Rejoining of gamma-radiation-induced single-strand breaks in plasmid DNA by human cell extracts: dependence on the concentration of the hydroxyl radical scavenger, Tris.
The rejoining of single-strand breaks induced by gamma irradiation in plasmid DNA under different scavenging conditions is described using human cell extracts. As the scavenging capacity of the irradiated solution increases from 1.5 x 10(7) to 3 x 10(8) s-1 using Tris-HCl as a scavenger, the ratio of single- to double-strand breaks is reduced from approximately 70:1 to 40:1. After irradiation, a proportion of DNA molecules have no initial strand breaks but contain damage that is converted to strand breaks when incubated either at 37 degrees C or in the presence of cellular extract. Repair of damage by the extracts is dependent upon the scavenging capacity of the irradiated solution. Optimal rejoining is observed when the scavenging capacity is < 1.5 x 10(7) s-1, and results in the repair of some initial strand breaks. As the scavenging capacity increases to 3 x 10(8) s-1 the proportion of breaks repaired is significantly reduced. The relative increase in the yield of double-strand breaks and reduced repairability of single-strand breaks at a scavenging capacity of 3 x 10(8) s-1 is consistent with the concept that the severity of damage increases upon increasing the scavenger concentration.
Medical Research Council Radiation and Genome Stability Unit, Harwell, Didcot, Oxon, United Kingdom.
P S
PS
Hodgkins
M P
MP
Fairman
P
P
O'Neill
D002460:Cell Line; D002474:Cell-Free System; D004249:DNA Damage; D004260:DNA Repair / Q000187:drug effects; D004307:Dose-Response Relationship, Radiation; D004492:Edetic Acid / Q000494:pharmacology; D004926:Escherichia coli / Q000235:genetics; D016166:Free Radical Scavengers / Q000494:pharmacology*; D005720:Gamma Rays; D006801:Humans; D017665:Hydroxyl Radical; D007700:Kinetics; D010957:Plasmids / Q000187:drug effects / Q000528:radiation effects*; D014325:Tromethamine / Q000494:pharmacology*
D016166:Free Radical Scavengers; D014325:Tromethamine; D017665:Hydroxyl Radical; D004492:Edetic Acid