Rat liver Cu,Zn-superoxide dismutase. Subcellular location in lysosomes

Rat liver Cu,Zn-superoxide dismutase, previously reported to be localized in the mitochondria intermembrane space in addition to the cytosol, is now found to be lysosomal. The enzyme has been shown to segregate coincidently with lysosomal enzymes and not mitochondrial enzymes from a subcellular granule fraction by three independent methods: 1) digitonin treatment, 2) hypotonic treatment, which selectively releases lysosomal enzymes into the supernatant prior to the release of mitochondrial enzymes, and 3) separation of mitochondria and lysosomes by isopycnic density centrifugation after in vivo loading of lysosomes with Triton WR-1339. The Cu,Zn-superoxide dismutase in lysosomes appears to be derived from cytosolic Cu,Zn-superoxide dismutase, since microinjection of the enzyme into HeLa cells results in a partial redistribution from cytosol to lysosomes. In the rat, the amount of superoxide dismutase in the lysosomes varies with the nutritional state of the animal. Fasted animals have about 8% and fed animals about 2% of the total cellular superoxide dismutase in the lysosomes. This increase in lysosomal Cu,Zn-superoxide dismutase upon fasting is consistent with what is known to occur with other soluble cytosolic proteins during autophagy. Cu,Zn-superoxide dismutase is probably not located in mitochondria, as the enzyme found associated with this organelle on isopycnic density gradients may be attributed to lysosomal contamination. However, the possibility that a small amount (less than 0.1% of total cellular) is located in the mitochondria cannot be excluded.