The effects of gamma-radiation on DNA.

In this study gamma-radiation-induced DNA strand breaks have been investigated using two systems: a plasmid-based assay, and a whole nuclei-, or whole cell-based, alkaline filter elution assay. Addition of alkali metal halides to DNA irradiated under frozen aqueous conditions were observed to have an effect on the radiosensitivity of the DNA. This, effect, which was not observed with DNA irradiated under fluid aqueous conditions, would appear to be due to two components; a physical component, and a chemical component which is dependent on the anions used. Addition of alkali metal halides appears to increase the volume of the hydrating layer of water which is formed around the DNA when it is frozen. This appears to increase the target volume when it is irradiated, with an observed increase in damage caused by H2O-+ and non-hydrated electrons from the ionisation of the hydration water. The chemical component, which may be a protective or a sensitising effect, is dependent on the anion in the system. The scavenging of electrons produced from the direct action of gamma radiation on DNA has been demonstrated using the intercalator, mitozantrone. This was demonstrated using plasmid DNA irradiated under frozen aqueous solutions, and compared with the e.s.r. spectroscopy results obtained by my coworkers. Finally, the protection of DNA by free-thiols has been investigated. Under indirect, dilute aqueous conditions, the amount of protection to plasmid DNA was observed to increase with increasing positive charge on the thiols. Under direct, frozen aqueous conditions, the radiosensitivity of plasmid DNA by the compounds used was less clear cut. Possible reasons for this contrast are discussed. The effect of a novel aminothiol with a +3 positive charge on the amount of damage to tissue culture cells was also investigated.