Oligonucleotides are potent antioxidants acting primarily through metal ion chelation

We report on a rather unknown feature of oligonucleotides, namely, their potent antioxidant activity. Previously, we showed that nucleotides are potent antioxidants in Fe^II/Cu^I/II-H202 systems. Here, we explored the potential of 2'-deoxyoligonucleotides as inhibitors of the Fe ^IIVCu^I/II-induced OH formation from H₂O ₂. The oligonucleotides [d(A)_5,7,20; d(T)_20; (2'-OMe-A)₅] proved to be highly potent antioxidants with IC _5o values of 5-17 or 4885 uM in inhibiting Fe^II/Cu ^II- or Cu^II-induced H₂O₂ decomposition, respectively, thus representing a 40-215fold increase in potency as compared with Trolox, a standard antioxidant. The antioxidant activity is only weakly dependent on the oligonucleotides' length or base identity. We analyzed by matrix-assisted laser desorption/ionization time of flight mass spectrometry and ¹H-NMR spectroscopy the composition of the d(A) ₅ solution exposed to the aforementioned oxidative conditions for 4 min or 24 h. We concluded that the primary (rapid) inhibition mechanism by oligonucleotides is metal ion chelation and the secondary (slow) mechanism is radical scavenging. We characterized the Cu^I-d(A)₅ and Cu^II-d(A)⁷ complexes by ¹H-NMR and ³¹P-NMR or frozen-solution ESR spectroscopy, respectively. Cu ¹ is probably coordinated to d(A)₅ via Nl and N7 of two adenine residues and possibly also via two phosphate/bridging water molecules. The ESR data suggest Cu¹¹ chelation through two nitrogen atoms of the adenine bases and two oxygen atoms (phosphates or water molecules). We conclude that oligonucleotides at micromolar concentrations prevent Fe ^II/Cu^I/II-induced oxidative damage, primarily through metal ion chelation. Furthermore, we propose the use of a short, metabolically stable oligonucleotide, (2'-OMe-A)₅, as a highly potent and relatively long lived (t_1/2 ∼ 20 h) antioxidant.