Antioxidant action of ubiquinol homologues with different isoprenoid chain length in biomembranes

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.

Preeclampsia is associated with a decrease in plasma coenzyme Q10 levels

Preeclampsia is a common ( approximately 7% of all pregnancies) disorder of human pregnancy in which the normal hemodynamic response to pregnancy is compromised. Despite many years of intensive research, the pathogenesis of preeclampsia is still not fully understood. The objective of the present study was to investigate the concentration of coenzyme Q10 in normal pregnancy and preeclampsia. Pregnant women (n = 18), women with preeclampsia (n = 12), and nonpregnant normotensive women (n = 22) were included. Plasma levels of coenzyme Q10 were measured by high-performance liquid chromatography. Plasma coenzyme Q10 levels were significantly higher in normal pregnant women (mean = 1.08, SEM = 0.08 umol/l; p <.005) in comparison to nonpregnant women (mean = 0.86, SEM = 0.16 umol/l) and women with preeclampsia (mean = 0.7, SEM = 0.03 umol/l; p <.0001). These results demonstrated that during preeclampsia there is a significant decrease in plasma levels of coenzyme Q10 compared to normal pregnant women, and compared to those who are not pregnant.

Bioenergetic approaches for neuroprotection in Parkinson's disease

There is considerable evidence suggesting that mitochondrial dysfunction and oxidative damage may play a role in the pathogenesis of Parkinson's disease (PD). This possibility has been strengthened by recent studies in animal models, which have shown that a selective inhibitor of complex I of the electron transport gene can produce an animal model that closely mimics both the biochemical and histopathological findings of PD. Several agents are available that can modulate cellular energy metabolism and that may exert antioxidative effects. There is substantial evidence that mitochondria are a major source of free radicals within the cell. These appear to be produced at both the iron-sulfur clusters of complex I as well as the ubiquinone site. Agents that have shown to be beneficial in animal models of PD include creatine, coenzyme Q(10), Ginkgo biloba, nicotinamide, and acetyl-L-carnitine. Creatine has been shown to be effective in several animal models of neurodegenerative diseases and currently is being evaluated in early stage trials in PD. Similarly, coenzyme Q(10) is also effective in animal models and has shown promising effects both in clinical trials of PD as well as in clinical trials in Huntington's disease and Friedreich's ataxia. Many other agents show good human tolerability. These agents therefore are promising candidates for further study as neuroprotective agents in PD.

Pulmonary arterial endothelial cells affect the redox status of coenzyme Q0

The pulmonary endothelium is capable of reducing certain redox-active compounds as they pass from the systemic venous to the arterial circulation. This may have important consequences with regard to the pulmonary and systemic disposition and biochemistry of these compounds. Because quinones comprise an important class of redox-active compounds with a range of physiological, toxicological, and pharmacological activities, the objective of the present study was to determine the fate of a model quinone, coenzyme Q0 (Q), added to the extracellular medium surrounding pulmonary arterial endothelial cells in culture, with particular attention to the effect of the cells on the redox status of Q in the medium. Spectrophotometry, electron paramagnetic resonance (EPR), and high-performance liquid chromatography (HPLC) demonstrated that, when the oxidized form Q is added to the medium surrounding the cells, it is rapidly converted to its quinol form (QH2) with a small concentration of semiquinone (Q*-) also detectable. The isolation of cell plasma membrane proteins revealed an NADH-Q oxidoreductase located on the outer plasma membrane surface, which apparently participates in the reduction process. In addition, once formed the QH2 undergoes a cyanide-sensitive oxidation by the cells. Thus, the actual rate of Q reduction by the cells is greater than the net QH2 output from the cells.

Distribution and breakdown of labeled coenzyme Q10 in rat

Radioactive coenzyme Q(10) ([(3)H]CoQ) was synthesized in a way that the metabolites produced retained the radioactivity. Administration of the lipid to rats intraperitoneally resulted in an efficient uptake into the circulation, with high concentrations found in spleen, liver, and white blood cells; lower concentrations in adrenals, ovaries, thymus, and heart; and practically no uptake in kidney, muscle, and brain. In liver homogenate most [(3)H]CoQ appeared in the organelles, but it was also present in the cytosol and transport vesicles. Mitochondria, purified on a metrizamide gradient, had a very low concentration of [(3)H]CoQ, which was mainly present in the lysosomes. All organs that took up the labeled lipid also contained water-soluble metabolites. The majority of metabolites excreted through the kidney and appeared in the urine. Some metabolites were also present in the feces, which further contained nonmetabolized [(3)H]CoQ, excreted through the bile. The major metabolites were purified from the urine, and the mass spectrometric fragmentation showed that these compounds, containing the ring with a short side chain, are phosphorylated. Thus, the results demonstrate that CoQ is metabolized in all tissues, the metabolites are phosphorylated in the cells, transported in the blood to the kidney, and excreted into the urine.

Cytoprotective and anticancer properties of coenzyme Q versus capsaicin

Coenzyme Q (CoQ) is an essential component of the mitochondrial electron transport chain and serves as an electron donor and acceptor in mitochondrial energy-linked respiration. CoQ1 was shown to prevent ROS formation and cell death in complex 1 inhibited cells. Low concentrations of capsaicin like CoQ1 inhibited ROS formation but CoQ1 was more effective at restoring the mitochondrial membrane potential collapse caused by complex 1 inhibitors such as rotenone. At low concentrations, capsaicin acts as a CoQ mimic by protecting against rotenone induced ROS formation and mitochondrial membrane potential collapse. Lipid peroxidation in isolated rat hepatocytes induced by cumene hydroperoxide and chloroacetaldehyde was also prevented. At higher concentrations, capsaicin and CoQ1 became cytotoxic. Hep G2 cells were more susceptible than hepatocytes. The cytotoxic mechanism for both capsaicin and CoQ1 was shown to involve a collapse of the mitochondrial membrane potential, however, only capsaicin caused ROS formation. The capsaicin side chain was required for capsaicin induced cytotoxicity. The anticancer properties of CoQ1 and capsaicin should prove useful for inducing tumor cell apoptosis.

Effect of analogs with modified prenyl side chains on growth of serum deficient HL60 cells

This study was organized by Professor Karl Folkers with the objective of finding derivatives of coenzyme Q which could be more effectively absorbed and would give better biomedical effects. In this series all the compounds are 2,3 dimethoxy, 5 methyl p benzoquinone with modified side chains in the 6 position. The modifications are primarily changes in chain length, unsaturation, methyl groups and addition of terminal phenyl groups. The test system evaluates the growth of serum deficient HL60, 3T3 and HeLa cells in the presence of coenzyme Q10 or coenzyme Q analogs. Short chain coenzyme Q homologues such as coenzyme Q2 give poor growth but compounds with saturated short aliphatic side chains from C10 to C18 produce good growth. Introduction of a single double bond at the 2' or 8' position in the aliphatic chain retains growth stimulation at low concentration but introduces inhibition at higher concentration. Introduction of a 3' methyl group in addition to the 2' enyl site in the side chain decreases the growth response and maintains inhibition. Addition of a terminal phenyl group to the side chain from C5 to C10 can produce analogs which give strong stimulation or strong inhibition of growth. The action of the analogs is in addition to the natural coenzyme Q in the cell and is not based on restoration of activity after depletion of normal coenzyme Q. The effects may be based on any of the sites in the cell where coenzyme Q functions. For example, coenzyme Q2 is known to decrease mitochondrial membrane potential whereas the analog with a 10C aliphatic side chain increases potential. Both of these compounds stimulate plasma membrane electron transport. Inhibition of apoptosis by coenzyme Q may also increase net cell proliferation and the 10C analog inhibits the permeability transition pore.

Hepatocyte metabolism of coenzyme Q1 (ubiquinone-5) to its sulfate conjugate decreases its antioxidant activity

Previous studies on the metabolism of coenyzme Q (CoQ) have focused on products found in the urine, bile or feces. However, the metabolites found in these samples were end products from a multitude of catabolic processes which did not necessarily reflect CoQ intracellular metabolism (e.g. in the liver, the major site of CoQ synthesis or metabolism). Using isolated rat hepatocytes, we have found that the sulfation of coenzyme Q1 (CoQ1) was the initial and dominant step following its reduction to the hydroquinone. This metabolic process is important as conjugation may occur on the hydroquinone metabolites of any coenzyme10 scission product retaining the quinone ring. By using rat liver cytosol, we were able to identify the monosulfated metabolite of CoQ1. The CoQ1 sulfate conjugate was identified by mass spectrometry followed by tandem mass spectrometry. The rate of formation of the CoQ1 sulfate conjugate was markedly increased by the addition of NADH and was prevented by dicumarol, a DT-diaphorase (NQO1) inhibitor. CoQ1 sulfate conjugate formation catalysed by cytosol was inhibited by the sulfotransferase 1A (SULT1A) inhibitor, pentachlorophenol (PCP) suggesting that sulfation was carried out by the SULT 1A isoform. CoQ1 sulfation in isolated hepatocytes and inversely CoQ1 hydroquinone formation were dependent on the concentration of inorganic sulfate in the media. Intracellular sulfation also decreased CoQ1 antioxidant and cytoprotective activity towards cumene hydroperoxide (CHP) induced cell death. Sulfotransferases may therefore play a significant role in endogenous CoQ metabolism following its degradation to short chain products.

Effect of MPTP on Dopamine metabolism in Ames dwarf mice

Hypopituitary dwarf mice exhibit a heightened antioxidative capacity and live extensively longer than age-matched controls. Importantly, dwarf mice resist peripheral oxidative stress induced by paraquat, and behaviorally, they maintain cognitive function and locomotor activity at levels above those observed in old wild-type animals. We assessed monoaminergic neurotransmitters in nigrostriatal tract and cerebellum after the administration of the dopaminergic neurotoxin, MPTP. There was no significant change in mitochondrial monoamine oxidase (MAO)-B and total MAO activity in the substantia nigra and nucleus caudatus putamen of wild-type and dwarf mice. Coenzymes Q-9 and Q-10 were present in similar quantities, as were dopamine, norepinephrine, and serotonin levels in the cerebellum and nigrostriatal tract. MPTP set off tremor, hind limb abduction, and straub tail behavior and induced significant dopamine depletion in the striatum of both dwarf and normal mice. This study shows that the MAO activity and the coenzyme content of dwarf mice are similar to those of their wild-type controls and hence susceptible to MPTP-induced toxicity.

Distribution of coenzyme Q homologues in brain

Ubiquinone (coenzyme Q10), in addition to its function as an electron and proton carrier in mitochondrial electron transport coupled to ATP synthesis, acts in its reduced form (ubiquinol) as an antioxidant, inhibiting lipid peroxidation in biological membranes and protecting mitochondrial inner-membrane proteins and DNA against oxidative damage accompanying lipid peroxidation. Tissue ubiquinone levels are subject to regulation by physiological factors that are related to the oxidative activity of the organism: they increase under the influence of oxidative stress, e.g. physical exercise, cold adaptation, thyroid hormone treatment, and decrease during aging. In the present study, coenzyme Q homologues were separated and quantified in the brains of mice, rats, rabbits, and chickens using high-performance liquid chromatography. In addition, the coenzyme Q homologues were measured in cells such as NG-108, PC-12, rat fetal brain cells and human SHSY-5Y and monocytes. In general, Q1 content was the lowest among the coenzyme homologues quantified in the brain. Q9 was not detectable in the brains of chickens and rabbits, but was present in the brains of rats and mice. Q9 was also not detected in human cell lines SHSY-5Y and monocytes. Q10 was detected in the brains of mice, rats, rabbits, and chickens and in cell lines. Since both coenzyme Q and vitamin E are antioxidants, and coenzyme Q recycles vitamins E and C, vitamin E was also quantified in mice brain using HPLC-electrochemical detector (ECD). The quantity of vitamin E was lowest in the substantia nigra compared with the other brain regions. This finding is crucial in elucidating ubiquinone function in bioenergetics; in preventing free radical generation, lipid peroxidation, and apoptosis in the brain; and as a potential compound in treating various neurodegenerative disorders.

Electron spin resonance investigation of semiquinone radicals formed from the reaction of ubiquinone 0 with human oxyhemoglobin

The redox properties and thiol reactivity of quinones play critical roles in their therapeutic and toxicological properties. The present study was undertaken to investigate the binding activity of ubiquinone 0 (UQ(0)) to human oxyhemoglobin (HbO(2)) using electron spin resonance (ESR). Addition of UQ(0) to HbO(2) resulted in the immediate detection of a five-line ESR spectrum characteristic of the semiquinone radical of UQ(0) (UQ(0)). With time the HbO(2) adduct with UQ(0), which was characterized by a broad immobilized ESR spectrum, was gradually formed. Matrix-assisted laser desorption/ionization time-of-flight mass spectra analysis showed that UQ(0) bound to the beta-chain of HbO(2). Superoxide dismutase dose-dependently suppressed the intensity of the broad spectrum and accelerated its formation. However, N-ethylmaleimide, a thiol-blocking agent, completely eliminated its formation. The nonspecific protease mixture pronase also prevented its formation and resulted in the gradual appearance of a 4-line spectrum from the 5-line spectrum of UQ(0). The structure of the species responsible for the 4-line spectrum was confirmed and identified by the reaction of UQ(0) with reduced glutathione. In human red blood cells, UQ(0) rapidly bound to glutathione but more slowly to HbO(2). These results suggest that UQ(0) reacted with both ferrous heme and the reactive beta-93 cysteinyl residue of HbO(2) to generate its corresponding semiquinone radical. Subsequently UQ(0) bound to the beta-93 cysteinyl residue of HbO(2) to form a covalent-binding adduct responsible for the broad spectrum.

Role of vitamins in skin care

Therapeutic (vitamins A and D, and their analogs) and antioxidant (vitamins C, E, and coenzyme Q) vitamins play an increasing role in skin care. Their benefits range from skin conditions such as acne and psoriasis to the protection against environmental insults.

Lung mitochondrial dysfunction in pulmonary hypertension syndrome. I. Site-specific defects in the electron transport chain

The main objectives of this study were to determine a) site-specific defects in the electron transport chain of lung mitochondria of broilers with pulmonary hypertension syndrome (PHS), b) if these defects are attenuated by high dietary vitamin E, and c) if these defects have a genetic basis. In Experiment 1, lung mitochondria were isolated from broilers with and without PHS fed diets containing 15 IU and 100 IU dl-alpha-tocopherol acetate/kg (VE); the four treatments were control, VE, PHS, and VE-PHS, respectively. Hydrogen peroxide (H2O2) generation in isolated lung mitochondria was monitored by dichlorofluorosein (DCF) fluorescence in response to chemicals that inhibit electron flow at specific sites on the electron transport chain using a 96-well microplate with Cytoflour (excitation/emission 480/530 nm). Basal H2O2 production was higher in PHS than in control mitochondria. Differences in H2O2 production between control and PHS were magnified by inhibition of Complexes I and III (Coenzyme Q) of the respiratory chain in mitochondria. Functional defects in PHS mitochondria were attenuated by high dietary VE. In Experiment 2, basal H2O2 production and that following inhibition of Complexes I and III were lower in lung mitochondria isolated from broilers selected for genetic resistance to PHS than in nonselected birds in the base population. The results of this study indicate that site-specific defects in Complexes I and III may underlie lung mitochondrial dysfunction in broilers with PHS, that these defects are attenuated by high dietary vitamin E, and that these defects may be related to genetic predisposition to PHS.

Incorporation of plastoquinone and ubiquinone into liposome membranes studied by HPLC analysis. The effect of side chain length and redox state of quinone

The efficiency of incorporation of plastoquinones and ubiquinones into phospholipid liposomes has been studied. The representatives of short (PQ1 and UQ1) middle (PQ4 and UQ4) and long (PQ9, UQ9 and UQ10) prenylquinones have been used to investigate the effect of quinone side chain length. The properties of hydroquinones have been also thoroughly examined in relation to the quinone forms. The extraction procedure was modified and further developed which enables removing of nonincorporated quinone by pentane washing and then determination of quinone content inside the lipid bilayer. The quantitatively evaluation of the amount of prenylquinone was assayed by means of HPLC analysis which offers much greater sensitivity and could be easily applied in case of hydroquinones. It has been found that PQ1 and UQ1 as well as their reduced forms were present mainly (about 80%) in the aqueous phase, when attempting to introduce them into phospholipid bilayer. In case of quinones having four and more isoprenyl units in side chain, a high level of quinone incorporation, ranging about 95%, was observed. The results pointed out that when comparing the effects of different exogenous quinones on membrane related processes, one has to consider the effectiveness of their incorporation within lipid bilayer.

Free radicals in the 1900's: from in vitro to in vivo

Remarkable progress has been achieved in the past 100 years in the field of free radical chemistry, biology and medicine since the discovery of free radicals in 1900. Free radical-mediated processes play a major role in the present industrial chemistry, but they also cause deleterious effects on rubber, plastics, oil products and foods. The importance of free radicals in vivo has been recognized increasingly from both positive and negative sides. Free radicals play an important role in phagocytosis, the production of some biologically essential compounds and possibly cell signaling. At the same time, they may cause oxidative modification of biological molecules, which leads to oxidative damage and eventually to various diseases, cancer and aging. The role and beneficial effects of antioxidants against such oxidative stress support this view. Furthermore, novel issues have been continuously found in this fascinating and yet controversial field of free radicals in biology. In this short article, the past work, present problems and future perspectives of free radicals in life science will be briefly discussed.

Anti-atherogenic effect of coenzyme Q10 in apolipoprotein E gene knockout mice

Oxidation of low-density lipoprotein (LDL) lipid is implicated in atherogenesis and certain antioxidants inhibit atherosclerosis. Ubiquinol-10 (CoQ10H2) inhibits LDL lipid peroxidation in vitro although it is not known whether such activity occurs in vivo, and, if so, whether this is anti-atherogenic. We therefore tested the effect of ubiquinone-10 (CoQ10) supplemented at 1% (w/w) on aortic lipoprotein lipid peroxidation and atherosclerosis in apolipoprotein E-deficient (apoE-/-) mice fed a high-fat diet. Hydroperoxides of cholesteryl esters and triacylglycerols (together referred to as LOOH) and their corresponding alcohols were used as the marker for lipoprotein lipid oxidation. Atherosclerosis was assessed by morphometry at the aortic root, proximal and distal arch, and the descending thoracic and abdominal aorta. Compared to controls, CoQ10-treatment increased plasma coenzyme Q, ascorbate, and the CoQ10H2:CoQ10 + CoQ10H2 ratio, decreased plasma alpha-tocopherol (alpha-TOH), and had no effect on cholesterol and cholesterylester alcohols (CE-OH). Plasma from CoQ10-supplemented mice was more resistant to ex vivo lipid peroxidation. CoQ10 treatment increased aortic coenzyme Q and alpha-TOH and decreased the absolute concentration of LOOH, whereas tissue cholesterol, cholesteryl esters, CE-OH, and LOOH expressed per bisallylic hydrogen-containing lipids were not significantly different. CoQ10-treatment significantly decreased lesion size in the aortic root and the ascending and the descending aorta. Together these data show that CoQ10 decreases the absolute concentration of aortic LOOH and atherosclerosis in apoE-/- mice.

Changes in superoxide dismutase activities and concentrations and myeloperoxidase activities in leukocytes from patients with diabetes mellitus

To investigate whether the two free-radical scavengers, Cu, Zn- and Mn-superoxide dismutase (SOD), are changed in leukocytes of diabetic patients, and the alteration of these enzymes correlates with the diabetic state, we measured the activity and concentration of these enzymes in leukocytes from diabetic patients. Both Cu, Zn-SOD and Mn-SOD activities in neutrophils and lymphocytes were significantly lower in patients with non-insulin-dependent diabetes mellitus than in healthy controls. The concentrations of these enzymes in leukocytes from diabetic patients, however, did not differ from those in controls. Cu, Zn-SOD and Mn-SOD activities in neutrophils inversely correlated with HbA(1c) concentrations. Myeloperoxidase activity in leukocytes was significantly reduced in NIDDM patients. These findings suggest that changes in these enzymes may affect the susceptibility to infection and immunocompetence of patients with diabetes.

Comparative study on dynamics of antioxidative action of alpha-tocopheryl hydroquinone, ubiquinol, and alpha-tocopherol against lipid peroxidation

Alpha-tocopheryl quinone is a metabolite of alpha-tocopherol (TOH) in vivo. The antioxidant action of its reduced form, alpha-tocopheryl hydroquinone (TQH2), has received much attention recently. In the present study, the antioxidative activity of TQH2 was studied in various systems in vitro and compared with that of ubiquinol-10 (UQH2) or TOH to obtain the basic information on the dynamics of the antioxidant action of TQH2. First, their hydrogen-donating abilities were investigated in the reaction with galvinoxyl, a stable phenoxyl radical, and TQH2 was found to possess greater second-order rate constant (1.0 x 10(4) M(-1) s(-1)) than UQH2 (6.0 x 10(3) M(-1) s(-1)) and TOH (2.4 x 10(3) M(-1) s(-1)) at 25 degrees C in ethanol. The stoichiometric numbers were obtained as 1.9, 2.0, and 1.0 for TQH2, UQH2, and TOH, respectively, in reducing galvinoxyl. Second, their relative reactivities toward peroxyl radicals were assessed in competition with N,N'-diphenyl-p-phenylenediamine (DPPD) and found to be 6.0 (TQH2), 1.9 (UQH2), and 1.0 (TOH). Third, their antioxidant efficacies were evaluated in the oxidation of methyl linoleate in organic solvents and in aqueous dispersions. The antioxidant potency decreased in the order TOH > UQH2 > TQH2, as assessed by either the extent of the reduction in the rate of oxidation or the duration of inhibition period. The reverse order of their reactivities toward radicals and their antioxidant efficacies was interpreted by the rapid autoxidation of TQH2 and UQH2, carried out by hydroperoxyl radicals. Although neither TQH2 nor UQH2 acted as a potent antioxidant by itself, they acted as potent antioxidants in combination with TOH. TQH2 and UQH2 reduced alpha-tocopheroxyl radical to spare TOH, whereas TOH suppressed the autoxidation of TQH2 and UQH2. In the micelle oxidation, the antioxidant activities of TQH2, UQH2, and TOH were similar, whereas 2,2,5,7,8-pentamethyl-6-chromanol exerted much more potent efficacy than TQH2, UQH2, or TOH. These results clearly show that the antioxidant potencies against lipid peroxidation are determined not only by their chemical reactivities toward radicals, but also by the fate of an antioxidant-derived radical and the mobility of the antioxidant at the microenvironment.

Comparisons of coenzyme Q bound to mitochondrial membrane proteins among different mammalian species

The objective of this study was to elucidate the mechanisms that govern the variations in the rates of mitochondrial superoxide anion radical (O2-*) generation in different species. The amounts of coenzyme Q (CoQ) associated with mitochondrial membrane proteins were compared in five different mammalian species, namely mouse, rat, rabbit, pig, and cow. Micelles of cardiac mitochondria were prepared using Triton X-100 or deoxycholate (DOC) as detergents, and the micelles containing mitochondrial proteins were sedimented by sucrose density ultracentrifugation. The amount of CoQ present in both types of micelles varied in different species, whereas alpha-tocopherol, another lipoidal molecule in mitochondrial membranes, could not be detected in the micelles of any of these species. The amounts of CoQ bound to mitochondrial proteins in DOC micelles were higher in those mammalian species where CoQ10 was the predominant CoQ homologue, and the amounts were found to be inversely correlated with the rate of mitochondrial 02-* generation among different species. Results also indicated that mitochondrial CoQ exists in at least two distinct pools, one of which is associated with the membrane proteins. The degree of association between CoQ and membrane proteins appears to be a factor determining the rate of mitochondrial O2-* generation.

Is ascorbic acid an antioxidant for the plasma membrane?

Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electrons to the alpha-tocopheroxyl radical and a trans-plasma membrane oxidoreductase activity. Ascorbate-derived reducing capacity is thus transmitted both into and across the plasma membrane. Recycling of alpha-tocopherol by ascorbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductase activity is evident in open membranes, ascorbate is the preferred electron donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic antioxidants such as alpha-tocopherol or ubiquinol-10. Further studies are needed to elucidate the structural components, mechanism, and physiological significance of this activity. Proposed functions for the oxidoreductase include stimulation of cell growth, reduction of the ascorbate free radical outside cells, recycling of alpha-tocopherol, reduction of lipid hydroperoxides, and reduction of ferric iron prior to iron uptake by a transferrin-independent pathway.

Effects of coenzyme Q10 on changes in the membrane potential and rate of generation of reactive oxygen species in hydrazine- and chloramphenicol-treated rat liver mitochondria

Effects of CoQ10 and cycloheximide (CHX) on hydrazine- and chloramphenicol (CP)-induced morphological and some functional changes of mitochondria using cultured rat hepatocytes and effects on the process of recovery from CP intoxication using mouse liver were examined. Results obtained are summarized as follows: (1) The formation of megamitochondria induced in the hepatocytes cultured for 22 h in the presence of 2 mM hydrazine or CP (300 microgram/ml) was suppressed by pretreatment of hepatocytes with CoQ10 (1 microM) or CHX (0.5 microgram/ml). This was proved by electron microscopic analysis of mitochondria. (2) Treatment of hepatocytes with hydrazine for 48 h or longer caused decreases in the membrane potential of mitochondria, which were suppressed by CoQ10. (3) Treatment of hepatocytes with hydrazine for 22 h or longer caused remarkable increases in intracellular levels of reactive oxygen species in hepatocytes, which were suppressed by CoQ10. (4) The process of recovery from the CP-induced changes of mitochondria in mouse liver was accelerated by CoQ10 and CHX.

The reaction of nitric oxide with ubiquinol: kinetic properties and biological significance

The reaction of nitric oxide (*NO) with ubiquinol-0 and ubiquinol-2, short-chain analogs of coenzyme Q, was examined in anaerobic and aerobic conditions in terms of formation of intermediates and stable molecular products. The chemical reactivity of ubiquinol-0 and ubiquinol-2 towards *NO differed only quantitatively, the reactions of ubiquinol-2 being slightly faster than those of ubiquinol-0. The ubiquinol/*NO reaction entailed oxidation of ubiquinol to ubiquinone and reduction of *NO to NO-, the latter identified by its reaction with metmyoglobin to form nitroxylmyoglobin and indirectly by measurement of nitrous oxide (N2O) by gas chromatography. Both the rate of ubiquinone accumulation and *NO consumption were linearly dependent on ubiquinol and *NO concentrations. The stoichiometry of *NO consumed per either ubiquinone formed or ubiquinol oxidized was 1.86 A 0.34. The reaction of *NO with ubiquinols proceeded with intermediate formation of ubisemiquinones that were detected by direct EPR. The second order rate constants of the reactions of ubiquinol-0 and ubiquinol-2 with *NO were 0.49 and 1.6 x 10(4) M(-1)s(-1), respectively. Studies in aerobic conditions revealed that the reaction of *NO with ubiquinols was associated with O2 consumption. The formation of oxyradicals - identified by spin trapping EPR- during ubiquinol autoxidation was inhibited by *NO, thus indicating that the O2 consumption triggered by *NO could not be directly accounted for in terms of oxyradical formation or H2O2 accumulation. It is suggested that oxyradical formation is inhibited by the rapid removal of superoxide anion by *NO to yield peroxynitrite, which subsequently may be involved in the propagation of ubiquinol oxidation. The biological significance of the reaction of ubiquinols with *NO is discussed in terms of the cellular O2 gradients, the steady-state levels of ubiquinols and *NO, and the distribution of ubiquinone (largely in its reduced form) in biological membranes with emphasis on the inner mitochondrial membrane.

Synthesis of [11C] coenzyme Q-related compounds for in vivo estimation of mitochondrial electron transduction and redox state in brain

We have studied the synthesis of [11C]2,3-dimethoxy-5-methyl-6-(10-hydroxy)-decyl-1,4-benzoquinone (idebenone) and [11C]2,3-dimethoxy-5-methyl-1,4-benzoquinone (CoQo) by methylation of their respective desmethyl precursors using [11C]CH3I for in vivo measurement of mitochondrial electron transfer and redox state. The [11C]idebenone was more lipophilic than [11C]CoQo; the latter became hydrophilic by reduction. Clearance of [11C]idebenone from mouse brain was more rapid than that of [11C]CoQo. The results indicated that modification of the isoprenoid side chain in [11C]CoQ is necessary to develop more suitable radiopharmaceuticals.

Vitamin E and coenzyme Q concentrations in the thyroid tissues of patients with various thyroid disorders

To clarify the different roles of free radical scavenging systems in various thyroid disorders, we measured the levels of alpha-, beta-, and gamma-tocopherols and coenzyme Q in the thyroid tissues of patients with thyroid tumors and Graves' disease using high-performance liquid chromatography. The levels of alpha-tocopherols and gamma-tocopherols in the thyroid tissue of patients with papillary carcinoma and the level of gamma-tocopherol in the thyroid tissue of patients with malignant lymphoma were elevated compared with those in normal thyroid tissues. The level of coenzyme Q was reduced in the thyroid tissue of patients with Graves' disease and follicular and papillary thyroid carcinomas. These findings imply that vitamin E and coenzyme Q as scavengers play some role in thyroid follicular cell hyperfunction or dysfunction.

Mitochondrial ubiquinone homologues, superoxide radical generation, and longevity in different mammalian species

Rates of mitochondrial superoxide anion radical (O-2) generation are known to be inversely correlated with the maximum life span potential of different mammalian species. The objective of this study was to understand the possible mechanism(s) underlying such variations in the rate of O-2 generation. The hypothesis that the relative amounts of the ubiquinones or coenzyme Q (CoQ) homologues, CoQ9 and CoQ10, are related with the rate of O-2 generation was tested. A comparison of nine different mammalian species, namely mouse, rat, guinea pig, rabbit, pig, goat, sheep, cow, and horse, which vary from 3.5 to 46 years in their maximum longevity, indicated that the rate of O-2 generation in cardiac submitochondrial particles (SMPs) was directly related to the relative amount of CoQ9 and inversely related to the amount of CoQ10, extractable from their cardiac mitochondria. To directly test the relationship between CoQ homologues and the rate of O-2 generation, rat heart SMPs, naturally containing mainly CoQ9 and cow heart SMPs, with high natural CoQ10 content, were chosen for depletion/reconstitution experiments. Repeated extractions of rat heart SMPs with pentane exponentially depleted both CoQ homologues while the corresponding rates of O-2 generation and oxygen consumption were lowered linearly. Reconstitution of both rat and cow heart SMPs with different amounts of CoQ9 or CoQ10 caused an initial increase in the rates of O-2 generation, followed by a plateau at high concentrations. Within the physiological range of CoQ concentrations, there were no differences in the rates of O-2 generation between SMPs reconstituted with CoQ9 or CoQ10. Only at concentrations that were considerably higher than the physiological level, the SMPs reconstituted with CoQ9 exhibited higher rates of O-2 generation than those obtained with CoQ10. These in vitro findings do not support the hypothesis that differences in the distribution of CoQ homologues are responsible for the variations in the rates of mitochondrial O-2 generation in different mammalian species.

Simultaneous detection of ubiquinol and ubiquinone in human plasma as a marker of oxidative stress

A method is described for the simultaneous detection of ubiquinol-10 and ubiquinone-10 in human plasma. In this procedure, heparinized human plasma was mixed with 5 vol of methanol and 10 vol of hexane. After vigorous shaking and centrifugation, an aliquot of the hexane phase (5 microl) was injected immediately and directly onto a reversed-phase HPLC to minimize the oxidation of ubiquinol to ubiquinone. A post-separation, on-line reduction column converts ubiquinone to ubiquinol which is quantified by electrochemical detection. The detection limit of plasma ubiquinol-10 and ubiquinone-10 is about 4 nM with excellent reproducibilities. Tocopherols, lycopene, and beta-carotene are also detectable in this method. In addition, free cholesterol, and cholesteryl esters can be quantified by their absorption at 210 nm. Using this method we have determined the ratio of ubiquinol to ubiquinone is about 95/5 in human plasma from healthy donors. We suggest that this method will be useful since the ratio of ubiquinol to ubiquinone has been suggested as a good marker of oxidative stress.

Mechanisms and dynamics of antioxidant action of ubiquinol

The mechanisms and dynamics of antioxidant action of ubiquinol have been studied. Ubiquinol scavenges peroxyl radical faster than alpha-tocopherol. However, it is autooxidized rapidly to give hydroperoxyl radical and/or superoxide and hence its antioxidant potency is smaller than that of alpha-tocopherol. The side chain of ubiquinol reduces the mobility between the membranes. It was concluded that ubiquinol acts as a potent antioxidant in combination with alpha-tocopherol.

Antioxidative activity of ubiquinol-10 at physiologic concentrations in human low density lipoprotein

Ubiquinol-10 is a powerful lipid-soluble antioxidant found in cell membranes and lipoproteins in vivo. Its mechanism of action on lipid peroxidation has been determined in model and biological systems. Data concerning antioxidative activity of ubiquinol-10 in lipoproteins, however, are still controversial. The present work examines its role in the prevention of low density lipoprotein (LDL) oxidation, specifically its influence on a copper-mediated oxidative modification of human LDL in vitro. We found that ubiquinol-10 incorporated in LDL in subnormal concentrations (0.05-0.13 mol/mol LDL incorporated in comparison with 0.10-1.20 mol/mol LDL reported as normally in human LDL) slightly but not significantly decreased production of lipid peroxidation products (lipid peroxides, conjugated dienes, thiobarbituric acid-reactive substances) during the first hours of oxidation. The extent of apolipoprotein B modification (LDL fluorescence at 360/430 nm) was also decreased. Increasing the ubiquinol-10 concentration in LDL to 0.55-1.48 mol/mol LDL made it significantly more resistant to copper-mediated oxidation than native LDL. Adding the same amounts of either ubiquinone-10 or alpha-tocopherol to the LDL suspension had almost no effect on its oxidation. Ubiquinol-10 decreased alpha-tocopherol consumption during LDL oxidation and was consumed more rapidly than the latter. These results demonstrate that LDL ubiquinol-10 content is an important factor influencing LDL susceptibility to oxidation by copper and suggest that it represents the first line of defense against oxidative modification in human LDL.

Reduction of ubiquinone in membrane lipids by rat liver cytosol and its involvement in the cellular defence system against lipid peroxidation

Rat liver homogenates reduced ubiquinone (UQ)-10 to ubiquinol (UQH2)-10 in the presence of NADPH rather than NADH. This NADPH-dependent UQ reductase (NADPH-UQ reductase) activity that was not inhibited by antimycin A and rotenone, was located mainly in the cytosol fraction and its activity accounted for 68% of that of the homogenates. Furthermore, the NADPH-UQ reductase from rat liver cytosol efficiently reduced both UQ-10 incorporated into egg yolk lecithin liposomes, and native UQ-9 residing in rat microsomes, to the respective UQH2 form in the presence of NADPH. The gross redox ratios of UQH2-9/(UQ-9 + UQH2-9) in individual tissues of rat correlated positively with the log of their respective cytosolic NADPH-UQ reductase activities, while the redox ratios in every intracellular fraction from liver were at about the same level, irrespective of NADPH-UQ reductase activities in the respective fractions. The combined addition of rat liver cytosol and NADPH inhibited to a great extent 2,2'-azobis(2,4-dimethyl-valeronitrile)-induced lipid peroxidation of UQ-10-fortified lecithin liposomes and completely inhibited such peroxidation in the liposomes in which UQH2-10 replaced UQ-10. The NADPH-UQ reductase activity was clearly separated from DT-diaphorase (EC 1.6.99.2) activity by means of Cibacron Blue-immobilized Bio-Gel A-5m chromatography. In conclusion, the NADPH-UQ reductase in cytosol, which is a novel enzyme to our knowledge, was presumed to be responsible for maintaining the steady-state redox levels of intracellular UQ and thereby to act as an endogenous antioxidant in protecting intracellular membranes from lipid peroxidation that is inevitably induced in aerobic metabolism.

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.

Free radical production by activated haem proteins: protective effect of coenzyme Q

The interaction of hydrogen peroxide with haem proteins leads readily to the formation of myoglobin and/or haemoglobin higher oxidation states (MbIV and/or HbIV), which are capable of promoting the oxidation of cellular costituents and are probably to blame for myocardic tissue damage in ischaemia/reperfusion. This study supports the evidence that the reduced form of Coenzyme Q, like other reducing agents, has an antioxidant activity exerted through the progressive reduction of ferryl forms (MbIV and/or HbIV) back to met and oxy forms (Mb and/or HbIIO2). Furthermore, the strong inactivation afforded by ferryl states of myoglobin on several enzymes, especially creatine kinase (CK), can be prevented by the addition of ubiquinol which protects the enzyme from the oxidative modifications. The ability of ubiquinol to recycle ferryl states of haem proteins provides a novel antioxidant mechanism for Coenzyme Q, besides its direct or indirect antiperoxidative activity, and may represent an important defense mechanism against oxidative tissue injury.

Distribution of ubiquinone and ubiquinol homologues in rat tissues and subcellular fractions

The oxidized (UQox) and reduced (UQred) forms of ubiquinone (UQ) homologues in rat tissues and subcellular fractions were analyzed to elucidate their distribution and physiological role. UQ-9 and UQ-10 were detected in all tissues studied, and UQ-9 was the predominant homologue. The total amount of UQox-10 and UQred-10 was 20-50% that of UQox-9 and UQred-9. The levels of these homologues were highest in heart with lesser amounts occurring in kidney, liver and other organs. In liver and blood plasma, the UQred homologue amounted to 70-80% of the total UQ (UQox + UQred = t-UQ). UQred was less than 30% of t-UQ in other tissues and blood cells. t-UQ was much higher in leukocytes and platelets in blood than in erythrocytes. In erythrocytes, t-UQ was exclusively located in the cell membranes. UQox and UQred were also found in all subcellular fractions isolated from liver and kidney in about the same ratio as UQred/t-UQ was present in the whole organ. The levels of UQox and UQred per mg protein in subcellular fractions from liver were highest in mitochondria, with lesser amounts present in plasma membranes, lysosomes, Golgi complex, nuclei, microsomes and cytosol. In the mitochondria, the outer membranes were richer in t-UQ than the inner membranes. In the Golgi complex, the light and intermediate fractions were rich in t-UQ when compared to the heavy fraction. The possible physiological role of UQox and UQred in tissues and subcellular fractions is discussed.

Antioxidant effect of coenzyme Q on hydrogen peroxide-activated myoglobin

In recent years increased attention has been focused on the reduced forms of coenzyme Q as antioxidant compounds inhibiting lipid peroxidation in model systems and in biological membranes, but in spite of extensive experimental evidences the molecular mechanisms responsible for the antioxidant activity of ubiquinones are still debated. Ferrylmyoglobin and/or its free radical form are regarded as powerful oxidizing agents capable of promoting oxidation of essential cellular constituents, particularly cell membranes. Therefore, we investigated the effects of ubiquinol on the formation and survival of ferryl species of myoglobin and on metmyoglobin itself. The addition of a threefold molar excess of hydrogen peroxide to a solution of metmyoglobin induces the rapid formation of a compound with the spectral characteristics of ferrylmyoglobin. The reaction is complete within 4 min, producing up to 76% of ferrylmyoglobin, which remains stable for at least 30 min. The addition of ubiquinol-1 to the same solution provokes a rapid and progressive reduction of ferrylmyoglobin to metmyoglobin and oxymyoglobin. Ubiquinol-1, furthermore, is also capable of protecting metmyoglobin against oxidation when added in the solution before hydrogen peroxide. Ubiquinol-1, indeed, is effective at both limiting the maximal ferrylmyoglobin level attained (59% inhibition) and accomplishing complete removal of the ferryl form (in about 15 min). The results demonstrate that ubiquinol is capable of reducing both ferrylmyoglobin and metmyoglobin to oxymyoglobin, providing a novel antioxidant mechanism for coenzyme Q.

Kinetic study of free-radical-scavenging action of biological hydroquinones (reduced forms of ubiquinone, vitamin K and tocopherol quinone) in solution

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.