Chemistry and biochemistry of superoxide dismutases

The univalent reduction of oxygen to the superoxide radical is a commonplace event in biological systems, and the superoxide dismutases, which catalytically scavenge this radical, are the primary defence against its potential cytotoxicity. The superoxide radical and hydrogen peroxide, can interact to generate the hydroxyl radical. Superoxide dismutases are metalloenzymes that can prevent the generation of hydroxyl radical by keeping the level of superoxide radical vanishingly low. Superoxide dismutases are essential for the survival of all oxygen metabolizing organisms. There are three types of superoxide dismutases: the manganese-containing, the iron-containing, and the copper-zinc-containing superoxide dismutases. The copper-zinc containing superoxide dismutases have generally been isolated from eukaryotic cells except for the enzyme from the symbiotic marine bacterium Photobacterium leiogenthi. The copper-zinc containing superoxide dismutases, from different sources, have a molecular weight of about 32,000, are composed of two identical subunits, and contain one atom of copper and one atom of zinc per subunit. The copper participates in the catalytic activity of the enzyme, while the zinc plays only a structural role. The enzyme has been resolved reversibly. Superoxide dismutases have been seen to provide protection against oxygen toxicity, against compounds that cause exacerbation of oxygen toxicity, against ionizing radiation, and also against the damaging sequelae of prolonged inflammation.