DNA single strand breakage by H2O2 and ferric or cupric ions: its modulation by histidine

The role of histidine on DNA breakage induced by hydrogen peroxide (H2O2) and ferric ions or by H2O2 and cupric ions was studied on purified DNA. L-histidine slightly reduced DNA breakage by H2O2 and Fe3+ but greatly inhibited DNA breakage by H2O2 and Cu2+. However, only when histidine was present, the addition of EDTA to H2O2 and Fe3+ exhibited a bimodal dose response curve depending on the chelator metal ratio. The enhancing effect of histidine on the rate of DNA degradation by H2O2 was maximal at a chelator metal ratio between 0.2 and 0.5, and was specific for iron. When D-histidine replaced L-histidine, the same pattern of EDTA dose response curve was observed. Superoxide dismutase greatly inhibited the rate of DNA degradation induced by H2O2, Fe3+, EDTA and L-histidine involving the superoxide radical. These studies suggest that the enhancing effect of histidine on the rate of DNA degradation by H2O2 and Fe3+ is mediated by an oxidant which could be a ferrous-dioxygen-ferric chelate complex or a chelate-ferryl ion.