**2. Major mitochondrial targets of ischemia/reperfusion (I/R)**

Proteins of the mitochondrial electron transport chain and oxidative phosphorylation are among the primary targets of ischemia and oxidative stress during reperfusion [3]. Decreases in activities of adenine nucleotide translocase (ANT) and ATP synthase are among the earliest events after the onset of cardiac ischemia [4–7]. Inhibition of NADH:ubiquinone dehydrogenase (complex I) and reduced cytochrome c content occur early during ischemia [5, 8], whereas the damage to ubiquinol:cytochrome c oxidoreductase (complex III) and cytochrome oxidase (complex IV) occurs in prolonged ischemia [5, 8]. Reduced activity of complex I is primarily caused by decreases in NADH dehydrogenase activity [5, 9] due to, in part, oxidative damage to the flavin mononucleotide (FMN) prosthetic groups, which results in electron leakage, superoxide production, and the generation of reactive oxygen species (ROS) in ischemic tissues [5, 10]. Ischemia decreases the activity of complex III by inactivating the iron-sulfur center, which contributes to electron leakage and superoxide production, and exacerbates oxidative stress originating from complex I [11]. The voltage-dependent anion channel (VDAC) and proteins of the mitochondrial permeability transition (MPT) pore are also targeted by I/R, which disrupts the transport of ions and solutes and the membrane potential for ATP synthesis [12, 13].

Mitochondria are dynamic organelles that regularly undergo fission (fragmentation) and fusion (formation of a network of mitochondria) [14]. Fission is mediated by the dynaminrelated protein 1 (Drp1) and mitochondrial fission protein 1 (Fis1) [14]. Fusion is regulated by the mitofusins (Mfn1 and Mfn2) and the optic atrophy protein 1 (Opa1) [14]. Drp1 and Opa1 are rapidly activated and translocated to mitochondria after ischemia in major organs. This promotes fission, the permeabilization of the mitochondrial outer membrane, and the release of proteins that initiate the intrinsic cascade of apoptosis [15, 16]. Inhibition of Drp1 inhibits fission and reduces cardiomyocyte death, the size of myocardial infarct, and acute kidney injury after ischemia [17]. Mitochondria also regulate the intrinsic cascade of apoptosis, which is activated after the release of mitochondrial proteins. Therefore, preservation of mitochondrial integrity and function is crucial for organ protection against I/R injury.
