Evidence for the Presence of the Ascorbate-Glutathione Cycle in Mitochondria and Peroxisomes of Pea Leaves

The presence of the enzymes of the ascorbate-glutathione cycle was investigated in mitochondria and peroxisomes purified from pea (Pisum sativum L.) leaves. All four enzymes, ascorbate peroxidase (APX; EC 1.11.1.11), monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2), were present in mitochondria and peroxisomes, as well as in the antioxidants ascorbate and glutathione. The activity of the ascorbate-glutathione cycle enzymes was higher in mitochondria than in peroxisomes, except for APX, which was more active in peroxisomes than in mitochondria. Intact mitochondria and peroxisomes had no latent APX activity, and this remained in the membrane fraction after solubilization assays with 0.2 M KCl. Monodehydroascorbate reductase was highly latent in intact mitochondria and peroxisomes and was membrane-bound, suggesting that the electron acceptor and donor sites of this redox protein are not on the external side of the mitochondrial and peroxisomal membranes. Dehydroascorbate reductase was found mainly in the soluble peroxisomal and mitochondrial fractions. Glutathione reductase had a high latency in mitochondria and peroxisomes and was present in the soluble fractions of both organelles. In intact peroxisomes and mitochondria, the presence of reduced ascorbate and glutathione and the oxidized forms of ascorbate and glutathione were demonstrated by high-performance liquid chromatography analysis. The ascorbate-glutathione cycle of mitochondria and peroxisomes could represent an important antioxidant protection system against H2O2 generated in both plant organelles.     

Kinetics and mechanisms of catalase in peroxisomes of the mitochondrial fraction

1. The primary intermediate of catalase and hydrogen peroxide was identified and investigated in peroxisome-rich mitochondrial fractions of rat liver. On the basis of kinetic constants determined in vitro, it is possible to calculate with reasonable precision the molecular statistics of catalase action in the peroxisomes. 2. The endogenous hydrogen peroxide generation is adequate to sustain a concentration of the catalase intermediate (p(m)/e) of 60-70% of the hydrogen peroxide saturation value. Total amount of catalase corresponds to 0.12-0.15nmol of haem iron/mg of protein. In State 1 the rate of hydrogen peroxide generation corresponds to 0.9nmol/min per mg of protein or 5% of the mitochondrial respiratory rate in State 4. 3. Partial saturation of the catalase intermediate with hydrogen peroxide (p(m)/e) in the mitochondrial fraction suggests its significant peroxidatic activity towards its endogenous hydrogen donor. A variation of this value (p(m)/e) from 0.3 in State 4 to 0 under anaerobic conditions is observed. 4. For a particular preparation the hydrogen peroxide generation rate in the substrate-supplemented State 4 corresponds to 0.17s(-1) (eqn. 6), the hydrogen peroxide concentration to 2.5nm and the hydrogen-donor concentration (in terms of ethanol) to 0.12mm. The reaction is 70% peroxidatic and 30% catalatic. 5. A co-ordinated production of both oxidizing and reducing substrates for catalase in the mitochondrial fraction is suggested by a 2.2-fold increase of hydrogen peroxide generation and a threefold increase in hydrogen-donor generation in the State 1 to State 4 transition. 6. Additional hydrogen peroxide generation provided by the urate oxidase system of peroxisomes (8-12nmol of uric acid oxidized/min per mg of protein) permits saturation of the catalase with hydrogen peroxide to haem occupancy of 40% compared with values of 36% for a purified rat liver catalase ofk(1)=1.7x10(7)m(-1).s(-1) and k'(4)=2.6x10(7)m(-1). s(-1)(Chance, Greenstein & Roughton, 1952). 7. The turnover of the catalase ethyl hydrogen peroxide intermediate (k'(3)) in the peroxisomes is initially very rapid since endogenous hydrogen peroxide acts as a hydrogen donor. k'(3) decreases fivefold in the uncoupled state of the mitochondria.     

Redox-dependent trafficking of 2,3,4,5, 6-pentafluorodihydrotetramethylrosamine, a novel fluorogenic indicator of cellular oxidative activity

The trafficking of 2,3,4,5,6-pentafluorodihydrotetramethylrosamine (PF-H(2)TMRos, also known as RedoxSensor Red), a new fluorogenic indicator for oxidative activity, was evaluated in a contact-inhibited cell line, normal rat kidney fibroblast (NRK-49F), using quantitative fluorescence microscopy. After cells were incubated with 1-5 microM dye at 37 degrees C for 10 to 30 min, fluorescent staining of its oxidized product (PF-TMRos) distributed in mitochondria and/or lysosomes. This distribution pattern varied depending on the proliferation state of cells. In proliferating cells, PF-H(2)TMRos was internalized through a nonendocytic pathway, then oxidized in the cytosol, followed by immediate targeting to active mitochondria, resulting in fluorescent staining in this organelle. Photo-oxidation experiments demonstrated that PF-H(2)TMRos is not directly transported to mitochondria. On the contrary, in contact-inhibited cells whose proliferation is inhibited, PF-H(2)TMRos enters cells and is transported to lysosomes before it is oxidized. This results in lysosomal rather than mitochondrial staining. In both proliferating and quiescent cell states, subcellular distribution of the oxidized dye PF-TMRos can be altered by treatment with an oxidant (hydrogen peroxide) or an antioxidant (N-acetyl-L-cysteine), indicating a regulatory relationship between cell proliferation and oxidative activity. In solution assay, this probe can be oxidized by a broad spectrum of oxidizing species including horseradish peroxidase, hydrogen peroxide and horseradish peroxidase, cytochrome c, cytochrome c and hydrogen peroxide, superoxide and hydrogen peroxide, nitric oxide (or nitrite), peroxynitrite, and lipid hydroperoxide. Based on its subcellular distribution and its oxidation by a broad range of oxidizing species, PF-H(2)TMRos is demonstrated to be a novel indicator for cellular oxidative stresses.     

Human mitochondrial peroxiredoxin 5 protects from mitochondrial DNA damages induced by hydrogen peroxide

Peroxiredoxin 5 is a thioredoxin peroxidase ubiquitously expressed in mammalian tissues. Peroxiredoxin 5 can be addressed intracellularly to mitochondria, peroxisomes, the cytosol and the nucleus. Here, we show that mitochondrial human peroxiredoxin 5 protects mitochondrial DNA (mtDNA) from oxidative attacks. In an acellular assay, recombinant peroxiredoxin 5 was shown to protect plasmid DNA from damages induced by metal-catalyzed generation of reactive oxygen species. In Chinese hamster ovary cells, overexpression of mitochondrial peroxiredoxin 5 significantly decreased mtDNA damages caused by exogenously added hydrogen peroxide. Altogether our results suggest that mitochondrial peroxiredoxin 5 may play an important role in mitochondrial genome stability.     

Alcohol oxidase and catalase in peroxisomes of methanol-grown Candida boidinii.

Microbodies, designated as peroxisomes because of their enzyme complement, have been isolated from methanol-grown cells of Candida boidinii. Spheroplast lysates were separated on non-continuous Ficoll density gradients, resulting in a mitochondrial fraction and a peroxisome fraction. Estimates of purity using the mitochondrial enzyme markers suggested that the contamination of mitochondria in the peroxisome fraction was about 2-3%. As shown by electron microscopy the peroxisomes were 0.4-0.6 mum in diameter and contained crystalloid inclusions. Alcohol oxidase and catalase, which catalyse the oxidation of methanol to formaldehyde in Candida boidinii, could be localized within the peroxisomes. Gel-electrophoretic studies of the peroxisome fraction demonstrated that it contained only two predominant protein bands consistent with alcohol oxidase and catalase. No alcohol oxidase and catalase activity was found in mitochondria.     

Role of the Ascorbate-Glutathione Cycle of Mitochondria and Peroxisomes in the Senescence of Pea Leaves

We investigated the relationship between H₂O₂ metabolism and the senescence process using soluble fractions, mitochondria, and peroxisomes from senescent pea (Pisum sativum L.) leaves. After 11 d of senescence the activities of Mn-superoxide dismutase, dehydroascorbate reductase (DHAR), and glutathione reductase (GR) present in the matrix, and ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) activities localized in the mitochondrial membrane, were all substantially decreased in mitochondria. The mitochondrial ascorbate and dehydroascorbate pools were reduced, whereas the oxidized glutathione levels were maintained. In senescent leaves the H₂O₂ content in isolated mitochondria and the NADH- and succinate-dependent production of superoxide (O₂^·−) radicals by submitochondrial particles increased significantly. However, in peroxisomes from senescent leaves both membrane-bound APX and MDHAR activities were reduced. In the matrix the DHAR activity was enhanced and the GR activity remained unchanged. As a result of senescence, the reduced and the oxidized glutathione pools were considerably increased in peroxisomes. A large increase in the glutathione pool and DHAR activity were also found in soluble fractions of senescent pea leaves, together with a decrease in GR, APX, and MDHAR activities. The differential response to senescence of the mitochondrial and peroxisomal ascorbate-glutathione cycle suggests that mitochondria could be affected by oxidative damage earlier than peroxisomes, which may participate in the cellular oxidative mechanism of leaf senescence longer than mitochondria.     

Mitochondrial and peroxisomal beta oxidation of the branched chain fatty acid 2-methylpalmitate in rat liver

A number of isoprenoids (e.g. pristanic acid and the side chains of fat soluble-vitamins) is degraded or shortened via beta oxidation. We synthesized 2-methyl-palmitate and 2-methyl[1-14C] palmitate as a model substrate for the study of the beta oxidation of branched (isoprenoid) fatty acids in rat liver. 2-Methylpalmitate was well oxidized by isolated hepatocytes and its oxidation was stimulated after treatment of the animals with a peroxisome proliferator. Subcellular fractionation of rat liver demonstrated that 2-methylpalmitate is activated to its CoA ester in endoplasmic reticulum, mitochondria, and peroxisomes and that mitochondria and peroxisomes are capable of beta-oxidizing 2-methylpalmitate. At low unbound 2-methylpalmitate concentrations and in the presence of competing straight chain fatty acids, a condition encountered in vivo, peroxisomal 2-methyl-palmitate oxidation was 2- to 4-fold more active than mitochondrial oxidation. Treatment of rats with a peroxisome proliferator markedly stimulated mitochondrial but only slightly peroxisomal 2-methylpalmitate oxidation. The same treatment dramatically induced palmitoyl-CoA oxidase but did not change 2-methyl-palmitoyl-CoA oxidase activity. Our results indicate 1) that in untreated rats peroxisomes contribute for an important part to the oxidation of 2-methylpalmitate; 2) that treatment with a peroxisome proliferator stimulates mainly the mitochondrial component of 2-methylpalmitate oxidation; and 3) that palmitoyl-CoA and 2-methylpalmitoyl-CoA are oxidized by different peroxisomal oxidases.     

Plant autophagy is responsible for peroxisomal transition and plays an important role in the maintenance of peroxisomal quality

In photosynthetic cells, a large amount of hydrogen peroxide is produced in peroxisomes through photorespiration, which is a metabolic pathway related to photosynthesis. Hydrogen peroxide, a reactive oxygen species, oxidizes peroxisomal proteins and membrane lipids, resulting in a decrease in peroxisomal quality. We demonstrate that the autophagic system is responsible for the elimination of oxidized peroxisomes in plant. We isolated Arabidopsis mutants that accumulated oxidized peroxisomes, which formed large aggregates. We revealed that these mutants were defective in autophagy-related (ATG) genes and that the aggregated peroxisomes were selectively targeted by the autophagic machinery. These findings suggest that autophagy plays an important role in the quality control of peroxisomes by the selective degradation of oxidized peroxisomes. In addition, the results suggest that autophagy is also responsible for the functional transition of glyoxysomes to leaf peroxisomes.     

Plant autophagy is responsible for peroxisomal transition and plays an important role in the maintenance of peroxisomal quality

In photosynthetic cells, a large amount of hydrogen peroxide is produced in peroxisomes through photorespiration, which is a metabolic pathway related to photosynthesis. Hydrogen peroxide, a reactive oxygen species, oxidizes peroxisomal proteins and membrane lipids, resulting in a decrease in peroxisomal quality. We demonstrate that the autophagic system is responsible for the elimination of oxidized peroxisomes in plant. We isolated Arabidopsis mutants that accumulated oxidized peroxisomes, which formed large aggregates. We revealed that these mutants were defective in autophagy-related (ATG) genes and that the aggregated peroxisomes were selectively targeted by the autophagic machinery. These findings suggest that autophagy plays an important role in the quality control of peroxisomes by the selective degradation of oxidized peroxisomes. In addition, the results suggest that autophagy is also responsible for the functional transition of glyoxysomes to leaf peroxisomes.     

Functional characterization of the mitochondrial uncoupling proteins from the white shrimp Litopenaeus vannamei

Mitochondrial uncoupling proteins (UCPs) play an essential role in dissipating the proton gradient and controlling the mitochondrial inner membrane potential. When active, UCPs promote proton leak across the inner membrane, oxidative phosphorylation uncoupling, oxygen uptake increase and decrease the ATP synthesis. Invertebrates possess only isoforms UCP4 and UCP5, however, the role of these proteins is not clear in most species since it may depend on the physiological needs of each animal. This study presents the first functional characterization of crustacean uncoupling proteins from the white shrimp Litopenaeus vannamei LvUCP4 and LvUCP5. Free radicals production in various shrimp organs/tissues was first evaluated, and mitochondria were isolated from shrimp pleopods. The oxygen consumption rate, membrane potential and proton transport of the isolated non-phosphorylating mitochondria were used to determine LvUCPs activation/inhibition. Results indicate that UCPs activity is stimulated in the presence of 4-hydroxyl-2-nonenal (HNE) and myristic acid, and inhibited by the purine nucleotide GDP. A hypoxia/re-oxygenation assay was conducted to determine whether UCPs participate in shrimp mitochondria response to oxidative stress. Isolated mitochondria from shrimp at re-oxygenation produced large quantities of hydrogen peroxide and higher levels of both LvUCPs were immunodetected. Results suggest that, besides the active response of the shrimp antioxidant system, UCP-like activity is activated after hypoxia exposure and during re-oxygenation. LvUCPs may represent a mild uncoupling mechanism, which may be activated before the antioxidant system of cells, to early control reactive oxygen species production and oxidative damage in shrimp.     

Effects of ethidium bromide on the respiratory chain and oligomycin-sensitive adenosine triphosphatase in purified mitochondria from the cellular slime mold Dicyostelium discoideum.

Mitochondria were isolated from the cellular slime mold. Dictyoostelium discoideum, and partially purified by sucrose density gradient fractionation. The most purified mitochondrial fraction from the gradient contained essentially no contaminating lysosomes and minimal amounts of contaminating peroxisomes as determined by the marker enzymes N-acetyl-glucosaminidase and catalase. A mitochondrial fraction with the same amount of lysosomal and peroxisomal contamination was also isolated from cells which had been treated with ethidium bromide for 5 days. The most purified mitochondrial fraction from control and ethidium bromide-treated cells had an identical buoyant density of 1.181 to 1.182 g per ml, suggesting that treatment with the drug does not result in any drastic structural changes in the mitochondrial membrane which would affect its density. In the purified mitochondria from ethidium bromide-treated cells, the content of cytochromes a-a3 was decreased over 80% and that of cytochrome oxidase and oligomycin sensitive ATPase were reduced approximately 50%. By contrast, the specific activities of NADH and succinate dehydrogenases were identical in the purified mitochondria from control and ethidium bromide-treated cells. Previously, we had reported that the specific activities of these two enzymes had nearly doubled in whole cells maintained in ethidium bromide for a time equivalent to six or seven generations after growth had stopped (Stuchell, R. N., Weinstein, B. I., and Beattie, D. S. (1973) Fed. Eur. Biochem. Coc Lett. 37, 23-26). These results suggest that continued formation of new mitochondrial membranes, with an identical complement of succinate and NADH dehydrogenases, must occur despite the cessation of cell growth which occurs as a result of the ethidium bromide induced loss of mitochondrial enzymes. Consequently, the amount of mitochondria, or mitochondrial protein per cell, calculated from the activity of NADH and succinate dehydrogenases has increased nearly 50%. Possible models to explain the control of mitochondrial biogenesis are discussed to explain these results.     

Antioxidant properties of UCP1 are evolutionarily conserved in mammals and buffer mitochondrial reactive oxygen species

Mitochondrial uncoupling reduces reactive oxygen species (ROS) production and appears to be important for cellular signaling/protection, making it a focus for the treatment of metabolic and age-related diseases. Whereas the physiological role of uncoupling protein 1 (UCP1) of brown adipose tissue is established for thermogenesis, the function of UCP1 in the reduction of ROS in cold-exposed animals is currently under debate. Here, we investigated the role of UCP1 in mitochondrial ROS handling in the Lesser hedgehog tenrec (Echinops telfairi), a unique protoendothermic Malagasy mammal with recently identified brown adipose tissue (BAT). We show that the reduction of ROS by UCP1 activity also occurs in BAT mitochondria of the tenrec, suggesting that the antioxidative role of UCP1 is an ancient mammalian trait. Our analysis shows that the quantity of UCP1 displays strong control over mitochondrial hydrogen peroxide release, whereas other factors, such as mild cold, nonshivering thermogenesis, oxidative capacity, and mitochondrial respiration, do not correlate. Furthermore, hydrogen peroxide release from recoupled BAT mitochondria was positively associated with mitochondrial membrane potential. These findings led to a model of UCP1 controlling mitochondrial ROS release and, presumably, being controlled by high membrane potential, as proposed in the canonical model of “mild uncoupling”. Our study further promotes a conserved role for UCP1 in the prevention of oxidative stress, which was presumably established during evolution before UCP1 was physiologically integrated into nonshivering thermogenesis.     

Preventive effect of dietary quercetin on disuse muscle atrophy by targeting mitochondria in denervated mice

Quercetin is a major dietary flavonoid in fruits and vegetables. We aimed to clarify the preventive effect of dietary quercetin on disuse muscle atrophy and the underlying mechanisms. We established a mouse denervation model by cutting the sciatic nerve in the right leg (SNX surgery) to lack of mobilization in hind-limb. Preintake of a quercetin-mixed diet for 14 days before SNX surgery prevented loss of muscle mass and atrophy of muscle fibers in the gastrocnemius muscle (GM). Phosphorylation of Akt, a key phosphorylation pathway of suppression of protein degradation, was activated in the quercetin-mixed diet group with and without SNX surgery. Intake of a quercetin-mixed diet suppressed the generation of hydrogen peroxide originating from mitochondria and elevated mitochondrial peroxisome proliferator-activated receptor-γ coactivator 1α mRNA expression as well as NADH dehydrogenase 4 expression in the GM with SNX surgery. Quercetin and its conjugated metabolites reduced hydrogen peroxide production in the mitochondrial fraction obtained from atrophied muscle. In C2C12 myotubes, quercetin reached the mitochondrial fraction. These findings suggest that dietary quercetin can prevent disuse muscle atrophy by targeting mitochondria in skeletal muscle tissue through protecting mitochondria from decreased biogenesis and reducing mitochondrial hydrogen peroxide release, which can be related to decreased hydrogen peroxide production and/or improvements on antioxidant capacity of mitochondria.     

Mitochondrial and peroxisomal metabolism of glutaryl-CoA

Using a fraction purified from liver peroxisomes, we demonstrate that products of the glutaryl-CoA oxidase reaction are glutaconyl-CoA and H2O2. No glutaconyl-CoA decarboxylation occurs with this fraction. In whole tissue homogenates, the handling of glutaryl-CoA by glutaryl-CoA dehydrogenase is inhibited when reoxidation of FADH2 is blocked. Under these conditions, glutaconyl-CoA decarboxylation, however, can still occur and 14CO2 is produced from labelled glutaryl-CoA in mole/mole ratio with H2O2. These data indicate that in the absence of its mitochondrial dehydrogenation, glutaryl-CoA is oxidized in peroxisomes to glutaconyl-CoA which is probably transferred to mitochondria where it is decarboxylated and further processed. This hypothesis allows coherent explanation for the observed organic aciduria in both glutaricaciduria types I and II.     

Mitochondrial glycerol-3-P acyltransferase 1 is most active in outer mitochondrial membrane but not in mitochondrial associated vesicles (MAV)

Glycerol 3-phosphate acyltransferase-1 (GPAT1), catalyzes the committed step in phospholipid and triacylglycerol synthesis. Because both GPAT1 and carnitine-palmitoyltransferase 1 are located on the outer mitochondrial membrane (OMM) it has been suggested that their reciprocal regulation controls acyl-CoA metabolism at the OMM. To determine whether GPAT1, like carnitine-palmitoyltransferase 1, is enriched in both mitochondrial contact sites and OMM, and to correlate protein location and enzymatic function, we used Percoll and sucrose gradient fractionation of rat liver to obtain submitochondrial fractions. Most GPAT1 protein was present in a vesicular membrane fraction associated with mitochondria (MAV) but GPAT specific activity in this fraction was low. In contrast, highest GPAT1 specific activity was present in purified mitochondria. Contact sites from crude mitochondria, which contained markers for both endoplasmic reticulum (ER) and mitochondria, also showed high expression of GPAT1 protein but low specific activity, whereas contact sites isolated from purified mitochondria lacked ER markers and expressed highly active GPAT1. To determine how GPAT1 is targeted to mitochondria, recombinant protein was synthesized in vitro and its incorporation into crude and purified mitochondria was assayed. GPAT1 was rapidly incorporated into mitochondria, but not into microsomes. Incorporation was ATP-driven, and lack of GPAT1 removal by alkali and a chaotropic agent showed that GPAT1 had become an integral membrane protein after incorporation. These results demonstrate that two pools of GPAT1 are present in rat liver mitochondria: an active one, located in OMM and a less active one, located in membranes (ER-contact sites and mitochondrial associated vesicles) associated with both mitochondria and ER.     

Oxidative stress causes a general, calcium-dependent degradation of mitochondrial polynucleotides

Oxidative stress has many effects on biological cells, including the modulation of gene expression. Reactive oxygen species are known to up-regulate and down-regulate RNA expression in prokaryotic and eukaryotic cells. We have previously reported that a preferential and calcium-dependent down-regulation of mitochondrial RNAs occurs when HA-1 hamster fibroblasts are exposed to hydrogen peroxide. Here we extend these studies to determine whether this down-regulation is specific to mitochondria RNA or involves general polynucleotide degradation. Degradation and associated decreases in the levels of 16S mitochondrial rRNA following exposure of cells to 400 μM hydrogen peroxide were found to be dependent on calcium at 2 and 5 h. Degradation of mitochondrial genomic DNA was also observed following peroxide exposure, and occurred at similar time points as for mitochondrial RNA degradation. As with mitochondrial RNA degradation, this mitochondrial genomic DNA degradation was dependent on calcium. These results indicate that there is a general, calcium-dependent degradation of mitochondrial polynucleotides following exposure of HA-1 fibroblasts to oxidative stress, and suggest that a dramatic shut-down in mitochondrial biosynthesis is an early-stage response to oxidative stress.     

Isolation and characterization of metabolically competent mitochondria from spinach leaf protoplasts

Intact mitochondria were prepared from spinach (Spinacia oleracea L. var. Kyoho) leaf protoplasts and purified by Percoll discontinuous gradient centrifugation. Assays of several marker enzymes showed that the final mitochondrial preparations obtained are nearly free from other contaminating organelles, e.g. chloroplasts, peroxisomes, and endoplasmic reticulum. These mitochondria oxidized malate, glycine, succinate, and NADH, tightly coupled to oxidative phosphorylation with high values of ADP to O ratio as well as respiratory control ratio. The rate of NADH oxidation was 331 nmoles O₂ per milligram mitochondrial protein per minute, which is comparable to that obtained by highly purified potato or mung bean mitochondria. However, the activity of glutamine synthetase was barely detectable in the isolated mitochondrial fraction. This finding rules out a hypothetical scheme (Jackson, Dench, Morris, Lui, Hall, Moore 1971 Biochem Soc Trans 7: 1122) dealing with the role of the mitochondrial glutamine synthetase in the reassimilation of NH₃, which is released during the step of photorespiratory glycine decarboxylation in green leaf tissues, but it is consistent with the photosynthetic nitrogen cycle (Keys, Bird, Cornelius, Lea, Wallsgrove, Miflin 1978 Nature (Lond) 275: 741), in which NH₃ reassimilation occurs outside the mitochondria.     

Mitochondria as source of reactive oxygen species under oxidative stress. Study with novel mitochondria-targeted antioxidants — the “Skulachev-ion” derivatives

Production of reactive oxygen species (ROS) in mitochondria was studied using the novel mitochondria-targeted antioxidants (SkQ) in cultures of human cells. It was shown that SkQ rapidly (1–2 h) and selectively accumulated in mitochondria and prevented oxidation of mitochondrial components under oxidative stress induced by hydrogen peroxide. At nanomolar concentrations, SkQ inhibited oxidation of glutathione, fragmentation of mitochondria, and translocation of Bax from cytosol into mitochondria. The last effect could be related to prevention of conformational change in the adenine nucleotide transporter, which depends on oxidation of critical thiols. Mitochondria-targeted antioxidants at nanomolar concentrations prevented accumulation of ROS and cell death under oxidative stress. These effects required 24 h or more (depending on the cell type) preincubation, and this was not related to slow induction of endogenous antioxidant systems. It is suggested that SkQ slowly accumulates in a small subpopulation of mitochondria that have decreased membrane potential and produce the major part of ROS under oxidative stress. This population was visualized in the cells using potential-sensitive dye. The possible role of the small fraction of “bad” mitochondria in cell physiology is discussed.     

Mitochondria in homeostasis of reactive oxygen species in cell, tissues, and organism

The recent knowledge on mitochondria as the substantial source of reactive oxygen species, namely superoxide and hydrogen peroxide efflux from mitochondria, is reviewed, as well as nitric oxide and subsequent peroxynitrite generation in mitochondria and their effects. The reactive oxygen species formation in extramitochondrial locations, in peroxisomes, by cytochrome P450, and NADPH oxidase reaction, is also briefly discussed. Conditions are pointed out under which mitochondria represent the major ROS source for the cell: higher percentage of non-phosphorylating and coupled mitochondria, in vivo oxygen levels leading to increased intensity of the reverse electron transport in the respiratory chain, and nitric oxide effects on the redox state of cytochromes. We formulate hypotheses on the crucial role of ROS generated in mitochondria for the whole cell and organism, in concert with extramitochondrial ROS and antioxidant defense. We hypothesize that a sudden decline of mitochondrial ROS production converts cells or their microenvironment into a “ROS sink” represented by the instantly released excessive capacity of ROS-detoxification mechanisms. A partial but immediate decline of mitochondrial ROS production may be triggered by activation of mitochondrial uncoupling, specifically by activation of recruited or constitutively present uncoupling proteins such as UCP2, which may counterbalance the mild oxidative stress.     

Selective targeting of a redox-active ubiquinone to mitochondria within cells: antioxidant and antiapoptotic properties

With the recognition of the central role of mitochondria in apoptosis, there is a need to develop specific tools to manipulate mitochondrial function within cells. Here we report on the development of a novel antioxidant that selectively blocks mitochondrial oxidative damage, enabling the roles of mitochondrial oxidative stress in different types of cell death to be inferred. This antioxidant, named mitoQ, is a ubiquinone derivative targeted to mitochondria by covalent attachment to a lipophilic triphenylphosphonium cation through an aliphatic carbon chain. Due to the large mitochondrial membrane potential, the cation was accumulated within mitochondria inside cells, where the ubiquinone moiety inserted into the lipid bilayer and was reduced by the respiratory chain. The ubiquinol derivative thus formed was an effective antioxidant that prevented lipid peroxidation and protected mitochondria from oxidative damage. After detoxifying a reactive oxygen species, the ubiquinol moiety was regenerated by the respiratory chain enabling its antioxidant activity to be recycled. In cell culture studies, the mitochondrially localized antioxidant protected mammalian cells from hydrogen peroxide-induced apoptosis but not from apoptosis induced by staurosporine or tumor necrosis factor-alpha. This was compared with untargeted ubiquinone analogs, which were ineffective in preventing apoptosis. These results suggest that mitochondrial oxidative stress may be a critical step in apoptosis induced by hydrogen peroxide but not for apoptosis induced by staurosporine or tumor necrosis factor-alpha. We have shown that selectively manipulating mitochondrial antioxidant status with targeted and recyclable antioxidants is a feasible approach to investigate the role of mitochondrial oxidative damage in apoptotic cell death. This approach will have further applications in investigating mitochondrial dysfunction in a range of experimental models.