Histone H1-and other protein- and amino acid-hydroperoxides can give rise to free radicals which oxidize DNA

Exposure of amino acids, peptides and proteins to radicals, in the presence of oxygen, gives high yields of hydroperoxides. These materials are readil y decomposed by transition metal ions to give further radicals. We hypothes ized that hydroperoxide formation on nuclear proteins, and subsequent decom position of these hydroperoxides to radicals, might result in oxidative dam age to associated DNA. We demonstrate here that exposure of histone HI and model compounds to gamma-radiation in the presence of oxygen gives hydroper oxides in a dose-dependent manner. These hydroperoxides decompose to oxygen - and carbon-centred radicals (detected by electron paramagnetic resonance spectroscopy) on exposure to Cu+ and other transition metal ions. These hyd roperoxide-derived radicals react readily with pyrimidine DNA. bases and nu cleosides to give adduct species (i.e. protein-DNA base cross-links). Produ ct analysis has demonstrated that radicals from histone H1-hydroperoxides, and other protein and amino acid hydroperoxides, can also oxidize both free 2'-deoxyguanosine and intact calf thymus DNA to give the mutagenic oxidize d base 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-hydroxy-2'-deoxyguanosine, 8- oxodG). The yield of 7,8-dihydro-8-oxo-2'-deoxyguanosine is proportional to the initial protein-hydroperoxide concentration, and corresponds (for hist one H1-hydroperoxide, 280 mu M) to approx. 1.4% conversion for free 2'-deox yguanosine (200 mu M), and 0.14% for 2'-deoxyguanosine in DNA (70 mu g/ml). Evidence has also been obtained with DNA for reaction at cytosine and thym ine, but not adenine; the lack of damage to the latter may result from dama ge transfer to 2'-deoxyguanosine residues. These studies demonstrate that i nitial radical-induced damage to nuclear proteins can give rise to subseque nt DNA damage; the latter includes both DNA-protein cross-links and formati on of oxidized DNA bases.