Combination reactions of superoxide with 8-oxo-7,8-dihydroguanine radicals in DNA: Kinetics and end products

Abstract

One of the major biomarkers of oxidative stress and oxidative damage of cellular DNA is 8-oxo-7,8-dihydroguanine (8-oxoGua), which is more easily oxidized than guanine to diverse oxidative products. In this work, we have investigated further oxidative transformations of 8-oxoGua in single- and double-stranded oligonucleotides to the dehydroguanidinohydantoin, oxaluric acid, and diastereomeric spiroiminodihydantoin lesions. The relative distributions of these end products were explored by a combined kinetic laser spectroscopy and mass spectrometry approach and are shown to depend markedly on the presence of superoxide radical anions. The 8-oxaGua radicals were produced by one-electron oxidation of 8-oxoGua by 2-aminopurine radicals generated by the two-photon ionization of 2-aminopurine residues site specifically positioned in 5′-d(CC[2-aminopurine]TC[8-oxoGua]CTACC). The hydrated electrons also formed in the photoionization process were trapped by dissolved molecular oxygen thus producing superoxide. A combination reaction between the 8-oxoGua and superoxide radicals occurs with the rate constant of (1.3 ± 0.2) × 108 M-1 S-1 and (1.0 ± 0.5) × 108 M-1 S-1 in single- and double-stranded DNA, respectively. The major end products of this reaction are the dehydroguanidinohydantoin lesions that slowly hydrolyze to oxaluric acid residues. In the presence of Cu,Zn-superoxide dismutase, an enzyme that induces the rapid catalytic dismutation of superoxide to the less reactive H 2O2 and O2, the yields of the dehydroguanidinohydantoin lesions become negligible. Under these conditions, the 8-oxoGua radicals do not exhibit any observable reactivities with oxygen (k < 102 M-1 S-1), decay on the time interval of several seconds, and the major reaction products are the spiroiminodihydantoin lesions. The possible biological implications of the 8-oxoGua oxidation are discussed. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.