Effects of ionizing radiation of DNA

The radiosensitivity of samples of bacterial DNA of several different base compositions was determined in aqueous solution. The number of single- and double-strand breaks induced by 60 'Co γ-rays was independent of composition over a range of 38-72% guanine plus cytosine. An endonuclease from M. luteus that converts nonbreak damage in irradiated DNA (probably base alteration) into easily-detectable strand breaks was used to assess the number of such lesions in the same DNA samples. There were 10-30% more sites susceptible to this enzyme in DNA rich in adenine and thymine; the extent of the difference depended on the irradiation conditions. Strand breakage was independent of base composition for all conditions tested. The significance of these results in relation to the reported increase in radiosensitivity of GC-rich organisms is discussed.

The covalent circular DNA from bacteriophage PM2 was used in a sedimentation analysis system to quantitate three distinct classes of lesions: (1) strand breaks, (2) alkali-labile bonds, and (3) sites susceptible to the y-endonuclease from M. luteus. This system was employed to study radiation damage to DNA (1) indirectly by the action of water radicals in dilute solution, (2) by direct deposition of photon energy in DNA, and (3) indirectly by secondary organic radicals. Each process probably contributes significantly to radiation action in vivo.

In dilute aqueous solution the OH-radical causes mostly strand breaks and alkali-labile bonds, but few if any enzyme sites. Conversely, the electron causes many enzyme sites, a few alkali-labile bonds and probably no strand breaks. The hydrogen atom accounts for a minor part of the total damage. The effect of oxygen in this condition is protection from radiation by scavenging of the aqueous electron.

In highly-protected conditions where the direct action of radiation becomes important the spectrum of damage is similar to the indirect effects, except that a new form of alkali-labile bonds (perhaps apurinic and apyrimidinic sites) that are susceptible to the γ-endonuclease are induced. Oxygen has no effect in this condition.

In the presence of thymidine all three classes of damage to DNA appear to be mediated by the thymidine-OH-radical adduct. Oxygen enhances each kind of damage 3- to 7-fold over the anoxic response. This oxygen effect apparently requires the aqueous electron, and is prevented by chloride. It seems likely that some critical sequence of reactions involving thymidine-peroxyl radicals is necessary for this potentiation of the oxygen effect. Irradiation in the presence of adenosine showed much less potentiation, and phenylalanine was completely unreactive in this respect. The probable importance of radiation damage mediated by organic radicals in vivo is discussed.