Formation of methyl ester of 2-methylglyceric acid from thymine glycol residues: a convenient new method for determining radiation damage to DNA

Determination of cis-thymine glycol in DNA by gas chromatography-mass spectrometry with selected ion recording and multiple reaction monitoring

A novel method for the determination of cis-thymine glycol in DNA has been developed, using gas chromatography-mass spectrometry with selected ion recording or multiple reaction monitoring. The procedure involves acidic hydrolysis of DNA in the presence of the internal standard cis-[2H3]thymine glycol, followed by derivatisation with N-methyl-N-(tert.-butyldimethylsilyl)trifluoroacetamide. The method was validated on DNA that had been oxidatively modified in vitro by radiation treatment, and was then applied to determine cis-thymine glycol in human placental DNA. Background levels of 5.45+/-2.98 ng cis-thymine glycol/mg DNA were observed in the human samples.

Gas chromatographic-mass spectrometric method for the assessment of oxidative damage to double-stranded dna by quantification of thymine glycol residues

A technique for the measurement of thymine glycol at parts per million concentrations in double-stranded polymeric DNA is described. The procedure utilizes base to ring-open DNA-bound thymine glycol in the presence of monomeric [(2)H4]thymine glycol as an internal standard, followed by reduction, solvolytic cleavage, and quantification of the characteristic methyl-2-methylglycerate released from polymeric DNA. Methyl-2-methyl-glycerate is derivatized to form the di-tert-butyldimethylsilyl [(TBDMS)2] ether to enhance its gas chromatographic properties and electron ionization detection. This assay was tested by measuring thymine gIyco1 levels in native, undamaged DNA (not purposefully oxidized). The measured quantities of thymine glycol are proportional to the amount of DNA analyzed. Components of DNA not containing oxidizable thymine do not contribute to the measured signal from methyl-2-methylglycerate-(TBDMS)2. These results indicate that there is approximately one thymine glycol per lo6 bases in undamaged DNA and that this value increases with storage of DNA in refrigerated aqueous solutions.

Thymine glycols and pyrimidine dimers in brain DNA during post-ischemic reperfusion

Free-radical reactions, known to occur in the reperfused brain, damage DNA in vitro. We therefore examined the hypothesis that thymine glycols and thymine dimers, which are known to block transcription and are formed by free radical mechanisms, are formed in brain DNA during reoxygenation following ischemia. Such biochemical lesions could account for the failure of protein synthesis that occurs following an ischemic insult. Large dogs were anesthetized, instrumented, and divided into four groups: (1) non-ischemic controls; (2) 20-min cardiac arrest without resuscitation; (3) 20-min cardiac arrest, resuscitation and 2 h reperfusion; and (4) 20-min cardiac arrest, resuscitation and 8 h reperfusion. Genomic DNA was isolated from the cerebral cortex. Thymine glycols were labeled by reduction with [3H]NaBH4. Pyrimidine dimers were determined by ELISA using antibody prepared against ultraviolet irradiated DNA. The data was evaluated by Kruskal-Wallis ANOVA with alpha = 0.05. The rabbit antibodies detected the thymine dimer content in 10 pg UV irradiated DNA but did not react with normal DNA. Borohydride labeling qualitatively detected thymine glycols generated by treatment of DNA with osmium tetroxide. There was no difference between the DNAs from the experimental groups in the content of thymine glycols or pyrimidine dimers (P = 0.608 and P = 0.219, respectively). We conclude that significant quantities of thymine glycols and thymine dimers are not formed in brain DNA during post-ischemic reperfusion. Therefore, the inhibition of brain protein synthesis during reperfusion, observed by other investigators, is unlikely to be caused by interruption of transcription by these species.

Brain nuclear DNA survives cardiac arrest and reperfusion

Iron-mediated peroxidation of brain lipids is known to occur during reperfusion following cardiac arrest. Since in vitro damage to DNA is caused by similar iron-dependent peroxidation, we tested whether free radical damage to genomic DNA also develops during reperfusion following cardiac arrest and resuscitation. Genomic DNA was isolated from the cerebral cortex in (i) normal dogs, (ii) dogs subjected to a 20-min cardiac arrest, and (iii) dogs resuscitated from a 20-min cardiac arrest and then allowed to reperfuse for 2 or 8 h. DNA strand nicks were evaluated by in vitro labeling of newly created 3' and 5' termini. DNA base damage was evaluated utilizing reaction with piperidine prior to labeling of 5' termini. The 3' DNA termini were labeled before and after digestion with exonuclease III, and the 5' DNA termini were labeled before and after treatment with piperidine. In vitro experiments with genomic DNA damaged by oxygen radicals verified that these labeling methods identified radical damage. In the experimental animal groups, terminal incorporation and electrophoretic mobility of brain nuclear DNA are not significantly changed either by 20 min of complete brain ischemia or during the first 8 h of reperfusion. We conclude that genomic DNA is not extensively damaged during cardiac arrest and early reperfusion, and therefore such DNA damage does not appear to be an important early aspect of the neurologic injury that accompanies cardiac arrest and resuscitation.

Yields of radiation-induced base products in DNA: effects of DNA conformation and gassing conditions

Gas chromatography-mass spectrometry with selected-ion monitoring was used to measure the yields of radiation-induced base products in aqueous solutions of native or heat-denatured DNA irradiated in the dose range 20-100 Gy. These DNA solutions were saturated with nitrous oxide, nitrogen, air or 20% oxygen in nitrous oxide during irradiation. The products measured were as follows: 5,6-dihydrothymine; 5-hydroxy-5,6-dihydrothymine; 5,6-dihydrothymine (thymine glycol); 5-hydroxy-5,6-dihydrocytosine; 5,6-dihydroxy-5,6- dihydrocytosine (cytosine glycol); 4,6-diamino-5-formamidopyrimidine; 7,8-dihydro-8-oxoadenine (8-hydroxyadenine); 2,6-diamino-4-hydroxy-5- formamidopyrimidine; and 7,8-dihydro-8-oxoguanine (8-hydroxyguanine). In oxygenated solutions, 5,6-dihydrothymine, 5-hydroxy-5,6-dihydrothymine and 5-hydroxy-5,6-dihydrocytosine were not formed. The yields of all products, other than 5,6-dihydrothymine, were greater in irradiated DNA samples from N2O-saturated solutions than from N2-saturated solutions. In N2-saturated solutions the yield of 8-hydroxyadenine was low and 8-hydroxyguanine was undetectable. Yields of pyrimidine products in heat-denatured DNA were greater than those in native DNA using all types of gases. However, the effects of DNA conformation on the yields of purine products were dependent on the type of gas used to saturate the irradiated DNA solutions. Yields of formamidopyrimidines were generally lower in solutions of DNA irradiated in the native than in the heat-denatured conformation. In air-saturated solutions of DNA, yields of 8-hydroxypurines were not influenced greatly by DNA conformation. In DNA solutions saturated with N2O/O2, 8-hydroxypurine formation was more favourable in the heat-denatured conformation than in the native conformation. On the other hand, in deoxygenated solutions, formation of 8-hydroxypurines was favoured in the native conformation. Data indicate that DNA conformation and the type of gas used to saturate the irradiated solutions have a profound influence on yields of base products in DNA.