Synthesis of cationic plastoquinone derivatives (SkQs) containing positively charged phosphonium or rhodamine moieties connected to plastoquinone by decane or pentane linkers is described. It is...

The mitochondria of various tissues from mice, naked mole rats (NMRs), and bats possess two mechanistically similar systems to prevent the generation of mitochondrial reactive oxygen species (mROS): hexokinases I and II and creatine kinase bound to mitochondrial membranes. Both systems operate in a manner such that one of the kinase substrates (mitochondrial ATP) is electrophoretically transported by the ATP/ADP antiporter to the catalytic site of bound hexokinase or bound creatine kinase without ATP dilution in the cytosol. One of the kinase reaction products, ADP, is transported back to the mitochondrial matrix via the antiporter, again through an electrophoretic process without cytosol dilution. The system in question continuously supports H⁺-ATP synthase with ADP until glucose or creatine is available. Under these conditions, the membrane potential, ∆ψ, is maintained at a lower than maximal level (i.e., mild depolarization of mitochondria). This ∆ψ decrease is sufficient to completely inhibit mROS generation. In 2.5-y-old mice, mild depolarization disappears in the skeletal muscles, diaphragm, heart, spleen, and brain and partially in the lung and kidney. This age-dependent decrease in the levels of bound kinases is not observed in NMRs and bats for many years. As a result, ROS-mediated protein damage, which is substantial during the aging of short-lived mice, is stabilized at low levels during the aging of long-lived NMRs and bats. It is suggested that this mitochondrial mild depolarization is a crucial component of the mitochondrial anti-aging system. Significance The mitochondria, organelles that produce the largest amounts of ATP and reactive oxygen species (mROS) in living cells, are equipped with a universal mechanism that can completely prevent mROS production. This mechanism consists of mild depolarization of the inner mitochondrial membrane to decrease the membrane potential to a level sufficient to form ATP but insufficient to generate mROS. In short-lived mice, aging is accompanied by inactivation of the mild depolarization mechanism, resulting in chronic poisoning of the organism with mROS. However, mild depolarization still functions for many years in long-lived naked mole rats and bats.

Plastoquinone, a very effective electron carrier and antioxidant of chloroplasts, was conjugated with decyltriphenylphosphonium to obtain a cation easily penetrating through membranes. This cation, called SkQ1, is specifically targeted to mitochondria by electrophoresis in the electric field formed by the mitochondrial respiratory chain. The respiratory chain also regenerates reduced SkQ1H2 from its oxidized form that appears as a result of the antioxidant activity of SkQ1H2. SkQ1H2 prevents oxidation of cardiolipin, a mitochondrial phospholipid that is especially sensitive to attack by reactive oxygen species (ROS). In cell cultures, SkQ1 and its analog plastoquinonyl decylrhodamine 19 (SkQR1) arrest H2O2-induced apoptosis. When tested in vivo, SkQs (i) prolong the lifespan of fungi, crustaceans, insects, fish, and mice, (ii) suppress appearance of a large number of traits typical for age-related senescence (cataract, retinopathies, achromotrichia, osteoporosis, lordokyphosis, decline of the immune system, myeloid shift of blood cells, activation of apoptosis, induction of β-galactosidase, phosphorylation of H2AX histones, etc.) and (iii) lower tissue damage and save the lives of young animals after treatments resulting in kidney ischemia, rhabdomyolysis, heart attack, arrhythmia, and stroke. We suggest that the SkQs reduce mitochondrial ROS and, as a consequence, inhibit mitochondriamediated apoptosis, an obligatory step of execution of programs responsible for both senescence and fast “biochemical suicide” of an organism after a severe metabolic crisis.

Mitochondrial dysfunction plays a crucial role in the macroautophagy/autophagy cascade. In a recently published study Sun et al. described the induction of autophagy by the membranophilic triphenylphosphonium (TPP)-based cation 10-(6'-ubiquinonyl) decyltriphenylphosphonium (MitoQ) in HepG2 cells (Sun C, et al. "MitoQ regulates autophagy by inducing a pseudo-mitochondrial membrane potential [PMMP]", Autophagy 2017, 13:730-738.). Sun et al. suggested that MitoQ adsorbed to the inner mitochondrial membrane with its cationic moiety remaining in the intermembrane space, adding a large number of positive charges and establishing a "pseudo-mitochondrial membrane potential," which blocked the ATP synthase. Here we argue that the suggested mechanism for generation of the "pseudo-mitochondrial membrane potential" is physically implausible and contradicts earlier findings on the electrophoretic displacements of membranophilic cations within and through phospholipid membranes. We provide evidence that TPP-cations dissipated the mitochondrial membrane potential in HepG2 cells and that the induction of autophagy in carcinoma cells by TPP-cations correlated with the uncoupling of oxidative phosphorylation. The mild uncoupling of oxidative phosphorylation by various mitochondria-targeted penetrating cations may contribute to their reported therapeutic effects via inducing both autophagy and mitochondria-selective mitophagy.