Mitochondrial Superoxide Dismutase

Mitochondria were isolated from chicken liver and were then separated into fractions containing the outer membrane, the soluble components of the intermembrane space, the inner membrane, and the soluble enzymes of the matrix space. The manganosuperoxide dismutase was found primarily in the matrix space whereas the cuprozinc superoxide dismutase, previously isolated from the cytosol, was found in the intermembrane space. No activity was associated with the membranous fractions. A strain of petite mutants of Saccharomyces cerevisiae which lacks mitochondrial DNA, was examined and found to contain both the mangano- and the cuprozinc types of superoxide dismutase. This result indicates that the manganoenzyme, although localized in the mitochondrial matrix, is actually made under the directions of the nuclear genome and that its biosynthesis therefore probably takes place in the cytosol.