DNA cleavage by hydroxyl radicals generated in the Cu,Zn-superoxide dismutase and hydrogen peroxide system.

To study the catalytic activity of Cu,Zn-superoxide dismutase (SOD) in the generation of hydroxyl radical (.OH) from H2O2, we investigated the mechanism of DNA cleavage mediated by human Cu,Zn-SOD and H2O2. When plasmid DNA was incubated with 5 microM Cu, Zn-SOD and 1.0 mM H2O2, DNA cleavage occurred within 15 min. A spectrophotometric study using a 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) showed that free .OH formation was proportional to concentrations of Cu,Zn-SOD and H2O2. .OH formation and DNA cleavage were inhibited by hydroxyl radical scavengers such as cyanide, azide, and formate. These results indicated that .OH generated early in the peroxidative reaction of Cu,Zn-SOD with H2O2 was implicated in DNA cleavage. Incubation with H2O2 resulted in a time-dependent release of copper ions from the Cu,Zn-SOD molecule. The released copper ions then likely participated in a Fenton-like reaction to produce .OH that may have caused DNA cleavage. Evidence that DTPA protected the DNA cleavage induced by the Cu,Zn-SOD/H2O2 system supports this mechanism. We suggest that DNA cleavage is mediated in the Cu,Zn-SOD/H2O2 system via the generation of .OH by a combination of the peroxidative reaction of Cu,Zn-SOD and the Fenton-like reaction of free copper ions released from oxidatively damaged SOD.