Detection and characterization of eukaryotic enzymes that recognize oxidative DNA damage

Deletion of p66Shc in mice increases the frequency of size-change mutations in the lacZ transgene

Upon oxidative challenge the genome accumulates adducts and breaks that activate the DNA damage response to repair, arrest, or eliminate the damaged cell. Thus, reactive oxygen species (ROS) generated by endogenous oxygen metabolism are thought to affect mutation frequency. However, few studies determined the mutation frequency when oxidative stress is reduced. To test whether in vivo spontaneous mutation frequency is altered in mice with reduced oxidative stress and cell death rate, we crossed p66Shc knockout (p66KO) mice, characterized by reduced intracellular concentration of ROS and by impaired apoptosis, with a transgenic line harboring multiple copies of the lacZ mutation reporter gene as part of a plasmid that can be recovered from organs into Escherichia coli to measure mutation rate. Liver and small intestine from 2- to 24-month-old, lacZ (p66Shc+/+) and lacZp66KO mice, were investigated revealing no difference in overall mutation frequency but a significant increase in the frequency of size-change mutations in the intestine of lacZp66KO mice. This difference was further increased upon irradiation of mice with X-ray. In addition, we found that knocking down cyclophilin D, a gene that facilitates mitochondrial apoptosis acting downstream of p66Shc, increased the size-change mutation frequency in small intestine. Size-change mutations also accumulated in death-resistant embryonic fibroblasts from lacZp66KO mice treated with H2 O2 . These results indicate that p66Shc plays a role in the accumulation of DNA rearrangements and suggest that p66Shc functions to clear damaged cells rather than affect DNA metabolism.

Impact of the oxidized guanine lesion spiroiminodihydantoin on the conformation and thermodynamic stability of a 15-mer DNA duplex

Spiroiminodihydantoin (Sp) is a hyperoxidized guanine base produced from oxidation of the mutagenic DNA lesion 7,8-dihydro-8-oxo-2'-deoxguanosine (8-oxoG) by a variety of species including peroxynitrite, singlet oxygen, and the high-valent metals Ir(IV) and Cr(V). In this study, the conformation and thermodynamic stability of a 15-mer DNA duplex containing an Sp lesion are examined using spectroscopic techniques and differential scanning calorimetry (DSC). The Sp lesion does not alter the global B-form conformation of the DNA duplex as determined by circular dichroism spectroscopy. Thermal denaturation experiments find that Sp significantly lowers the thermal stability of the duplex by approximately 20 degrees C. The enthalpies, entropies, and free energies of duplex formation for 15-mers containing guanine, 8-oxoG, and Sp were determined by performing DSC experiments as well as van't Hoff analysis of UV melting spectroscopic data. The thermodynamic stability of the Sp duplex is significantly reduced compared to that of both the 8-oxoG and parent G duplexes, with the thermodynamic destabilization being enthalpic in origin. The thermodynamic impact of the Sp lesion is compared to what is found for other types of DNA base damage and discussed in relation to how the presence of this lesion could affect cellular processes, in particular the recognition and repair of these adducts by the base excision repair enzymes.

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.

Bioavailability and antioxidant effects of orange juice components in humans

Seven healthy females and six males consumed daily 256 mg of vitamin C, 271 mg of flavanones (mainly as glycosides), 6 mg of carotenoids (mainly xanthophylls and cryptoxanthins), and 0.16 mg of folate by incorporation of daily three times 236 mL of not from concentrate orange juice (OJ) into their habitual diet. At the end of 3 weeks, mean vitamin C, folate, carotenoid, and flavanone plasma concentrations increased significantly relative to baseline by 59% (p < 0.001), 46% (p = 0.018), and 22% (p < 0.001), and 8-fold (p = 0.045), respectively. Flavanones were excreted in urine 9-fold more at the end of the intervention (p = 0.01) but returned to baseline 2 days after study completion. After the 3 week intervention, plasma concentrations of vitamins A and E did not change. 8-Hydroxydeoxyguanosine in white blood cells declined by 16% (p = 0.38; n = 11), and in individuals with high baseline concentrations by 29% (p = 0.36; n = 7), respectively. Low-density lipoprotein (LDL)-/high-density lipoprotein (HDL)-cholesterol ratios decreased but cholesterol (HDL, LDL, total) and thiobarbituric acid reactive substance plasma concentrations did not change significantly. We conclude from this pilot study that OJ is an excellent food source to enhance circulating concentrations of valuable hydrophilic as well as lipophilic phytochemicals.

Urinary excretion of 3,N4-etheno-2'-deoxycytidine in humans as a biomarker of oxidative stress: association with cigarette smoking

Smokers are known to have elevated levels of lipid peroxidation, a form of oxidative stress. Etheno DNA adduct formation can originate from endogenous lipid peroxidation or from exogenous exposure of carcinogens. Using a modified stable isotope dilution GC/negative ion chemical ionization/MS assay originally developed for urinary 3,N(4)-ethenocytosine (epsilonCyt), the nucleoside 3,N(4)-etheno-2'-deoxycytidine (epsilondCyd) was detected for the first time in human urine. The presence of epsilondCyd in human urine was confirmed by LC/electrospray ionization/tandem MS. Concentrations of epsilondCyd in the 24 h urine samples from healthy individuals not occupationally exposed to industrial chemicals were in the range between 0 and 0.80 nM. A statistically significant correlation was established between cigarette smoking and urinary excretion of epsilondCyd after being adjusted for creatinine (p = 0.004). Furthermore, the urinary total antioxidant capacity was found to correlate inversely with the epsilondCyd levels (r = -0.50, p = 0.02). The results indicate that urinary epsilondCyd may provide a valuable noninvasive biomarker for oxidative DNA damage.

In vitro nucleotide misinsertion opposite the oxidized guanosine lesions spiroiminodihydantoin and guanidinohydantoin and DNA synthesis past the lesions using Escherichia coli DNA polymerase I (Klenow fragment)

The low redox potential of 8-oxo-7,8-dihydroguanine (OG), a molecule regarded as a marker of oxidative damage in cells, makes it an easy target for further oxidation. Using a temperature-dependent method of synthesis, the oxidation products of OG, guanidinohydantoin (Gh) and/or its isomer iminoallantoin (Ia) as well as spiroiminodihydantoin (Sp), have been site-specifically incorporated into DNA oligomers. Single nucleotide insertion and primer extension experiments using Escherichia coli Kf exo(-) DNA polymerase were carried out under "standing start" and "running start" conditions in various sequence contexts. dAMP and dGMP were found to be inserted opposite these OG oxidation products. Steady-state kinetic studies show that the Gh/Ia.G base pair yields a lower K(m) value compared to the Sp.G pair or X.A (X = Gh/Ia or Sp). Running start experiments using oxidized and unoxidized OG-containing templates showed enhanced full extension in the presence of all four dNTPs. A sequence preference for efficiency of extension was found when Gh/Ia and Sp are present in the DNA template, possibly leading to primer misalignment. Full extension is more efficient for the templates containing two Gs immediately 3' to the lesions compared to two As. Although these lesions cause a significant block for DNA elongation, results show that they are more easily bypassed by the polymerase when situated in the appropriate sequence context. UV melting studies carried out on duplexes mimicking the template/primer systems were used to characterize thermal stability of the duplexes. These experiments suggest that both Gh/Ia and Sp destabilize the duplex to a much greater extent than OG, with Sp being most severe.

Antioxidative activities of oolong tea

While the antioxidative properties of green and black tea have been extensively studied, less attention has been given to these properties in oolong tea. The reducing powers, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activities, the amount of total phenolic compounds, the inhibitory effect on FeCl(2)/H(2)O(2) (Fenton reaction system)-induced DNA damage, and the inhibitory effect on erythrocyte hemolysis of an oolong tea water extract (OTE) were evaluated in the present study. The OTE was found to have strong antioxidative activities in all of the model systems tested. When the OTE was separated into different fractions according to molecular weight, it was found that the fractions with higher amounts of phenolic compounds (lower molecular weight) have stronger antioxidative activities. The present results support the concept that oolong tea contains several low molecular weight antioxidants that may have health promotion activities.

N-Nitrosodimethylamine-Mediated Formation of Oxidized and Methylated DNA Bases in a Cytochrome P450 2E1 Expressing Cell Line

The metabolic activation of N-nitrosodimethylamine (NDMA) to reactive metabolites is a critical step for the expression of its toxic and carcinogenic potential. We have previously reported that a P450 2E1 expressing cell line, GM2E1, can metabolize NDMA to toxic reactive metabolites and cause apoptotic cell death. To investigate whether DNA is a critical target for the reactive metabolites of NDMA, we measured the levels of DNA adducts in untreated and NDMA-treated GM2E1 cells. 8-Hydroxydeoxyguanosine (8-OHdG), a biomarker for oxidative DNA damage, was analyzed following enzymatic hydrolysis of DNA. 7-Methylguanine (7-mGua), the most suitable marker for the DNA adducts formed by methylating agents, was released by thermal depurination of DNA. The modified guanine adducts were separated by HPLC and quantified using electrochemical detection. The levels of 8-OHdG and 7-mGua in GM2E1 cells treated with NDMA increased up to approximately 4- and 100-fold over those in the untreated cells, respectively. The addition of ascorbic acid, an antioxidant, to the NDMA-treated cells resulted in a significant decrease in the cytotoxicity with a concomitant decrease in the levels of 8-OHdG, but not the levels of 7-mGua. Our results demonstrate that the metabolism of NDMA in GM2E1 cells causes both DNA methylation and oxidation and support the hypothesis that NDMA-mediated DNA damage may play an important role in its toxic effects.

Characterization of the alkaline degradation products of an oligodeoxynucleotide containing 8-oxo-7,8-dihydro-2'-deoxyguanosine by electrospray ionization mass spectrometry

Oligodeoxynucleotides containing 8-oxo-7,8-dihydro-2'-deoxyguanosine exhibit alkaline sensitivity and undergo cleavage of the phosphodiester backbone. Identification of the major degradation products and unstable intermediates formed in concentrated ammonia was accomplished by HPLC isolation and characterization by electrospray ionization mass spectrometry. Unstable intermediates were reduced in situ with NaBH4 prior to isolation and mass analysis. This technique produced accurate mass data for an oligonucleotide intermediate containing an abasic site, a strand cleavage, product containing the 3'-terminus, and two products with the 5'-terminus. 8-Oxoguanine was not present in the product HPLC chromatogram, suggesting rearrangement or degradation of this moiety prior to glycosidic bond cleavage. A scheme for the decomposition of 7,8-dihydro-2'-deoxyguanosine-containing oligonucleotides in 28% ammonia solution is presented.