The oxidative inactivation of mitochondrial electron transport chain components and ATPase

Bovine heart submitochondrial particles (SMP) were exposed to continuous fluxes of hydroxyl radical (^•OH) alone, superoxide anion radical (O^- ₂) alone, or mixtures of ^•OH and O^- ₂, by γ radiolysis in the presence of 100% N₂O (^•OH exposure), 100% O₂ + formate (O^- ₂ exposure), or 100% O₂ alone (^•OH + O^- ₂ exposure). Hydrogen peroxide effects were studied by addition of pure H₂O₂. NADH dehydrogenase, NADH oxidase, succinate dehydrogenase, succinate oxidase, and ATPase activities (V_max) were rapidly inactivated by ^•OH (10% inactivation at 15-40 nmol of ^•OH/mg of SMP protein, 50-90% inactivation at 600 nmol of ^•OH/mg of SMP protein) and by ^•OH + O^- ₂ (10% inactivation at 20-80 nmol of ^•OH + O^- ₂/mg of SMP protein, 45-75% inactivation at 600 nmol of ^•OH + O^- ₂/mg of SMP protein). Importantly, O^- ₂ was a highly efficient inactivator of NADH dehydrogenase, NADH oxidase, and ATPase (10% inactivation at 20-50 nmol of O^- ₂/mg of SMP protein, 40% inactivation at 600 nmol of O^- ₂/mg of SMP protein), a mildly efficient inactivator of succinate dehydrogenase (10% inactivation at 150 nmol of O^- ₂/mg of SMP protein, 30% inactivation at 600 nmol of O^- ₂/mg of SMP protein), and a poor inactivator of succinate oxidase (<10% inactivation at 600 nmol of O^- ₂/mg of SMP protein). H₂O₂ partially inactivated NADH dehydrogenase, NADH oxidase, and cytochrome oxidase, but even 10% loss of these activities required at least 500-600 nmol of H₂O₂/mg of SMP protein. Cytochrome oxidase activity (oxygen consumption supported by ascorbate + N,N,N′,N′-tetramethyl-p-phenylenediamine) was remarkably resistant to oxidative inactivation, with less than 20% loss of activity evident even at ^•OH, O^- ₂, ^•OH + O^- ₂, or H₂O₂ concentrations of 600 nmol/mg of SMP protein. Cytochrome c oxidase activity, however (oxidation of, added, ferrocytochrome c), exhibited more than a 40% inactivation at 600 nmol of ^•OH/mg of SMP protein. The ^•OH-dependent inactivations reported above were largely inhibitable by the ^•OH scavenger mannitol. In contrast, the O^- ₂-dependent inactivations were inhibited by active superoxide dismutase, but not by denatured superoxide dismutase or catalase. Membrane lipid peroxidation was evident with ^•OH exposure but could be prevented by various lipid-soluble antioxidants which did not protect enzymatic activities at all. We conclude that both ^•OH and O^- ₂ are effective inactivators of specific proteins in the respiratory chain, but that the patterns of inactivation are quite distinct for the two active oxygen species: in contrast, the respiratory chain appears to be far more resistant to H₂O₂. Generalized lipid peroxidation, although a simple and widely used indicator of oxidative stress, appears to be unrelated to electron transport chain inactivation. The pattern of results presented here may prove useful in probing molecular mechanisms of oxidative damage to mitochondria, and in the development of effective antioxidant strategies.