The effect of serum albumin on the radiolysis of DNA studied by constant field electrophoresis and compared to alterations caused by low molecular weight OH. scavengers.

After radiolysis of calf thymus DNA in 10(-2) mol dm-3 phosphate buffer at pH7 under N2, N2O and air the yields of double-strand breaks (dsb) have been determined by constant field electrophoresis. Double-strand (dsb) breaks were formed according to a linear-quadratic relationship with dose showing a lower G-value under aerobic than under anaerobic conditions (G (air) = 1.4 nmolJ-1; G (N2) = 2.1 nmolJ-1; G (N2O) = 4.9 nmolJ-1). To test the reliability of this system the effect of low molecular weight OH. scavengers which were already used in comparable work with plasmid DNA were studied. The results with plasmid DNA and calf thymus DNA obtained by different techniques of electrophoresis agreed quite well. Under N2 more protection was obtained with ethanol than with DMSO or with t-butanol. Under air, double-strand breakage was further decreased and reached the same level with all of these scavengers. Furthermore the constant field electrophoresis gives similar results as the low-angle light scattering technique for radiation induced double strand breakage of calf thymus DNA. When BSA was used at the same scavenger capacity as the low molecular weight scavengers, the protection against double strand breakage was less if radiolysis was carried out in the presence of proteins. Under anaerobic conditions the protection factor was 13 in the presence of BSA, while with DMSO or t-butanol this factor was about 100 and with ethanol 300. In contrast to the low molecular weight OH. scavengers oxygen enhanced radiation-induced double-strand breakage with BSA. It is assumed that protein peroxyl radicals may cause strand breakage.