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Cytotoxic superoxide radical(O_2^-) is a common intermediate of oxygen reduction reaction in the respiring cells and therefore these cells should be protected against the cytotoxic effect of superoxide radicals. At the present time, three types of superoxide dismutase, copper- and zinc-containing, iron-containing and manganese- containing superoxide dismutase, have been isolated from both eukaryotic and prokaryotic cells. The purification and properties of cytosolic superoxide dismutase of human term placenta have been reported previously from this laboratory. In this study, therefore, the experiment was carried out to characterize mitochondrial .superoxide dismutase of the placenta. Mitochondria of human placenta were isolated from human term placenta cell homogenate by differential centrifugation and the mitochondrial superoxide dismutase was purified by DEAE-cellulose column chromatography. The results were summarized as follows: 1. The distribution of superxode dismutase in terms of activity, in the cytosolic and mitochondrial fractions was found being 85%(7.4 units/g) and (1.3 units/g) respectively. 2. The distribution of mitochondrial superoxide dismutase in intermembrane space and matrix of mitochondria was found being 47% and 53% respectivly. 3. The mitochondrial superoxide dismutase was purified approximately 52 fold. The molecular weight of the purified enzyme was estimated to be 85,000 by Sephadex G-100 gel filtration method. 4. Purified mitochondrial superoxide dismutase contained 1.82 atoms of manganese per molecule and was very similar to the superoxide dismutase previously isolated from other eukaryotes. 5. The activity of manganese-containing superoxide dismutase was inhibited about 10% by 1mM cyanide but it is inhibited as much as 88% by ethanol-chloroform(5:1, v/v) mixture. 6. The ultraviolet absorption spectrum of purified mitochondrial superoxide dismutase was also similar to those of the previously prepared eukaryotic enzyme. 7. The activity of xanthine oxidase of human term placenta was found to be 23.6 units/g and the relative activity in the cytosolic and mitochondrial xanthine oxidase was 83% and 17%, respectively.
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