Measurements of nucleic bases released after gamma irradiation of DNA in solution in air

The release of unaltered nucleic bases from gamma-irradiated DNA in a dilute buffered aqueous solution was studied in both salmon sperm and superhelical viral DNA. Analyses of freed bases were made by high-performance liquid chromatography. An elution protocol was developed for maximum separation of the four nucleic bases and nucleosides with a sensitivity of 10-20 pmol of nucleic base. It was found that: (i) both prompt and delayed release of bases postirradiation occur in both types of DNA; (ii) these yields (G-values) were measured to be 10-15 times higher for the salmon sperm DNA in comparison to the SV40 DNA; (iii) the A-T/G-C ratio in the DNA was not reflected in the ratios of the released base; and (iv) based on measurements made by us of DNA strand breaks in SV40 DNA (unpublished results), less than half of all breaks result in the release of an undamaged base