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 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 alphaDSB and betaDSB. 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), alphaDSB predominate over DSB. On the other hand, if sigma<2x10(8)s(-1), the measured G(alphaDSB) decreased in parallel with G(SSB), i.e., G(alphaDSB)/G(SSB) was a constant. When sigma>2x10(8) s(-1). G(alphaDSB) decreased slightly so that the ratio of alphaDSB to SSB evidently increased. Therefore, alphaDSB 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 approximately 10(5) to approximately 10(9) s(-)1, which shows that the DNA steric structure was not influenced by mannitol.
Shao
C
C
Laboratory of Ion Beam Bioengineering, Institute of Plasmid Physics, Academia Sinica, Hefei, PR China. CLShao@mail.ipp.ac.cn
Saito
M
M
Yu
Z
Z
D004249:DNA Damage / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D016166:Free Radical Scavengers / Q000737:chemistry / Q000378:metabolism; D005609:Free Radicals; D005720:Gamma Rays; D066298:In Vitro Techniques; D016014:Linear Models; D008353:Mannitol / Q000737:chemistry / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism / Q000528:radiation effects*
D016166:Free Radical Scavengers; D005609:Free Radicals; D008353:Mannitol