Extracellular iron (II) can protect cells from hydrogen peroxide

We hypothesized that exposure of cells to H2O2 plus Fe2+ would increase formation of cell-derived lipid peroxides that would inactivate prostaglandin H synthase, resulting in decreased prostaglandin synthesis. Therefore, we treated human endothelial cells with 0-100 microM H2O2 followed immediately by addition of 0-200 microM Fe2+. After oxidant exposure, cells were stimulated with 20 microM arachidonic acid to induce prostaglandin I2 (PGI2) synthesis. Adding 100 microM H2O2 prior to arachidonic acid decreased PGI2 synthesis more than 80%. However, to our surprise, the addition of Fe2+, in increasing amounts, progressively protected PGI2 synthesis against the harmful effects of H2O2. A ratio of one part H2O2 to two parts Fe2+ offered almost complete protection, whereas Fe3+ did not protect PGI2 synthesis from H2O2. We found that 100 microM H2O2 was not cytolytic; however, 250 microM H2O2 was cytolytic; Fe2+ protected against this cytotoxicity. In addition, extracellular Fe2+ prevented the rise in intracellular calcium caused by H2O2 and extracellular Fe2+ preserved intracellular glutathione in H2O2-exposed cells. Electron paramagnetic resonance spin trapping demonstrated that extracellular Fe2+ generated the hydroxyl free radical, HO. outside the cell. We speculate that extracellular Fe2+ protects the intracellular space from H2O2 by initiating the Fenton reaction outside the cell. This reductive cleavage of H2O2 generates HO. in the extracellular space, where much of the HO. will react with noncellular components, thereby protecting the cell interior.