Serial Review: The Powerhouse Takes Control of the Cell: the Role of Mitochondria in Signal TransductionMitochondrial superoxide: production, biological effects, and activation of uncoupling proteins☆
Introduction
The main function of mitochondria is ATP synthesis. Electrons from reduced substrates are passed from complexes I and II of the electron transport chain through complexes III and IV to oxygen, forming water and causing protons to be pumped across the mitochondrial inner membrane. The proton motive force set up by proton pumping drives protons back through the ATP synthase in the inner membrane, forming ATP from ADP and phosphate. There are two major side reactions that are relevant here: electrons may leak from the respiratory chain and react inappropriately with oxygen to form superoxide, and pumped protons may leak back across the inner membrane, diverting the conserved energy away from ATP synthesis and into heat production.
Mitochondrial superoxide production is a major cause of the cellular oxidative damage that may underlie degradative diseases and aging. In this review we outline our current picture of the sites and determinants of superoxide production by the electron transport chain and then consider its cellular and physiological effects, with particular emphasis on one of the recently identified targets: the proton conductance of mitochondrial uncoupling proteins. These considerations lead to a model for the function of uncoupling proteins in attenuating mitochondrial superoxide production. We consider the evidence for this model and the physiological and pathological situations to which it may be relevant.
Section snippets
Methods for measuring superoxide production
Measurement of mitochondrial superoxide production is reviewed in [1], [2]. Superoxide production by isolated respiratory chain complexes or submitochondrial particles is usually followed spectrophotometrically by monitoring reduction of epinephrine or of acetylated cytochrome c. Direct measurement of superoxide production in intact isolated mitochondria is more problematic due to its inability to cross the inner membrane and its rapid conversion to hydrogen peroxide by superoxide dismutase
The significance of superoxide in biological systems
Superoxide is a reactive molecule but it can be converted to hydrogen peroxide by superoxide dismutase (Mn-SOD in the matrix, Cu/Zn-SOD in the cytosol) and then to oxygen and water by catalase or glutathione peroxidase. However, these antioxidant systems are not perfect, and superoxide that evades them (together with the secondary reactive oxygen species it generates) can damage proteins, lipids, and DNA directly. Although the hydrogen peroxide produced by SOD is relatively unreactive, it can
Superoxide activation of the proton conductance of uncoupling proteins (UCPs)
An initial connection between UCPs and reactive oxygen species was suggested in 1997 when GDP, the classic nucleotide inhibitor of UCP1, was shown to increase membrane potential and hydrogen peroxide production in mitochondria containing UCP1 or UCP2, but not in mitochondria lacking UCPs [59]. Reactive oxygen species were first implicated as potential activators of uncoupling by UCPs in the late 1990s when studies suggested that they caused ATP-inhibited uncoupling in plant mitochondria [60],
Mitochondrial uncoupling proteins have an ancestral function of protection against superoxide production and consequent damage
The thermogenic function of UCP1 has been well characterized, but a function for its homologs (UCP2, UCP3, avian UCP, and plant UCP) has yet to be unambiguously defined. A possible physiological function for UCPs has been proposed [68], [69], [70] based on two observations discussed above. First, matrix superoxide production from complex I is highly dependent on the magnitude of the proton motive force, and second, matrix superoxide activates the proton conductance of UCPs and induces mild
Conclusions
Although much is known about superoxide production by mitochondria, we lack a consensus on where it is produced in the electron transport chain, on the mechanism of its production, and on how it is regulated. There is a vast literature about effects of superoxide and other ROS on cells, but there is still no unambiguous mechanistic description of how mitochondrial superoxide production may cause disease or aging. Research into the interactions of superoxide with mitochondrial uncoupling
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This article is part of a series of reviews on “The Powerhouse Takes Control of the Cell: the Role of Mitochondria in Signal Transduction.” The full list of papers may be found on the home page of the journal.