Mitochondrial superoxide decreases yeast survival in stationary phase

Yeast lacking mitochondrial superoxide dismutase (MnSOD) display shortened stationary-phase survival and provide a good model system for studying mito chondrial oxidative damage. We observed a marked decrease in respiratory fu nction preceding stationary-phase death of yeast lacking MnSOD (sod2 Delta) . Agents (mitochondrial inhibitors) that are known to increase or decrease superoxide production in submitochondrial particles affected stationary-pha se survival in a manner inversely correlated with their effects on superoxi de production, implicating superoxide in this mitochondrial disfunction, Si milar but less-dramatic effects were observed in wild-type yeast. The activ ities of certain mitochondrial enzymes were particularly affected. In sod2 Delta yeast the activity of aconitase, a 4Fe-4S-cluster-containing enzyme l ocated in the matrix, was greatly and progressively decreased as the cells established stationary phase. Succinate dehydrogenase activity also decreas ed in MnSOD mutants; cytochrome oxidase and ATPase activities did not. Acon itase could be reactivated by addition of materials required for cluster as sembly (Fe3+ and a sulfur source), both in extracts and in vivo, indicating that inactivation of the enzyme was by disassembly of the cluster. Our res ults support the conclusion that superoxide is generated in the mitochondri a in vivo and under physiological conditions and that MnSOD is the primary defense against this toxicity. When the balance between superoxide generati on and MnSOD activity is disrupted, superoxide mediates iron release from m itochondrial iron-sulfur clusters, leading first to loss of mitochondrial f unction and then to death, independently of mtDNA damage. These results rai se the possibility that similar processes may occur in higher eukaryotes, ( C) 1999 Academic Press.