Oxygen-effect on strand breaks and specific end-groups in DNA of irradiated thymocytes

Thymocytes were irradiated with fst electrons up to 6 Mrad in the presence and absence of oxygen. The cells were treated before irradiation with a cold shock to prevent any repair reaction durng irradiation. The DNA isolated subsequently was analysed for double-strand breaks (dsb), actual single-strand breaks (asb) and aklali-induced strand breaks (alisb). We observed a linear increase of all types of lesions with dose and an o.e.r. for dsb of 3.6, for ssb of 4.9 and for alisb of 2.1. The data do not deviate significantly from those, measured on thymocytes irradiated without cold shock. In DNA of irradiated thymocytes, the frequency of 3' and 5' hydroxyl and 5' phosphate end-groups was analyzed enzymatically. In both the ssb and alisb, about 11 percent of the terminals carried 5'OH end-groups and 20 to 40 per cent 5' phosphate groups. On the 3' terminals, 60 to 68 per cent of the ssb were identified as 3'OH end-groups, whereas on the alisb only a small amount of 3'OH end-groups was found. The frequency of characterized end-groups showed the same oxygen effect as the corresponding strand breaks. Therefore, in the presence and absence of oxygen, the same mechanism may be responsible for formation of DNA strand breaks in vivo. (author)