ABSTRACT
Superoxide can be generated either enzymatically or by autoxidation of small molecules. Evolution has taken many liberties with members of both superoxide dismutases (SOD) families. A complex and ever-changing picture has emerged with respect to the targeting of SODs to organelles and across cell membranes. The amino-acid sequences of these proteins show clear homology with the MnSODs of bacteria. Functional mitochondrial MnSODs exist as homotetramers, in contrast with typical bacterial MnSODs, which function as homodimers. The Tetrahymena pyriformis protein is one of only a few characterized eukaryotic FeSODs and may well have been derived relatively recently from an MnSOD. The superoxide-based theory of oxygen toxicity led to two testable hypotheses concerning the role and importance of superoxide dismutase. The hypothesis was based on the distribution of SOD activity among aerobic, aerotolerant, and anaerobic bacteria.
