Abstract
The protective effect of ascorbic acid (AA) against double-strand breaks (DSBs) in DNA caused by various sources of radiation was evaluated by single-molecule observation of giant DNA (T4 DNA; 166kbp) through the use of fluorescence microscopy. Samples were exposed to three different forms of radiation: visible light, γ-ray and ultrasound. With regard to irradiation with visible light in the presence of a photo-sensitizer to induce reactive oxygen, 1 mM AA reduced the damage down to ca.30%. The same concentration of AA decreased the damage caused by γ-ray to ca.70%. In contrast, AA had almost no protective effect against the damage caused by ultrasound. Summarizing the results and discussion we may conclude as that: 1) The significant protective effect of AA against photo-induced damage may reflect the effective diminish of reactive oxygen species (ROS) by AA. 2) As for γ-ray, there exist the protective effect by AA but a little bit weaker than the case of photo irradiation. This may be due to the generation of plural number of ROS by single photon of γ-ray. Surviving ROS against the diminishment effect by AA may cause DSBs. 3) As for the DSBs by ultrasound, physical damage caused by the shockwave through the generation of cavitation dominates. Thus, the chemical effect of AA is considered to be negligible small for the protection of DSBs.
