The L-histidine-mediated enhancement of hydrogen peroxide-induced DNA double strand breakage and cytotoxicity does not involve metabolic processes.

The cytotoxic response of Chinese hamster ovary (CHO) cells to challenge with hydrogen peroxide was highly dependent upon the temperature of exposure, being markedly higher at 37 degrees than at 4 degrees C. Increasing intracellular levels of L-histidine prior to challenge with hydrogen peroxide increased the toxicity elicited by the oxidant at both physiologic and ice-bath temperatures. The effect of the amino acid, however, was more pronounced under conditions at 4 degrees C, as compared to 37 degrees C. Indeed, at 4 degrees C the oxidant was nontoxic at submillimolar levels and pre-exposure to L-histidine restored cytotoxicity to levels slightly higher than those observed after treatment at 37 degrees C (in the micromolar range). Pre-exposure to the amino acid increased the production of DNA double-strand breaks (DSBs) elicited by treatment with the oxidant both at 37 degrees and 4 degrees C. A remarkable correlation was found when the level of this lesion was plotted against the cytotoxic response observed using different concentrations of L-histidine or hydrogen peroxide, or treating the cells with the oxidant either at 37 degrees or 4 degrees C, thus suggesting the existence of a cause-effect relationship. The overlapping correlation curves obtained with cells challenged with the oxidant at 4 degrees or 37 degrees C also suggest that similar molecular mechanisms mediate the formation of DNA DSBs under both experimental conditions. Two lines of evidence provide experimental support for this inference: (1) the kinetics of repair of DNA DSBs generated at 37 degrees or 4 degrees C were virtually superimposable; this would suggest that the same repair pathway(s) is/are responsible for the removal of DNA DSBs generated at the two temperatures; and (2) the size distribution of double-stranded DNA fragments produced under the two treatment conditions, resulting in a similar cytotoxic response, was basically identical. This is indicative of remarkable similarities in the topology of chromosomal domains where DSBs are generated. Overall, the results presented in this paper provide further experimental evidence supporting the notion that DNA DSBs are responsible for the L-histidine-mediated enhancement of hydrogen peroxide-induced cytotoxicity, and demonstrate that the mechanism whereby the amino acid enhances the ability of hydrogen peroxide to produce DNA double strand breakage and cell killing does not depend on cellular metabolism and/or energy-dependent reactions.