The Influence of Glutathione on Single-strand Breakage in Single-stranded DNA Irradiated in Aqueous Solution in the Absence and Presence of Oxygen

Summary

The yields of strand break formation (G_ssb) in single-stranded DNA (ssDNA) initiated by radiation-generated OH radicals have been determined using the method of low-angle laser light scattering (LALLS). The irradiations were carried out in aqueous, N₂O-saturated solutions in the absence and presence of oxygen and at different concentrations of glutathione (GSH). GSH exhibits a protective effect, which is shown to be mainly due to OH radical scavenging. To quantify this, the rate constants for the reactions of OH with GSH and DNA have been redetermined under our experimental conditions. The values obtained were 9·0 × 10⁹ and 4·5 × 10⁸ dm³ mol⁻¹ s⁻¹, respectively. From the G_ssb values obtained under anoxic conditions it is concluded that GSH protects against strand breakage (in addition to OH scavenging) by reacting with DNA radicals in competition to strand break formation. The rate constant of the repair reaction is 8·1 × 10⁴ dm³ mol⁻¹ s⁻¹ at room temperature. For irradiations carried out in the presence of oxygen the rate of strand break formation is determined by the decay of DNA peroxyl radicals. Under these conditions we observed no protective effect of GSH apart from OH radical scavenging. The results are compatible with those that are expected from the oxygen-fixation hypothesis.