title: Mechanisms underlying production of double-strand breaks in plasmid DNA after decay of 125I-Hoechst.
abstract: Previously, the kinetics of strand break production by (125)I-labeled m-iodo-p-ethoxyHoechst 33342 ((125)IEH) in supercoiled (SC) plasmid DNA had demonstrated that approximately 1 DSB is produced per (125)I decay both in the presence and absence of the hydroxyl radical scavenger DMSO. In these experiments, an (125)IEH:DNA molar ratio of 42:1 was used. We now hypothesize that this DSB yield (but not the SSB yield) may be an overestimate due to subsequent decays occurring in any of the 41 (125)IEH molecules still bound to nicked (N) DNA. To test our hypothesis, (125)IEH was incubated with SC pUC19 plasmids ((125)IEH:DNA ratio of approximately 3:1) and the SSB and DSB yields were quantified after the decay of (125)I. As predicted, the number of DSBs produced per (125)I decay is one-half that reported previously ( approximately 0.5 compared to approximately 1, +/- DMSO) whereas the number of SSBs ( approximately 3/(125)I decay) is similar to that obtained previously ( approximately 90% are generated by OH radicals). Direct visualization by atomic force microscopy confirms formation of L and N DNA after (125)IEH decays in SC DNA and supports the strand break yields reported. These findings indicate that although SSB production is independent of the number of (125)IEH bound to DNA, the DSB yield can be augmented erroneously by (125)I decays occurring in N DNA. Further analysis indicates that 17% of SSBs and 100% of DSBs take place within the plasmid molecule in which an (125)IEH molecule decays, whereas 83% of SSBs are formed in neighboring plasmid DNA molecules.
authors.affiliation: Department of Radiology, Harvard Medical School, Boston, Massachusetts 02115, USA.
authors.forename: Pichumani
authors.initials: P
authors.lastname: Balagurumoorthy
authors.forename: Kai
authors.initials: K
authors.lastname: Chen
authors.forename: Ralph C
authors.initials: RC
authors.lastname: Bash
authors.forename: S James
authors.initials: SJ
authors.lastname: Adelstein
authors.forename: Amin I
authors.initials: AI
authors.lastname: Kassis
mesh_terms: D001562:Benzimidazoles / Q000737:chemistry* / Q000494:pharmacology*; D004247:DNA / Q000737:chemistry* / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D004249:DNA Damage / Q000187:drug effects*; D004121:Dimethyl Sulfoxide / Q000494:pharmacology; D004587:Electrophoresis, Agar Gel; D007457:Iodine Radioisotopes; D018625:Microscopy, Atomic Force; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D010957:Plasmids / Q000737:chemistry* / Q000187:drug effects* / Q000648:ultrastructure
chemical_list: D001562:Benzimidazoles; D007457:Iodine Radioisotopes; D004247:DNA; C017807:bisbenzimide ethoxide trihydrochloride; D004121:Dimethyl Sulfoxide