Nickel(II)- and cobalt(II)-dependent damage by hydrogen peroxide to the DNA bases in isolated human chromatin

Nickel compounds are known to be carcinogenic to humans and animals. Cobalt compounds produce tumors in animals and are probably carcinogenic to humans. The mechanisms of the carcinogenicity of these metal compounds, however, have remained elusive. In the present work, we have investigated the ability of Ni(II) and Co(II) ions in the presence of H2O2 to cause chemical changes in DNA bases in chromatin extracted from cultured cells of human origin. Eleven modified DNA bases in chromatin were identified and quantitated by the use of gas chromatography-mass spectrometry. 2-Hydroxyadenine (isoguanine), which has not previously been shown to occur DNA or chromatin, was also identified. Products identified were typical hydroxyl radical-induced products of DNA bases, suggesting that the hydroxyl radical was involved in their formation. This idea was supported by partial inhibition of product formation by typical scavengers of hydroxyl radical. Partial inhibition of product formation indicated a possible "site-specific" formation of hydroxyl radical by unchelated Ni(II) and Co(II) ions bound to chromatin. Although treatment of chromatin for 1 h with Co(II)/H2O2 caused formation of significant amounts of products, treatment with Ni(II)/H2O2 required incubation times of more than 5 h and an increase in Ni(II) concentration before increases in product amounts above background levels became detectable. In both cases, ascorbic acid did not increase product yields. Glutathione at a physiologically relevant concentration had little overall effect on DNA base modification. Superoxide dismutase increased the yields of most products. Chelation of Ni(II) and Co(II) ions with EDTA almost completely inhibited product formation. Ni(II) in the presence of H2O2 produced greater base damage to the DNA in chromatin than to isolated DNA, unlike other metal ions tested. DNA damage in chromatin caused by Ni(II) and Co(II) ions in the presence of H2O2 may contribute to the established genotoxicity and carcinogenicity of these metal ions.