Mitochondrial respiratory chain and thioredoxin reductase regulate intermembrane Cu,Zn-superoxide dismutase activity: implications for mitochondrial energy metabolism and apoptosis
Résumé
Intermembrane space Cu,Zn-superoxide dismutase (IMS Cu,Zn-SOD) is inactive in isolated intact rat liver mitochondria and is activated following oxidative modification of its critical thiol groups. The present study aimed to identify biochemical pathways implicated in the regulation of IMS Cu,Zn-SOD activity and to assess the impact of its functional state on key mitochondrial events. Exogenous H 2}O 2} (5 µM) activated Cu,Zn-SOD in intact mitochondria. However, neither H 2}O 2} alone nor H 2}O 2} in the presence of mitochondrial peroxiredoxin III activated Cu,Zn-SOD, which was purified from mitochondria and subsequently reduced by DTT to an inactive state. The reduced enzyme was activated following incubation with superoxide generating system, xanthine and xanthine oxidase. In intact mitochondria, the extent and duration of Cu,Zn-SOD activation was inversely correlated with mitochondrial superoxide production. Presence of thioredoxin reductase-1 (TxrR-1) was demonstrated in the mitochondrial IMS by western blotting. Inhibitors of TxrR-1, CDNB or auranofin, prolonged duration of H 2}O 2}-induced Cu,Zn-SOD activity in intact mitochondria. TxrR-1 inactivated Cu,Zn-SOD purified from mitochondria in an active oxidized state. Activation of IMS Cu,Zn-SOD by exogenous H 2}O 2} delayed CaCl 2}-induced loss of trans-membrane potential, decreased cytochrome c release, and markedly prevented superoxide-induced loss of aconitase activity in intact mitochondria respiring at state-3. These findings suggest that IMS Cu,Zn-SOD activity is reversibly regulated by H 2}O 2}, superoxide, and TxrR-1 and that the active enzyme is implicated in protecting vital mitochondrial functions.
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