DNA Strand Breakage Is Correlated with Unaltered Base Release after Gamma Irradiation

Unaltered base release is correlated with strand breakage for γ-irradiated bacteriophage PM2 DNA in aqueous solution at pH 7.4. The yield of DNA strand breaks is determined by the agarose gel electrophoresis method. High-performance liquid chromatography (HPLC) is used to assay the release of unaltered nucleic bases. Previously reported HPLC methods have been updated. Unaltered base release is linear with dose up to 424 Gy, where up to 0.2% of all DNA bases are released. No detectable amounts of unaltered nucleosides are released and, besides unaltered bases, only one other product released from DNA is observed. Base release yields do not reflect the PM2 GC content of 43%. Only 76% of all prompt strand breaks appear to be associated with the release of an unaltered free base, whereby the guanine, cytosine, adenine and thymine yields are 9, 27, 18 and 22% of the prompt strand break yield, respectively. Postirradiation incubation at 37°C for 24 h increases the strand break yield 1.38-fold and the unaltered base release yield 1.76-fold such that 97% of the final strand breaks appear to be associated with the release of an unaltered base, whereby the guanine, cytosine, adenine and thymine yields are 10, 36, 23 and 28% of the final strand break yield, respectively. These data indicate that, given proper conditions, nearly every strand break leads to a base release. The bearing of these results on OH radical attack leading to strand breakage and base release is discussed.