Zinc(II) protects against metal-mediated free radical induced damage: studies on single and double-strand DNA breakage.

M13 DNA was used as a source for single and double-stranded DNA. Free radical-induced damage to single and double stranded DNA was caused by ascorbate/iron and ascorbate/copper oxidative systems. The degree of breakage was estimated by running samples on an agarose gel and staining with ethidium bromide, followed by photographic analysis. DNA breakage was dependent on time and concentration of iron or copper ions. Zinc ions protected against damage caused by iron/ascorbate both to single-stranded and double-stranded DNA. In contrast, in the copper/ascorbate system zinc ions protected only against the double-stranded DNA (replicative form of M13) breakage, and not against copper-mediated single-stranded DNA breakages. It seemed to amplify the efficiency of breakage. The protection provided to the replicative form in the copper/ascorbate system is much less effective than the protection to DNA in the iron/ascorbate system. These results support the notion that redox-inactive metal ions, that compete for iron or copper binding sites, could provide protection against transition metal-mediated and free radical-induced damage.