A critical appraisal of the mitochondrial coenzyme Q pool

COVID-19 during Gestation: Maternal Implications of Evoked Oxidative Stress and Iron Metabolism Impairment

COVID-19 has reached pandemic proportions worldwide, with considerable consequences for both health and the economy. In pregnant women, COVID-19 can alter the metabolic environment, iron metabolism, and oxygen supply of trophoblastic cells, and therefore have a negative influence on essential mechanisms of fetal development. The purpose of this study was to investigate, for the first time, the effects of COVID-19 infection during pregnancy with regard to the oxidative/antioxidant status in mothers’ serum and placenta, together with placental iron metabolism. Results showed no differences in superoxide dismutase activity and placental antioxidant capacity. However, antioxidant capacity decreased in the serum of infected mothers. Catalase activity decreased in the COVID-19 group, while an increase in 8-hydroxy-2’-deoxyguanosine, hydroperoxides, 15-FT-isoprostanes, and carbonyl groups were recorded in this group. Placental vitamin D, E, and Coenzyme-Q10 also showed to be increased in the COVID-19 group. As for iron-related proteins, an up-regulation of placental DMT1, ferroportin-1, and ferritin expression was recorded in infected women. Due to the potential role of iron metabolism and oxidative stress in placental function and complications, further research is needed to explain the pathogenic mechanism of COVID-19 that may affect pregnancy, so as to assess the short-term and long-term outcomes in mothers’ and infants’ health.

Mitochondrial proton leaks and uncoupling proteins

Non-shivering thermogenesis in brown adipose tissue is mediated by uncoupling protein 1 (UCP1), which provides a carefully regulated proton re-entry pathway across the mitochondrial inner membrane operating in parallel to the ATP synthase and allowing respiration, and hence thermogenesis, to be released from the constraints of respiratory control. In the 40 years since UCP1 was first described, an extensive, and frequently contradictory, literature has accumulated, focused on the acute physiological regulation of the protein by fatty acids, purine nucleotides and possible additional factors. The purpose of this review is to examine, in detail, the experimental evidence underlying these proposed mechanisms. Emphasis will be placed on the methodologies employed and their relation to the physiological constraints under which the protein functions in the intact cell. The nature of the endogenous, UCP1-independent, proton leak will also be discussed. Finally, the troubled history of the putative novel uncoupling proteins, UCP2 and UCP3, will be evaluated.

Oxidative stress and mitochondrial dysfunction in early-onset and late-onset preeclampsia

Preeclampsia is a pregnancy-specific syndrome with multisystem involvement which leads to foetal, neonatal, and maternal morbidity and mortality. This syndrome is characterized by the onset of clinical signs and symptoms and delivery before (early-onset preeclampsia, eoPE), or after (late-onset preeclampsia, loPE), the 34 weeks of gestation. Preeclampsia is a mitochondrial disorder where its differential involvement in eoPE and loPE is unclear. Mitochondria regulate cell metabolism and are a significant source of reactive oxygen species (ROS). The syncytiotrophoblast in eoPE and loPE show altered mitochondrial structure and function resulting in ROS overproduction, oxidative stress, and cell damage and death. Mitochondrial dysfunction in eoPE may result from altered expression of several molecules, including dynamin-related protein 1 and mitofusins, compared with loPE where these factors are either reduced or unaltered. Equally, mitochondrial fusion/fission dynamics seem differentially modulated in eoPE and loPE. It is unclear whether the electron transport chain and oxidative phosphorylation are differentially altered in these two subgroups of preeclampsia. However, the activity of complex IV (cytochrome c oxidase) and the expression of essential proteins involved in the electron transport chain are reduced, leading to lower oxidative phosphorylation and mitochondrial respiration in the preeclamptic placenta. Interventional studies in patients with preeclampsia using the coenzyme Q₁₀, a key molecule in the electron transport chain, suggest that agents that increase the antioxidative capacity of the placenta may be protective against preeclampsia development. In this review, the mitochondrial dysfunction in both eoPE and loPE is summarized. Therapeutic approaches are discussed in the context of contributing to the understanding of mitochondrial dysfunction in eoPE and loPE.

Mitochondria and Coenzyme Q10 in the Pathogenesis of Preeclampsia

Hypertensive disorders during pregnancy constitute one of the main causes of maternal and perinatal morbidity and mortality across the world and particularly in developing countries such as Ecuador. However, despite its impact on public health, the primary pathophysiological processes involved are yet to be elucidated. It has been proposed, among other theories, that an abnormal placentation may induce an endothelial dysfunction, which is ultimately responsible for the final clinical manifestations. Mitochondria, particularly from trophoblastic cells, are responsible for the production of energy, which is extremely important for normal placentation. The malfunction in this supply of energy may produce higher levels of free radicals. In both production of energy and free radicals, coenzyme Q10 (CoQ10) plays a crucial role in electron transport. As such, the role of CoQ10 in the genesis and prevention of preeclampsia has become the focus of a number of research groups, including that of the authors. Developing an in-depth understanding of these mechanisms might allow us to design new and feasible strategies with which we can reduce preeclampsia, particularly in the Latin-American countries.

Important roles for membrane lipids in haloarchaeal bioenergetics

Recent advances in lipidomic analysis in combination with various physiological experiments set the stage for deciphering the structure-function of haloarchaeal membrane lipids. Here we focused primarily on changes in lipid composition of Haloferax volcanii, but also performed a comparative analysis with four other haloarchaeal species (Halobacterium salinarum, Halorubrum lacusprofundi, Halorubrum sodomense and Haloplanus natans) all representing distinctive cell morphologies and behaviors (i.e., rod shape vs. pleomorphic behavior). Common to all five haloarchaea, our data reveal an extraordinary high level of menaquinone, reaching up to 72% of the total lipids. This ubiquity suggests that menaquinones may function beyond their ordinary role as electron and proton transporter, acting simultaneously as ion permeability barriers and as powerful shield against oxidative stress. In addition, we aimed at understanding the role of cations interacting with the characteristic negatively charged surface of haloarchaeal membranes. We propose for instance that by bridging the negative charges of adjacent anionic phospholipids, Mg²⁺ acts as surrogate for cardiolipin, a molecule that is known to control curvature stress of membranes. This study further provides a bioenergetic perspective as to how haloarchaea evolved following oxygenation of Earth's atmosphere. The success of the aerobic lifestyle of haloarchaea includes multiple membrane-based strategies that successfully balance the need for a robust bilayer structure with the need for high rates of electron transport - collectively representing the molecular basis to inhabit hypersaline water bodies around the planet.

Distribution and dynamics of quinones in the lipid bilayer mimicking the inner membrane of mitochondria

Quinone and its analogues (Q) constitute an important class of compounds that perform key electron transfer reactions in oxidative- and photo-phosphorylation. In the inner membrane of mitochondria, ubiquinone molecules undergo continuous redox transitions enabling electron transfer between the respiratory complexes. In such a dynamic system undergoing continuous turnover for ATP synthesis, an uninterrupted supply of substrate molecules is absolutely necessary. In the current work, we have performed atomistic molecular dynamics simulations and free energy calculations to assess the structure, dynamics, and localization of quinone and its analogues in a lipid bilayer, whose composition mimics the one in the inner mitochondrial membrane. The results show that there is a strong tendency of both quinone and quinol molecules to localize in the vicinity of the lipids' acyl groups, right under the lipid head group region. Additionally, we observe a second location in the middle of the bilayer where quinone molecules tend to stabilize. Translocation of quinone through a lipid bilayer is very fast and occurs in 10-100ns time scale, whereas the translocation of quinol is at least an order of magnitude slower. We suggest that this has important mechanistic implications given that the localization of Q ensures maximal occupancy of the Q-binding sites or Q-entry points in electron transport chain complexes, thereby maintaining an optimal turnover rate for ATP synthesis.

Border between natural product and drug: comparison of the related benzoquinones idebenone and coenzyme Q10

Coenzyme Q₁₀ is a ubiquitous component of cellular membranes and belongs to the class of benzoquinones that mainly differ with regards to the length and composition of their hydrophobic tail. The characteristic quinone group can accept electrons from various biological sources and is converted by a one electron transfer to the unstable semiquinone or by a two electron transfer to the more stable hydroquinone. This feature makes CoQ₁₀ the bona fide cellular electron transfer molecule within the mitochondrial respiratory chain and also makes it a potent cellular antioxidant. These activities serve as justification for its popular use as food supplement. Another quinone with similarities to the naturally occurring CoQ₁₀ is idebenone, which shares its quinone moiety with CoQ₁₀, but at the same time differs from CoQ₁₀ by the presence of a much shorter, less lipophilic tail. However, despite its similarity to CoQ₁₀, idebenone cannot be isolated from any natural sources but instead was synthesized and selected as a pharmacologically active compound in the 1980s by Takeda Pharmaceuticals purely based on its pharmacological properties. Several recent clinical trials demonstrated some therapeutic efficacy of idebenone in different indications and as a consequence, many practitioners question if the freely available CoQ₁₀ could not be used instead. Here, we describe the molecular and pharmacological features of both molecules that arise from their structural differences to answer the question if idebenone is merely a CoQ₁₀ analogue as frequently perpetuated in the literature or a pharmaceutical drug with entirely different features.

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The benzoquinones CoQ₁₀ and idebenone have vastly different solubility.

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Both molecules need to get activated by cellular reductases.

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Due to their solubility both molecules are in different cellular compartments.

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Therefore, both quinones are activated by different enzymes.

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Thus, their solubility strongly determines their biological activities.

Coenzyme q and the respiratory chain: coenzyme q pool and mitochondrial supercomplexes

Two alternative models of organization of the mitochondrial electron transport chain (mETC) have been alternatively favored or questioned by the accumulation evidences of different sources, the solid model or the random collision model. Both agree in the number of respiratory complexes (I-IV) that participate in the mETC, but while the random collision model proposes that Complexes I-IV do not interact physically and that electrons are transferred between them by coenzyme Q and cytochrome c, the solid model proposes that all complexes super-assemble in the so-called respirasome. Recently, the plasticity model has been developed to incorporate the solid and the random collision model as extreme situations of a dynamic organization, allowing super-assembly free movement of the respiratory complexes. In this review, we evaluate the supporting evidences of each model and the implications of the super-assembly in the physiological role of coenzyme Q.

Mitochondrial respiration without ubiquinone biosynthesis

Ubiquinone (UQ), a.k.a. coenzyme Q, is a redox-active lipid that participates in several cellular processes, in particular mitochondrial electron transport. Primary UQ deficiency is a rare but severely debilitating condition. Mclk1 (a.k.a. Coq7) encodes a conserved mitochondrial enzyme that is necessary for UQ biosynthesis. We engineered conditional Mclk1 knockout models to study pathogenic effects of UQ deficiency and to assess potential therapeutic agents for the treatment of UQ deficiencies. We found that Mclk1 knockout cells are viable in the total absence of UQ. The UQ biosynthetic precursor DMQ9 accumulates in these cells and can sustain mitochondrial respiration, albeit inefficiently. We demonstrated that efficient rescue of the respiratory deficiency in UQ-deficient cells by UQ analogues is side chain length dependent, and that classical UQ analogues with alkyl side chains such as idebenone and decylUQ are inefficient in comparison with analogues with isoprenoid side chains. Furthermore, Vitamin K2, which has an isoprenoid side chain, and has been proposed to be a mitochondrial electron carrier, had no efficacy on UQ-deficient mouse cells. In our model with liver-specific loss of Mclk1, a large depletion of UQ in hepatocytes caused only a mild impairment of respiratory chain function and no gross abnormalities. In conjunction with previous findings, this surprisingly small effect of UQ depletion indicates a nonlinear dependence of mitochondrial respiratory capacity on UQ content. With this model, we also showed that diet-derived UQ10 is able to functionally rescue the electron transport deficit due to severe endogenous UQ deficiency in the liver, an organ capable of absorbing exogenous UQ.

Dietary macronutrients modulate the fatty acyl composition of rat liver mitochondrial cardiolipins

The interaction of dietary fats and carbohydrates on liver mitochondria were examined in male FBNF1 rats fed 20 different low-fat isocaloric diets. Animal growth rates and mitochondrial respiratory parameters were essentially unaffected, but mass spectrometry-based mitochondrial lipidomics profiling revealed increased levels of cardiolipins (CLs), a family of phospholipids essential for mitochondrial structure and function, in rats fed saturated or trans fat-based diets with a high glycemic index. These mitochondria showed elevated monolysocardiolipins (a CL precursor/product of CL degradation), elevated ratio of trans-phosphocholine (PC) (18:1/18:1) to cis-PC (18:1/18:1) (a marker of thiyl radical stress), and decreased ubiquinone Q9; the latter two of which imply a low-grade mitochondrial redox abnormality. Extended analysis demonstrated: i) dietary fats and, to a lesser extent, carbohydrates induce changes in the relative abundance of specific CL species; ii) fatty acid (FA) incorporation into mature CLs undergoes both positive (>400-fold) and negative (2.5-fold) regulation; and iii) dietary lipid abundance and incorporation of FAs into both the CL pool and specific mature tetra-acyl CLs are inversely related, suggesting previously unobserved compensatory regulation. This study reveals previously unobserved complexity/regulation of the central lipid in mitochondrial metabolism.

Atypical cristae morphology of human syncytiotrophoblast mitochondria: role for complex V

Mitochondrial complexes I, III(2), and IV from human cytotrophoblast and syncytiotrophoblast associate to form supercomplexes or respirasomes, with the following stoichiometries: I(1):(III(2))(1) and I(1):(III(2))(1-2):IV(1-4). The content of respirasomes was similar in both cell types after isolating mitochondria. However, syncytiotrophoblast mitochondria possess low levels of dimeric complex V and do not have orthodox cristae morphology. In contrast, cytotrophoblast mitochondria show normal cristae morphology and a higher content of ATP synthase dimer. Consistent with the dimerizing role of the ATPase inhibitory protein (IF(1)) (García, J. J., Morales-Ríos, E., Cortés-Hernandez, P., and Rodríguez-Zavala, J. S. (2006) Biochemistry 45, 12695-12703), higher relative amounts of IF(1) were observed in cytotrophoblast when compared with syncytiotrophoblast mitochondria. Therefore, there is a correlation between dimerization of complex V, IF(1) expression, and the morphology of mitochondrial cristae in human placental mitochondria. The possible relationship between cristae architecture and the physiological function of the syncytiotrophoblast mitochondria is discussed.

NQO1-dependent redox cycling of idebenone: effects on cellular redox potential and energy levels

Short-chain quinones are described as potent antioxidants and in the case of idebenone have already been under clinical investigation for the treatment of neuromuscular disorders. Due to their analogy to coenzyme Q10 (CoQ10), a long-chain quinone, they are widely regarded as a substitute for CoQ10. However, apart from their antioxidant function, this provides no clear rationale for their use in disorders with normal CoQ10 levels. Using recombinant NAD(P)H:quinone oxidoreductase (NQO) enzymes, we observed that contrary to CoQ10 short-chain quinones such as idebenone are good substrates for both NQO1 and NQO2. Furthermore, the reduction of short-chain quinones by NQOs enabled an antimycin A-sensitive transfer of electrons from cytosolic NAD(P)H to the mitochondrial respiratory chain in both human hepatoma cells (HepG2) and freshly isolated mouse hepatocytes. Consistent with the substrate selectivity of NQOs, both idebenone and CoQ1, but not CoQ10, partially restored cellular ATP levels under conditions of impaired complex I function. The observed cytosolic-mitochondrial shuttling of idebenone and CoQ1 was also associated with reduced lactate production by cybrid cells from mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) patients. Thus, the observed activities separate the effectiveness of short-chain quinones from the related long-chain CoQ10 and provide the rationale for the use of short-chain quinones such as idebenone for the treatment of mitochondrial disorders.

Ethylene glycol monomethyl ether-induced toxicity is mediated through the inhibition of flavoprotein dehydrogenase enzyme family

Ethylene glycol monomethyl ether (EGME) is a widely used industrial solvent known to cause adverse effects to human and other mammals. Organs with high metabolism and rapid cell division, such as testes, are especially sensitive to its actions. In order to gain mechanistic understanding of EGME-induced toxicity, an untargeted metabolomic analysis was performed in rats. Male rats were administrated with EGME at 30 and 100 mg/kg/day. At days 1, 4, and 14, serum, urine, liver, and testes were collected for analysis. Testicular injury was observed at day 14 of the 100 mg/kg/day group only. Nearly 1900 metabolites across the four matrices were profiled using liquid chromatography-mass spectrometry/mass spectrometry and gas chromatography-mass spectrometry. Statistical analysis indicated that the most significant metabolic perturbations initiated from the early time points by EGME were the inhibition of choline oxidation, branched-chain amino acid catabolism, and fatty acid β-oxidation pathways, leading to the accumulation of sarcosine, dimethylglycine, and various carnitine- and glycine-conjugated metabolites. Pathway mapping of these altered metabolites revealed that all the disrupted steps were catalyzed by enzymes in the primary flavoprotein dehydrogenase family, suggesting that inhibition of flavoprotein dehydrogenase-catalyzed reactions may represent the mode of action for EGME-induced toxicity. Similar urinary and serum metabolite signatures are known to be the hallmarks of multiple acyl-coenzyme A dehydrogenase deficiency in humans, a genetic disorder because of defects in primary flavoprotein dehydrogenase reactions. We postulate that disruption of key biochemical pathways utilizing flavoprotein dehydrogenases in conjugation with downstream metabolic perturbations collectively result in the EGME-induced tissue damage.

Tissue-, substrate-, and site-specific characteristics of mitochondrial reactive oxygen species generation

Reactive oxygen species are a by-product of mitochondrial oxidative phosphorylation, derived from a small quantity of superoxide radicals generated during electron transport. We conducted a comprehensive and quantitative study of oxygen consumption, inner membrane potentials, and H(2)O(2) release in mitochondria isolated from rat brain, heart, kidney, liver, and skeletal muscle, using various respiratory substrates (alpha-ketoglutarate, glutamate, succinate, glycerol phosphate, and palmitoyl carnitine). The locations and properties of reactive oxygen species formation were determined using oxidative phosphorylation and the respiratory chain modulators oligomycin, rotenone, myxothiazol, and antimycin A and the uncoupler CCCP. We found that in mitochondria isolated from most tissues incubated under physiologically relevant conditions, reactive oxygen release accounts for 0.1-0.2% of O(2) consumed. Our findings support an important participation of flavoenzymes and complex III and a substantial role for reverse electron transport to complex I as reactive oxygen species sources. Our results also indicate that succinate is an important substrate for isolated mitochondrial reactive oxygen production in brain, heart, kidney, and skeletal muscle, whereas fatty acids generate significant quantities of oxidants in kidney and liver. Finally, we found that increasing respiratory rates is an effective way to prevent mitochondrial oxidant release under many, but not all, conditions. Altogether, our data uncover and quantify many tissue-, substrate-, and site-specific characteristics of mitochondrial ROS release.

From protons to OXPHOS supercomplexes and Alzheimer's disease: structure-dynamics-function relationships of energy-transducing membranes

By the elucidation of high-resolution structures the view of the bioenergetic processes has become more precise. But in the face of these fundamental advances, many problems are still unresolved. We have examined a variety of aspects of energy-transducing membranes from large protein complexes down to the level of protons and functional relevant picosecond protein dynamics. Based on the central role of the ATP synthase for supplying the biological fuel ATP, one main emphasis was put on this protein complex from both chloroplast and mitochondria. In particular the stoichiometry of protons required for the synthesis of one ATP molecule and the supramolecular organisation of ATP synthases were examined. Since formation of supercomplexes also concerns other complexes of the respiratory chain, our work was directed to unravel this kind of organisation, e.g. of the OXPHOS supercomplex I(1)III(2)IV(1), in terms of structure and function. Not only the large protein complexes or supercomplexes work as key players for biological energy conversion, but also small components as quinones which facilitate the transfer of electrons and protons. Therefore, their location in the membrane profile was determined by neutron diffraction. Physico-chemical features of the path of protons from the generators of the electrochemical gradient to the ATP synthase, as well as of their interaction with the membrane surface, could be elucidated by time-resolved absorption spectroscopy in combination with optical pH indicators. Diseases such as Alzheimer's dementia (AD) are triggered by perturbation of membranes and bioenergetics as demonstrated by our neutron scattering studies.

Impedance spectroscopy of bacterial membranes: coenzyme-Q diffusion in a finite diffusion layer

The inner membrane of Escherichia coli, overexpressing an ubiquinol oxidase, cytochrome bo3 (cbo3), was "tethered" in a planar configuration to a gold electrode. Electron transfer to cbo3 was achieved via native ubiquinol-8 or added ubiquinol-10, and impedance spectroscopy was used to characterize the diffusion properties of the ubiquinol/ubiquinone in the tethered membrane system. Spectra were obtained at varying direct current (DC) potentials covering the potential window in which the voltammetric catalytic wave of cbo3 is visible. These spectra were compared to those obtained after addition of a potent inhibitor of cbo3, cyanide, and the difference in impedance was analyzed using a derived equivalent circuit, which is similar to that of open finite-length diffusion (OFLD) or the finite Warburg circuit, but with the boundary conditions modified to account for the fact that ubiquinol reoxidation is limited by enzyme activity. Analysis of the impedance spectra of the tethered membrane system gave kinetic parameters that are consistent with values obtained using cyclic voltammetry. Importantly, the diffusion rate of ubiquinone (10(-13)-10(-12) cm2/s) was found to be orders of magnitude lower than accepted values for lateral diffusion (10(-8)-10(-7) cm2/ s). It is hypothesized that this result represent perpendicular diffusion of quinone across the membrane, corresponding to a "flip" time between 0.05 and 1 s.

Ultrastructure of the mitochondrion and its bearing on function and bioenergetics

The recently ascertained network and dynamic organization of the mitochondrion, as well as the demonstration of energy proteins and metabolites subcompartmentalization, have led to a reconsideration of the relationships between organellar form and function. In particular, the impact of mitochondrial morphological changes on bioenergetics is inseparable. Several observations indicate that mitochondrial energy production may be controlled by structural rearrangements of the organelle both interiorly and globally, including the remodeling of cristae morphology and elongation or fragmentation of the tubular network organization, respectively. These changes are mediated by fusion or fission reactions in response to physiological signals that remain unidentified. They lead to important changes in the internal diffusion of energy metabolites, the sequestration and conduction of the electric membrane potential (Delta Psi), and possibly the delivery of newly synthesized ATP to various cellular areas. Moreover, the physiological or even pathological context also determines the morphology of the mitochondrion, suggesting a tight and mutual control between mitochondrial form and bioenergetics. In this review, we delve into the link between mitochondrial structure and energy metabolism.

Lipidomic analysis and electron transport chain activities in C57BL/6J mouse brain mitochondria

The objective of this study was to characterize the lipidome and electron transport chain activities in purified non-synaptic (NS) and synaptic (Syn) mitochondria from C57BL/6J mouse cerebral cortex. Contamination from subcellular membranes, especially myelin, has hindered past attempts to accurately characterize the lipid composition of brain mitochondria. An improved Ficoll and sucrose discontinuous gradient method was employed that yielded highly enriched mitochondrial populations free of myelin contamination. The activities of Complexes I, II, III, and II/III were lower in Syn than in NS mitochondria, while Complexes I/III and IV activities were similar in both populations. Shotgun lipidomics showed that levels of cardiolipin (Ptd(2)Gro) were lower, whereas levels of ceramide and phosphatidylserine were higher in Syn than in NS mitochondria. Coenzyme Q(9) and Q(10) was also lower in Syn than in NS mitochondria. Gangliosides, phosphatidic acid, sulfatides, and cerebrosides were undetectable in brain mitochondria. The distribution of Ptd(2)Gro molecular species was similar in both populations and formed a unique pattern, consisting of seven major molecular species groups, when arranged according to mass to charge ratios. Remodeling involving choline and ethanolamine phosphoglycerides could explain Ptd(2)Gro heterogeneity. NS and Syn mitochondrial lipidomic heterogeneity could influence energy metabolism, which may contribute to metabolic compartmentation of the brain.

Evaluation of the mutagenic and genotoxic potential of ubiquinol

Ubiquinol (the reduced form of coenzyme Q(10)) is the two-electron reduction product of ubiquinone (the oxidized form of coenzyme Q(10)), and has been shown to be an integral part of living cells, where it functions as an antioxidant in both mitochondria and lipid membranes. To provide information to enable a Generally Regarded as Safe (GRAS) evaluation for the use of ubiquinol in selected foods, a series of Organisation of Economic Cooperation and Development (OECD) and good laboratory practice (GLP) toxicological studies was conducted to evaluate the mutagenic and genotoxic potential of Kaneka QH brand of ubiquinol. Ubiquinol did not induce reverse mutations in Salmonella typhimurium strains TA100, TA1535, TA98, and TA1537 and Escherichia coli WP2uvrA at concentrations up to 5000 mu g/plate, in either the absence and presence of exogenous metabolic activation by rat liver S9. Likewise, ubiquinol did not induce chromosome aberrations in Chinese hamster lung fibroblast (CHL/IU) cells in short-term (6-h) tests with or without rat liver S9 at concentrations up to 5000 mu g/ml or in a continuous (24-h) treatment test at concentrations up to 1201 mu g/ml. Finally, no mortalities, no abnormal clinical signs, and no significant increase in chromosome damage were observed in an in vivo micronucleus test when administered orally at doses up to 2000 mg/kg/day. Thus, ubiquinol was evaluated as negative in the bacterial reverse mutation, chromosomal aberration, and rat bone marrow micronucleus tests under the conditions of these assays.

Subchronic Oral Toxicity of Ubiquinol in Rats and Dogs

Ubiquinol is the two-electron reduction product of ubiquinone (coenzyme Q10or CoQ10) and functions as an antioxidant in both mitochondria and lipid membranes. In humans and most mammals, including dogs, the predominant form of coenzyme Q is coenzyme Q10, whereas the primary form in rodents is coenzyme Q9(CoQ9). Therefore, the subchronic toxicity of ubiquinol was evaluated and compared in Sprague-Dawley rats and beagle dogs. In the initial rat study, males and females were given ubiquinol at doses of 0, 300, 600, or 1200 mg/kg or ubiquinone at 1200 mg/kg by gavage for 13 weeks. This was followed by the second study, where females were given with doses of 75, 150, 200, or 300 mg/kg/day in order to determine a no observed adverse effect level (NOAEL). In the dog study, the test material was administered to males and females at dose levels of 150, 300, and 600 mg/kg, and ubiquinone was included at 600 mg/kg. Clinical observations, mortality, body weights, food and water consumption, ophthalmoscopy, urinalysis, hematology, blood biochemistry, gross findings, organ weights, and histopathological findings were examined. In both species, determination of plasma and liver ubiquinol concentrations, measured as total coenzyme Q10, were performed. There were no deaths or test article–related effects in body weight, food consumption, ophthalmology, urinalysis, or hematology in rats. Histopathological examinations revealed test article–related effects on the liver, spleen, and mesenteric lymph node in female rats but not in male rats. In the liver, fine vacuolation of hepatocytes was observed in the ubiquinol groups at 200 mg/kg and above. These changes were judged to be of no toxicological significance because they were not considered to induce cytotoxic changes. Microgranuloma and focal necrosis with accumulation of macrophages were observed in the ubiquinol groups at 300 mg/kg and above. These findings were accompanied by slight increases in blood chemistry enzymes (aspartate aminotransferase [AST], alanine aminotransferase [ALT], and lactate dehydrogenase [LDH]), which was suggestive of either potential hepatotoxicity or a normal physiological response to ubiguinol loading. Microgranuloma, and focal necrosis were judged to be only adverse effects induced by test article based on their incidence and pathological characteristics. These changes observed in liver were thought due to uptake of the administered ubiquinol by the liver as an adaptive response to xenobiotics, and the microgranulomas and focal necrosis were considered the results of excessive uptake of ubiquinol, which exceeded the capacity for adaptive response. Based on these findings the NOAEL in rats was conservatively estimated to be 600 mg/kg/day for males and 200 mg/kg/day for females. In dogs, there were no deaths or ubiquinol-related toxicity findings during the administration period. No test article–related effects were observed in body weight, food consumption, ophthalmology, electrocardiogram, urinalysis, hematology, or blood chemistry. Histopathological examination revealed no effects attributable to administration of ubiquinol or ubiquinone in any organs examined. Based on these findings, a NOAEL for ubiquinol in male and female dogs was estimated to be more than 600 mg/kg/day under the conditions of this study.

Structure of glycerol-3-phosphate dehydrogenase, an essential monotopic membrane enzyme involved in respiration and metabolism

Sn-glycerol-3-phosphate dehydrogenase (GlpD) is an essential membrane enzyme, functioning at the central junction of respiration, glycolysis, and phospholipid biosynthesis. Its critical role is indicated by the multitiered regulatory mechanisms that stringently controls its expression and function. Once expressed, GlpD activity is regulated through lipid-enzyme interactions in Escherichia coli. Here, we report seven previously undescribed structures of the fully active E. coli GlpD, up to 1.75 A resolution. In addition to elucidating the structure of the native enzyme, we have determined the structures of GlpD complexed with substrate analogues phosphoenolpyruvate, glyceric acid 2-phosphate, glyceraldehyde-3-phosphate, and product, dihydroxyacetone phosphate. These structural results reveal conformational states of the enzyme, delineating the residues involved in substrate binding and catalysis at the glycerol-3-phosphate site. Two probable mechanisms for catalyzing the dehydrogenation of glycerol-3-phosphate are envisioned, based on the conformational states of the complexes. To further correlate catalytic dehydrogenation to respiration, we have additionally determined the structures of GlpD bound with ubiquinone analogues menadione and 2-n-heptyl-4-hydroxyquinoline N-oxide, identifying a hydrophobic plateau that is likely the ubiquinone-binding site. These structures illuminate probable mechanisms of catalysis and suggest how GlpD shuttles electrons into the respiratory pathway. Glycerol metabolism has been implicated in insulin signaling and perturbations in glycerol uptake and catabolism are linked to obesity in humans. Homologs of GlpD are found in practically all organisms, from prokaryotes to humans, with >45% consensus protein sequences, signifying that these structural results on the prokaryotic enzyme may be readily applied to the eukaryotic GlpD enzymes.

Respiratory chain components involved in the glycerophosphate dehydrogenase-dependent ROS production by brown adipose tissue mitochondria

Involvement of mammalian mitochondrial glycerophosphate dehydrogenase (mGPDH, EC 1.1.99.5) in reactive oxygen species (ROS) generation was studied in brown adipose tissue mitochondria by different spectroscopic techniques. Spectrofluorometry using ROS-sensitive probes CM-H2DCFDA and Amplex Red was used to determine the glycerophosphate- or succinate-dependent ROS production in mitochondria supplemented with respiratory chain inhibitors antimycin A and myxothiazol. In case of glycerophosphate oxidation, most of the ROS originated directly from mGPDH and coenzyme Q while complex III was a typical site of ROS production in succinate oxidation. Glycerophosphate-dependent ROS production monitored by KCN-insensitive oxygen consumption was highly activated by one-electron acceptor ferricyanide, whereas succinate-dependent ROS production was unaffected. In addition, superoxide anion radical was detected as a mGPDH-related primary ROS species by fluorescent probe dihydroethidium, as well as by electron paramagnetic resonance (EPR) spectroscopy with DMPO spin trap. Altogether, the data obtained demonstrate pronounced differences in the mechanism of ROS production originating from oxidation of glycerophosphate and succinate indicating that electron transfer from mGPDH to coenzyme Q is highly prone to electron leak and superoxide generation.

Remote Position Substituents as Modulators of Conformational and Reactive Properties of Quinones. Relevance of the π/π Intramolecular Interaction

Several studies have described that quinoid rings with electron-rich olefins at remote position experience changes in their redox potential. Since the original description of these changes, different approaches have been developed to describe the properties of the binding sites of ubiquinones. The origin of this phenomenon has been attributed to lateral chain flexibility and its effect on the recognition between proteins and substrates associated with their important biological activity. The use of electrochemical-electron spin resonance (EC-ESR) assays and theoretical calculations at MP2/6-31G(d,p) and MP2/6-31++G(d,p)//MP2/6-31G(d,p) levels of several conformers of perezone [(2-(1,5-dimethyl-4-hexenyl)-3-hydroxy-5-methyl-1,4-benzoquinone] established that a weak pi-pi interaction controls not only the molecular conformation but also its diffusion coefficient and electrochemical properties. An analogous interaction can be suggested as the origin of similar properties of ubiquinone Q10. The use of nuclear magnetic resonance rendered, for the first time, direct evidence of the participation of different perezone conformers in solution and explained the cycloaddition process observed when the aforementioned quinone is heated to form pipitzols, sesquiterpenes with a cedrene skeleton. The fact that biological systems can modulate the redox potential of this type of quinones depending on the conformer recognized by an enzyme during a biological transformation is of great relevance.

Supramolecular Structure of the Mitochondrial Oxidative Phosphorylation System

The protein complexes of the mitochondrial oxidative phosphorylation system were recently reported to form supramolecular assemblies termed respiratory supercomplexes or respirasomes. These supercomplexes are considered to be of great functional importance. Here we review new insights into supercomplex structure and physiology.

Structure of electron transfer flavoprotein-ubiquinone oxidoreductase and electron transfer to the mitochondrial ubiquinone pool

Electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) is a 4Fe4S flavoprotein located in the inner mitochondrial membrane. It catalyzes ubiquinone (UQ) reduction by ETF, linking oxidation of fatty acids and some amino acids to the mitochondrial respiratory chain. Deficiencies in ETF or ETF-QO result in multiple acyl-CoA dehydrogenase deficiency, a human metabolic disease. Crystal structures of ETF-QO with and without bound UQ were determined, and they are essentially identical. The molecule forms a single structural domain. Three functional regions bind FAD, the 4Fe4S cluster, and UQ and are closely packed and share structural elements, resulting in no discrete structural domains. The UQ-binding pocket consists mainly of hydrophobic residues, and UQ binding differs from that of other UQ-binding proteins. ETF-QO is a monotopic integral membrane protein. The putative membrane-binding surface contains an alpha-helix and a beta-hairpin, forming a hydrophobic plateau. The UQ-flavin distance (8.5 A) is shorter than the UQ-cluster distance (18.8 A), and the very similar redox potentials of FAD and the cluster strongly suggest that the flavin, not the cluster, transfers electrons to UQ. Two possible electron transfer paths can be envisioned. First, electrons from the ETF flavin semiquinone may enter the ETF-QO flavin one by one, followed by rapid equilibration with the cluster. Alternatively, electrons may enter via the cluster, followed by equilibration between centers. In both cases, when ETF-QO is reduced to a two-electron reduced state (one electron at each redox center), the enzyme is primed to reduce UQ to ubiquinol via FAD.

TCDD decreases ATP levels and increases reactive oxygen production through changes in mitochondrial F(0)F(1)-ATP synthase and ubiquinone

Mitochondria generate ATP and participate in signal transduction and cellular pathology and/or cell death. TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) decreases hepatic ATP levels and generates mitochondrial oxidative DNA damage, which is exacerbated by increasing mitochondrial glutathione redox state and by inner membrane hyperpolarization. This study identifies mitochondrial targets of TCDD that initiate and sustain reactive oxygen production and decreased ATP levels. One week after treating mice with TCDD, liver ubiquinone (Q) levels were significantly decreased, while rates of succinoxidase and Q-cytochrome c oxidoreductase activities were increased. However, the expected increase in Q reduction state following TCDD treatment did not occur; instead, Q was more oxidized. These results could be explained by an ATP synthase defect, a premise supported by the unusual finding that TCDD lowers ATP/O ratios without concomitant changes in respiratory control ratios. Such results suggest either a futile cycle in ATP synthesis, or hydrolysis of newly synthesized ATP prior to release. The TCDD-mediated decrease in Q, concomitant with an increase in respiration, increases complex 3 redox cycling. This acts in concert with glutathione to increase membrane potential and reactive oxygen production. The proposed defect in ATP synthase explains both the greater respiratory rates and the lower tissue ATP levels.

Kinetics of bimolecular reactions in model bilayers and biological membranes. A critical review

The quantitative study of the probability of molecular encounters giving rise to a reaction in membranes is a challenging discipline. Model systems, model in the sense that they use model bilayers and model reactants, have been widely used for this purpose, but the methodologies employed for the analysis of the results obtained in experiments, and for experimental design, are so disparate that a concerned experimentalist has difficulty in deciding about the value of each approach. This review intends to examine the several approaches that can be found in the literature showing, when feasible, the weakness, strengths and limits of application of each of them. There is not, so far, a full experimental validation of the most promising theories for the analysis of reactions in two dimensions, what leaves open a large field for new research. The major challenge resides in the time range in which the processes take place, but the possibilities of the existing techniques for these studies are far from exhausted. We review also the attempts of several authors to quantitatively analyze the kinetics of reactions in biological membranes. Especially in this field, the recently developed microspectroscopies enclose a still unexplored potential.

The mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase of Trypanosomatidae and the glycosomal redox balance of insect stages of Trypanosoma brucei and Leishmania spp

The genes for the mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase were identified in Trypanosoma brucei and Leishmania major genomes. We have expressed the L. major gene in Saccharomyces cerevisiae and confirmed the subcellular localization and activity of the produced enzyme. Using cultured T. brucei procyclic and Leishmania mexicana promastigote cells with a permeabilized plasma membrane and containing intact glycosomes, it was shown that dihydroxyacetone phosphate is converted into pyruvate, and stimulates oxygen consumption, indicating that all components of the glycerol 3-phosphate/dihydoxyacetone phosphate shuttle between glycosomes and mitochondrion are present in these insect stages of both organisms. A computer model has been prepared for the energy and carbohydrate metabolism of these cells. It was used in an elementary mode analysis to get insight into the metabolic role of the shuttle in these insect-stage parasites. Our analysis suggests that the shuttle fulfils important roles for these organisms, albeit different from its well-known function in the T. brucei bloodstream form. It allows (1) a high yield of further metabolizable glycolytic products by decreasing the need to produce a secreted end product of glycosomal metabolism, succinate; (2) the consumption of glycerol and glycerol 3-phosphate derived from lipids; and (3) to keep the redox balance of the glycosome finely tuned due to a highly flexible and redundant system.

Mitochondrial Complex I: structural and functional aspects

This review examines two aspects of the structure and function of mitochondrial Complex I (NADH Coenzyme Q oxidoreductase) that have become matter of recent debate. The supramolecular organization of Complex I and its structural relation with the remainder of the respiratory chain are uncertain. Although the random diffusion model [C.R. Hackenbrock, B. Chazotte, S.S. Gupte, The random collision model and a critical assessment of diffusion and collision in mitochondrial electron transport, J. Bioenerg. Biomembranes 18 (1986) 331-368] has been widely accepted, recent evidence suggests the presence of supramolecular aggregates. In particular, evidence for a Complex I-Complex III supercomplex stems from both structural and kinetic studies. Electron transfer in the supercomplex may occur by electron channelling through bound Coenzyme Q in equilibrium with the pool in the membrane lipids. The amount and nature of the lipids modify the aggregation state and there is evidence that lipid peroxidation induces supercomplex disaggregation. Another important aspect in Complex I is its capacity to reduce oxygen with formation of superoxide anion. The site of escape of the single electron is debated and either FMN, iron-sulphur clusters, and ubisemiquinone have been suggested. The finding in our laboratory that two classes of hydrophobic inhibitors have opposite effects on superoxide production favours an iron-sulphur cluster (presumably N2) is the direct oxygen reductant. The implications in human pathology of better knowledge on these aspects of Complex I structure and function are briefly discussed.

Influence of the chain length of ubiquinones on their interaction with DPPC in mixed monolayers

The thermodynamic behavior of representative short (UQ2), middle (UQ4 and UQ6) and long-chain (UQ10) ubiquinones (UQ) mixed with dipalmitoyl-phosphatidylcholine (DPPC) was studied in monolayers at the air-water interface. The influence of isoprenoid chain-length of UQ on miscibility of both lipids was investigated by analysis of surface pressure-area isotherms and using fluorescence microscopy. Analysis of excess areas (A(ex)) and free energies of mixing (DeltaGm), calculated from compression isotherms in the full range of ubiquinones concentrations, has given evidences for UQ-rich constant-size (UQ6, UQ10) or less growth limited (UQ2, UQ4) microdomains formation within mixed films. Fluorescence microscopy observation revealed that ubiquinones are preferentially soluble in the expanded phase. When lateral pressure increased, concomitant evolutions of A(ex) and DeltaGm parameters, and composition dependence of collapse surface pressures, argue for an evolution towards a total segregation, never reached due to expulsion of ubiquinones from the film. The possible significance of these observations is discussed in relation to ubiquinones organization and similar chain length effects in membranes.

New insights into structure and function of mitochondria and their role in aging and disease

This review covers some novel findings on mitochondrial biochemistry and discusses diseases due to mitochondrial DNA mutations as a model of the changes occurring during physiological aging. The random collision model of organization of the mitochondrial respiratory chain has been recently challenged on the basis of findings of supramolecular organization of respiratory chain complexes. The source of superoxide in Complex I is discussed on the basis of laboratory experiments using a series of specific inhibitors and is presumably iron sulfur center N2. Maternally inherited diseases due to mutations of structural genes in mitochondrial DNA are surveyed as a model of alterations mimicking those occurring during normal aging. The molecular defects in senescence are surveyed on the basis of the "Mitochondrial Theory of Aging", establishing mitochondrial DNA somatic mutations, caused by accumulation of oxygen radical damage, to be at the basis of cellular senescence. Mitochondrial production of reactive oxygen species increases with aging and mitochondrial DNA mutations and deletions accumulate and may be responsible for oxidative phosphorylation defects. Evidence is presented favoring the mitochondrial theory, with primary mitochondrial alterations, although the problem is made more complex by changes in the cross-talk between nuclear and mitochondrial DNA.

Mitochondrial lipid abnormality and electron transport chain impairment in mice lacking alpha-synuclein

The presynaptic protein alpha-synuclein, implicated in Parkinson disease (PD), binds phospholipids and has a role in brain fatty acid (FA) metabolism. In mice lacking alpha-synuclein (Snca-/-), total brain steady-state mass of the mitochondria-specific phospholipid, cardiolipin, is reduced 22% and its acyl side chains show a 51% increase in saturated FAs and a 25% reduction in essential n-6, but not n-3, polyunsaturated FAs. Additionally, 23% reduction in phosphatidylglycerol content, the immediate biosynthetic precursor of cardiolipin, was observed without alterations in the content of other brain phospholipids. Consistent with these changes, more ordered lipid head group and acyl chain packing with enhanced rotational motion of diphenylhexatriene (DPH) about its long axis were demonstrated in time-resolved DPH fluorescence lifetime experiments. These abnormalities in mitochondrial membrane properties were associated with a 15% reduction in linked complex I/III activity of the electron transport chain, without reductions in mitochondrial number, complex II/III activity, or individual complex I, II, III, or IV activity. Reduced complex I activity is thought to be a critical factor in the development of PD. Thus, altered membrane composition and structure and impaired complex I/III function in Snca-/- brain suggest a relationship between alpha-synuclein's role in brain lipid metabolism, mitochondrial function, and PD.

Localization of coenzyme Q10 in the center of a deuterated lipid membrane by neutron diffraction

Quinones (e.g., coenzyme Q, CoQ10) are best known as carriers of electrons and protons during oxidative phosphorylation and photosynthesis. A myriad of mostly more indirect physical methods, including fluorescence spectroscopy, electron-spin resonance, and nuclear magnetic resonance, has been used to localize CoQ10 within lipid membranes. They have yielded equivocal and sometimes contradictory results. Seeking unambiguous evidence for the localization of ubiquinone within lipid bilayers, we have employed neutron diffraction. CoQ10 was incorporated into stacked bilayers of perdeuterated dimyristoyl phosphatidyl choline doped with dimyristoyl phosphatidyl serine containing perdeuterated chains in the natural fluid-crystalline state. Our data show CoQ10 at the center of the hydrophobic core parallel to the membrane plane and not, as might be expected, parallel to the lipid chains. This localization is of importance for its function as a redox shuttle between the respiratory complexes and, taken together with our recent result that squalane is in the bilayer center, may be interpreted to show that all natural polyisoprene chains lie in the bilayer center. Thus ubiquinone, in addition to its free radical scavenging and its well-known role in oxidative phosphorylation as a carrier of electrons and protons, might also act as an inhibitor of transmembrane proton leaks.

Specificity of coenzyme Q10 for a balanced function of respiratory chain and endogenous ubiquinone biosynthesis in human cells

Coenzyme Q (Q) is an obligatory component of both respiratory chain and uncoupling proteins. Also, Q acts as an antioxidant in cellular membranes. Several neurodegenerative diseases are associated with modifications of Q10 levels. For these reasons, therapies based on Q supplementation in the diet are currently studied in order to mitigate the symptoms of these diseases. However, the incorporation of exogenous Q also affects aging process in nematodes probably affecting reactive oxygen species (ROS) production. The aim of the present work is to clarify if supplementation with both Q10 and Q6 isoforms affects mitochondrial Q10 content, respiratory chain activity and ROS levels in human cells. Cells incorporated exogenously added Q10 and Q6 isoforms into mitochondria that produced changes in mitochondrial activity depending on the side chain length. Supplementation with Q10, but not with Q6, increased mitochondrial Q-dependent activities. However, Q6 affected the mitochondrial membrane potential, ROS production, and increased the protein levels of both catalase and Mn-superoxide dismutase (Mn-SOD). Also, Q6 induced a transient decrease in endogenous mitochondrial Q10 levels by increasing its catabolism. These results show that human cells supplemented with Q6 undergo a mitochondrial impairment, which is not observed with Q10 supplementation.

Mitochondrial Oxidative Phosphorylation Is Defective in the Long-lived Mutant clk-1

The long-lived mutant of Caenorhabditis elegans, clk-1, is unable to synthesize ubiquinone, CoQ(9). Instead, the mutant accumulates demethoxyubiquinone(9) and small amounts of rhodoquinone(9) as well as dietary CoQ(8). We found a profound defect in oxidative phosphorylation, a test of integrated mitochondrial function, in clk-1 mitochondria fueled by NADH-linked electron donors, i.e. complex I-dependent substrates. Electron transfer from complex I to complex III, which requires quinones, is severely depressed, whereas the individual complexes are fully active. In contrast, oxidative phosphorylation initiated through complex II, which also requires quinones, is completely normal. Here we show that complexes I and II differ in their ability to use the quinone pool in clk-1. This is the first direct demonstration of a differential interaction of complex I and complex II with the endogenous quinone pool. This study uses the combined power of molecular genetics and biochemistry to highlight the role of quinones in mitochondrial function and aging.

Respiratory chain supercomplexes in plant mitochondria

Supercomplexes are defined associations of protein complexes, which are important for several cellular functions. This "quintenary" organization level of protein structure recently was also described for the respiratory chain of plant mitochondria. Except succinate dehydrogenase (complex II), all complexes of the oxidative phosphorylation (OXPOS) system (complexes I, III, IV and V) were found to form part of supercomplexes. Compositions of these supramolecular structures were systematically investigated using digitonin solubilizations of mitochondrial fractions and two-dimensional Blue-native (BN) polyacrylamide gel electrophoresis. The most abundant supercomplex of plant mitochondria includes complexes I and III at a 1:2 ratio (I1 + III2 supercomplex). Furthermore, some supercomplexes of lower abundance could be described, which have I2 + III4, V2, III2 + IV(1-2), and I1 + III2 + IV(1-4) compositions. Supercomplexes consisting of complexes I plus III plus IV were proposed to be called "respirasome", because they autonomously can carry out respiration in the presence of ubiquinone and cytochrome c. Plant specific alternative oxidoreductases of the respiratory chain were not associated with supercomplexes under all experimental conditions tested. However, formation of supercomplexes possibly indirectly regulates alternative respiratory pathways in plant mitochondria on the basis of electron channeling. In this review, procedures to characterize the supermolecular organization of the plant respiratory chain and results concerning supercomplex structure and function are summarized and discussed.

Bioenergetics of mitochondrial diseases associated with mtDNA mutations

This mini-review summarizes our present view of the biochemical alterations associated with mitochondrial DNA (mtDNA) point mutations. Mitochondrial cytopathies caused by mutations of mtDNA are well-known genetic and clinical entities, but the biochemical pathogenic mechanisms are often obscure. Leber's hereditary optic neuropathy (LHON) is due to three main mutations in genes for complex I subunits. Even if the catalytic activity of complex I is maintained except in cells carrying the 3460/ND1 mutation, in all cases there is a change in sensitivity to complex I inhibitors and an impairment of mitochondrial respiration, eliciting the possibility of generation of reactive oxygen species (ROS) by the complex. Neurogenic muscle weakness, Ataxia and Retinitis Pigmentosa (NARP), is due to a mutation in the ATPase-6 gene. In NARP patients ATP synthesis is strongly depressed to an extent proportional to the mutation load; nevertheless, ATP hydrolysis and ATP-driven proton translocation are not affected. It is suggested that the NARP mutation affects the ability of the enzyme to couple proton transport to ATP synthesis. A point mutation in subunit III of cytochrome c oxidase is accompanied by a syndrome resembling MELAS: however, no major biochemical defect is found, if we except an enhanced production of ROS. The mechanism of such enhancement is at present unknown. In this review, we draw attention to a few examples in which the overproduction of ROS might represent a common step in the induction of clinical phenotypes and/or in the progression of several human pathologies associated with mtDNA point mutations.

The mitochondrial respiratory chain is partially organized in a supercomplex assembly: kinetic evidence using flux control analysis

The model of the respiratory chain in which the enzyme complexes are independently embedded in the lipid bilayer of the inner mitochondrial membrane and connected by randomly diffusing coenzyme Q and cytochrome c is mostly favored. However, multicomplex units can be isolated from mammalian mitochondria, suggesting a model based on direct electron channeling between complexes. Kinetic testing using metabolic flux control analysis can discriminate between the two models: the former model implies that each enzyme may be rate-controlling to a different extent, whereas in the latter, the whole metabolic pathway would behave as a single supercomplex and inhibition of any one of its components would elicit the same flux control. In particular, in the absence of other components of the oxidative phosphorylation apparatus (i.e. ATP synthase, membrane potential, carriers), the existence of a supercomplex would elicit a flux control coefficient near unity for each respiratory complex, and the sum of all coefficients would be well above unity. Using bovine heart mitochondria and submitochondrial particles devoid of substrate permeability barriers, we investigated the flux control coefficients of the complexes involved in aerobic NADH oxidation (I, III, IV) and in succinate oxidation (II, III, IV). Both Complexes I and III were found to be highly rate-controlling over NADH oxidation, a strong kinetic evidence suggesting the existence of functionally relevant association between the two complexes, whereas Complex IV appears randomly distributed. Moreover, we show that Complex II is fully rate-limiting for succinate oxidation, clearly indicating the absence of substrate channeling toward Complexes III and IV.

The mitochondrial production of reactive oxygen species in relation to aging and pathology

Mitochondria are known to be strong producers of reactive oxygen species (ROS) and, at the same time, particularly susceptible to the oxidative damage produced by their action on lipids, proteins, and DNA. In particular, damage to mtDNA induces alterations to the polypeptides encoded by mtDNA in the respiratory complexes, with consequent decrease of electron transfer, leading to further production of ROS and thus establishing a vicious circle of oxidative stress and energetic decline. This deficiency in mitochondrial energetic capacity is considered the cause of aging and age-related degenerative diseases. Complex I would be the enzyme most affected by ROS, since it contains seven of the 13 subunits encoded by mtDNA. Accordingly, we found that complex I activity is significantly affected by aging in rat brain and liver mitochondria as well as in human platelets. Moreover, due to its rate control over aerobic respiration, such alterations are reflected on the entire oxidative phosphorylation system. We also investigated the role of mitochondrial complex I in superoxide production and found that the one-electron donor to oxygen is most probably the Fe-S cluster N2. Short chain coenzyme Q (CoQ) analogues enhance ROS formation, presumably by mediating electron transfer from N2 to oxygen, both in bovine heart SMP and in cultured HL60 cells. Nevertheless, we have accumulated much evidence of the antioxidant role of reduced CoQ(10) in several cellular systems and demonstrated the importance of DT-diaphorase and other internal cellular reductases to reduce exogenous CoQ(10) after incorporation.

Metabolism and function of coenzyme Q

Coenzyme Q (CoQ) is present in all cells and membranes and in addition to be a member of the mitochondrial respiratory chain it has also several other functions of great importance for the cellular metabolism. This review summarizes the findings available to day concerning CoQ distribution, biosynthesis, regulatory modifications and its participation in cellular metabolism. There are a number of indications that this lipid is not always functioning by its direct presence at the site of action but also using e.g. receptor expression modifications, signal transduction mechanisms and action through its metabolites. The biosynthesis of CoQ is studied in great detail in bacteria and yeast but only to a limited extent in animal tissues and therefore the informations available is restricted. However, it is known that the CoQ is compartmentalized in the cell with multiple sites of biosynthesis, breakdown and regulation which is the basis of functional specialization. Some regulatory mechanisms concerning amount and biosynthesis are established and nuclear transcription factors are partly identified in this process. Using appropriate ligands of nuclear receptors the biosynthetic rate can be increased in experimental system which raises the possibility of drug-induced upregulation of the lipid in deficiency. During aging and pathophysiological conditions the tissue concentration of CoQ is modified which influences cellular functions. In this case the extent of disturbances is dependent on the localization and the modified distribution of the lipid at cellular and membrane levels.

Evaluation of ubiquinone concentration and mitochondrial function relative to cerivastatin-induced skeletal myopathy in rats

As a class, hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors can potentially cause skeletal myopathy. One statin, cerivastatin, has recently been withdrawn from the market due to an unacceptably high incidence of rhabdomyolysis. The mechanism underlying statin-induced myopathy is unknown. This paper sought to investigate the relationship among statin-induced myopathy, mitochondrial function, and muscle ubiquinone levels. Rats were administered cerivastatin at 0.1, 0.5, and 1.0 (mg/kg)/day or dose vehicle (controls) by oral gavage for 15 days. Samples of type I-predominant skeletal muscle (soleus) and type II-predominant skeletal muscle [quadriceps and extensor digitorum longus (EDL)], and blood were collected on study days 5, 10, and 15 for morphological evaluation, clinical chemistry, mitochondrial function tests, and analysis of ubiquinone levels. No histological changes were observed in any of the animals on study days 5 or 10, but on study day 15, mid- and high-dose animals had necrosis and inflammation in type II skeletal muscle. Elevated creatine kinase (CK) levels in blood (a clinical marker of myopathy) correlated with the histopathological diagnosis of myopathy. Ultrastructural characterization of skeletal muscle revealed disruption of the sarcomere and altered mitochondria only in myofibers with degeneration, while adjacent myofibers were unaffected and had normal mitochondria. Thus, mitochondrial effects appeared not to precede myofiber degeneration. Mean coenzyme Q9 (CoQ9) levels in all dose groups were slightly decreased relative to controls in type II skeletal muscle, although the difference was not significantly different in most cases. Mitochondrial function in skeletal muscle was not affected by the changes in ubiquinone levels. The ubiquinone levels in high-dose-treated animals exhibiting myopathy were not significantly different from low-dose animals with no observable toxic effects. Furthermore, ubiquinone levels did not correlate with circulating CK levels in treated animals. The results of this study suggest that neither mitochondrial injury, nor a decrease in muscle ubiquinone levels, is the primary cause of skeletal myopathy in cerivastatin-dosed rats.

Theoretical Study of the Trapping of the OOH Radical by Coenzyme Q

The reactivity of the hydroperoxyl radical with coenzyme Q, as a prototypical chemical reaction involved in biological antioxidant actions, was studied theoretically. Two pathways were analyzed: the hydrogen abstraction reaction from the phenolic hydrogen on the reduced form (ubiquinol), and OOH addition on the oxidized form (ubiquinone). Optimized geometries, harmonic vibrational frequencies, and energies of the stationary points (reactants, intermediate complexes, transition states, and products) for each pathway were calculated at the BHandHLYP/6-31G level of theory. The reaction paths for the two mechanisms were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory with multidimensional small-curvature tunneling. We found that the reactivity of the OOH radical is dominated by the hydrogen abstraction mechanism on ubiquinol, with a rate constant of 5.32 x 10(5) M(-1) s(-1), at 298 K. This result strongly contrasts with that, also obtained by our group, for the more reactive OH radical, which attacks ubiquinone by an addition mechanism, with a diffusion-controlled rate of 6.25 x 10(10) M(-1) s(-1), at 298 K.