Heterochromatic proteins specifically enhance nickel-induced 8-oxo-dG formation

7,8-Dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) was measured as an indicator of nickel-induced oxidative base damage in the presence of H2O2. Heterochromatic proteins isolated from Chinese hamster liver cells enhanced the formation of 8-oxo-dG induced by NiCl2 and H2O2 in vitro, whereas euchromatic proteins inhibited this reaction. The inhibitory effect of euchromatic proteins on dG oxidation may be due to the oxygen radical scavenging effects of low molecular weight protein-rich fractions. Gel electrophoresis confirmed that histone H1 was present at a higher concentration in heterochromatin than in euchromatin. It is believed that the presence of nickel-protein complexes in cells is crucial for the formation of reactive oxygen species (ROS). We found that Ni2+ binds to histone H1 and core histones as determined by 63Ni autoradiography of proteins on nitrocellulose membranes. In vitro studies showed that commercially purified histone H1, and to a considerably lesser extent core histones, enhanced the NiCl2 and H2O2 catalyzed formation of 8-oxo-dG in a reaction containing free dG base. Since histone H1 is a lysine- and alanine-rich protein, the levels of 8-oxo-dG induced by NiCl2 and H2O2 were studied in the presence of these amino acids and found to be enhanced by them. These results suggest that nickel may specifically produce oxidative DNA damage in heterochromatin because of the nature of its binding to histone H1 and core histones. This selective oxidation of genetically inactive heterochromatin may explain why nickel compounds which generate oxygen radicals and oxidize DNA bases are inactive in most gene mutation assays.