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)-H(2)O(2) systems. Here, we explored the potential of 2'-deoxyoligonucleotides as inhibitors of the Fe(II)/Cu(I/II)-induced *OH formation from H(2)O(2). The oligonucleotides [d(A)(5,7,20); d(T)(20); (2'-OMe-A)(5)] proved to be highly potent antioxidants with IC(50) values of 5-17 or 48-85 microM in inhibiting Fe(II)/Cu(I)- or Cu(II)-induced H(2)O(2) decomposition, respectively, thus representing a 40-215-fold 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 (1)H-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 (1)H-NMR and (31)P-NMR or frozen-solution ESR spectroscopy, respectively. Cu(I) is probably coordinated to d(A)(5) via N1 and N7 of two adenine residues and possibly also via two phosphate/bridging water molecules. The ESR data suggest Cu(II) 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) approximately 20 h) antioxidant.