PROTONATION OF THE IMIDAZOLE RING PREVENTS THE MODULATION BY HISTIDINE OF OXIDATIVE DNA-DEGRADATION

When supercoiled DNA is incubated with Fe(II) at pH 7 in the presence of hydrogen peroxide, the rate of nicking first increases with increas ing H2O2 concentration to reach a maximum, then decreases and eventual ly increases again. When 0.1 mM histidine is added at neutral pH at lo w H2O2 concentration (< 3 mM), it hinders the nicking of DNA; when it is added at high H2O2 concentrations (> 10 mM), it enhances the rate o f nicking. When similar experiments are performed at slightly acidic p H (4.5) the biphasic behavior is maintained, independent of the presen ce of histidine. One can conclude that the protonation of imidazole (p K=5.9) abolishes the capability of histidine to modulate the oxidative degradation of DNA. Results of electron spin resonance experiments su ggest that at low H2O2 concentration, the protective effect of histidi ne could be the consequence of its capability to bind OH. radicals.