Partially reduced species of oxygen, including the superoxide anion radical, hydroxyl radical, and hydrogen peroxide, are generated intracellularly as by-product of oxygen metabolism. These reactive oxygen species cause peroxidation af membrane lipids, denaturation of proteins, and modification of DNA, all of which ultimately can lead to cell death. In mammals, cell damage induced by partially reduced oxygen species can also initiate local inflammatory responses, which then lead to further oxidant-mediated tissue injury.