Antioxidant activity for the ten sulfur compounds was also determined for iron-mediated DNA damage using similar gel electrophoresis methods with FeII and H2O2. The gel in Fig. 4 shows that GSSG inhibits iron-mediated DNA damage (lanes 9 and 10), with a maximum inhibition of 50% of oxidative DNA damage (p = 0.003) at 10,000 lM. In addition to GSSG, GSH and 3-carboxypropyl disulfide also inhibit iron-mediated DNA damage: GSH inhibits 23% of oxidative damage at 10,000 lM (p = 0.04), and 3carboxypropyl disulfide inhibits 22% of iron-mediated DNA damage at 1000 lM (p = 0.03). The significant inhibition of ironmediated DNA damage by both reduced and oxidized glutathione is surprising, since the other amino acid compounds have no effect on this type of damage. These results suggest an additional reason for the presence of large concentrations of glutathione in vivo, since this antioxidant would prevent both copper- and iron-mediated oxidative damage at high concentrations. The observed antioxidant activity of 3-carboxypropyl disulfide with FeII/H2O2 was unexpected, since this compound did not prevent oxidative DNA damage with CuI/H2O2