Mitochondrial respiration scavenges extramitochondrial superoxide anion via a nonenzymatic mechanism

Abstract

We determined that mitochondrial respiration reduced cytosolic oxidant stress in vivo and scavenged extramitochondrial superoxide anion (O2/-·) in vitro. First, Saccharomyces cerevisiae deficient in both the cytosolic antioxidant cuprozinc superoxide dismutase (Cu,Zn-SOD) and electron transport (Rho0 state) grew poorly (P < 0.05) in 21% O2 compared with parent yeast and yeast deficient only in electron transport or Cu,Zn-SOD, whereas anaerobic growth was the same (P > 0.05) in all yeast. Second, isolated yeast and mammalian mitochondria scavenged extramitochondrial O2/-· generated by xanthine/xanthine oxidase. Yeast mitochondria scavenged 42% more (P < 0.05) extramitochondrial O2/-· during pyruvate/malate-induced respiration than in the resting state. Addition of either antimycin (respiratory chain inhibitor) or FCCP (respiratory chain uncoupler) prevented increased O2/- · scavenging. Mitochondria isolated from yeast deficient in the mitochondrial manganous superoxide dismutase (Mn-SOD) increased (P < 0.05) O2/-· scavenging 56% during respiration. This apparent SOD activity, expressed in units of SOD activity per milligram of mitochondrial protein, was the same (9±0.6 vs. 10±1.0; P = 0.43) as the O2/-· scavenging of mitochondria with Mn-SOD, suggesting that respiration-dependent mitochondrial O2/-· scavenging was nonenzymatic. Finally, isolated rat liver and lung mitochondria also increased (P < 0.05) O2/-· scavenging during respiration. We speculate that respiring mitochondria, via the protonmotive pump, present a polarized, proton-rich surface that enhances nonenzymatic dismutation of extramitochondrial O2/-· and that this is a previously unrecognized function of mitochondrial respiration with potential physiological ramifications.