Oxidatively Generated Damage to Cellular DNA: Mechanistic Aspects

In this chapter emphasis is placed on recent aspects of the oxidative formation of several classes of modified bases in cellular DNA that arise from the reaction of the hydroxyl radical (•OH), singlet oxygen and hypochlorous acid. Degradation compounds are detected quantitatively and specifically after suitable DNA hydrolysis into either nucleosides or bases by HPLC-tandem mass spectrometry. Thus, 6 oxidized nucleosides including: the four cis and trans diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine, 5-(hydroxymethyl)-2′-deoxyuridine and 5-formyl-2′-deoxyuridine are found to be formed as the result of •OH radical mediated oxidation of thymidine. In addition, γ-irradiation of cellular DNA was found to generate 8-oxo-7,8-dihydropurine derivatives and related formamidopyrimidine compounds resulting from •OH radical oxidation of the guanine and adenine bases. Furthermore, singlet oxygen oxidation of guanine was found to give rise exclusively to 8-oxo-7,8-dihydro-2′-deoxyguanosine while HOCl reaction with cytosine, adenine and guanine led to the formation of 5-chlorocytosine, 8-chloroadenine and 8-chloroguanine nucleosides respectively in the DNA and RNA of human white blood cells. Interestingly, formation of these various degradation products has been rationalized in terms of existing mechanisms that were proposed previously from model studies, mostly involving free nucleosides.