Labile iron pool (LIP) constitutes a crossroad of metabolic pathways of iron-containing compounds and is midway between the cellular need for iron, its uptake and storage. In this study we investigated oxidative DNA damage in relation to the labile iron pool in a pair of mouse lymphoma L5178Y (LY) sublines (LY-R and LY-S) differing in sensitivity to hydrogen peroxide. The LY-R cells, which are hydrogen peroxide-sensitive, contain 3 times more labile iron than the hydrogen peroxide-resistant LY-S cells. Using the comet assay, we compared total DNA breakage in the studied cell lines treated with hydrogen peroxide (25 microM for 30 min at 4 degrees C). More DNA damage was found in LY-R cells than in LY-S cells. We also compared the levels of DNA lesions sensitive to specific DNA repair enzymes in both cell lines treated with H(2)O(2). The levels of endonuclease III-sensitive sites and Fapy-DNA glycosylase-sensitive sites were found to be higher in LY-R cells than in LY-S cells. Our data suggest that the sensitivity of LY-R cells to H(2)O(2) is partially caused by the higher yield of oxidative DNA damage, as compared to that in LY-S cells. The critical factor appears to be the availability of transition metal ions that take part in the OH radical-generating Fenton reaction (very likely in the form of LIP).