A comparison of DNA damages produced under conditions of direct and indirect action of radiation.

Studies using a 32P-postlabelling assay reveal that at both low and high concentrations of hydroxyl radical scavenger, DNA damage from alpha-irradiation is similar to that from gamma-irradiation. At low concentrations of scavenger the identified damages are predominantly thymine glycol (Tg) and phosphoglycolate (pg) lesions, indicative of indirect hydroxyl radical (.OH) action. However, at high concentrations of scavenger (i.e. where direct effects are expected to dominate) a somewhat different pattern of damage is observed after alpha- and gamma-irradiation, with several novel lesions detected. Further experiments, in which the results of DMSO scavenging of irradiated DNA solutions are compared with glycerol scavenging and irradiation of 'dry' solid DNA, demonstrate that both direct effects and scavenger-derived secondary radicals contribute to the different spectra of lesions observed upon irradiation of highly scavenged DNA systems. Generally, irradiation under conditions that favour direct damage result in a lower relative yield of pg, whilst for the highly scavenged systems Tg damages persist. To account for this we propose that Tg are efficiently produced by direct action in these systems. Differences seen upon irradiation of the 'dry' system, compared with those of dilute aqueous solution, suggest that direct action yields a distinct spectrum of lesions.