DIFFERENTIAL-EFFECTS OF HISTIDINE ON HYDROGEN PEROXIDE-INDUCED BACTERIAL KILLING AND DNA NICKING IN-VITRO

The hydrogen peroxide dose-response curves for Escherichia coli killin g and DNA nicking in vitro display remarkably similar bimodal patterns . The concentrations of the oxidant resulting in maximum mode one kill ing, however, exceeds by two orders of magnitude those resulting in th e mode one DNA nicking response. Addition of histidine differentially affects the experimental curves describing the dose-dependency for bac terial killing and DNA damage in vitro. Indeed, the lethal effect elic ited by the oxidant in the presence of the amino acid is strictly conc entration-dependent and thus the inactivation curve loses its bimodal character. In marked contrast, histidine abolishes DNA damage generate d by low concentrations of hydrogen peroxide(< 100 mu M) in the in vit ro system (the mode one DNA nicking response) but greatly increases DN A damage produced by concentrations of the oxidant higher than 1 mM (t he mode two DNA nicking response). Experimental results also suggest t hat treatment of covalently closed circular double-stranded supercoile d DNA with hydrogen peroxide, in the presence of both histidine and ir on, may result in the formation of DNA double strand breakage, a type of lesion which is not efficiently produced by the oxidant in the abse nce of the amino acid. Taken together, the above results indicate that histidine differentially affects the in vitro DNA cleavage and E. col i lethality induced by hydrogen peroxide and suggest that different mo lecular events mediate mode one DNA nicking in vitro and mode one kill ing of bacterial cells.