Mitochondrial complex II can generate reactive oxygen species at high rates in both the forward and reverse reactions.

Respiratory complex II oxidizes succinate to fumarate as part of the Krebs cycle and reduces ubiquinone in the electron transport chain. Previous experimental evidence suggested that complex II is not a significant contributor to the production of reactive oxygen species (ROS) in isolated mitochondria or intact cells unless mutated. However, we find that when complex I and complex III are inhibited and succinate concentration is low, complex II in rat skeletal muscle mitochondria can generate superoxide or H(2)O(2) at high rates. These rates approach or exceed the maximum rates achieved by complex I or complex III. Complex II generates these ROS in both the forward reaction, with electrons supplied by succinate, and the reverse reaction, with electrons supplied from the reduced ubiquinone pool. ROS production in the reverse reaction is prevented by inhibition of complex II at either the ubiquinone-binding site (by atpenin A5) or the flavin (by malonate), whereas ROS production in the forward reaction is prevented by malonate but not by atpenin A5, showing that the ROS from complex II arises only from the flavin site (site II(F)). We propose a mechanism for ROS production by complex II that relies upon the occupancy of the substrate oxidation site and the reduction state of the enzyme. We suggest that complex II may be an important contributor to physiological and pathological ROS production.

Is defective electron transport at the hub of aging?

The bulwark of the mitochondrial theory of aging is that a defective respiratory chain initiates the death cascade. The increased production of superoxide is suggested to result in progressive oxidant damage to cellular components and particularly to mtDNA that encodes subunits assembled in respiratory complexes. Earlier studies of respiration in muscle mitochondria obtained from large cohorts of patients supported this notion by showing that either singly or in combinations, the respiratory complexes exhibited decreased activity in the elderly. The following critique of the most cited publications over the past decade points out the systematic errors that put earlier work at odds with recent findings. These later investigations indicate that aging has no overt effect on either the electron transport system or oxidative phosphorylation.

Cytopathies involving mitochondrial complex II.

Complex II (succinate-ubiquinone oxidoreductase) is the smallest complex in the respiratory chain and contains four nuclear-encoded subunits SdhA, SdhB, SdhC, and SdhD. It functions both as a respiratory chain component and an essential enzyme of the TCA cycle. Electrons derived from succinate can thus be directly transferred to the ubiquinone pool. Major insights into the workings of complex II have been provided by crystal structures of closely related bacterial enzymes, which have also been genetically manipulated to answer questions of structure-function not approachable using the mammalian system. This information, together with that accrued over the years on bovine complex II and by recent advances in understanding in vivo synthesis of the non-heme iron co-factors of the enzyme, is allowing better recognition of improper functioning of human complex II in diseased states. The discussion in this review is thus limited to cytopathies arising because the enzyme itself is defective or depleted by lack of iron-sulfur clusters. There is a clear dichotomy of effects. Enzyme depletion and mutations in SDHA compromise TCA activity and energy production, whereas mutations in SDHB, SDHC, and SDHD induce paraganglioma. SDHC and SDHD are the first tumor suppressor genes of mitochondrial proteins.

Mitochondrial reactive oxygen species in mice lacking superoxide dismutase 2: attenuation via antioxidant treatment.

Mice that lack the mitochondrial form of superoxide dismutase (SOD2) incur severe pathologies and mitochondrial deficiencies, including major depletion of complex II, as a consequence of buildup of endogenous reactive oxygen species (Melov, S., Coskun, P., Patel, M., Tuinstra, R., Cottrell, B., Jun, A. S., Zastawny, T. H., Dizdaroglu, M., Goodman, S. I., Huang, T. T., Miziorko, H., Epstein, C. J., and Wallace, D. C. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 846-851 and Li, Y., Huang, T. T., Carlson, E. J., Melov, S., Ursell, P. C., Olson, J. L., Noble, L. J., Yoshimura, M. P., Berger, C., Chan, P. H., Wallace, D. C., and Epstein, C. J. (1995) Nat. Genet. 11, 376-381). These problems can be greatly attenuated or rescued by synthetic antioxidant treatment, such as with the catalytic antioxidant EUK189 (Hinerfeld, D., Traini, M. D., Weinberger, R. P., Cochran, B., Doctrow, S. R., Harry, J., and Melov, S. (2004) J. Neurochem. 88, 657-667). We have used heart mitochondria from sod2 null mice to better understand mitochondrial reactive oxygen species production both in the absence of SOD2 and following in vivo antioxidant treatment. Isolated heart mitochondria from 5-day-old sod2 null animals respiring on the complex II substrate succinate exhibited statistically significant higher levels of mitochondrial O2* (157%, p < 0.01) but significantly less H2O2 (33%, p < 0.001) than wild type littermates. Treatment of sod2 nullizygous mice with EUK189 proportionately increased the levels of complex II and H2O2. Increased production of O2* resulting from complex II normalization had no effect on steady state levels due to the rapid conversion to H2O2, a process presumably aided by the presence of the EUK189, an SOD mimetic.