HETEROCHROMATIC PROTEINS SPECIFICALLY ENHANCE NICKEL-INDUCED 8-OXO-DGFORMATION

7,8-Dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) was measured as an indi cator of nickel-induced oxidative base damage in the presence of H2O2. Heterochromatic proteins isolated from Chinese hamster liver cells en hanced the formation of 8-oxo-dG induced by NiCl2 and H2O2 in vitro, w hereas euchromatic proteins inhibited this reaction, The inhibitory ef fect of euchromatic proteins on dG oxidation may be due to the oxygen radical scavenging effects of low molecular weight protein-rich fracti ons, Gel electrophoresis confirmed that histone H-1 was present at a h igher concentration in heterochromatin than in euchromatin. It is beli eved 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 H-1 and core histones as determined by Ni-63 autora diography of proteins on nitrocellulose membranes. In vitro studies sh owed that commercially purified histone H-1, and to a considerably les ser extent core histones, enhanced the NiCl2 and H2O2 catalyzed format ion of 8-oxo-dG in a reaction containing free dG base, Since histone H -1 is a lysine- and alanine-rich protein, the levels of 8-oxo-dG induc ed 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 m ay specifically produce oxidative DNA damage in heterochromatin becaus e of the nature of its binding to histone H-1 and core histones, This selective oxidation of genetically inactive heterochromatin may explai n why nickel compounds which generate oxygen radicals and oxidize DNA bases are inactive in most gene mutation assays.