Ubiquinone-10 Research Articles

Location and interaction of idebenone and mitoquinone in a membrane similar to the inner mitochondrial membrane. Comparison with ubiquinone 10

Oxygen is essential for aerobic life on earth but it is also the origin of harmful reactive oxygen species (ROS). Ubiquinone is par excellence the endogenous cellular antioxidant, but a very hydrophobic one. Because of that, other molecules have been envisaged, such as idebenone (IDE) and mitoquinone (MTQ), molecules having the same redox active benzoquinone moiety but higher solubility. We have used molecular dynamics to determine the location and interaction of these molecules, both in their oxidized and reduced forms, with membrane lipids in a membrane similar to that of the mitochondria. Both IDE and reduced IDE (IDOL) are situated near the membrane interface, whereas both MTQ and reduced MTQ (MTQOL) locate in a position adjacent to the phospholipid hydrocarbon chains. The quinone moieties of both ubiquinone 10 (UQ10) and reduced UQ10 (UQOL10) in contraposition to the same moieties of IDE, IDOL, MTQ and MTQOL, located near the membrane interphase, whereas the isoprenoid chains remained at the middle of the hydrocarbon chains. These molecules do not aggregate and their functional quinone moieties are located in the membrane at different depths but near the hydrophobic phospholipid chains whereby protecting them from ROS harmful effects.

The location of coenzyme Q10 in phospholipid membranes made of POPE: a small-angle synchrotron X-ray diffraction study.

The location of coenzyme Q10 (Q10) inside the inner mitochondrial membrane is a topic of research aiming at a deeper understanding of the function of the mitochondrial respiratory chain. We investigated the location of Q10 inside model membranes made of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine by means of small-angle synchrotron X-ray diffraction. Q10, which stands for ubiquinone-10 (UQ) or ubihydroquinone-10 (UH), did not remarkably influence the main phase transition temperature, but significantly decreased the lamellar-inverse hexagonal phase transition temperature (T(h)). The effect of UH on T(h) was stronger than the effect of UQ and the effect of liquid Q10 on T(h) was stronger than the effect of crystalline Q10. In the presence of Q10, the lattice parameters of the lamellar phases remained unchanged, whereas the H II lattice parameter was clearly influenced: While UQ had an increasing effect, UH had a decreasing effect. Furthermore, Q10 prevented the formation of cubic phases. The results give new evidence that the headgroup of Q10 is distant from the center of the membrane, which might be important for the function of the mitochondrial respiratory chain.

Electrochemistry of LB films of mixed MGDG:UQ on ITO

The electrochemical behaviour of biomimetic monolayers of monogalactosyldiacylglycerol (MGDG) incorporating ubiquinone-10 (UQ) has been investigated. MGDG is the principal component in the thylakoid membrane and UQ seems a good substitute for plastoquinone-9, involved in photosynthesis chain. The monolayers have been performed using the Langmuir and Langmuir–Blodgett (LB) techniques and the redox behaviour of the LB films, transferred at several surface pressures on a glass covered with indium–tin oxide (ITO), has been characterized by cyclic voltammetry. The cyclic voltammograms show that UQ molecules present two redox processes (I and II) at high UQ content and high surface pressures, and only one redox process (I) at low UQ content and low surface pressures. The apparent rate constants calculated for processes I and II indicate a different kinetic control for the reduction and the oxidation of UQ/UQH2 redox couple, being kRapp(I)=2.2·10−5s−1, kRapp(II)=5.1·10−14 kOapp(I)=3.3·10−3s−1 and kOapp(II)=6.1·10−6s−1, respectively. The correlation of the redox response with the physical states of the LB films allows determining the positions of the UQ molecules in the biomimetic monolayer, which change with the surface pressure and the UQ content. These positions are known as diving and swimming.

Efficient dye-sensitized solar cells employing highly environmentally-friendly ubiquinone 10 based I2-free electrolyte inspired by photosynthesis

A highly environmentally-friendly ubiquinone 10 (UQ10) based I2-free electrolyte, which is inspired by photosynthesis, is employed in dye-sensitized solar cells (DSSCs) under 100 mW cm−2(AM 1.5G) illumination.

Erythrobacter jejuensis sp. nov., isolated from seawater

A Gram-staining-negative, yellow-pigmented, non-motile, strictly aerobic, rod-shaped bacterium, designated strain CNU001(T), was isolated from seawater collected on the coast of Jeju Island, South Korea, and subjected to a polyphasic taxonomic study. The temperature, pH and NaCl ranges for growth were 10-30 °C, pH 6.0-10.0 and 2.0-5.0 %, respectively. Phylogenetic analyses based on 16S rRNA gene sequences showed that strain CNU001(T) belonged to the genus Erythrobacter in the family Erythrobacteraceae, with Erythrobacter longus DSM 6997(T) (96.6 % sequence similarity), Erythrobacter gaetbuli SW-161(T) (96.3 %), Erythrobacter vulgaris 022 2-10(T) (96.2 %), Erythrobacter nanhaisediminis T30(T) (96.1 %) and other members of the genus Erythrobacter (<96.0 %) identified as the novel strain's closest relatives. The major cellular fatty acids were C18 : 1ω7c and C17 : 1ω6c. The polar lipids were found to be diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine, phosphatidylglycerol, one sphingoglycolipid, one unidentified aminolipid and six other unidentified lipids. The major respiratory quinone was ubiquinone-10 (UQ-10) and the genomic DNA G+C content of the novel strain was 58.9 mol%. On the basis of phenotypic, phylogenetic and genotypic data, strain CNU001(T) represents a novel species within the genus Erythrobacter, for which the name Erythrobacter jejuensis sp. nov. is proposed. The type strain is CNU001(T) ( = KCTC 23090(T) = JCM 16677(T)).

Biomimetic monolayer films of monogalactosyldiacylglycerol incorporating ubiquinone

Ubiquinone and plastoquinone are two of the main electron and proton shuttle molecules in biological systems, and monogalactosyldiacylglycerol (MGDG) is the most abundant lipid in the thylakoid membrane of chloroplasts. Saturated MGDG, ubiquinone-10 (UQ) and MGDG:UQ mixed monolayers at the air/water interface have been studied using surface pressure–area isotherms and Brewster Angle Microscopy. Moreover, the transferred Langmuir–Blodgett films have been observed by Atomic Force Microscopy. The results show that MGDG:UQ mixtures present more fluid phase than pure MGDG, indicating a higher order degree for the later. It is also observed an important influence of UQ on the MGDG matrix before UQ collapse pressure and a low influence after this event, due to UQ expulsion from the MGDG matrix. This expulsion leads to a similar remaining UQ content for all the tested mixtures, indicating a limiting content of this molecule in the MGDG matrix at high surface pressures. The thermodynamic studies confirm the stability of the MGDG:UQ mixtures at low surface pressures, although presenting a non-ideal behaviour. Results point to consider UQ as a good candidate for studies of artificial photosynthesis.

Electron Transport in Supported and Tethered Lipid Bilayers Modified with Bioelectroactive Molecules

Tethered bilayer lipid membranes (tBLMs) are commonly used as model membranes. However, in biophysical studies, free-standing membranes (“black” lipid membranes) are more realistic models of cellular processes. In this article, we discuss the rates of electron transfer in both types of bilayer lipid membranes. These BLMs were then modified using two very important mitochondrial membrane-associated molecules: ubiquinone-10 (UQ10) and α-tocopherol (VitE). The electron transfer rates in the unmodified films were studied with three redox couples, Fe(CN)6(3-/4-), Ru(NH3)6(3+/2+), and NAD+/NADH, using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The rate of electron transfer in the modified films was studied using the biologically relevant NAD+/NADH electroactive couple using the same methods. It is shown that when the BLMs are modified with only UQ10, it is possible to observe electron transfer. However, when the antioxidant VitE is added to the modification, the electron transfer provided by UQ10 is inhibited. Following initial studies using CV, a comparison of electron transfer theory and data was used to investigate this phenomenon in more detail, using EIS data. The standard rate constant caused by electron tunneling across the film, k(th)(0), depends on the value of β used. Two different values of the potential independent electron tunneling coefficient, β, were fitted, and it is shown that a β value half of those usually reported in literature (refereed here as β(app)) gives better agreement between the theory and the experimental results. The unmodified films present k(th)(0) values on the order of 10(-15) cm s(-1) when β = 0.72 Å(-1) and k(th)(0) values on the order of 10(-9) cm s(-1) when β(app) = 0.38 Å(-1). For the modified films, the values of k(th)(0) are on the order of 10(-15) cm s(-1) when β = 0.72 Å(-1) and 10(-9) cm s(-1) for β(app) = 0.38 Å(-1). The experimental electron transfer rate constant, k(app)(0), is on the order of 10(-8) cm s(-1) for unmodified and modified (with (i) UQ10, (ii) VitE, and (iii) UQ10 + VitE) films.

Electrochemical behaviour of mixed LB films of ubiquinone – DPPC

The structure and the electrochemical behaviour of Langmuir and Langmuir–Blodgett (LB) films of the biological ubiquinone-10 (UQ) and a mixture of dipalmytoilphosphatidylcholine (DPPC) and UQ at the molar ratios DPPC:UQ 5:1 and 10:1 have been investigated. The surface pressure-area isotherms of the Langmuir films and the AFM images of the LB films show the formation of a monolayer in the DPPC:UQ mixture till a certain surface pressure is attained, and then at higher surface pressures the UQ is progressively expelled. The cyclic voltammograms of DPPC:UQ LB films formed on indium tin oxide, ITO, at different surface pressures show one reduction and one oxidation peak at low surface pressures, but two or even more reduction and oxidations peaks at medium and high surface pressures. The electrochemical behaviour is correlated with the film structure.

Gel-Phase Microdomains and Lipid Rafts in Monolayers Affect the Redox Properties of Ubiquinone-10

The redox properties of ubiquinone-10 (UQ) were examined in monolayers of mixtures of dioleoylphosphatidylcholine, palmitoylsphingomyelin, and cholesterol of different compositions, self-assembled on a mercury electrode, over the pH range from 7.5 to 9.5. A detailed analysis of the cyclic voltammograms of UQ in the above lipid environments points to a mechanism consisting of an elementary electron transfer step followed by two protonation (or deprotonation) steps in quasiequilibrium and by a further electron transfer step. In a lipid environment of solid-ordered (so) microdomains in a liquid-disordered (ld) matrix, electron transport across the lipid monolayer takes place in the ld phase. In a pure so phase, UQ tends to segregate into UQ-rich pools, exhibiting reversible electron transfer steps. In a lipid environment consisting of liquid-ordered (lo) microdomains (lipid rafts) in an ld matrix, UQ molecules tend to localize along the edge of the lipid rafts. However, in a lipid environment consisting exclusively of lo and so microdomains, UQ molecules tend to segregate into UQ-rich pools. In all lipid environments, electron transport by UQ occurs with the quinone moiety localized on the solution side with respect to the ester linkages of the dioleoylphosphatidylcholine molecules.

Coenzyme Q10 in human blood: Native levels and determinants of oxidation during processing and storage

Coenzyme Q10 (Q10) is present in the circulation mainly in its reduced form (ubiquinol-10; UL10), but oxidizes quickly ex vivo to ubiquinone-10 (UN10). Therefore, native UL10:UN10 ratios, used as markers of redox status and disease risk, are difficult to measure. We established an RP-(U)HPLC method with coulometric detection to measure natively circulating UL10 and UN10 concentrations by adding a ubiquinol/ubiquinone mixture as an internal standard immediately after plasma preparation. This allowed adjustment for unavoidable artificial UL10 oxidation as well as for total losses (or gains) of analytes during sample storage, processing, and analysis because the internal standards exactly paralleled the chemical behavior of Q10. This technique applied to blood ( n = 13) revealed Q10 levels of 680–3300 nM with a mean UL10:UN10 ratio of 95:5, which was inversely associated with total Q10 ( r = − 0.69; p = 0.004). The oxidation of UL10 to UN10 was equimolar, increased by O 2, and decreased by lower temperatures or various degassing methods. Although UL10 was stable in blood or when pure in organic solvents at 22 °C, its oxidation was catalyzed dose dependently by α-tocopherol and butylated hydroxytoluene, particularly when present in combination. Key structural features for the catalytic pro-oxidant properties of phenolic antioxidants included two substituents vicinal to the phenolic hydroxyl group.

Labrys portucalensis sp. nov., a fluorobenzene-degrading bacterium isolated from an industrially contaminated sediment in northern Portugal

A detailed classification of a novel bacterial strain, designated F11(T), capable of degrading fluorobenzene as a sole carbon and energy source, was performed by using a polyphasic approach. This Gram-negative, rod-shaped, non-motile, non-spore-forming, aerobic bacterium was isolated from a sediment sample collected from an industrially contaminated site in northern Portugal. The predominant whole-cell fatty acids were C(19 : 0) cyclo omega8c, C(16 : 0), C(18 : 1)omega7c, C(18 : 0), C(18 : 0) 3-OH and C(16 : 0) 3-OH. The G+C content of the DNA was 62.9 mol% and the major respiratory quinone was ubiquinone 10 (UQ-10). 16S rRNA gene sequence analysis revealed that strain F11(T) was a member of the class Alphaproteobacteria and was phylogenetically related to the genus Labrys, having sequence similarities of 95.6 and 93.1 % to the type strains of Labrys monachus and Labrys methylaminiphilus, respectively. DNA-DNA hybridization experiments revealed levels of relatedness of <70 % between strain F11(T) and the type strains of L. monachus and L. methylaminiphilus (38.6 and 34.1 %, respectively), justifying the classification of strain F11(T) as representing a novel species of the genus Labrys. The name Labrys portucalensis sp. nov. is proposed for this organism. The type strain is F11(T) (=LMG 23412(T)=DSM 17916(T)).

Ubichromanol: A prodrug to support mitochondrial ubiquinone functions?

Ubichromanol-9 (UCa9), with a side chain consisting of nine isoprene units) is a reductive cyclization product of ubiquinone-10 (UQ10). It acts as a radical scavenging antioxidant and is about half as effective as alpha-tocopherol. Already decades ago its one-electron oxidation product, the ubichromanoxyl radical had been identified. However, nothing was known so far about the two-electron oxidation product of this antioxidant and its bioactivity. This study proves that ubichromanol can be oxidized to a ubiquinone-like compound with a hydroxyl-substituted side chain (UQ10OH), a metabolite that is naturally present in bovine liver mitochondria. The bioactivity of this ubiquinone derivative in its reduced form as substrate for mitochondrial complex III (cytochrome bc1 complex) was slightly below that of native ubiquinol, but significantly higher than that of reduced alpha-tocopheryl quinone. Since ubiquinone-like molecules (UQ10OH, UQ10) were identified as oxidation products of UCa9 during lipid peroxidation, this ubiquinone derivative could provide a possibility to combine antioxidant properties of chromanols and bioenergetic benefits of UQ10.

Does different orientation of the methoxy groups of ubiquinone-10 in the reaction centre of Rhodobacter sphaeroides cause different binding at QA and QB?

The different roles of ubiquinone-10 (UQ10) at the primary and secondary quinone (QA and QB) binding sites of Rhodobacter sphaeroides R26 reaction centres are governed by the protein microenvironment. The 4C=O carbonyl group of QA is unusually strongly hydrogen-bonded, in contrast to QB. This asymmetric binding seems to determine their different functions. The asymmetric hydrogen-bonding at QA can be caused intrinsically by distortion of the methoxy groups or extrinsically by binding to specific amino-acid side groups. Different X-ray-based structural models show contradictory orientations of the methoxy groups and do not provide a clear picture. To elucidate if distortion of the methoxy groups induces this hydrogen-bonding, their (ring-)C-O vibrations were assigned by use of site-specifically labelled [5-13C]UQ10 and [6-13C]UQ10 reconstituted at either the QA or the QB binding site. Two infrared bands at 1288 cm(-1) and 1264 cm(-1) were assigned to the methoxy vibrations. They did not shift in frequency at either the QA or QB binding sites, as compared with unbound UQ10. As the frequencies of these vibrations and their coupling are sensitive to the conformations of the methoxy groups, different conformations of the C(5) and C(6) methoxy groups at the QA and QB binding sites can now be excluded. Both methoxy groups are oriented out of plane at QA and QB. Therefore, hydrogen-bonding to His M219 combined with electrostatic interactions with the Fe2+ ion seems to determine the strong asymmetric binding of QA.

Radical Pair Dynamics and Interactions in Quinone-Reconstituted Photosynthetic Reaction Centers of Rb. Sphaeroides R26: A Multifrequency Magnetic Resonance Study

The secondary photoinduced radical pair state P+• QA-• in Zn-substituted and quinone-reconstituted photosynthetic reaction centers of Rhodobacter sphaeroides R26 is investigated with transient EPR spectroscopy at X- (9.3 GHz), Q- (34.7 GHz), and D-band (130.0 GHz) microwave frequencies. Novel D-band P+• QA-• electron spin-polarized spectra are presented for three different reconstituted reaction centers. The shape of the electron spin-polarized P+• QA-• EPR spectrum strongly depends on the lifetime and magnetic properties of its precursor radical pair P+• ΦA-•, whose lifetime was manipulated by replacing the native ubiquinone-10 (UQ-10) with duroquinone (DQ) or 2-ethyl anthraquinone (AQ). From spectral simulations incorporating the transfer of spin-correlation between the two radical pairs, information about the magnetic interactions and dynamics of the intermediate primary P+• ΦA-• radical pair was obtained. When the lifetime of P+• ΦA-• is longer than a few nanoseconds, the influence of the magnitude an...

Half-life of ubiquinone and plastoquinone in spinach cells

The half-life of plastoquinone (PQ), ubiquinone-9 (UQ-9) and ubiquinone-10 (UQ-10) in spinach tissue was determined. This was achieved by monitoring the decay of radioactivity incorporated into these lipids from a labeled precursor. The half-life of PQ was 15 h while for UQ-9 and UQ-10 it was longer, i.e. 30 h. The values of half-lives of PQ and UQ suggest a high rate of turnover of these lipids in spinach cells.

Interaction of Ubiquinone-10 with Dipalmitoylphosphatidylcholine and Their Formation of Small Dispersed Particles

Stable aqueous dispersions of ubiquinone-10 (UQ) were obtained by cosonication with dipalmitoylphosphatidylcholine (DPPC) in the UQ mole fraction range 0.1–0.7. To clarify the dispersal mechanism, the dispersed particles were characterized, and the interaction between UQ and DPPC was investigated using several physicochemical techniques. Dynamic light scattering (DLS) measurements showed that the diameter of the dispersed particles was 50–70 nm. A limited amount of UQ was incorporated into DPPC bilayer membranes (approximately 5 mol%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein, and the volume in the UQ/DPPC particles decreased remarkably with the addition of UQ into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO4. These results indicate that the excess UQ separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer.

Radical Pair Dynamics and Interactions in Quinone-Reconstituted Photosynthetic Reaction Centers of Rb. sphaeroides R26: A Multifrequency Magnetic Resonance Study

The secondary photoinduced radical pair state P+• QA-• in Zn-substituted and quinone-reconstituted photosynthetic reaction centers of Rhodobacter sphaeroides R26 is investigated with transient EPR spectroscopy at X- (9.3 GHz), Q- (34.7 GHz), and D-band (130.0 GHz) microwave frequencies. Novel D-band P+• QA-• electron spin-polarized spectra are presented for three different reconstituted reaction centers. The shape of the electron spin-polarized P+• QA-• EPR spectrum strongly depends on the lifetime and magnetic properties of its precursor radical pair P+• ΦA-•, whose lifetime was manipulated by replacing the native ubiquinone-10 (UQ-10) with duroquinone (DQ) or 2-ethyl anthraquinone (AQ). From spectral simulations incorporating the transfer of spin-correlation between the two radical pairs, information about the magnetic interactions and dynamics of the intermediate primary P+• ΦA-• radical pair was obtained. When the lifetime of P+• ΦA-• is longer than a few nanoseconds, the influence of the magnitude and sign of the exchange interaction JPΦ between P+• and ΦA-• on the shape of the observed ESP spectrum is significant. This effect is even more pronounced at the relatively high D-band microwave frequency, facilitating accurate determination of JPΦ = −0.9 ± 0.1 mT, for both DQ and AQ-reconstituted RCs, a value not significantly different from that determined here for the UQ-10 reconstituted sample (JPΦ = −0.7 ± 0.5 mT). The singlet and triplet rate constants (kS and kT) for the intermediate radical pair P+• ΦA-• in the duroquinone- or 2-ethyl anthraquinone-reconstituted samples were (2 ± 1) × 107 s-1 and (2 ± 1) × 108 s-1, respectively. The exchange interaction between P+• and ΦA-• is JPQ = −0.5 ± 0.3 μT, −0.2 ± 0.1 μT, and −0.5 ± 0.2 μT for UQ-10, AQ, and DQ, respectively. X-band ESEEM experiments showed that the dipolar interaction between P+• and ΦA-• is −0.12 ± 0.01 mT, independent of the quinone, corresponding to a center-to-center distance of 28.5 ± 0.8 Å.

Voltammetric Probing of Molecular Assemblies of Ubiquinone-10 at the Air–Water Interfaces

The redox properties of ubiquinone 10 (UQ10) placed at the air–water interface were studied using the horizontal touching method with a thin mercury film (TMFE) working electrode and cyclic voltammetry. Changes of pH of the subphase affected the formal potential of the ubiquinone/ubiquinol system exhibiting the participation of protons in the overall reduction of UQ10. The protonation of the semiquinone transition product was found to be the rate determining step. This explains the dependence of the rate constant value on pH. The highest values of rate constants were found at pH over 13. Under these conditions the mechanism of the process is different. The concentration of protons is small, and the availability of the counter ions (i.e., K+) becomes crucial for the kinetics of reduction. Their role is to neutralize the negative charge of the redox group following its reduction. The logarithm of rate constants was found to decrease linearly with the increase of surface concentration of ubiquinone. This reflects the influence of intermolecular interactions in the monolayer on the kinetics of the electrode process.

Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH 2) as well as that of UQH 2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH 2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH 2 ⋅+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH ⋅ neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH 2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ ⋅− resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ ⋅− uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.

Effect of arylthiolated 2,3-dimethoxy-1,4-benzoquinones on respiratory activity and lipid peroxidation in bovine heart mitochondria.

A series of arylthiolated 2,3-dimethoxy-1,4-benzoquinones was synthesized and tested for the effect on the respiratory system and the lipid peroxidation in bovine heart mitochondria (BHM). These quinones showed intense inhibitory activities on the respiratory system in BHM. Their inhibitory activity in the succinate oxidase system was greater than that in the NADH oxidase system. No difference between the difference spectra, with and without these quinones, of the reduced minus oxidized forms of cytochromes (cyt.) suggested that these quinones inhibit at the site after cyt. a+a3 in the respiratory chain. Moreover, these quinones were as efficient as exogenous ubiquinone-10 (UQ-10) for the inhibition of lipid peroxidation. 5- And 5,6-di-arylthio groups on the quinone ring were found to be favorable for inhibition of the respiratory system and lipid peroxidation. Our results suggest that arylthiolated 2,3-dimethoxy-1,4-benzoquinones act as antioxidants by increasing the amount of endogenous reduced UQ-10 in BHM.