Ubiquinol-3 and ubiquinol-7 exhibit similar antioxidant activity in model membranes

Impact of Antioxidants on Cardiolipin Oxidation in Liposomes: Why Mitochondrial Cardiolipin Serves as an Apoptotic Signal?

Molecules of mitochondrial cardiolipin (CL) get selectively oxidized upon oxidative stress, which triggers the intrinsic apoptotic pathway. In a chemical model most closely resembling the mitochondrial membrane-liposomes of pure bovine heart CL-we compared ubiquinol-10, ubiquinol-6, and alpha-tocopherol, the most widespread naturally occurring antioxidants, with man-made, quinol-based amphiphilic antioxidants. Lipid peroxidation was induced by addition of an azo initiator in the absence and presence of diverse antioxidants, respectively. The kinetics of CL oxidation was monitored via formation of conjugated dienes at 234 nm. We found that natural ubiquinols and ubiquinol-based amphiphilic antioxidants were equally efficient in protecting CL liposomes from peroxidation; the chromanol-based antioxidants, including alpha-tocopherol, were 2-3 times less efficient. Amphiphilic antioxidants, but not natural ubiquinols and alpha-tocopherol, were able, additionally, to protect the CL bilayer from oxidation by acting from the water phase. We suggest that the previously reported therapeutic efficiency of mitochondrially targeted amphiphilic antioxidants is owing to their ability to protect those CL molecules that are inaccessible to natural hydrophobic antioxidants, being trapped within respiratory supercomplexes. The high susceptibility of such occluded CL molecules to oxidation may have prompted their recruitment as apoptotic signaling molecules by nature.

Antioxidant properties and free radical-scavenging reactivity of a family of hydroxynaphthalenones and dihydroxyanthracenones

This study was undertaken to investigate the free radical-scavenging and antioxidant activities of various structurally related hydroquinones including hydroxynaphthalenones and dihydroxyanthracenones. Electron spin resonance spectroscopy and spin trapping techniques were used to evaluate the ability of hydroquinones to scavenge hydroxyl, diphenylpicrylhydrazyl, and galvinoxyl radicals. In addition, the oxygen radical absorbing capacity assay using fluorescein (ORAC-FL) was used to obtain the relative antioxidant capacity of these radicals. The rate constants of the first H atom abstraction by 2,2-diphenyl-2-picrylhydrazyl (k(2)), were obtained under pseudo-first-order conditions. The free radical-scavenging activities and k(2) values discriminate well between hydroxynaphthalenones and dihydroxyanthracenones, showing that the latter have better antioxidant properties. The aforementioned experimental data agree with quantum-chemical results demonstrating the relevance of intramolecular H bonding to radical-scavenging activities.

Therapeutic uses of antioxidant liposomes

This review will focus on the therapeutic uses of antioxidant liposomes. Antioxidant liposomes have a unique ability to deliver both lipid- and water-soluble antioxidants to tissues. This review will detail the varieties of antioxidants which have been incorporated into liposomes, their modes of administration, and the clinical conditions in which antioxidant liposomes could play an important therapeutic role. Antioxidant liposomes should be particularly useful for treating diseases or conditions in which oxidative stress plays a significant pathophysiological role because this technology has been shown to suppress oxidative stress. These diseases and conditions include cancer, trauma, irradiation, retinotherapy or prematurity, respiratory distress syndrome, chemical weapon exposure, and pulmonary infections.

Substituted p-hydroquinones as inhibitors of lipid peroxidation

The technique based on monitoring oxygen consumption was applied to study 12 alkyl- and methoxy-substituted p-hydroquinones (QH(2)) as a chain-breaking antioxidant during the oxidation of styrene and methyl linoleate (ML) in bulk as well as ML oxidation in micellar solution of sodium dodecyl sulfate (SDS) at 37 degrees C. The antioxidant activities of QH(2) were characterized by two parameters: the rate constant k(1) for reaction of QH(2) with the peroxy radical LO(2)*: QH(2)+LO(2)*-->QH*+LOOH and the stoichiometric factor of inhibition, f, which shows how many kinetic chains may be terminated by one molecule of QH(2). In the case of styrene and ML oxidation in bulk, f values never exceed two; for the majority of QH(2), f was found to be significantly less than two due to the interaction of QH* with molecular oxygen. In the absence of superoxide dismutase (SOD), all the studied QH(2) displayed a very moderate if any antioxidant capability during ML oxidation in SDS micelles. When 20U/ml SOD was added, the majority of QH(2) showed a pronounced ability to inhibit ML oxidation, f parameter being ca. one. The features of QH(2) as an antioxidant in aqueous environment are suggested to associate with the reactivity of semiquinone (Q*(-)). Q*(-) reacts readily with molecular oxygen with formation of superoxide (O(2)*(-)); further reactions of O(2)*(-) result in fast depleting QH(2) and chain propagation. The addition of SOD results in purging a reaction mixture from O(2)*(-) and, as a corollary, in depressing undesirable reactions with the participation of O(2)*(-). With all the oxidation models, QH(2) were found to be very reactive to LO(2)*. The rate constants k(1) decreased progressively when going from the oxidation of styrene to ML oxidation in bulk and further to ML oxidation in SDS micelles.

The biomolecule ubiquinone exerts a variety of biological functions

The chemistry of ubiquinone allows reversible addition of single electrons and protons. This unique property is used in nature for aerobic energy gain, for unilateral proton accumulation, for the generation of reactive oxygen species involved in physiological signaling and a variety of pathophysiological events. Since several years ubiquinone is also considered to play a major role in the control of lipid peroxidation, since this lipophilic biomolecule was recognized to recycle alpha-tocopherol radicals back to the chain-breaking form, vitamin E. Ubiquinone is therefore a biomolecule which has increasingly focused the interest of many research groups due to its alternative pro- and antioxidant activity. We have intensively investigated the role of ubiquinone as prooxidant in mitochondria and will present experimental evidences on conditions required for this function, we will also show that lysosomal ubiquinone has a double function as proton translocator and radical source under certain metabolic conditions. Furthermore, we have addressed the antioxidant role of ubiquinone and found that the efficiency of this activity is widely dependent on the type of biomembrane where ubiquinone exerts its chain-breaking activity.

Therapeutic uses of antioxidant liposomes

This chapter focuses on the use of antioxidant liposomes in the general area of free radical biology and medicine. The term antioxidant liposome is relatively new and refers to liposomes containing lipid-soluble chemical antioxidants, water-soluble chemical antioxidants, enzymatic antioxidants, or combinations of these various antioxidants. The role of antioxidants in health and disease has been extensively discussed, and many excellent reviews and books are available (1–3). Antioxidant liposomes hold great promise in the treatment of many diseases in which oxidative stress plays a prominent role. Oxidative stress is a physiological condition in which the production of damaging free radicals exceeds the in vivo capacity of antioxidant protection mechanisms to prevent pathophysiology. Free radicals are molecules with unpaired electrons, often highly reactive and damaging to biological systems. The biological membranes of subcellular organelles are a major site of free radical damage but proteins and DNA are also significant targets. Moreover, free radicals can alter cellular signal transduction pathways and stimulate the synthesis of inflammatory cytokines. Oxygen radicals and other reactive oxygen species (ROS) arise from the single electron reductions of oxygen.

Antioxidants and total peroxyl radical-trapping ability of olive and seed oils

The presence of the oxidized and reduced forms of ubiquinones Q(9) and Q(10) was determined in commercial extra virgin olive and seed oils, where the amounts of alpha- and gamma-tocopherols and beta-carotene were also quantitated. Very high concentrations of ubiquinones were found in soybean and corn oils. Furthermore, the total antioxidant capability of each oil was evaluated by measuring total radical-trapping antioxidant parameters (TRAP) in tert-butyl alcohol and using egg lecithin as the oxidizable substrate. These values decreased in the order sunflower > corn > peanut > olive; the highest TRAP, which was found in sunflower oil, was related to the very high amount of alpha-tocopherol. Olive oil, because of the low content of alpha-tocopherol, exhibited a TRAP value approximately one-third that of sunflower oil. TRAP values of corn and soybean oils, in which low amounts of alpha-tocopherol but very high contents of gamma-tocopherol and reduced ubiquinones were present, were intermediate. gamma-Tocopherol exhibited a poor ability of trapping peroxyl radicals in tert-butyl alcohol. This behavior was probably due to the effects of the solvent on the rate of hydrogen abstraction from this phenol.

Superoxide dismutase enhances chain-breaking antioxidant capability of hydroquinones

2-tert-butyl-(1), 2,6-dimethyl-(2), 2,5-dimethyl-(3), trimethyl-(4), and 2,3-dimethoxy-5-methyl-(5) substituted p-hydroquinones (QH2) were tested as a chain-breaking antioxidant during the oxidation of methyl linoleate (ML) in dodecyl sulfate micellar solution, pH 7.40, at 37 degrees C. In the absence of superoxide dismutase (SOD), all the studied QH2 displayed very moderate if any antioxidant capability. When 5-25 U/ml SOD was added, QH2 showed a pronounced ability to inhibit ML oxidation. The stoichiometric factor of inhibition was found to be about one for all the tested QH2 in the presence of SOD. The reactivities of QH2 to the ML peroxy radical increase in the order QH2 5 < QH2 3 < QH2 1 approximately QH2 2 < QH2 4; reactivity of QH2 4 exceeds that reported for the majority of phenolic antioxidants. The features of QH2 as an antioxidant in aqueous environment is likely associated with the reactivity of semiquinone (O.-) formed due to attack of the peroxy radical to QH2. O.- reacts readily with molecular oxygen with formation of superoxide (O2.-); in turn, O2.- attacks both to QH2 and ML (likely, as HO2.) that results in fast depleting QH2 and chain propagation, respectively. The addition of SOD results in purging a reaction mixture from O2.- and, as a corollary, in depressing undesirable reactions with the participation of O2.-. Under these conditions, QH2 displays the theoretically highest inhibitory activity which is determined solely by the reactivity of QH2 to the peroxy radical.

DT-Diaphorase maintains the reduced state of ubiquinones in lipid vesicles thereby promoting their antioxidant function

The activity of purified DT-diaphorase in the reduction of ubiquinone homologues of different side-chain length incorporated in uni- and multilamellar vesicles was determined. The direct relationship between the reduced state of ubiquinones and the inhibition of lipid autoxidation induced by thermolabile azocompounds was also demonstrated. Results demonstrate that DT-diaphorase is able to generate and to maintain the reduced, antioxidant form of ubiquinones in both types of vesicles. Furthermore, the results reported herein show that, in the presence of nicotinamide adenine dinucleotide (NADH) and DT-diaphorase, ubiquinol-containing multilamellar vesicles exposed to a lipophilic azocompound did not undergo lipid peroxidation, whereas in vesicles lacking either NADH or DT-diaphorase, thiobarbituric acid reactive substances (TBARS) formation occurred. It is suggested that DT-diaphorase may be responsible for maintaining the reduced state of ubiquinones in various nonmitochondrial cellular membranes.

Antioxidant properties of plastoquinol and other biological prenylquinols in liposomes and solution

Oxidation of biological prenylquinols, like plastoquinol-9 (PQH2-9), ubiquinol-10 (UQH2-10), reduced vitamins K1 (VK1H2) and K2 (VK2H2), alpha-tocopherol quinol (alpha-TQH2) and alpha-tocopherol (alpha-T) was followed by their fluorescence during sonication of egg yolk lecithin/prenylquinol liposomes. The order of magnitude of oxidation of the prenylquinols by free radicals generated during sonication was UQH2-10 > VK2H2 > VK1H2 > alpha-TQH2 > PQH2-9 > alpha-T. It was shown that egg yolk lecithin undergoes degradation even when sonicated briefly under atmosphere of nitrogen and at 0 degree C. A kinetic study of free radical scavenging action of the prenylquinols in solvents of different polarity was performed. The pseudo-first-order rate constants, k, for the reaction of the prenylquinols with 1,1-diphenyl-2-picrylhydrazyl (DPPH) in hexane showed that their scavenging activity changes in the order VK2H2 > VK1H2 > alpha-TQH2 > PQH2-9 > alpha-T > UQH2-10, being the highest in hexane and methanol, whereas in acetone and ethyl acetate the scavenging activity appeared much lower. The reaction rate constants, k, were apparently not dependent on the solvent polarity. The antioxidant activity of the prenylquinols in natural membranes is discussed.

Characterization of large unilamellar vesicles as models for studies of lipid peroxidation initiated by azocompounds

The aim of this work was to characterize large unilamellar vesicles (LUVETs) prepared by a hand-driven extrusion device in order to use them for studies of lipid peroxidation and antioxidant activity. Vesicle structure and size were examined by electron microscopy. Lipid and antioxidant content was determined before and after the extrusion procedure. Then LUVETs were subjected to autoxidation initiated by both the lipid-soluble 2,2'-azobis(2,4-dimethylvaleronitrile) and the water-soluble 2,2'-azobis(2-amidinopropane hydrochloride) azocompounds. The results demonstrated that: i) LUVETs prepared with lipid concentrations ranging between 25 and 150 mM were essentially unilamellar and reasonably homogeneous, with an average diameter of 90 nm; ii) the phospholipid, cholesterol and antioxidant amounts retained by filters were about 10-15%; iii) LUVETs were suitable for autoxidation studies initiated by the water-soluble azocompound both in the absence and presence of antioxidants. The lipid-soluble azocompound could be used only at low concentrations and its vesicle content had to be determined since part of the initiator was not incorporated into the lipid bilayer. These data suggest that LUVETs seem to be recommended for studies of lipid peroxidation and antioxidant activity.

The relative essentiality of the antioxidative function of coenzyme Q--the interactive role of DT-diaphorase

This paper will address two aspects regarding the antioxidative role of coenzyme Q (CoQ): (1) Is the antioxidant function of CoQ primary or secondary (coincidental), i.e. was this molecule selected during evolution to function primarily as an essential functional component of the mitochondrial electron transfer chain and oxidative phosphorylation processes, is its antioxidative capability merely a coincidence of its hydroquinone structure, or was its synthetic enzyme sequence selected on the basis of the advantage to the evolving organism of both functions of CoQ? (2) What is the mechanism whereby the hydroquinone (antioxidant) form of CoQ (CoQH2) is maintained in high proportion in the various and many membranes in which it resides, and in which an obvious electron transfer mechanism to reduce it is not present? The essentiality of the antioxidative role of CoQH2 will be explored and compared to other primary and secondary antioxidants. Recent evidence implicating the two-electron quinone reductase, DT-diaphorase, in the maintenance of the reduced, antioxidant state of CoQ during the oxidative stress of exhaustive exercise will be presented, and a hypothesis concerning the evolutionary significance of DT-diaphorase will be offered.

Autoxidation and antioxidant activity of ubiquinol homologues in large unilamellar vesicles

The antioxidant activity of ubiquinol homologues with different side-chain length such as ubiquinol-3 and ubiquinol-7 was compared with that of alpha-tocopherol when peroxidation was induced by the water-soluble initiator 2,2'-azobis-(2-amidinopropane hydrochloride). In large unilamellar vesicles containing equal amounts of alpha-tocopherol, ubiquinol-3 and ubiquinol-7 the rates of inhibition were very similar but the stoichiometric factor of quinols was approximately 1. To explain this low value, which is one-half of that found when the autoxidation was performed in apolar solvents (Chem. Phys. Lipids (1992) 61, 121-130), the oxidation of alpha-tocopherol and ubiquinol-3 initiated by the azocompound was studied both in methanol and in dimiristoyl-lecithin vesicles. The results obtained show that the ubiquinol homologues undergo a radical chain reaction taking place at the polar interface and suggest that the average preferred location of both quinol headgroups is near to the outer surface of the bilayer.