Reactive Oxygen Species Generated from the Reaction of Copper(II) Complexes with Biological Reductants Cause DNA Strand Scission

The generation of hydroxyl radicals (•OH) from the reaction of Cu(II) complexes with biological reductants such as ascorbic acid, glutathione, acetylcysteine, and hydroquinone was confirmed by spin-trapping experiments using electron spin resonance (ESR).The following Cu(II) complexes were used: Cu(II)-(CyHH)2(CyHH, cyclo(l-histidyl-l-histidyl)), Cu(II)(OP)2(OP,o-phenanthroline), Cu(II)(HGG) (HGG,l-histidylglycylglycine), and Cu(II)(en)2(en, ethylenediamine). The methyl radical adduct of α-(pyridyl-4-N-oxide)-N-tert-butylnitrone (POBN–CH3) was obtained from the reaction of ascorbic acid with all Cu(II) complexes used here in the presence of a spin trap, POBN, and dimethyl sulfoxide, indicating the generation of•OH. Glutathione,N-acetylcysteine, and hydroquinone reacted with both Cu(II)(CyHH)2and Cu(II)(OP)2to generate POBN-CH3, while these reductants did not react with either Cu(II)(HGG) or Cu(II)(en)2. Interestingly, the formation of POBN–CH3in the reaction of Cu(II)-(CyHH)2with glutathione orN-acetylcysteine was found only at a Cu(II)(CyHH)2/glutathione or Cu(II)-(CyHH)2/N-acetylcysteine ratio of 1. The DNA strand scission caused by reaction mixtures of Cu(II) complexes with reductants was investigated under thesame conditions as the ESR spin-trapping experiments. Addition of ascorbic acid to mixtures of these four Cu(II) complexes and DNA resulted in DNA strand breakage. Hydroquinone plus Cu(II)(CyHH)2also caused DNA strand scission. In addition, DNA strand breakage was observed with the reaction of Cu(II)(OP)2with glutathione,N-acetylcysteine, and hydroquinone. In contrast, reaction mixtures of glutathione,N-acetylcysteine, or hydroquinone with Cu(II)-(HGG) or Cu(II)(en)2did not cause DNA strand scission within the concentration range used. The results obtained here suggest that there is a good correlation between POBN-CH3formation and DNA strand scission. Thus, DNA strand scission may be caused by•OH generated from the reaction of some Cu(II) complexes with biological reductants under aerobic conditions. Since ascorbic acid, glutathione, andN-acetylcysteine are present in living cells, some Cu(II) complexes may be capable of initiating DNA damage in the presence of these reductants.