The effect of superoxide anion (O
2-) on killing of cells was studied.
E.
coli was killed by a non-enzymatical O
2--generating system,
i.e. a phenazine methosulfate (PMS)-NADH system. Both
polA and
recA repair-deficient mutants were more sensitive to O
2- than wild type cells. Furthermore, using the
polA mutant strain, it was clearly shown that incubation in growing medium increased survival, indicating the involvement of a
rec-dependent repair system. It was concluded that the direct cause of cell death induced by O
2- was DNA damage. When a scavenger of the active oxygen, such as superoxide dismutase (SOD) or catalase, was present in the reaction buffer during O
2- treatment, the survival of cells increased. Of especial note, cells were completely protected by addition of catalase. It was suggested that H
2O
2 produced from O
2- and H
2O was a molecule more toxic to
E. coli cells than the O
2- itself in this system. When SOD and catalase were induced inside cells by adding methyl-viologen in the medium, a protective effect against O
2- appeared. Cells treated with methyl-viologen were also resistant, although slightly, to aerobic γ-ray irradiation, suggesting that the scavenger of active oxygen in cells was partly responsible for protection against γ-ray irradiation. It is deduced that O
2- and/or H
2O
2 generated in cells may be one of the causes of cell damage by aerobic γ-ray irradiation.
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