Superoxide dismutases (SODs) are metal-containing enzymes that catalyze the dismutation of superoxide radicals to oxygen and hydrogen peroxide. Three classes of SODs,Fe-SOD, Mn-SOD and CuZn-SOD, have been distinguished according to their bound metals.The Fe-SOD is essentially a prokaryotic enzyme,while the CuZn-SOD is essentially an eukaryotic enzyme. The Mn-SOD is present in both prokaryotes and eukaryotes. The Fe- and MnSODs appear to highly similar in the primary structure, three-dimensional structure and other physiochemical properties, but have no resemblance to CuZn-SOD. The Fe- and Mn-SOD can be traced to a common ancestor, whereas CuZn-SOD has evolved independently at a later time.