Action of Hydrogen Peroxide on Human Fibroblast in Culture

Human fibroblasts in culture lose the capacity of proliferating when exposed to hydrogen peroxide in the concentration range of 1 to 10 μ M . The toxicity of H 2 O 2 to xeroderma pigmentosum cells (XP12RO). defective in excision repair of lesions produced by UV‐irradiation, was about twice as high as to cells proficient in excision repair (VA13). This compound produces single‐strand breaks in intracellular DNA but not in purified DNA. These breaks are in situ physical discontinuities rather than alkali‐labile bonds, and their generation occurs at the same extent at 4°C and 37° indicating that they are not produced by an endonuclease. The results favor the hypothesis that H 2 O 2 reacts in the cell producing a radical species which brings about the formation of DNA single‐strand breaks. These breaks are effectively repaired by both XP12RO and VA13 fibroblasts. The possible reason for the lethality of H 2 O 2 is discussed.