Radioprotection of DNA by Thiols: Relationship between the Net Charge on a Thiol and Its Ability to Protect DNA

Release of free bases from calf thymus DNA upon irradiation in aerated$0.1\ {\rm mol}\ {\rm dm}^{-3}\ {\rm NaClO}_{4}$ at pH 7 has been measured by HPLC and shown to be markedly influenced by the presence of thiols during irradiation. The ability of thiols to protect DNA was shown to depend upon the net charge (Z) at pH 7 in the order WR1065 (Z = +2) > cysteamine (Z = +1) > 2-mercaptoethanol (Z = 0) ≅ dithiothreitol (Z = 0) > GSH (Z = -1) ≅ 2-mercaptoethanesulfonic acid (Z = -1) ≅ 2-mercaptosuccinate (Z = -2). A similar dependence of protection upon net charge was found for disulfides: cystamine (Z = +2) > 2-mercaptoethyl disulfide (Z = 0) > GSSG (Z = -2). Protection by WR1065, but not by 2-mercaptoethanol or GSH, was found to decrease significantly with increasing ionic strength. Protection by WR1965 and GSH was not markedly dependent upon pH between pH 6 and 8. The results are explained in terms of electrostatic interaction of the thiols with DNA, leading to high concentrations of cations near DNA, which allow them to scavenge hydroxyl radicals and repair DNA radicals effectively and to low concentrations of anionic thiols near DNA, which limit their effectiveness as protectors. Poly(dG,dC) and calf thymus DNA exhibited comparable release of G and C upon changing from 0.1 to$0.7\ {\rm mol}\ {\rm dm}^{-3}\ {\rm MgSO}_{4}$. Since this change causes poly(dG,dC), but not calf thymus DNA, to undergo a change from the B-form to the Z-form of DNA, both forms must have a comparable susceptibility to radiation-induced base release.