Exposure of amino acids, peptides and proteins to radicals, in the presence of oxygen, gives high yields of hydroperoxides. These materials are readily decomposed by transition metal ions to give further radicals. We hypothesized that hydroperoxide formation on nuclear proteins, and subsequent decomposition of these hydroperoxides to radicals, might result in oxidative damage to associated DNA. We demonstrate here that exposure of histone H1 and model compounds to γ-radiation in the presence of oxygen gives hydroperoxides 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 hydroperoxide-derived radicals react readily with pyrimidine DNA bases and nucleosides to give adduct species (i.e. protein-DNA base cross-links). Product 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 oxidized 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 histone H1-hydroperoxide, 280 μM) to approx. 1.4% conversion for free 2′-deoxyguanosine (200 μM), and 0.14% for 2′-deoxyguanosine in DNA (70 μg/ml). Evidence has also been obtained with DNA for reaction at cytosine and thymine, but not adenine; the lack of damage to the latter may result from damage transfer to 2′-deoxyguanosine residues. These studies demonstrate that initial radical-induced damage to nuclear proteins can give rise to subsequent DNA damage; the latter includes both DNA-protein cross-links and formation of oxidized DNA bases.
Macromolecular complexes of polyampholytes
