Iron ion-dependent modification of bases in DNA by the superoxide radical-generating system hypoxanthine/xanthine oxidase.

Damage to the bases in DNA produced by the hypoxanthine/xanthine oxidase system in the presence of iron ions was studied. The base products in DNA were measured using gas chromatography-mass spectrometry with selected ion monitoring after acidic hydrolysis of DNA and trimethylsilylation. Products identified were cytosine glycol, thymine glycol, 5,6-dihydroxycytosine, 4,6-diamino-5-formamidopyrimidine, 8-hydroxyadenine, 2,6-diamino-4-hydroxy-5-formamidopyrimidine, and 8-hydroxyguanine. These are typical hydroxyl radical-induced products of the bases in DNA. 2,6-Diamino-4-hydroxy-5-formamidopyrimidine was the major product, followed by 8-hydroxyguanine, in DNA treated with hypoxanthine/xanthine oxidase/Fe3+-EDTA. The use of Fe3+ did not cause as much damage to the bases in DNA as did the use of Fe3+-EDTA. In both systems, the formation of the products was inhibited by superoxide dismutase, catalase, dimethyl sulfoxide, mannitol, and desferrioxamine, but inhibitions were much stronger in the systems containing EDTA. Hence formation of hydroxyl radicals by a superoxide radical-assisted Fenton reaction is proposed to account for the results obtained. 2,6-Diamino-4-hydroxy-5-formamidopyrimidine, 5,6-dihydroxycytosine, 4,6-diamino-5-formamidopyrimidine, and 8-hydroxyguanine were proposed as the products in DNA to measure if one aims to measure DNA products as indices of oxidative DNA damage involving hydroxyl radicals in vivo.