The extent of conversion of supercoiled pBR322 plasmid DNA to the open circular and linear forms can be measured by HPLC on a Waters Gen Pak FAX column following in vitro γ irradiation of the DNA. This radiation effect has proven to be useful for the study of the radioprotection of DNA by thiols and other drugs. This system was used with γ irradiation in air at pH 7.0 and physiological ionic strength to compare radioprotection by a series of thiols, disulfides, and thioethers, all having <tex-math>$\sim 10^{8}\ {\rm s}^{-1}$</tex-math> effective hydroxyl radical scavenging rate <tex-math>$(10\ {\rm mm}\ {\rm dm}^{-3}\ \text{drug})$</tex-math> and having net charge (Z) ranging from -2 to +3. All sulfur compounds exhibited substantial protection due to scavenging of hydroxyl radicals in bulk solution but thiols exhibited a 24-fold variation in relative ability to protect the plasmid DNA from strand breaks, as assessed from the dose-response curves: mercaptosuccinate (Z = -2), 0.53; GSH (Z = -1), 0.67; 3-mercaptopropionate (Z = -1) 0.80; mercaptoethanol (Z = 0), 1.00; dithiothrietol (Z = 0), 1.5; cysteamine (Z = +1), 3.7; N-(2-mercaptoethyl)-1,3-diaminopropane (WR-1065, Z = +2), 6.7; <tex-math>$N^{1}\text{-}(2\text{-mercaptoethyl})\text{spermidine}$</tex-math> (WR-35980, Z = +3), 12. Comparison of these results with those obtained using disulfide and thioether radioprotectors indicated that local scavenging of hydroxyl radicals near DNA increases slightly with Z, apparently as a result of variations in thiol concentration near DNA, but this accounts for only a small fraction of the change with Z found for cationic thiols. The marked increase in protection found for cationic thiols was attributed to chemical repair of DNA radicals and was in accord with predictions based upon recently measured rates for chemical repair of DNA radicals and was in accord with predictions based upon recently measured rates for chemical repair of pBR322 radicals. It is concluded that chemical repair of DNA radicals by anionic thiols does not compete with the oxygen fixation reaction in air and that protection by these thiols occurs primarily via the scavenging of hydroxyl radicals. However, chemical repair of DNA radicals is significantly enhanced by counterion condensation for cationic thiols and becomes a significant factor in their ability to protect DNA against radiation damage under aerobic conditions.

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