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 approximate to 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. (C) 1996 by Radiation Research Society