To determine whether oxidative stress after cerebral ischemia-reperfusion affects genetic stability in the brain, we studied mutagenesis after forebrain ischemia-reperfusion in Big Blue transgenic mice (male C57BL/6 strain) containing a reporter lacI gene, which allows detection of mutation frequency. The frequency of mutation in this reporter lacI gene increased from 1.5 to 7.7 (per 100,000) in cortical DNA after 30 min of forebrain ischemia and 8 hr of reperfusion and remained elevated at 24 hr reperfusion. Eight DNA lesions that are characteristic of DNA damage mediated by free radicals were detected. Four mutagenic lesions (2,6-diamino-4-hydroxy-5-formamidopyrimidine, 8-hydroxyadenine, 5-hydroxycytosine, and 8-hydroxyguanine) examined by gas chromatography/mass spectrometry and one corresponding 8-hydroxy-2'-deoxyguanosine by a method of HPLC with electrochemical detection increased in cortical DNA two- to fourfold (p < 0.05) during 10-20 min of reperfusion. The damage to gamma-actin and DNA polymerase-beta genes was detected within 20 min of reperfusion based on the presence of formamidopyrimidine DNA N-glycosylase-sensitive sites. These genes became resistant to the glycosylase within 4-6 hr of reperfusion, suggesting a reduction in DNA damage and presence of DNA repair in nuclear genes. These results suggest that nuclear genes could be targets of free radicals.

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For understanding of the role of oxidative DNA damage in biological processes such as mutagenesis and carcinogenesis, it is essential to identify and quantify this type of DNA damage in cells. This can be achieved by gas chromatography/mass spectrometry. The present study describes the quantification of modified bases m DNA by isotope-dilution mass spectrometry with the use of stable isotope-labeled analogues as internal standards. A number of isotopically labeled DNA bases were synthesized. The mass spectra of their trimethylsilyl derivatives were recorded. Calibration plots were obtained for known quantities of modified bases and their isotope-labeled analogues. Quantification of various modified DNA bases by isotope-dilution mass spectrometry was demonstrated in isolated chromatin exposed to ionizing radiation. The results indicate that gas chromatography/stable isotope-dilution mass spectrometry is an ideally suited technique for selective and sensitive quantification of modified bases in DNA.

IntroductionLeukocyte-induced DNA damage may partially account for the known association between chronic inflammation and malignancy.

DNA base damage was assayed using gas chromatography/ mass spectrometry with selected ion monitoring (GC/MS-SIM) in renal and hepatic chromatin of male F344 rats up to 14 days after a single i.p. injection of 90 micromol Ni(II) acetate/kg body wt. Ten different damaged bases were quantified. No damage was found in either organ 12 h after Ni(II) treatment. The damage became significant only from day 1, with magnitude and persistence depending on the organ and base. In livers, levels of five DNA base products were significantly elevated over those in control rats. They were: 8-oxoguanine (by 46% at day 1 postinjection); 2,6-diamino-4-hydroxy-5-formamidopyrimidine (by 107% at day 1); 5-(hydroxymethyl)uracil (by 94% at day 1); 5,6-dihydroxyuracil (by 128% at day 1); and 5-hydroxyhydantoin (by 39% in terms of the overall adjusted means for days 1-14 post-injection). The elevation was highest at day 1 post-injection followed by a decrease at later days, except for 5-hydroxyhydantoin. In kidneys, the levels of only three damaged bases, 8-oxoguanine, 5-hydroxyhydantoin and 5,6-dihydroxyuracil were increased significantly (by 31, 73 and 60%, respectively) and one base, 8-oxoadenine, was increased by 26%, just below significance, all in terms of overall adjusted means for days 1-14 post-injection. Hence, unlike those in the liver, the renal increases persisted for 14 days. The results reveal a tissue specific response to Ni(II)-mediated oxidative DNA base damage with apparently faster DNA repair in liver than in kidney, the main target of Ni(II) carcinogenicity.