Physiological Functions of Mitochondrial Reactive Oxygen Species
Free Radical Medicine and Biology [Working Title]
Mitochondria are the major energy producers within a cell in the form of adenosine triphosphate by oxidative phosphorylation. Normal mitochondrial metabolism inevitably generates reactive oxygen species (ROS), which have been considered to solely cause cellular damage. Increase of oxidative stress has been linked to various pathologies. Thus, mitochondrial ROS (mROS) were basically proposed as byproducts of oxidative metabolism, which undergo normalized by antioxidant enzymes. However, the mROS
... . However, the mROS have extensively been esteemed to function as signalling molecules to regulate a wide variety of physiology. These phenomena are indeed dependent on mitochondrial redox status, which is dynamically altered under different physiological and pathological conditions. The oxidative stress is incurred by which the redox status is inclined to exceeded oxidation or reduction. Here, we attempt to integrate the recent advances in our understanding of the physiological functions of mROS. Keywords: mitochondrial ROS, oxidative stress, oxidative metabolism, redox signaling, mitochondrial physiology Free Radical Medicine and Biology 2 reactive metabolites of O 2 , including superoxide anion (O 2 ·− ) and hydrogen peroxide (H 2 O 2 ), formed by one-and two-electron reductions of O 2 , respectively . In the presence of transition metal ions, the more reactive hydroxyl radical (OH · ) is formed. The O 2 ·− is rapidly dismutated to H 2 O 2 by two dismutases including Cu/ Zn-superoxide dismutase (Cu/ZnSOD) in mitochondrial intermembrane space and manganese-dependent superoxide dismutase (MnSOD) in mitochondrial matrix. Unless the dismutation of O 2 ·− is catalyzed into H 2 O 2 , the radical oxidant promotes DNA damage, protein oxidation and lipid peroxidation in many types of cells. H 2 O 2 is also cell damaging molecule to be degraded to water by catalase  . Although the O 2 ·− generation by respiratory complexes is a well-established phenomenon, it is still poorly understood in mechanism  . Mitochondria have been implicated in the regulation of a number of physiological and pathological processes, including proliferation, differentiation, programmed cell death, innate immunity, autophagy, redox signalling, calcium homeostasis, hypoxic stress responses and stem cell reprogramming        . The mROS production contributes to mitochondrial damage in a range of pathologies, which is also is closely related to redox signalling in the cell [4, 17] . However, accumulating evidences show that mROS are not only deleterious molecules derived from the cellular metabolism but also indispensable participants in diverse cellular signalling and regulations    . In this chapter, we briefly summarize recent developments in our understanding of the involvement of mROS as signalling mediators in redox biology, rather than pathological stress, underlying physiological conditions.