Redox activation of mitochondrial intermembrane space Cu,Zn-superoxide dismutase

These findings suggest that intermembrane space Cu,Zn-superoxide dismutase is inactive in intact mitochondria and that an oxidative modification of its critical thiol groups is necessary for its activation



Scholarcy highlights

  • Mitochondria are major cellular sources of superoxide anion and hydrogen peroxide
  • The following were added to the reaction mixture: bovine Cu,Zn-superoxide dismutase; intact mitochondria; intermembrane space preparation; KCN
  • Consistent with these findings, incubation of the mitochondrial intermembrane preparation with 4 mM dithiothreitol for 2 h at 37 ◦C completely inactivated intermembrane space Cu,Zn-superoxide dismutase. Thiol alkylating agents, such as IAM or NEM, elicited two different effects on mitochondrial Cu,Zn-superoxide dismutase activity depending on the mitochondrial preparation: treatment of intact mitochondria with alkylating agents in the presence of digitonin substantially decreased the activity of intermembrane space Cu,Zn-superoxide dismutase; this is expected when considering that the enzyme occurs in the reduced state in intact mitochondria; treatment of mitochondrial intermembrane preparation with either NEM or IAM did not affect intermembrane space Cu,Zn-superoxide dismutase activity, because the enzyme was already in its oxidized state. These findings suggest that thiol groups are critical in the activation process of intermembrane space Cu,Zn-superoxide dismutase during disruption of the mitochondrial outer membrane, and that once intermembrane space Cu,Znsuperoxide dismutase is activated, the thiol groups are no longer sensitive to the alkylating agents
  • Recent studies confirmed the localization of Cu,Zn-superoxide dismutase in the mitochondrial intermembrane space of both rat liver and yeast, and, thereby, ended a 30-year debate following the initial report on the presence of this enzyme in the intermembrane space
  • The biochemical pathways for oxidation of critical thiols of the intermembrane space Cu,Zn-superoxide dismutase are not known; it appears that thiol oxidation, rather than metal insertion, is the limiting step in the activation of Cu,Zn-superoxide dismutase, for the enzyme is inactive in intact mitochondria and is activated upon rupture of the mitochondrial outer membrane
  • Bacitracin, a well known inhibitor of protein disulphide-isomerase, partially inhibited intermembrane space Cu,Zn-superoxide dismutase activation following outer membrane disruption, suggesting that protein disulphide-isomerase may constitute a significant part of the thioldisulphide oxidoreductase activities that are possibly located in the cytosolic side of outer mitochondrial membrane
  • It is an intriguing possibility that the reversible activation of Cu,Zn-superoxide dismutase in the intermembrane space may function to divert O2− from its reaction with cytochrome c to form a more diffusible oxidant, H2O2, which can act as a redox signal both in mitochondria and cytosol

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