Formation of single- and double-strand breaks of pBR322 plasmid irradiated in the presence of scavengers

By the method of gel electrophoresis, radiation-induced DNA single- and double-strand breaks (SSB, DSB) were studied with a model system of pBR322 solution in vitro in the presence of (OH)-O-. radical scavengers, mannitol and TE (10(-2) mol dm(-3) Tris and 10(-3) mol dm(-3) ethylene diamine tetra-acetic acid). Experiments showed that SSB resulted from one-hit events of radiation energy deposition and DSB resulted from both one-hit and two-hit energy deposition events and so were distinguished into two classes of alpha DSB and beta DSB. Moreover, alpha/beta, where alpha is the number of DSB per unit dose induced in one irradiation event and beta the number of DSB per unit squared dose induced by the combination of two independent SSB, was related to the scavenging capacity, sigma, and for sigma>10(8) s(-1), alpha DSB predominate over DSB. On the other hand, if sigma<2x10(8) s(-1), the measured G(alpha DSB) decreased in parallel with G(SSB), i.e., G(alpha DSB)/G(SSB) was a constant. When sigma>2x10(8) s(-1), G(alpha DSB) decreased slightly so that the ratio of alpha DSB to SSB evidently increased. Therefore, alpha DSB could be induced by the radical transfer mechanism for sigma<2x10(8) s(-1) and contrarily produced by the local multiply damaged sites (LMDS) mechanism for sigma larger than this value. In addition, the distance for two independent complementary SSB forming a DSB was deduced, but no apparent variation of it was found in the wide sigma range from similar to 10(5) to similar to 10(9) s(-1). which shows that the DNA steric structure was not influenced by mannitol.