-OH-Gua, as estimated by the nicking assay, was significantly higher in blood leukocytes of healthy volunteers (44.6 +/- 20.21 and 37.54 +/- 13.43 pmol/h/mg protein for smokers and nonsmokers, respectively) than in the leukocytes of lung cancer patients (24.56 +/- 11.28 pmol/h/mg protein). Because oxidative DNA insult represented by urinary excretion of oxidative DNA lesions was similar in both groups of subjects with similar smoking status, it appears likely that a higher rate of generation of oxidative damage in cellular DNA of lung cancer patients is a result of deficiency of the repair mechanism(s) in this group.

Decreased repair activities of 1,N(6)-ethenoadenine and 3,N(4)-ethenocytosine in lung adenocarcinoma patients

To assess the role of oxidative stress and lipid peroxidation (LPO) in the pathogenesis of lung cancer, we measured the levels of 1,N(6)-ethenoadenine (epsilonA) and 3,N(4)-ethenocytosine (epsilonC) in the DNA by immunoaffinity/(32)P postlabeling (33 cases). We also measured the capacity for epsilonA and epsilonC repair (by the nicking assay) in normal and tumor lung tissues, as well as in blood leukocytes of lung cancer patients (56 cases). Repair activities for epsilonA and epsilonC were also assayed in leukocytes of healthy volunteers, matched with cancer patients for age, sex, and smoking habit (25 individuals). Up to 10-fold variations among individuals were observed both in adducts level and repair activities. No differences in epsilonA and epsilonC levels between tumor and nonaffected lung tissues were recorded. However, leukocytes accumulated a significantly higher number of DNA adducts than the lung tissues. Repair activities for both epsilonA and epsilonC were significantly higher in tumor than in normal lung tissue. No significant differences in epsilonA and epsilonC repair activities were associated with age, sex, or smoking habit. However, a significant difference in repair capacity was observed between two histological types of lung cancer, squamous cell carcinoma (SQ) and adenocarcinoma (AD). In individuals suffering from lung AD, epsilonA- and epsilonC-repair activities in normal lung and blood leukocytes were significantly lower than in SQ patients. Moreover, in nonaffected lung tissue of AD patients, the ratio epsilonA/epsilonC adducts was lower than in SQ patients. Differences have also been found between epsilonA and epsilonC repair activities of cancer patients and healthy volunteers. Repair capacity for epsilonA was significantly lower in blood leukocytes of lung cancer patients than in leukocytes of healthy volunteers (P = 0.012). This difference was even larger between healthy volunteers and patients developing inflammation-related AD (P = 0.00033). Repair activities for epsilonC were the same in leukocytes of healthy controls, all lung cancer patients, and SQ patients. However, individuals with ADs revealed significantly lower epsilonC-repair activity (P = 0.013). These results suggest that oxidative stress-mediated lipid peroxidation might contribute to induction and/or progression of lung cancer. Decreased activity of base excision repair pathway for epsilonA and epsilonC is associated particularly with inflammation-related lung AD.

The pyrimidine ring-opened derivative of 1,N6-ethenoadenine is excised from DNA by the Escherichia coli Fpg and Nth proteins

It was previously shown that 1,N(6)-ethenoadenine (epsilonA) in DNA rearranges into a pyrimidine ring-opened derivative of 20-fold higher mutagenic potency in Escherichia coli (AB1157 lacDeltaU169) than the parental epsilonA (Basu, A. K., Wood, M. L., Niedernhofer, L. J., Ramos, L. A., and Essigmann, J. M. (1993) Biochemistry 32, 12793-12801). We have found that at pH 7.0, the stability of the N-glycosidic bond in epsilondA is 20-fold lower than in dA. In alkaline conditions, but also at neutrality, epsilondA depurinates or converts into products: epsilondA --> B --> C --> D. Compound B is a product of water molecule addition to the C(2)-N(3) bond, which is in equilibrium with a product of N(1)-C(2) bond rupture in epsilondA. Compound C is a deformylated derivative of ring-opened compound B, which further depurinates yielding compound D. Ethenoadenine degradation products are not recognized by human N-alkylpurine-DNA glycosylase, which repairs epsilonA. Product B is excised from oligodeoxynucleotides by E. coli formamidopyrimidine-DNA glycosylase (Fpg) and endonuclease III (Nth). Repair by the Fpg protein is as efficient as that of 7,8-dihydro-8-oxoguanine when the excised base is paired with dT and dC but is less favorable when paired with dG and dA. Ethenoadenine rearrangement products are formed in oligodeoxynucleotides also at neutral pH at the rate of about 2-3% per week at 37 degrees C, and therefore they may contribute to epsilonA mutations.

Endogenous oxidative DNA base modifications analysed with repair enzymes and GC/MS technique

GC/MS technique was used to identify endogenous levels of oxidatively modified DNA bases. To avoid possible artefact formation we used Fpg and Endo III endonucleases instead of acid hydrolysis to liberate the base products from unmodified DNA samples. Several different DNA preparations were used: (i) commercial calf thymus DNA, (ii) DNA isolated from rat liver, (iii) DNA isolated from human lymphocytes and (iv) nuclei isolated from rat liver. In all DNA samples used in our assays the most efficiently removed bases by Fpg protein are FapyG and FapyA although 8-oxoG was also detected in all preparations. The amount of 8-oxoG in human lymphocytes and in rat liver DNA was 3 and 2 per 10(7)bases, respectively. It is reasonable to postulate that the presented method is one of the techniques which should be used to reveal the enigma of endogenous, oxidative DNA damage.

Fapyadenine is a moderately efficient chain terminator for prokaryotic DNA polymerases

Hypoxanthine¿xanthine oxidase¿Fe3+¿ethylenediaminetetraacetate (EDTA) was used to modify ss M13 mp18 phage DNA. The dominant base modifications found by GC/IDMS-SIM were FapyGua, FapyAde, 8-hydroxyguanine, and thymine glycol. Analysis of in vitro DNA synthesis on oxidatively modified template by three DNA polymerases revealed that T7 DNA polymerase and Klenow fragment of polymerase I from Escherichia coli were blocked mainly by oxidized pyrimidines in the template whereas some purines that were easily bypassed by the prokaryotic polymerases constituted a block for DNA polymerase beta from calf thymus. DNA synthesis by T7 polymerase on poly(dA) template, where FapyAde content increased 16-fold on oxidation, yielded a final product with a discrete ladder of premature termination bands. When DNA synthesis was performed on template from which FapyAde, FapyGua, and 8OHGua were excised by the Fpg protein new chain terminations at adenine and guanine sites appeared or existing ones were enhanced. This suggests that FapyAde, when present in DNA, is a moderately toxic lesion. Its ability to arrest DNA synthesis depends on the sequence context and DNA polymerase. FapyGua might possess similar properties.