MODULATION BY L-HISTIDINE OF H2O2-MEDIATED DAMAGE OF CELLULAR AND ISOLATED DNA

The mechanism by which L-histidine modulates H2O2-induced damage to DN A has been investigated by alkaline and neutral gel electrophoresis of cellular DNA, by measuring the conversion of purified supercoiled DNA to its relaxed and linear forms and by the ESR spin-trapping techniqu e. L-Histidine greatly increased the amount of H2O2-mediated DNA singl e-strand breaks. DNA double-strand breaks were produced only in cells exposed to H2O2 and L-histidine. The addition of a cell permeable chel ator such as o-phenanthroline (unlike EDTA, DTPA and desferrioxamine) prevented both DNA single- and double-strand breakage induced by H2O2 plus L-histidine. In vitro, the profile of the dose-response curve for the ferrous iron-mediated, H2O2-dependent DNA nicking was modified by the addition of L-histidine. At low H2O2 concentrations, correspondin g to the maximum extent of DNA cleavage, L-histidine was protective. A t higher H2O2 concentrations L-histidine enhanced the formation of DNA single-stand breaks and produced DNA double-strand breaks. The increa se in H2O2-mediated DNA nicking by L-histidine depended on the L-histi dine:Fe(II) ratio, the maximal rate occurring at a molar ratio of 10(3 ):1 and being independent of the concentration of DNA. Thus, it appear ed that intracellular iron mediated both DNA single- and double-strand breaks induced by H2O2 plus L-histidine. Results of ESR experiments s eemed to rule out the involvement of the hydroxyl radical by itself in DNA cleavage mediated by the L-histidine:Fe(II):H2O2 system.