Extracellular reduction of ubiquinone-1 and -10 by human Hep G2 and blood cells

Effects of oral administration of common antioxidant supplements on the energy metabolism of red blood cells. Attenuation of oxidative stress-induced changes in Rett syndrome erythrocytes by CoQ10

Nutritional supplements are traditionally employed for overall health and for managing some health conditions, although controversies are found concerning the role of antioxidants-mediated benefits in vivo. Consistently with its critical role in systemic redox buffering, red blood cell (RBC) is recognized as a biologically relevant target to investigate the effects of oxidative stress. In RBC, reduction of the ATP levels and adenylate energy charge brings to disturbance in intracellular redox status. In the present work, several popular antioxidant supplements were orally administrated to healthy adults and examined for their ability to induce changes on the energy metabolism and oxidative status in RBC. Fifteen volunteers (3 per group) were treated for 30 days per os with epigallocatechin gallate (EGCG) (1 g green tea extract containing 50% EGCG), resveratrol (325 mg), coenzyme Q10 (CoQ10) (300 mg), vitamin C (1 g), and vitamin E (400 U.I.). Changes in the cellular levels of high-energy compounds (i.e., ATP and its catabolites, NAD and GTP), GSH, GSSG, and malondialdehyde (MDA) were simultaneously analyzed by ion-pairing HPLC. Response to oxidative stress was further investigated through the oxygen radical absorptive capacity (ORAC) assay. According to our experimental approach, (i) CoQ10 appeared to be the most effective antioxidant inducing a high increase in ATP/ADP, ATP/AMP, GSH/GSSG ratio and ORAC value and, in turn, a reduction of NAD concentration, (ii) EGCG modestly modulated the intracellular energy charge potential, while (iii) Vitamin E, vitamin C, and resveratrol exhibited very weak effects. Given that, the antioxidant potential of CoQ10 was additionally assessed in a pilot study which considered individuals suffering from Rett syndrome (RTT), a severe X-linked neuro-developmental disorder in which RBC oxidative damages provide biological markers for redox imbalance and chronic hypoxemia. RTT patients (n = 11), with the typical clinical form, were supplemented for 12 months with CoQ10 (300 mg, once daily). Level of lipid peroxidation (MDA production) and energy state of RBCs were analyzed at 2 and 12 months. Our data suggest that CoQ10 may significantly attenuate the oxidative stress-induced damage in RTT erythrocytes.

Extracellular coenzyme Q10 (CoQ10) is reduced to ubiquinol-10 by intact Hep G2 cells independent of intracellular CoQ10 reduction

Coenzyme Q₁₀ (CoQ₁₀) is an essential factor in the mitochondrial respiratory chain and is closely associated with ATP production in humans. It is known that orally administered CoQ₁₀ in humans is rapidly reduced, and most is detected as a reduced form, ubiquinol-10 (CoQ₁₀H₂), in serum. However, the mechanism of exogenous CoQ₁₀ reduction in vivo is unclear. Therefore, in order to clarify how CoQ₁₀ is reduced to CoQ₁₀H₂, we conducted a study using human liver cancer cell line Hep G2 cells, which show strong intracellular CoQ₁₀-reducing activity. When intact cells were incubated with CoQ₁₀, the exogenously added CoQ₁₀ was incorporated into the cells, time-, concentration-, and temperature-dependently, and 50-80% of that was detected as CoQ₁₀H₂. On the other hand, a part of the extracellular CoQ₁₀ was also detected as CoQ₁₀H₂, and the amount was greater than that of the intracellular CoQ₁₀H₂. Furthermore, the CoQ₁₀-loaded cells did not leak the intracellular CoQ₁₀H₂ (or CoQ₁₀) to the outside of the cells, and modulation of the extracellular CoQ₁₀H₂ amount had little effect on the intracellular CoQ₁₀ or CoQ₁₀H₂ contents, suggesting the existence of an individual mechanism of CoQ₁₀ reduction inside and outside the cells. Moreover, intact cells could reduce CoQ₁₀ in low-density lipoprotein to CoQ₁₀H₂. Therefore, we concluded that a novel CoQ₁₀-reducing mechanism may exist in the plasma membrane, probably the outer surface, of Hep G2 cells, and it may work to reduce extracellular CoQ₁₀ and/or maintain extracellular CoQ₁₀H₂.

Catalytic Formation of Hydrogen Peroxide from Coenzyme NADH and Dioxygen with a Water-Soluble Iridium Complex and a Ubiquinone Coenzyme Analogue

A ubiquinone coenzyme analogue (Q0: 2,3-dimethoxy-5-methyl-1,4-benzoquinone) was reduced by coenzyme NADH to yield the corresponding reduced form of Q0 (Q0H2) in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex (1: [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H2O)]2SO4) in water at ambient temperature as observed in the respiratory chain complex I (Complex I). In the catalytic cycle, the reduction of 1 by NADH produces the corresponding iridium hydride complex that in turn reduces Q0 to produce Q0H2. Q0H2 reduced dioxygen to yield hydrogen peroxide (H2O2) under slightly basic conditions. Catalytic generation of H2O2 was made possible in the reaction of O2 with NADH as the functional expression of NADH oxidase in white blood cells utilizing the redox cycle of Q0 as well as 1 for the first time in a nonenzymatic homogeneous reaction system.

Ubiquinol-10 supplementation activates mitochondria functions to decelerate senescence in senescence-accelerated mice

AIM

The present study was conducted to define the relationship between the anti-aging effect of ubiquinol-10 supplementation and mitochondrial activation in senescence-accelerated mouse prone 1 (SAMP1) mice.

RESULTS

Here, we report that dietary supplementation with ubiquinol-10 prevents age-related decreases in the expression of sirtuin gene family members, which results in the activation of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a major factor that controls mitochondrial biogenesis and respiration, as well as superoxide dismutase 2 (SOD2) and isocitrate dehydrogenase 2 (IDH2), which are major mitochondrial antioxidant enzymes. Ubiquinol-10 supplementation can also increase mitochondrial complex I activity and decrease levels of oxidative stress markers, including protein carbonyls, apurinic/apyrimidinic sites, malondialdehydes, and increase the reduced glutathione/oxidized glutathione ratio. Furthermore, ubiquinol-10 may activate Sirt1 and PGC-1α by increasing cyclic adenosine monophosphate (cAMP) levels that, in turn, activate cAMP response element-binding protein (CREB) and AMP-activated protein kinase (AMPK).

INNOVATION AND CONCLUSION

These results show that ubiquinol-10 may enhance mitochondrial activity by increasing levels of SIRT1, PGC-1α, and SIRT3 that slow the rate of age-related hearing loss and protect against the progression of aging and symptoms of age-related diseases.

Involvement of thermoplasmaquinone-7 in transplasma membrane electron transport of Entamoeba histolytica trophozoites: a key molecule for future rational chemotherapeutic drug designing

The quinone composition of the transplasma membrane electron transport chain of parasitic protozoa Entamoeba histolytica was investigated. Purification of quinone from the plasma membrane of E. histolytica and its subsequent structural elucidation revealed the structure of the quinone as a methylmenaquinone-7 (thermoplasmaquinone-7), a napthoquinone. Membrane bound thermoplasmaquinone-7 can be destroyed by UV irradiation with a concomitant loss of plasma membrane electron transport activity. The abilities of different quinones to restore transplasma membrane electron transport activity in UV irradiated trophozoites were compared. The lost activity was recovered completely by the addition of thermoplasmaquinone-7, but ubiquinones are unable to restore the same. These findings clearly indicate that thermoplasmaquinone-7 acts as a lipid shuttle in the plasma membrane of the parasite to mediate electron transfer between cytosolic reductant and non permeable electron acceptors. This thermoplasmaquinone-7 differs from that of the mammalian host and can provide a novel target for future rational chemotherapeutic drug designing.

Coenzyme Q1 redox metabolism during passage through the rat pulmonary circulation and the effect of hyperoxia

The objective was to evaluate the pulmonary disposition of the ubiquinone homolog coenzyme Q(1) (CoQ(1)) on passage through lungs of normoxic (exposed to room air) and hyperoxic (exposed to 85% O(2) for 48 h) rats. CoQ(1) or its hydroquinone (CoQ(1)H(2)) was infused into the arterial inflow of isolated, perfused lungs, and the venous efflux rates of CoQ(1)H(2) and CoQ(1) were measured. CoQ(1)H(2) appeared in the venous effluent when CoQ(1) was infused, and CoQ(1) appeared when CoQ(1)H(2) was infused. In normoxic lungs, CoQ(1)H(2) efflux rates when CoQ(1) was infused decreased by 58 and 33% in the presence of rotenone (mitochondrial complex I inhibitor) and dicumarol [NAD(P)H-quinone oxidoreductase 1 (NQO1) inhibitor], respectively. Inhibitor studies also revealed that lung CoQ(1)H(2) oxidation was via mitochondrial complex III. In hyperoxic lungs, CoQ(1)H(2) efflux rates when CoQ(1) was infused decreased by 23% compared with normoxic lungs. Based on inhibitor effects and a kinetic model, the effect of hyperoxia could be attributed predominantly to 47% decrease in the capacity of complex I-mediated CoQ(1) reduction, with no change in the other redox processes. Complex I activity in lung homogenates was also lower for hyperoxic than for normoxic lungs. These studies reveal that lung complexes I and III and NQO1 play a dominant role in determining the vascular concentration and redox status of CoQ(1) during passage through the pulmonary circulation, and that exposure to hyperoxia decreases the overall capacity of the lung to reduce CoQ(1) to CoQ(1)H(2) due to a depression in complex I activity.

Evaluation of the mutagenic and genotoxic potential of ubiquinol

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

Subchronic Oral Toxicity of Ubiquinol in Rats and Dogs

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

Safety assessment of coenzyme Q10 (CoQ10)

Coenzyme Q10 (CoQ10) is a naturally occurring component present in living cells. Its physiological function is to act as an essential cofactor for ATP production, and to perform important antioxidant activities in the body. In most countries, CoQ10 has been widely used as a dietary supplement for more than 20 years. Recently, the use of CoQ10 as a dietary supplement has grown with a corresponding increase in daily dosage. The present review describes the safety profile of CoQ10 on the basis of animal and human data. The published reports concerning safety studies indicate that CoQ10 has low toxicity and does not induce serious adverse effects in humans. The acceptable daily intake (ADI) is 12mg/kg/day, calculated from the no-observed-adverse-effect level (NOAEL) of 1200 mg/kg/day derived from a 52-week chronic toxicity study in rats, i.e., 720 mg/day for a person weighing 60 kg. Risk assessment for CoQ10 based on various clinical trial data indicates that the observed safety level (OSL) for CoQ10 is 1200 mg/day/person. Evidence from pharmacokinetic studies suggest that exogenous CoQ10 does not influence the biosynthesis of endogenous CoQ9/CoQ10 nor does it accumulate into plasma or tissues after cessation of supplementation. Overall, these data from preclinical and clinical studies indicate that CoQ10 is highly safe for use as a dietary supplement. Additionally, analysis of CoQ10 bioavailability or its pharmacokinetics provides the pertinent safety evaluation for CoQ10.

Role of mitochondrial electron transport complex I in coenzyme Q1 reduction by intact pulmonary arterial endothelial cells and the effect of hyperoxia

The objective was to determine the impact of intact normoxic and hyperoxia-exposed (95% O(2) for 48 h) bovine pulmonary arterial endothelial cells in culture on the redox status of the coenzyme Q(10) homolog coenzyme Q(1) (CoQ(1)). When CoQ(1) (50 microM) was incubated with the cells for 30 min, its concentration in the medium decreased over time, reaching a lower level for normoxic than hyperoxia-exposed cells. The decreases in CoQ(1) concentration were associated with generation of CoQ(1) hydroquinone (CoQ(1)H(2)), wherein 3.4 times more CoQ(1)H(2) was produced in the normoxic than hyperoxia-exposed cell medium (8.2 +/- 0.3 and 2.4 +/- 0.4 microM, means +/- SE, respectively) after 30 min. The maximum CoQ(1) reduction rate for the hyperoxia-exposed cells, measured using the cell membrane-impermeant redox indicator potassium ferricyanide, was about one-half that of normoxic cells (11.4 and 24.1 nmol x min(-1) x mg(-1) cell protein, respectively). The mitochondrial electron transport complex I inhibitor rotenone decreased the CoQ(1) reduction rate by 85% in the normoxic cells and 44% in the hyperoxia-exposed cells. There was little or no inhibitory effect of NAD(P)H:quinone oxidoreductase 1 (NQO1) inhibitors on CoQ(1) reduction. Intact cell oxygen consumption rates and complex I activities in mitochondria-enriched fractions were also lower for hyperoxia-exposed than normoxic cells. The implication is that intact pulmonary endothelial cells influence the redox status of CoQ(1) via complex I-mediated reduction to CoQ(1)H(2), which appears in the extracellular medium, and that the hyperoxic exposure decreases the overall CoQ(1) reduction capacity via a depression in complex I activity.

Nonenzymatic reduction of thymoquinone in physiological conditions

Thymoquinone (TQ) is the bioactive constituent of the volatile oil of Nigella sativa L. and has been shown to exert antioxidant antineoplastic and anti-inflammatory effects. During the study of its possible mechanism of action, we found that TQ reacts chemically (i.e. nonenzymatically) with glutathione (GSH), NADH and NADPH. A combination of liquid chromatography/UV-Vis spectrophotometry/Mass spectrometry analyses was used to identify the products of these reactions. The reaction that occur in physiological conditions indicates the formation of only two products, glutathionyl-dihydrothymoquinone after rapid reaction with GSH, and dihydrothymoquinone (DHTQ) after slow reaction time with NADH and NADPH. Measurement of the antioxidant activity of reduced compounds against organic radicals such as 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) also revealed a potential scavenging activity for glutathionyl-dihydrothymoquinone similar to that of DHTQ. Under our experimental conditions, TQ shows lower scavenging activities than glutathionyl-dihydrothymoquinone and DHTQ; it is very interesting to observe that the reduced compounds apparently show an antioxidant capacity equivalent to Trolox. The results indicate a possible intracellular nonenzymatic metabolic activation of TQ dependent on GSH, NADH or NADPH that may represent a "cellular switch" able to modulate cellular antioxidant defences.

Study on safety and bioavailability of ubiquinol (Kaneka QH™) after single and 4-week multiple oral administration to healthy volunteers

The safety and bioavailability of ubiquinol (the reduced form of coenzyme Q(10)), a naturally occurring lipid-soluble nutrient, were evaluated for the first time in single-blind, placebo-controlled studies with healthy subjects after administration of a single oral dose of 150 or 300 mg and after oral administration of 90, 150, or 300 mg for 4 weeks. No clinically relevant changes in results of standard laboratory tests, physical examination, vital signs, or ECG induced by ubiquinol were observed in any dosage groups. The C(max) and AUC(0-48 h) derived from the mean plasma ubiquinol concentration-time curves increased non-linearly with dose from 1.88 to 3.19 micro g/ml and from 74.61 to 91.76 micro g h/ml, respectively, after single administration. Trough concentrations had nearly plateaued at levels of 2.61 micro g/ml for 90 mg, 3.66 micro g/ml for 150 mg, and 6.53 micro g/ml for 300 mg at day 14, and increased non-linearly with dose in the 4-week study. In conclusion, following single or multiple-doses of ubiquinol in healthy volunteers, significant absorption of ubiquinol from the gastrointestinal tract was observed, and no safety concerns were noted on standard laboratory tests for safety or on assessment of adverse events for doses of up to 300 mg for up to 2 weeks after treatment completion.

Preliminary evidence on existence of transplasma membrane electron transport in Entamoeba histolytica trophozoites: a key mechanism for maintaining optimal redox balance

Entamoeba histolytica, an amitochondriate parasitic protist, was demonstrated to be capable of reducing the oxidized form of alpha-lipoic acid, a non permeable electron acceptor outside the plasma membrane. This transmembrane reduction of non permeable electron acceptors with redox potentials ranging from -290 mV to +360 mV takes place at neutral pH. The transmembrane reduction of non permeable electron acceptors was not inhibited by mitochondrial electron transport inhibitors such as antimycin A, rotenone, cyanide and azide. However, a clear inhibition with complex III inhibitor, 2-(n-heptyl)-4-hydroxyquinoline-N-oxide; modifiers of sulphydryl groups and inhibitors of glycolysis was revealed. The iron-sulphur centre inhibitor thenoyltrifluoroacetone failed to inhibit the reduction of non permeable electron acceptors whereas capsaicin, an inhibitor of energy coupling NADH oxidase, showed substantial inhibition. p-trifluromethoxychlorophenylhydrazone, a protonophore uncoupler, resulted in the stimulation of alpha-lipoic acid reduction but inhibition in oxygen uptake. Mitochondrial electron transport inhibitors substantially inhibited the oxygen uptake in E. histolytica. Transmembrane reduction of alpha-lipoic acid was strongly stimulated by anaerobiosis and anaerobic stimulation was inhibited by 2-(n-heptyl)-4-hydroxyquinoline-N-oxide. Transmembrane redox system of E. histolytica was also found to be sensitive to UV irradiation. All these findings clearly demonstrate the existence of transplasma membrane electron transport system in E. histolytica and possible involvment of a naphthoquinone coenzyme in transmembrane redox of E. histolytica which is different from that of mammalian host and therefore can provide a novel target for future rational chemotherapeutic drug designing.

Adaptations to oxidative stress induced by vitamin E deficiency in rat liver

Vitamin E deficiency in rats led to a sequence of antioxidant defense adaptations in the liver. After three weeks, alpha-tocopherol concentration was 5% of control, but ascorbate and ubiquinol concentrations were 2- to 3-fold greater than control. During the early phase of adaptation no differences in markers of lipid peroxidation were observed, but the activities of both cytochrome b5 reductase and glucose-6-phosphate dehydrogenase were significantly greater in deficient livers. By nine weeks, accumulation of lipid peroxidation end products began to occur along with declining concentrations of ascorbate, and higher NQO1 activities. At twelve weeks, rat growth ceased, and both lipid peroxidation products and cytosolic calcium-independent phospholipase A2 reached maximum concentrations. Thus, in growing rats the changes progressed from increases in both ubiquinol and quinone reductases through accumulation of lipid peroxidation products and loss of endogenous antioxidants to finally induction of lipid metabolizing enzymes and cessation of rat growth.

Characterization of the redox components of transplasma membrane electron transport system from Leishmania donovani promastigotes

An investigation has been made of the points of coupling of four nonpermeable electron acceptors e.g., alpha-lipoic acid (ALA), 5,5'-dithiobis (2-nitroaniline-N-sulphonic acid) (DTNS), 1,2-naphthoquinone-4-sulphonic acid (NQSA) and ferricyanide which are mainly reduced via an interaction with the redox sites present in the plasma membrane of Leishmania donovani promastigotes. ALA, DTNS, NQSA and ferricyanide reduction and part of O2 reduction is shown to take place on the exoplasmic face of the cell, for it is affected by external pH and agents that react with the external surface. Redox enzymes of the transplasma membrane electron transport system orderly transfer electron from one redox carrier to the next with the molecular oxygen as the final electron acceptor. The redox carriers mediate the transfer of electrons from metabolically generated reductant to nonpermeable electron acceptors and oxygen. At a pH of 6.4, respiration of Leishmania cells on glucose substrate shut down almost completely upon addition of an uncoupler FCCP and K+-ionophore valinomycin. The most pronounced effects on O2 uptake were obtained by treatment with antimycin A, 2-heptadecyl-4-hydroxyquinone-N-oxide, paracholoromercuribenzene sulphonic acid and trifluoperazine. Relatively smaller effects were obtained by treatment with potassium cyanide. Inhibition observed with respect to the reduction of the electron acceptors ALA, DTNS, NQSA and ferricyanide was not similar in most cases. The redox chain appears to be branched at several points and it is suggested that this redox chain incorporate iron-sulphur center, b-cytochromes, cyanide insensitive oxygen redox site, Na+ and K+ channel, capsaicin inhibited energy coupling site and trifluoperazine inhibited energy linked P-type ATPase. We analyzed the influence of ionic composition of the medium on reduction of electron acceptors in Leishmania donovani promastigotes. Our data suggest that K+ have some role for ALA reduction and Na+ for ferricyanide reduction. No significant effects were found with DTNS and NQSA reduction when Na+ or K+ was omitted from the medium. Stimulation of ALA, DTNS, NQSA and ferricyanide reduction was obtained by omitting Cl- from the medium. We propose that this redox system may be an energy source for control of membrane function in Leishmania cells.

The Effect of HMG-CoA Reductase Inhibitors on Coenzyme Q10

The HMG-CoA reductase inhibitors, also known as statins, have an enviable safety profile; however, myotoxicity and to a lesser extent hepatotoxicity have been noted in some patients following treatment. Statins target several tissues, depending upon their lipophilicity, where they competitively inhibit HMG-CoA reductase, the rate-limiting enzyme for mevalonic acid synthesis and subsequently cholesterol biosynthesis. HMG-CoA reductase is also the first committed rate-limiting step for the synthesis of a range of other compounds including steroid hormones and ubidecarenone (ubiquinone), otherwise known as coenzyme Q(10) (CoQ(10)). Recent interest has focused on the possible role CoQ(10) deficiency may have in the pathophysiology of the rare adverse effects of statin treatment. Currently, there is insufficient evidence from human studies to link statin therapy unequivocally to pathologically significantly decreased tissue CoQ(10) levels. Although statin treatment has been reported to lower plasma/serum CoQ(10) status, few human studies have assessed tissue CoQ(10) status. The plasma/serum CoQ(10) level is influenced by a number of physiological factors and, therefore, has limited value as a means of assessing intracellular CoQ(10) status. In those limited studies that have assessed the effect of statin treatment upon tissue CoQ(10) levels, none have shown evidence of a fall in CoQ(10) levels. This may reflect the doses of statins used, since many appear to have been used at doses below those recommended for their maximum therapeutic effects. Moreover, the poor bioavailability in those peripheral tissues tested may not reflect the effects the agents are having in liver and muscle, the tissues commonly affected in those patients who do not tolerate statins. This article reviews the biochemistry of CoQ(10), its role in cellular metabolism and the available evidence linking possible CoQ(10) deficiency to statin therapy.

Impact of pulmonary arterial endothelial cells on duroquinone redox status

The study objective was to use pulmonary arterial endothelial cells to examine kinetics and mechanisms contributing to the disposition of the quinone 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ) observed during passage through the pulmonary circulation. The approach was to add DQ, durohydroquinone (DQH2), or DQ with the cell membrane-impermeant oxidizing agent, ferricyanide (Fe(CN)6(3)-), to the cell medium, and to measure the medium concentrations of substrates and products over time. Studies were carried out under control conditions and with dicumarol, to inhibit NAD(P)H:quinone oxidoreductase 1 (NQO1), or cyanide, to inhibit mitochondrial electron transport. In control cells, DQH2 appears in the extracellular medium of cells incubated with DQ, and DQ appears when the cells are incubated with DQH2. Dicumarol blocked the appearance of DQH2 when DQ was added to the cell medium, and cyanide blocked the appearance of DQ when DQH2 was added to the cell medium, suggesting that the two electron reductase NQO1 dominates DQ reduction and mitochondrial electron transport complex III is the predominant route of DQH2 oxidation. In the presence of cyanide, the addition of DQ also resulted in an increased rate of appearance of DQH2 and stimulation of cyanide-insensitive oxygen consumption. As DQH2 does not autoxidize-comproportionate over the study time course, these observations suggest a cyanide-stimulated one-electron DQ reduction and durosemiquinone (DQ*-) autoxidation. The latter processes are apparently confined to the cell interior, as the cell membrane impermeant oxidant, ferricyanide, did not inhibit the DQ-stimulated cyanide-insensitive oxygen consumption. Thus, regardless of whether DQ is reduced via a one- or two-electron reduction pathway, the net effect in the extracellular medium is the appearance of DQH2. These endothelial redox functions and their apposition to the vessel lumen are consistent with the pulmonary endothelium being an important site of DQ reduction to DQH2 observed in the lungs.

Plasma and CSF markers of oxidative stress are increased in Parkinson's disease and influenced by antiparkinsonian medication

We determined systemic oxidative stress in Parkinson's disease (PD) patients, patients with other neurological diseases (OND) and healthy controls by measurement of in vitro lipoprotein oxidation and levels of hydro- and lipophilic antioxidants in plasma and cerebrospinal fluid (CSF). Additionally, we investigated the influence of levodopa (LD) and dopamine agonist therapy (DA) on the oxidative status in PD patients. We found increased oxidative stress, seen as higher levels of lipoprotein oxidation in plasma and CSF, decrease of plasma levels of protein sulfhydryl (SH) groups and lower CSF levels of alpha-tocopherol in PD patients compared to OND patients and controls. Levodopa treatment did not significantly change the plasma lipoprotein oxidation but LD monotherapy tended to result in an increase of autooxidation and in a decrease of plasma antioxidants with significance for ubiquinol-10. DA monotherapy was significantly associated with higher alpha-tocopherol levels. Patients with DA monotherapy or co-medication with DA showed a trend to lower lipoprotein oxidation. These data support the concept of oxidative stress as a factor in the pathogenesis of PD and might be an indicator of a potential prooxidative role of LD and a possible antioxidative effect of DA in PD treatment.

Plasma levels and redox status of coenzyme Q10 in infants and children

INTRODUCTION

Increased attention has been paid to the role of lipophilic antioxidants in childhood nutrition and diseases during recent years. The lipophilic antioxidant coenzyme Q10 (CoQ10) is known as an effective inhibitor of oxidative damage. In contrast to other lipophilic antioxidants like alpha-tocopherol the plasma concentrations of CoQ10 in childhood are poorly researched. The aim of this study was to determine plasma level and redox status (oxidized form in total CoQ10 in %) of CoQ10 in clinically healthy infants, preschoolers and school-aged children.

METHODS

Plasma level and redox status of CoQ10 were measured by HPLC in 199 clinically healthy children, three groups of infants [1st-4th month (n = 35), 5th-8th month (n = 25), 9th-12th month (n = 25) ], preschoolers (n = 60) and school-aged children (n = 54). The CoQ10 plasma levels were related to plasma cholesterol concentrations. The median and the 5th and 95th percentile were calculated.

RESULTS

Plasma levels and redox status of CoQ10 in infants were significantly higher than in preschoolers and school-aged children. The CoQ10 redox status in the 1st-4th month was significantly increased when compared to the remaining subgroups of infants. In elder children the CoQ10 redox status stabilized.

CONCLUSIONS

This is the first study concerning age-related values of plasma level and redox status of CoQ10 in apparently healthy children. Decreased CoQ10 values could be involved in various pathological conditions affecting childhood. Therefore, the application of age-adjusted reference values may provide more specific criteria to define threshold values for CoQ10 deficiency in plasma.

Duroquinone reduction during passage through the pulmonary circulation

The lungs can substantially influence the redox status of redox-active plasma constituents. Our objective was to examine aspects of the kinetics and mechanisms that determine pulmonary disposition of redox-active compounds during passage through the pulmonary circulation. Experiments were carried out on rat and mouse lungs with 2,3,5,6-tetramethyl-1,4-benzoquinone [duroquinone (DQ)] as a model amphipathic quinone reductase substrate. We measured DQ and durohydroquinone (DQH2) concentrations in the lung venous effluent after injecting, or while infusing, DQ or DQH2 into the pulmonary arterial inflow. The maximum net rates of DQ reduction to DQH2 in the rat and mouse lungs were approximately 4.9 and 2.5 micromol. min(-1).g dry lung wt(-1), respectively. The net rate was apparently the result of freely permeating access of DQ and DQH2 to tissue sites of redox reactions, dominated by dicumarol-sensitive DQ reduction to DQH2 and cyanide-sensitive DQH2 reoxidation back to DQ. The dicumarol sensitivity along with immunodetectable expression of NAD(P)H-quinone oxidoreductase 1 (NQO1) in the rat lung tissue suggest cytoplasmic NQO1 as the dominant site of DQ reduction. The effect of cyanide on DQH2 oxidation suggests that the dominant site of oxidation is complex III of the mitochondrial electron transport chain. If one envisions DQ as a model compound for examining the disposition of amphipathic NQO1 substrates in the lungs, the results are consistent with a role for lung NQO1 in determining the redox status of such compounds in the circulation. For DQ, the effect is conversion of a redox-cycling, oxygen-activating quinone into a stable hydroquinone.

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.

Intracellular redox status affects transplasma membrane electron transport in pulmonary arterial endothelial cells

Pulmonary arterial endothelial cells possess transplasma membrane electron transport (TPMET) systems that transfer intracellular reducing equivalents to extracellular electron acceptors. As one aspect of determining cellular mechanisms involved in one such TPMET system in pulmonary arterial endothelial cells in culture, glycolysis was inhibited by treatment with iodoacetate (IOA) or by replacing the glucose in the cell medium with 2-deoxy-D-glucose (2-DG). TPMET activity was measured as the rate of reduction of the extracellular electron acceptor polymer toluidine blue O polyacrylamide. Intracellular concentrations of NADH, NAD(+), NADPH, and NADP(+) were determined by high-performance liquid chromatography of KOH cell extracts. IOA decreased TPMET activity to 47% of control activity concomitant with a decrease in the NADH/NAD(+) ratio to 34% of the control level, without a significant change in the NADPH/NADP(+) ratio. 2-DG decreased TPMET activity to 53% of control and decreased both NADH/NAD(+) and NADPH/NADP(+) ratios to 51% and 55%, respectively, of control levels. When lactate was included in the medium along with the inhibitors, the effects of IOA and 2-DG on both TPMET activity and the NADPH/NADP(+) ratios were prevented. The results suggest that cellular redox status is a determinant of pulmonary arterial endothelial cell TPMET activity, with TPMET activity more highly correlated with the poise of the NADH/NAD(+) redox pair.

Transplasma membrane electron transport in Leishmania donovani promastigotes

Leishmania donovani promastigotes are capable of reducing certain electron acceptors with redox potential at pH 7 down to -125 mV; outside the plasma membrane promastigotes can reduce ferricyanide. Ferricyanide has been used as an artificial electron acceptor probe for studying the mechanism of transplasma membrane electron transport. Transmembrane ferricyanide reduction by L. donovani promastigotes was not inhibited by such mitochondrial inhibitors as antimycin A or cyanide, but it responded to inhibitors of glycolysis. Transmembrane ferricyanide reduction by Leishmania appears to involve a plasma membrane electron transport chain dissimilar to that of hepatocyte cells. As with other cells, transmembrane electron transport is associated with proton release, which may be involved in internal pH regulation. The Leishmania transmembrane redox system differs from that of mammalian cells in being 4-fold less sensitive to chloroquine and 12-fold more sensitive to niclosamide. Sensitivities to these drugs suggest that transplasma membrane electron transport and associated proton pumping may be targets for the drugs used against leishmaniasis.

Evidence for the extracellular reduction of alpha-lipoic acid by Leishmania donovani promastigotes: a transplasma membrane redox system

Leishmania donovani cells, capable of reducing certain electron acceptors with redox potentials at pH 7.0 down to -290 mV, outside the plasma membrane, can reduce the oxidised form of alpha-lipoic acid. alpha-Lipoic acid has been used as natural electron acceptor probe for studying the mechanism of transplasma membrane electron transport. Transmembrane alpha-lipoic acid reduction by Leishmania was not inhibited by mitochondrial inhibitors as azide, cyanide, rotenone or antimycin A, but responded to hemin, modifiers of sulphhydryl groups and inhibitor of glycolysis. The protonophores carbonyl cyanide chlorophenylhydrazone and 2,4-dinitrophenol showed inhibition of alpha-lipoic acid reduction. This transmembrane redox system differs from that of mammalian cells in respect to its sensitivity of UV irradiation and stimulation by diphenylamine. Thus a naphthoquinone coenzyme appears to be involved in alpha-lipoic acid reduction by Leishmania cells.

NADH and NADPH-dependent reduction of coenzyme Q at the plasma membrane

High affinity for NADH, and low affinity for NADPH, for reduction of endogenous coenzyme Q10 (CoQ10) by pig liver plasma membrane is reported in the present work. CoQ reduction in plasma membrane is carried out, in addition to other mechanisms, by plasma membrane coenzyme Q reductase (PMQR). We show that PMQR-catalyzed reduction of CoQ0 by both NADH and NADPH is accompanied by generation of CoQ0 semiquinone radicals in a superoxide-dependent reaction. In the presence of a water-soluble vitamin E homologue, Trolox, this reduction leads to quenching of the Trolox phenoxyl radicals. The involvement of PMQR versus DT-diaphorase under the conditions of vitamin E and selenium sufficiency and deficiency was evaluated for CoQ reduction by plasma membranes. The data presented here suggest that both nucleotides (NADH and NADPH) can be accountable for CoQ reduction by PMQR on the basis of their physiological concentrations within the cell. The enzyme is primarily responsible for CoQ reduction in plasma membrane under normal (nonoxidative stress-associated) conditions.

Tocopherol-mediated peroxidation of lipoproteins: implications for vitamin E as a potential antiatherogenic supplement

The 'oxidation theory' of atherosclerosis proposes that oxidation of low density lipoprotein (LDL) contributes to atherogenesis. Although little direct evidence for a causative role of 'oxidized LDL' in atherogenesis exists, several studies show that, in vitro, oxidized LDL exhibits potentially proatherogenic activities and lipoproteins isolated from atherosclerotic lesions are oxidized. As a consequence, the molecular mechanisms of LDL oxidation and the actions of alpha-tocopherol (alpha-TOH, vitamin E), the major lipid-soluble lipoprotein antioxidant, have been studied in detail. Based on the known antioxidant action of alpha-TOH and epidemiological evidence, vitamin E is generally considered to be beneficial in coronary artery disease. However, intervention studies overall show a null effect of vitamin E on atherosclerosis. This confounding outcome can be rationalized by the recently discovered diverse role for alpha-TOH in lipoprotein oxidation; that is, alpha-TOH displays neutral, anti-, or, indeed, pro-oxidant activity under various conditions. This review describes the latter, novel action of alpha-TOH, termed tocopherol-mediated peroxidation, and discusses the benefits of vitamin E supplementation alone or together with other antioxidants that work in concert with alpha-TOH in ameliorating lipoprotein lipid peroxidation in the artery wall and, hence, atherosclerosis.

A role for reduced coenzyme Q in atherosclerosis?

Substantial evidence implicates oxidative modification of low density lipoprotein (LDL) as an important event contributing to atherogenesis. As a result, the elucidation of the molecular mechanisms by which LDL is oxidized and how such oxidation is prevented by antioxidants has been a significant research focus. Studies on the antioxidation of LDL lipids have focused primarily on alpha-tocopherol (alpha-TOH), biologically and chemically the most active form of vitamin E and quantitatively the major lipid-soluble antioxidant in extracts prepared from human LDL. In addition to alpha-TOH, plasma LDL also contains low levels of ubiquinol-10 (CoQ10H2; the reduced form of coenzyme Q10). Recent studies have shown that in oxidizing plasma lipoproteins alpha-TOH can exhibit anti- or pro-oxidant activities for the lipoprotein's lipids exposed to a vast array of oxidants. This article reviews the molecular action of alpha-TOH in LDL undergoing "mild" radical-initiated lipid peroxidation, and discusses how small levels of CoQ10H2 can represent an efficient antioxidant defence for lipoprotein lipids. We also comment on the levels alpha-TOH, CoQ10H2 and lipid oxidation products in the intima of patients with coronary artery disease and report on preliminary studies examining the effect of coenzyme Q10 supplementation on atherogenesis in apolipoprotein E knockout mice.

Plasma ubiquinol-10 as a marker for disease: is the assay worthwhile?

Ubiquinol-10 and ubiquinone-10 were measured in plasma of patients with several pathologies known to be associated with increased oxidative stress. Plasma ubiquinol-10, expressed as a percentage of total ubiquinol-10 + ubiquinone-10, was found to be significantly lower in hyperlipidaemic patients and in patients with liver diseases than in age-matched control subjects. In contrast, no decrease in ubiquinol-10 was detected in plasma of patients with coronary heart disease and Alzheimer's disease. Except for ubiquinol-10, no other lipophilic antioxidant was found to be decreased in patients with liver diseases. These data suggest that the level of ubiquinol-10 in human plasma may serve as a marker for liver dysfunction, reflecting its diminished reduction by the liver rather than increased consumption by oxidants.

Analysis of ubiquinone and tocopherol levels in normal and hyperlipidemic human plasma

The type and amount of lipophilic antioxidants in plasma of hyperlipidemic patients are of great importance since they play a central role in preventing deleterious oxidation of blood lipids and proteins. Isolation and quantitation of lipophilic antioxidants from hyperlipidemic plasma samples meet great obstacles because of increased levels of various intermediary lipid products. This study was designed to develop a rapid and efficient extraction and separation procedure for simultaneous analysis of ubiquinone-9 and -10 as well as alpha-, delta-, and gamma-tocopherol isomers. The levels of ubiquinone-10, alpha- and gamma-tocopherol were analyzed in human plasma samples using high-performance liquid chromatography. Lipid extraction was performed by petroleum ether/methanol/water. After phase separation, ubiquinone was reduced to ubiquinol by sodium borohydride and the lipids were separated on a C18 column. A binary gradient with solvents containing lithium perchlorate was used, and an electrochemical detector was employed for quantitation. This procedure was also efficient for the analysis of antioxidant lipids in samples containing a large number of accumulated and interfering lipid intermediates. Thus, the procedure described here is useful for efficient and rapid quantitation of ubiquinones and tocopherols in human plasma samples, especially those originating from hyperlipidemic patients.

Alpha-tocopheryl hydroquinone is an efficient multifunctional inhibitor of radical-initiated oxidation of low density lipoprotein lipids

As the oxidation of low density lipoprotein (LDL) lipids may be a key event in atherogenesis, there is interest in antioxidants as potential anti-atherogenic compounds. Here we report that alpha-tocopheryl hydroquinone (alpha-TQH2) strongly inhibited or completely prevented the (per)oxidation of ubiquinol-10 (CoQ10H2), alpha-tocopherol (alpha-TOH), and both surface and core lipids in LDL exposed to either aqueous or lipophilic peroxyl radicals, Cu2+, soybean lipoxygenase, or the transition metal-containing Ham's F-10 medium in the absence or presence of human monocyte-derived macrophages. The antioxidant activity of alpha-TQH2 was superior to that of several other lipophilic hydroquinones, including endogenous CoQ10H2, which is regarded as LDL's first line of antioxidant defence. At least three independent activities contributed to the antioxidant action of alpha-TQH2. First, alpha-TQH2 readily associated with LDL and instantaneously reduced the lipoprotein's ubiquinone-10 to CoQ10H2, thereby maintaining this antioxidant in its active form. Second, alpha-TQH2 directly intercepted aqueous peroxyl radicals, as indicated by the increased rate of its consumption with increasing rates of radical production, independent of LDL's content of CoQ10H2 and alpha-TOH. Third, alpha-TQH2 rapidly quenched alpha-tocopheroxyl radical in oxidizing LDL, as demonstrated directly by electron paramagnetic resonance spectroscopy. Similar antioxidant activities were also seen when alpha-TQH2 was added to high-density lipoprotein or the protein-free Intralipid, indicating that the potent antioxidant activity of alpha-TQH2 was neither lipoprotein specific nor dependent on proteins. These results suggest that alpha-TQH2 is a candidate for a therapeutic lipid-soluble antioxidant. As alpha-tocopherylquinone is formed in vivo at sites of oxidative stress, including human atherosclerotic plaque, and biological systems exist that reduce the quinone to the hydroquinone, our results also suggest that alpha-TQH2 could be a previously unrecognized natural antioxidant.

Oxidation of LDL and their subfractions: kinetic aspects and CoQ10 content

Coenzyme Q10 in its reduced form, ubiquinol-10, although present in LDL at concentrations considerably lower than that of alpha-tocopherol, exerts a potent antioxidant action in this class of lipoproteins. Previous studies indicated that the content of CoQ10 is the lowest in the densest subfraction of LDL, i.e. LDL3, which is commonly regarded as the most peroxidizable and atherogenic one. These levels were associated with the highest levels of hydroperoxides detectable in the three subclasses. Enrichment of LDL with CoQ10, by means of exogenous supplementation, resulted in a significant increase of CoQ10 in LDL, mainly in LDL3, and in a lower extent of peroxidizability. Spontaneous oxidation of ubiquinol was monitored in plasma and in LDL of unsupplemented and of supplemented subjects and the time-course of oxidation was found considerably slower in CoQ10-enriched LDL. The lagphase of conjugated dienes formation upon induced oxidation was significantly correlated with the absolute content of ubiquinol-10. Distribution of CoQ10 among different classes of plasma lipoproteins was also studied: about 60% of plasma CoQ10 was found associated with LDL.

Inhibition of LDL oxidation by ubiquinol-10. A protective mechanism for coenzyme Q in atherogenesis?

The oxidation of low density lipoprotein (LDL) is now commonly regarded as an important early event in atherogenesis. As such there is considerable interest in the ability of antioxidant supplementation to attenuate LDL oxidation and hence atherosclerosis. A majority of studies on LDL antioxidation have focused on alpha-tocopherol (alpha-TOH), biologically and chemically the most active form of vitamin E and quantitatively the major lipid-soluble antioxidant in extracts prepared from human LDL. In addition to alpha-TOH, circulating LDL also contains low levels of ubiquinol-10 (CoQ10H2; the reduced form of coenzyme Q). Recent studies have shown that in intact, isolated LDL, alpha-TOH can act as either an anti- or prooxidant for the lipoprotein's lipids. This article reviews the molecular action of alpha-TOH in LDL undergoing radical-initiated oxidation, and how the presence of CoQ10H2 suppresses the pro-oxidant or complements the antioxidant activity of the vitamin. We also comment on the plasma and intimal levels of alpha-TOH and CoQ10H2 in patients suffering from coronary artery disease and discuss the potential implications of these results for atherogenesis.

Extracellular ascorbate stabilization as a result of transplasma electron transfer in Saccharomyces cerevisiae

The presence of yeast cells in the incubation medium prevents the oxidation of ascrobate catalyzed by copper ions. Ethanol increases ascorbate retention. Pyrazole, an alcohol dehydrogenase inhibitor, prevents ascorbate stabilization by cells. Chelation of copper ions does not account for stabilization, since oxidation rates with broken or boiled cells or conditioned media are similar to control rates in the absence of cells. Protoplast integrity is needed to reach optimal values of stabilization. Chloroquine, a known inhibitor of plasma membrane redox systems, inhibits the ascorbate stabilization, the inhibition being partially reversed by coenzyme Q6. Chloroquine does not inhibit ferricyanide reduction. Growth of yeast in iron-deficient media to increase ferric ion reductase activity also increases the stabilization. In conclusion, extracellular ascorbate stabilization by yeast cells can reflect a coenzyme Q dependent transplasmalemma electron transfer which uses NADH as electron donor. Iron deficiency increases the ascorbate stabilization but the transmembrane ferricyanide reduction system can act independently of ascorbate stabilization.

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.

Is alpha-tocopherol a reservoir for alpha-tocopheryl hydroquinone?

The products of oxidation of the alpha-tocopherol model compound, 2,2,5,7,8-pentamethyl-6-chromanol (PH) by t-butyl hydroperoxide in chloroform varied with the amount of water present. In the presence of a trace of water, the main products were the spirodimer (PSD) and spirotrimer (PST). As the content of water increased, the main product became 2-(3-hydroxy-3-methylbutyl)-3,5,6-trimethyl-1,4-benzoquinone (PQ). Oxidation of PH in aqueous liposome suspension also produced PQ as the major product. These results suggested that, in aqueous solutions, the major oxidation product of PH would be PQ and of alpha-tocopherol (TH) would be alpha-tocopheryl quinone (TQ). The ease of reduction of PQ and TQ was studied in chemical and biological systems. PQ, TQ, and ubiquinone-10 (UQ) were rapidly reduced to their respective hydroquinones (PQH2, TQH2, and UQH2) at pH 7.3 by NADH plus FAD. Whole blood reduced PQ rapidly at 37 degrees C to PQH2 but did not reduce TQ to TQH2. Human peripheral blood mononuclear cells took up TQ from a bovine serum albumin complex and reduced it to TQH2. Ingestion of TQ (350 mg) by one of us (PSK) resulted in the formation of TQH2 during a 5 h period. These results demonstrate that several biological systems are able to reduce TQ to TQH2 and that it is a reaction that may occur normally in vivo.

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.

Human blood cells support the reduction of low-density-lipoprotein-associated cholesteryl ester hydroperoxides by albumin-bound ebselen

Ebselen, a glutathione peroxidase mimic capable of reducing simple as well as complex hydroperoxides, including those of phospholipids and cholesteryl esters in intact oxidized low-density lipoprotein (LDLox), requires the presence of low-molecular-mass thiols to be active. In plasma, the drug is thought to be transported as an inactive albumin complex. As formation of LDLox is likely to occur extracellularly, we tested under which conditions ebselen can support reduction of LDLox-associated cholesteryl ester hydroperoxides outside cells. We observed that addition of albumin-bound ebselen to whole blood, but not plasma, resulted in reduction of LDLox-associated cholesteryl linoleate hydroperoxides to the corresponding hydroxides. The observed reduction was rapid and its extent increased with increasing concentrations of ebselen. Physical contact of blood cells with LDLox was not required for this reducing activity. These results demonstrate that, in the presence of blood cells, extracellular ebselen is catalytically active. They suggest that ebselen may be considered as a drug for extracellular targets.

Metabolic implications of coenzyme Q10 in red blood cells and plasma lipoproteins

Plasma coenzyme Q10 (CoQ10) is currently assayed in our laboratory for its well-known diagnostic meaning; in fact plasma CoQ10 levels are inversely related to metabolic demand. Definite levels of CoQ10 are also found in white and red blood cell components, as well as in platelets. Plasma and erythrocyte CoQ10 has a well assessed antioxidant role, which was demonstrated through a series of experiments. Erythrocytes previously enriched with exogenous CoQ10 were found more resistant to a hemolysis induced by a free radical initiator. Several enzymatic activities of erythrocyte ghosts were also protected by different side chain CoQ homologues, both when reduced and, although at a lesser extent, in the oxidized state. CoQ was not effective in preventing metal-catalyzed oxidation of erythrocyte membrane enzymes, and this effect is likely to be due to lack of interaction of CoQ with the metal target. Moreover CoQ was able to protect isolated enzymes and erythrocyte membrane bound enzymes from the inactivating effect of free radicals generated by water sonolysis or radiolysis. As far as plasma lipoproteins are concerned it is well known that LDL isolated from healthy volunteers supplemented with CoQ10 are more resistant to peroxidation induced by an azoinitiator. We started to systematically investigate CoQ10 and vitamin E levels in isolated human LDL and HDL. Both CoQ10 and vitamin E concentrations, referred to protein, were found higher in LDL than in HDL. Susceptibility to exogenously applied peroxidation did not correlate with the endogeneous content of the two antioxidants, possibly on the basis of different lipid content of these lipoproteins.