ubiquinone and tocopherylquinone

Isoprenoid quinones are one of the most important groups of compounds occurring in membranes of living organisms. These compounds are composed of a hydrophilic head group and an apolar isoprenoid side chain, giving the molecules a lipid-soluble character. Isoprenoid quinones function mainly as electron and proton carriers in photosynthetic and respiratory electron transport chains and these compounds show also additional functions, such as antioxidant function. Most of naturally occurring isoprenoid quinones belong to naphthoquinones or evolutionary younger benzoquinones. Among benzoquinones, the most widespread and important are ubiquinones and plastoquinones. Menaquinones, belonging to naphthoquinones, function in respiratory and photosynthetic electron transport chains of bacteria. Phylloquinone K(1), a phytyl naphthoquinone, functions in the photosynthetic electron transport in photosystem I. Ubiquinones participate in respiratory chains of eukaryotic mitochondria and some bacteria. Plastoquinones are components of photosynthetic electron transport chains of cyanobacteria and plant chloroplasts. Biosynthetic pathway of isoprenoid quinones has been described, as well as their additional, recently recognized, diverse functions in bacterial, plant and animal metabolism.

Topics: Benzoquinones; Electron Transport; Naphthoquinones; Plastoquinone; Ubiquinone; Vitamin E; Vitamin K 1; Vitamin K 2

Alpha-tocopherol (Toc) is an efficient lipophilic antioxidant present in all mammalian lipid membranes. This chromanol is metabolized by two different pathways: excessive dietary Toc is degraded in the liver by side chain oxidation, and Toc acting as antioxidant is partially degraded to alpha-tocopheryl quinone (TQ). The latter process and the similarity between TQ and ubiquinone (UQ) prompted us to study the distribution of TQ in rat liver mitochondrial membranes and the interference of TQ with the activity of mitochondrial and microsomal redox enzymes interacting with UQ. In view of the contradictory literature results regarding Toc, we determined the distribution of Toc, TQ, and UQ over inner and outer membranes of rat liver mitochondria. Irrespective of the preparation method, the TQ/Toc ratio tends to be higher in mitochondrial inner membranes than in outer membranes suggesting TQ formation by respiratory oxidative stress in vivo. The comparison of the catalytic activities using short-chain homologues of TQ and UQ showed decreasing selectivity in the order complex II (TQ activity not detected)>Q(o) site of complex III>Q(i) site of complex III>complex I approximately cytochrome b(5) reductase>cytochrome P-450 reductase (comparable reactivity of UQ and TQ). TQ binding to some enzymes is comparable to UQ despite low activities. These data show that TQ arising from excessive oxidative degradation of Toc can potentially interfere with mitochondrial electron transfer. On the other hand, both microsomal and mitochondrial enzymes contribute to the reduction of TQ to tocopheryl hydroquinone, which has been suggested to play an antioxidative role itself.

Topics: Animals; Cytochrome-B(5) Reductase; Electron Transport; Electron Transport Complex III; Kinetics; Male; Microsomes; Mitochondrial Membranes; NADPH-Ferrihemoprotein Reductase; Quinone Reductases; Rats; Rats, Sprague-Dawley; Ubiquinone; Vitamin E

Topics: Binding Sites; Binding, Competitive; Electron Transport Complex III; Mitochondria; Oxidation-Reduction; Ubiquinone; Vitamin E

A sensitive procedure is described for the simultaneous determination of vitamin E and coenzyme Q homologues and alpha-tocopherol oxidation products using two-isocratic step high pressure liquid chromatography (HPLC) and electrochemical detection in the oxidative mode. Zinc-catalyzed reduction in a post-column reactor allows the detection of alpha-tocopherolquinone, epoxy-tocopherolquinone, and ubiquinones. This technique was used to quantify lipophilic antioxidants in the liver tissue of rats treated or not with alpha-tocopherolquinone and in a plant oil. Alpha-tocopherolquinone and its epoxide derivatives, formed from alpha-tocopherol during iron-catalyzed phospholipid peroxidation, were also determined in a liposome suspension. The high selectivity and sensitivity of the coulometric detection system enabled use of low oxidation potentials giving little baseline noise, while a fast isolation procedure and quantitative recoveries of all oxidized and reduced forms made it possible to measure a high ubiquinol/ubiquinone ratio in liver tissue. Administration of alpha-tocopherolquinone to rats did not alter the antioxidant status of the liver, despite strong accumulation of both this quinone and its reduced form, alpha-tocopherolhydroquinone. These results indicate the presence of an efficient reductase and suggest that it could contribute to the protection of cellular membranes from oxidative stress.

Topics: Animals; Antioxidants; Chromatography, High Pressure Liquid; Drug Stability; Iron; Lipid Peroxidation; Liposomes; Liver; Male; Plant Oils; Rats; Rats, Wistar; Reproducibility of Results; Ubiquinone; Vitamin E; Zinc

Kinetic study of free-radical-scavenging (FRS) action of eight kinds of biologically important hydroquinones (HQ's) (ubiquinol-10 (UQ10H2 1), ubiquinol-0 (UQ0H2 2), vitamin K1 HQ (VK1H2 3), vitamin K3 HQ (VK3H2 4), alpha-, beta-, gamma-tocopherol HQ's (alpha-, beta-, gamma-TQH2 5, 6, 7), and 2,3,5-trimethyl-1,4-HQ (TMQH2 8)) has been performed. The second-order rate constants, k3, for the reaction of HQ's 1-8 with substituted phenoxyl radical (PhO.) in ethanol, diethyl ether, benzene, and n-hexane have been measured with a stopped-flow spectrophotometer, as a model reaction of HQ's with unstable free radicals (LOO., LO., and HO.) in biological systems. The rate constant of UQ10H2 1 is similar to that of alpha-tocopherol in ethanol. The HQ's 3-8 showed higher reactivity than alpha-tocopherol in ethanol. Especially, the rate constants of VK1H2 3 and VK3H2 4 were found to be 31- and 21-fold larger than that of alpha-tocopherol, respectively, which has the highest reactivity among natural tocopherols. The rate constant of these HQ's increased by decreasing the polarity of solvents. The approximate order of magnitude of k3 value was (i) VK1H2 and VK3H2 > (ii) alpha-, beta-, and gamma-TQH2's and TMQH2 > (iii) alpha-tocopherol > (iv) UQ10H2 and UQ0H2 in solution. The result suggests that these biological HQ's also scavenge the active oxygen free radicals and prevent lipid peroxidation in various tissues and membranes. On the other hand, the reaction between substituted phenoxyl and biological quinones has not been observed.

Topics: Free Radical Scavengers; Hydroquinones; Kinetics; Oxidation-Reduction; Ubiquinone; Vitamin E; Vitamin K

A kinetic study of the regeneration reaction of vitamin E (tocopherol) with eight biological hydroquinones (HQs) (ubiquinol-10 (Q10H2 1); ubiquinol-0 (Q0H2 2); vitamin K1 HQ (VK1H2 3); vitamin K3 HQ (VK3H2 4); alpha-, beta-, and gamma-tocopherol-HQs (alpha-, beta-, and gamma-TQH2 5-7); and 2,3,5-trimethyl-1,4-HQ (TMQH2 8)) in solution was performed. The second-order rate constants (k4) for the reaction of HQs 1-8 with alpha-tocopheroxyl and 5,7-diisopropyltocopheroxyl radicals in ethanol, benzene, and isopropyl alcohol/water (5:1, v/v) solutions were measured with a stopped-flow spectrophotometer. The order of magnitude of k4 values obtained for HQs is VK1H2 > VK3H2 > alpha-TQH2 > beta-TQH2 approximately gamma-TQH2 approximately TMQH2 > Q10H2 > Q0H2, being independent of the kinds of tocopheroxyl radicals and the polarity of the solvents. The log of the k4 values obtained for HQs was found to correlate with their peak oxidation potentials. Comparing the k2 value (2.68 x 10(6) M-1 s-1 obtained for the reaction of alpha-tocopheroxyl with vitamin C (sodium ascorbate) with those (k4 = 2.54 x 10(5) and 8.15 x 10(5) M-1 s-1) obtained for the reaction of alpha-tocopheroxyl with Q10H2 and alpha-TQH2 in isopropyl alcohol/water mixtures, the former is approximately 11 and 3 times as reactive as the latter, respectively. On the other hand, the k2 value obtained for sodium ascorbate is smaller than the k4 values obtained for VK1H2 and VK3H2. These results suggest that mixtures of vitamin E and these HQs (as well as those of vitamins E and C) may function synergistically as antioxidants in various tissues and mitochondria.

Topics: Ascorbic Acid; Free Radicals; Hydroquinones; In Vitro Techniques; Kinetics; Lipid Peroxides; Oxidation-Reduction; Solutions; Ubiquinone; Vitamin E; Vitamin K

Coenzyme Q added to culture media stimulates the growth of HeLa and Balb/3T3 cells in serum free conditions. The stimulation by coenzyme Q is additive to the stimulation by ferricyanide, an impermeable electron acceptor for the transplasma membrane electron transport. alpha Tocopherylquinone can also stimulate cell growth, but vitamin K1 is inactive or inhibitory. The response to coenzyme Q and ferricyanide is enhanced with insulin. A contribution to plasma membrane NADH oxidation or modification of the membrane quinone redox balance can be a basis for the growth stimulation.

Topics: 3T3 Cells; Animals; Cell Division; Coenzymes; Culture Media, Serum-Free; Dose-Response Relationship, Drug; Ferricyanides; HeLa Cells; Humans; Insulin; Kinetics; Mice; Ubiquinone; Vitamin E; Vitamin K 1

The antioxidant effect of alpha-tocopherolquinone and alpha-tocopherolhydroquinone was studied in liposomes and rat liver submitochondrial particles. Both alpha-tocopherolquinone and alpha-tocopherolhydroquinone inhibit lipid peroxidation induced by ascorbate/Fe2+ in liposomes and by cumene hydroperoxide in submitochondrial particles. Alpha-tocopherolhydroquinone is much more effective than alpha-tocopherolquinone in inhibiting lipid peroxidation. Submitochondrial particles, depleted of ubiquinones and reincorporated with alpha-tocopherolquinone, are protected from lipid peroxidation only in the presence of succinate. Alpha-tocopherolquinone cannot replace endogenous ubiquinones in the respiratory chain function, nevertheless it can be reduced by the mitochondrial respiratory chain substrates, presumably through the reduced ubiquinones.

Topics: alpha-Tocopherol; Animals; Ascorbic Acid; Benzene Derivatives; Cattle; Electron Transport; Ferrous Compounds; Lipid Peroxides; Liposomes; Malondialdehyde; Mitochondria, Liver; Oxidation-Reduction; Rats; Submitochondrial Particles; Succinates; Succinic Acid; Ubiquinone; Vitamin E

Topics: Myocardium; Quinones; Ubiquinone; Vitamin E