Inhibition of Mammalian 15-Lipoxygenase-Dependent Lipid Peroxidation in Low-Density Lipoprotein by Quercetin and Quercetin Monoglucosides

Lipoxygenase is suggested to be involved in the early event of atherosclerosis by inducing plasma low-density lipoprotein (LDL) oxidation in the subendothelial space of the arterial wall. Since flavonoids such as quercetin are recognized as lipoxygenase inhibitors and they occur mainly in the glycoside form, we assessed the effect of quercetin and its glycosides (quercetin 3-O-β-glucopyranoside, Q3G; quercetin 4′-O-β-glucopyranoside, Q4′G; quercetin 7-O-β-glucopyranoside, Q7G) on rabbit reticulocyte 15-lipoxygenase (15-Lox)-induced human LDL lipid peroxidation and compared it with the inhibition obtained by ascorbic acid and α-tocopherol, the main water-soluble and lipid-soluble antioxidants in blood plasma, respectively. Quercetin inhibited the formation of cholesteryl ester hydroperoxides (CE-OOH) and endogenous α-tocopherol consumption effectively throughout the incubation period of 6 h. Ascorbic acid exhibited an effective inhibition only in the initial stage and LDL preloaded with fivefold α-tocopherol did not affect the formation of CE-OOH compared with the native LDL. CE-OOH formation was inhibited by both quercetin and quercetin monoglucosides in a concentration-dependent manner. Quercetin, Q3G, and Q7G exhibited a higher inhibitory effect than Q4′G (IC₅₀: 0.3–0.5 μM for quercetin, Q3G, and Q7G and 1.2 μM for Q4′G). While endogenous α-tocopherol was completely depleted after 2 h of LDL oxidation, quercetin, Q7G, and Q3G prevented the consumption of α-tocopherol. Quercetin and its monoglucosides were also exhausted during the LDL oxidation. These results indicate that quercetin glycosides as well as its aglycone are capable of inhibiting lipoxygenase-induced LDL oxidation more efficiently than ascorbic acid and α-tocopherol.     

Inhibitory effect of flavonoids on low-density lipoprotein peroxidation catalyzed by mammalian 15-lipoxygenase

Lipoxygenase-dependent low-density lipoprotein (LDL) oxidation is believed to be involved in atherogenesis. Inhibition of lipoxygenase-induced lipid peroxidation might, therefore, be an important mode to suppress the development of atherosclerosis. Because dietary antioxidants inhibit LDL oxidation in vitro and their intake is inversely associated with coronary heart diseases, we compared the inhibitory effect of three typical flavonoids-quercetin, epicatechin, and flavone-with alpha-tocopherol and ascorbic acid against human LDL oxidation catalyzed by mammalian 15-lipoxygenase. The oxidative modification of LDL was monitored by measurement of cholesteryl ester hydroperoxide (CE-OOH) formation and consumption of antioxidants by using HLPC. Quercetin and epicatechin were the strongest inhibitors of LDL oxidation catalyzed by 15-lipoxygenase; ascorbic acid was an effective inhibitor in the first 3 h of oxidation; and fivefold alpha-tocopherol-enriched LDL showed a partial inhibition of CE-OOH formation only after 4-6 h of incubation. Flavone had no effect. Quercetin, ascorbic acid, and alpha-tocopherol were consumed in the first 3 h of incubation. Consumption of LDL alpha-tocopherol was partially inhibited by ascorbic acid and quercetin, whereas epicatechin and flavone were without effect. These results emphasize the inhibitory effect of the flavonoids quercetin and epicatechin on 15-lipoxygenase-mediated LDL lipid peroxidation. At similar concentrations, they are stronger antioxidants than ascorbic acid, alpha-tocopherol, and flavone.     

Attenuation of lipid peroxidation and hyperlipidemia by quercetin glucoside in the aorta of high cholesterol-fed rabbit

Antioxidative activity of dietary flavonoids is suggested to be, at least partly, responsible for a wide variety of their biological effects relating to anti-atherosclerosis. However, it is not known whether dietary flavonoids reach to the target site and act as antioxidants. In this study, we tried to evaluate the antioxidative effect of quercetin 3-O-beta-D-glucoside (Q3G), a typical flavonoid present in vegetables, in rabbit aorta. New Zealand White rabbits were fed a control diet (control group), 2.0% cholesterol diet (HC group) and 2.0% cholesterol plus 0.1% Q3G (HC + Q3G group) for one month. The amounts of total cholesterol, triacylglycerol and total fatty acids in both the plasma and aorta were significantly lower in the HC + Q3G group as compared with the HC group. Quercetin was detected in the aorta of the HC + Q3G group after enzymatic deconjugation, indicating that quercetin accumulated as conjugated metabolites in the aorta. The contents of TBA-reacting substances (TBARS) and cholesteryl ester hydroperoxides (CEOOH) in the aorta of the HC + Q3G group were significantly lower than those in the HC group. The aorta of HC + Q3G group was more resistant than that of HC group in copper ion-induced lipid peroxidation ex vivo. HC + Q3G group accumulated a higher amount of vitamin E per total cholesterol than HC group in the aorta. These results strongly suggest that quercetin glucosides accumulate in the aorta as their metabolites and attenuate lipid peroxidation occurring in the aorta, along with the attenuation of hyperlipidemia.     

Inhibition of human low density lipoprotein oxidation by flavonols and their glycosides

Antioxidative effects of the flavonols and their glycosides, i.e., quercetin (Q), quercetin galactopyranoside (QG), quercetin rhamnolpyranoside (QR), rutin (R), morin (MO), myrecetin (MY), kaempferol (K) and kaempferol glucoside (KG), against free radical initiated peroxidation of human low density lipoprotein (LDL) were studied. The peroxidation was initiated either by a water-soluble initiator 2,2'-azobis(2-amidino propane hydrochloride) (AAPH), or by cupric ion (Cu2+). The reaction kinetics were monitored either by the uptake of oxygen and the depletion of alpha-tocopherol (TOH) presented in the native LDL, or by the formation of thiobarbituric acid reactive substances (TBARS). Kinetic analysis of the antioxidation process demonstrates that these flavonols and their glycosides are effective antioxidants against AAPH- and Cu(2+)-initiated LDL peroxidation, the flavonols bearing ortho-dihydroxyl groups possess significantly higher antioxidant activity than those bearing no such functionalities, and the glycosides are less active than their parent aglycones.     

Protective effect of quercetin against cigarette tar extract-induced impairment of erythrocyte deformability

Quercetin (3,3',4',5,7-pentahydroxyflavone) is one of the most abundant flavonol-type flavonoids rich in diet and suggested to possess a beneficial role in blood circulation. This study was conducted to know the effect of quercetin aglycone and one of its possible metabolite, quercetin-3-O-beta-D-glucuronide on cigarette tar extract-induced impairment of erythrocyte deformability. Erythrocyte suspension containing quercetin aglycone, quercetin-3-O-beta-D-glucuronide or quercetin-3-O-beta-D-glucoside was forced to flow through microchannels with equivalent diameter of 7 &mgr;m and its transit time was measured as an index of erythrocyte deformability using microchannel array method. Both quercetin aglycone and quercetin-3-O-beta-D-glucuronide, but not quercetin-3-O-beta-D-glucoside, substantially increased erythrocyte deformability indicating that the former two compounds affect the physicochemical state of erythrocyte by interacting with its membranes. Aqueous cigarette tar extract caused marked decrease in erythrocyte deformability with concomitant increase of membranous lipid peroxidation. In that case, quercetin aglycone suppressed the impairment of erythrocyte deformability as well as membranous lipid peroxidation. The same effect was found in quercetin-3-O-beta-D-glucuronide, eventhough its effect was lower than that of quercetin aglycone. Thus, not only quercetin aglycone but also its conjugate metabolite protects erythrocyte membrane from the damage of smoking by scavenging reactive oxygen species generated from cigarette tar. Intake of quercetin-rich food may be helpful to protect membranous damage in erythrocytes from smoking.     

Glucuronidated quercetin lowers blood pressure in spontaneously hypertensive rats via deconjugation

Background

Chronic oral quercetin reduces blood pressure and restores endothelial dysfunction in hypertensive animals. However, quercetin (aglycone) is usually not present in plasma, because it is rapidly metabolized into conjugated, mostly inactive, metabolites. The aim of the study is to analyze whether deconjugation of these metabolites is involved in the blood pressure lowering effect of quercetin.

Methodology/Principal Findings

We have analyzed the effects on blood pressure and vascular function in vitro of the conjugated metabolites of quercetin (quercetin-3-glucuronide, Q3GA; isorhamnetin-3-glucuronide, I3GA; and quercetin-3′-sulfate, Q3''S) in spontaneously hypertensive rats (SHR). Q3GA and I3GA (1 mg/kg i.v.), but not Q3''S, progressively reduced mean blood pressure (MBP), measured in conscious SHR. The hypotensive effect of Q3GA was abolished in SHR treated with the specific inhibitor of β-glucuronidase, saccharic acid 1,4-lactone (SAL, 10 mg/ml). In mesenteric arteries, unlike quercetin, Q3GA had no inhibitory effect in the contractile response to phenylephrine after 30 min of incubation. However, after 1 hour of incubation Q3GA strongly reduced this contractile response and this effect was prevented by SAL. Oral administration of quercetin (10 mg/Kg) induced a progressive decrease in MBP, which was also suppressed by SAL.

Conclusions

Conjugated metabolites are involved in the in vivo antihypertensive effect of quercetin, acting as molecules for the plasmatic transport of quercetin to the target tissues. Quercetin released from its glucuronidated metabolites could be responsible for its vasorelaxant and hypotensive effect.     

Protection by quercetin and quercetin 3-O-beta-D-glucuronide of peroxynitrite-induced antioxidant consumption in human plasma low-density lipoprotein.

Effect of quercetin and its conjugated metabolite quercetin 3-O-beta-D-glucuronide (Q3GA), on peroxynitrite-induced consumption of lipophilic antioxidants in human plasma low-density lipoprotein (LDL) was measured to estimate the role of dietary flavonoids in the defense system against oxidative modification of LDL based on the reaction of nitric oxide and superoxide anion. Synthesized peroxynitrite-induced consumption of endogenous lycopene beta-carotene and alpha-tocopherol was effectively suppressed by adding quercetin aglycone into LDL solution. Q3GA also inhibited the consumption of these antioxidants effectively. These results indicate that dietary quercetin is capable of inhibiting peroxynitrite-induced oxidative modification of LDL in association with lipophilic antioxidants present within this lipoprotein particle.     

Accumulation of quercetin conjugates in blood plasma after the short-term ingestion of onion by women

Quercetin is a typical flavonoid present mostly as glycosides in plant foods; it has attracted much attention for its potential beneficial effects in disease prevention. In this study, we examined human volunteers after the short-term ingestion of onion, a vegetable rich in quercetin glucosides. The subjects were served diets containing onion slices (quercetin equivalent: 67.6-93.6 mg/day) with meals for 1 wk. Quercetin was only found in glucuronidase-sulfatase-treated plasma, and its concentration after 10 h of fasting increased from 0.04 +/- 0.04 microM before the trial to 0.63 +/- 0.72 microM after the 1-wk trial. The quercetin content in low-density lipoprotein (LDL) after glucuronidase-sulfatase treatment corresponded to <1% of the alpha-tocopherol content. Human LDL isolated from the plasma after the trial showed little improvement of its resistance to copper ion-induced oxidation. It is therefore concluded that conjugated metabolites of quercetin accumulate exclusively in human blood plasma in the concentration range of 10(-7) approximately 10(-6) M after the short-term ingestion of vegetables rich in quercetin glucosides, although these metabolites are hardly incorporated into plasma LDL.     

Quercetin metabolites inhibit copper ion-induced lipid peroxidation in rat plasma

The oxidative susceptibility of plasma obtained from rats after intragastric administration of quercetin was studied to know whether or not quercetin acts as an in vivo antioxidant after metabolic conversion. Quercetin was raised in the rat blood plasma essentially as glucuronide and/or sulfate conjugates. The plasma obtained from rats after quercetin administration was more resistant against copper sulfate-induced lipid peroxidation than the control plasma on the basis of the accumulation of cholesteryl ester hydroperoxides and the consumption of α-tocopherol. The results strongly suggest that some conjugated metabolites of quercetin act as effective antioxidants when plasma is subject to metal ion-induced lipid peroxidation.     

Identification of quercetin 3-O-beta-D-glucuronide as an antioxidative metabolite in rat plasma after oral administration of quercetin

The potential beneficial effect of dietary quercetin (3,3',4',5,7-pentahydroxyflavone) has attracted much attention in relation to the prevention of cardiovascular disease. It is generally recognized that dietary quercetin is subject to metabolic conversion resulting in conjugated forms during absorption and circulation. However, no quercetin conjugates have yet been identified from biological fluids or tissues. In the present study, we isolated and characterized two quercetin conjugates from the plasma of quercetin-administered rats. The blood plasma was collected from 26 rats 30 min after oral administration of quercetin (250 mg/kg body weight), concentrated, dissolved in 2% acetic acid aqueous solution (pH 2.65), and extracted with ethyl acetate. Two compounds (P2, P3) were obtained from the extract by repeated reversed-phase HPLC. On the other hand, two quercetin glucuronides were synthesized chemically and identified as quercetin 3-O-beta-D-glucuronide (Q3GA) and quercetin 4'-O-beta-D-glucuronide (Q4'GA), as determined from FABMS, 1H- and 13C-NMR, and HMBC data. The retention times of P2 and P3 in the HPLC chromatogram corresponded to those of Q3GA and Q4'GA, respectively. FABMS data demonstrated that P2 and P3 are quercetin monoglucuronides. 1H-NMR data for P2 were completely in agreement with those for Q3GA. P2 was therefore identified as Q3GA. This is, to our knowledge, the first evidence that Q3GA accumulates in vivo after oral administration of quercetin. Q3GA is likely to act as an effective antioxidant in blood plasma low-density lipoprotein, because this conjugated metabolite was found to possess a substantial antioxidant effect on copper ion-induced oxidation of human plasma low-density lipoprotein as well as 1,1-diphenyl-2-picrylhydrazyl radical-scavenging activity.     

Protection by quercetin and quercetin 3-O-β-D-glucuronide of peroxynitrite-induced antioxidant consumption in human plasma low-density lipoprotein

Effect of quercetin and its conjugated metabolite quercetin 3-O-β-D-glucuronide (Q3GA), on peroxynitrite-induced consumption of lipophilic antioxidants in human plasma low-density lipoprotein (LDL) was measured to estimate the role of dietary flavonoids in the defense system against oxidative modification of LDL based on the reaction of nitric oxide and superoxide anion. Synthesized peroxynitrite-induced consumption of endogenous lycopene β-carotene and α-tocopherol was effectively suppressed by adding quercetin aglycone into LDL solution. Q3GA also inhibited the consumption of these antioxidants effectively. These results indicate that dietary quercetin is capable of inhibiting peroxynitrite-induced oxidative modification of LDL in association with lipophilic antioxidants present within this lipoprotein particle.     

Effect of quercetin and its conjugated metabolite on the hydrogen peroxide-induced intracellular production of reactive oxygen species in mouse fibroblasts

To clarify the antioxidative role of quercetin metabolites in cellular oxidative stress, we measured the inhibitory effects of the quercetin aglycon and quercetin 3-O-beta-D-glucuronide (Q3GA), which is one of the quercetin metabolites in the blood after an intake of quercetin-rich food, on the production of hydrogen peroxide (H2O2)-induced intracellular reactive oxygen species in mouse fibroblast 3T3 cultured cells. When the cells were exposed to H2O2 in the presence of quercetin or Q3GA, Q3GA was found to be less effective than quercetin. In the case of a pretreatment with quercetin or Q3GA before the exposure, Q3GA, but not the quercetin aglycon, exerted an inhibitory effect, although its cellular uptake was unlikely. The quercetin aglycon appeared to fail in its antioxidative effect due to metabolic conversion into isorhamnetin conjugates, with substantial oxidative degradation resulting from the pretreatment. It is, therefore, suggested that quercetin metabolites take part in the protection of intracellular oxidative stress induced by the extraneous attack of H2O2.     

Effect of quercetin and its glucuronide metabolite upon 6-hydroxydopamine-induced oxidative damage in Neuro-2a cells

Quercetin is ubiquitously distributed in plant foods. This antioxidative polyphenol is mostly converted to conjugated metabolites in the body. Parkinson disease (PD) has been suggested to be related to oxidative stress derived from abnormal dopaminergic activity. We evaluated if dietary quercetin contributes to the antioxidant network in the central nervous system from the viewpoint of PD prevention. A neurotoxin, 6-hydroxydopamine (6-OHDA), was used as a model of PD. 6-OHDA-induced H?O? production and cell death in mouse neuroblastoma, Neuro-2a. Quercetin aglycone suppressed 6-OHDA-induced H?O? production and cell death, although aglycone itself reduced cell viability at higher concentration. Quercetin 3-O-β-D-glucuronide (Q3GA), which is an antioxidative metabolite of dietary quercetin, was little incorporated into the cell resulting in neither suppression of 6-OHDA-induced cell death nor reduction of cell viability. Q3GA was found to be deconjugated to quercetin by microglial MG-6 cells. These results indicate that quercetin metabolites should be converted to their aglycone to exert preventive effect on damage to neuronal cells.     

Inhibition of human low density lipoprotein oxidation by active principles from spices

Spice components and their active principles are potential antioxidants. In this study we examined the effect of phenolic and non-phenolic active principles of common spices on copper ion-induced lipid peroxidation of human low density lipoprotein (LDL) by measuring the formation of thiobarbituric acid reactive substance (TBARS) and relative electrophoretic mobility (REM) of LDL on agarose gel. Curcurriin, capsaicin, quercetin, piperine, eugenol and allyl sulfide inhibited the formation of TBARS effectively through out the incubation period of 12 h and decreased the REM of LDL. Spice phenolic active principles viz. curcumin, quercetin and capsaicin at 10 M produced 40–85% inhibition of LDL oxidation at different time intervals while non-phenolic antioxidant allyl sulfide was less potent in inhibiting oxidation of LDL. However, allyl sulfide, eugenol and ascorbic acid showed pro-oxidant activity at lower concentrations (10 M) and antioxidant activity at higher concentrations (50 M) only. Among the spice principles tested quercetin and curcumin showed the highest inhibitory activity while piperine showed least antioxidant activity at equimolar concentration during initiation phase of oxidation of LDL. The inhibitory effect of curcumin, quercetin and capsaicin was comparable to that of BHA, but relatively more potent than ascorbic acid. Further, the effect of curcurnin, quercetin, capsaicin and BHA on initiation and propagation phases of LDL oxidation showed that curcurnin significantly inhibited both initiation and propagation phases of LDL oxidation, while quercetin was found to be ineffective at propagation phase. These data suggest that the above spice active principles, which constitute about 1–4% of above spices, are effective antioxidants and offer protection against oxidation of human LDL.     

Screening for free radical scavenging and cell aggregation inhibitory activities by secondary metabolites from Turkish Verbascum species

Free radical scavenging and cell aggregation inhibitory activities of 36 secondary metabolites isolated from the methanolic extracts of Verbascum cilicicum Boiss., V. lasianthum Boiss. ex Bentham, V pterocalycinum var. mutense Hub.-Mor., and V. salviifolium Boiss. (Scrophulariaceae) were investigated. The isolated compounds, 6-O-vaniloyl ajugol (1), ilwensisaponin A (2), ilwensisaponin C (3), verbascoside (4), beta-hydroxyacteoside (5), martynoside (6), poliumoside (7), forsythoside B (8), angoroside A (9), dehydrodiconiferyl alcohol-9-O-beta-D-glucopyranoside (10), dehydrodiconiferyl alcohol-9'-O-beta-D-glucopyranoside (11), apigenin 7-O-beta-glucopyranoside (12), luteolin 7-O-beta-glucopyranoside (13), luteolin 3'-O-beta-glucopyranoside (14) and chrysoeriol 7-O-beta-glucopyranoside (15), exhibited a dose-dependent inhibition of bioautographic and spectrophotometric DPPH activities. Verbascoside (4) was the most active (IC50 4.0 microg/ml) comparing it to vitamin C (IC50 4.4 microg/ml) to inhibit phorbol 12-myristate 13-acetate (PMA)-induced peroxide-catalyzed oxidation of 2',7'-dichlorofluorescein (DCFH) by reactive oxygen species (ROS) within human promyelocytic HL-60 cells. Ilwensisaponin A (2) (MIC 6.9 microg/ml) showed moderate in vitro activity on lymphocyte-associated antigen-1 (LFA-1)/intercellular adhesion molecule-1 (ICAM-1)-mediated aggregation using the HL-60 cell line [positive control was cytochalasin B (MIC 2.3 microg/ml)]. None of the other compounds showed free radical scavenging and cell aggregation inhibitory activities.     

Structural Aspects of The Inhibitory Effect of Glabridin on LDL Oxidation

The inhibitory effects of glabridin, an isoflavan isolated from licorice (Glycyrrhiza glabra) root, and its derivatives on the oxidation of LDL induced by copper ions or mediated by macrophages were studied, in order to evaluate the contribution of the different parts of the isoflavan molecule to its antioxidant activity. The peak potential (E_1/2) of the isoflavan derivatives, their radical scavenging capacity toward 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical and their ability to chelate heavy metals were also analyzed and compared to their inhibitory activity on LDL oxidation. In copper ion-induced LDL oxidation, glabridin (1), 4′-O-methylglabridin (2), hispaglabridin A (3), and hispaglabridin B (4), which have two hydroxyl groups at positions 2′ and 4′ or one hydroxyl at position 2′ on ring B, successfully inhibited the formation of conjugated dienes, thiobarbituric acid reactive substances (TBARS) and lipid peroxides, and inhibited the electrophoretic mobility of LDL under oxidation. Compounds 1–3 exhibited similar activities, whereas compound 4 was less active. In macrophage-mediated LDL oxidation, the TBARS formation was also inhibited by these isoflavans (1–4) at a similar order of activity to that obtained in copper ion-induced LDL oxidation. On the other hand, 2′-O-methylglabridin (5), a synthesized compound, whose hydroxyl at 2′-position is protected and the hydroxyl at 4′-position is free, showed only minor inhibitory activity in both LDL oxidation systems. 2′,4′-O-Dimethylglabridin (6), whose hydroxyls at 2′- and 4′-positions are both protected, was inactive. Resorcinol (7), which is identical to the phenolic B ring in glabridin, presented low activity in these oxidation systems. The isoflavene glabrene (8), which contains an additional double bond in the heterocyclic C ring, was the most active compound of the flavonoid derivatives tested in both oxidation systems. The peak potential of compounds 1–5 (300 μM), tested at pH 7.4, was similar (425–530 mV), and that for compound 6 and 8 was 1078 and 80 mV, respectively. Within 30 min of incubation, compounds 1, 2, 3, 4, 8 scavenged 31%, 16%, 74%, 51%, 86%, respectively, of DPPH radical, whereas compounds 5 and 6, which almost did not inhibit LDL oxidation, also failed to scavenge DPPH. None of the isoflavan derivatives nor the isoflavene compound were able to chelate iron, or copper ions. These results suggest that the antioxidant effect of glabridin on LDL oxidation appears to reside mainly in the 2′ hydroxyl, and that the hydrophobic moiety of the isoflavan is essential to obtain this effect. It was also shown that the position of the hydroxyl group at B ring significantly affected the inhibitory efficiency of the isoflavan derivatives on LDL oxidation, but did not influence their ability to donate an electron to DPPH or their peak potential values.     

Metabolic conversion of dietary flavonoids alters their anti-inflammatory and antioxidant properties

The notion that dietary flavonoids exert beneficial health effects in humans is often based on in vitro studies using the glycoside or aglycone forms of these flavonoids. However, flavonoids are extensively metabolized in humans, resulting in the formation of glucuronide, methyl, and sulfate derivatives, which may have different properties than their parent compounds. The goal of this study was to investigate whether different chemical modifications of the same flavonoid molecule affect its biological and antioxidant activities. Hence, we studied the anti-inflammatory effects of several major human metabolites of quercetin and (-)-epigallocatechin-3-O-gallate (EGCG) by assessing their inhibitory effects on tumor necrosis factor α (TNFα)-induced protein expression of cellular adhesion molecules in human aortic endothelial cells (HAEC). HAEC were incubated with 1-30 μM quercetin, 3'- or 4'-O-methyl-quercetin, quercetin-3-O-glucuronide, and quercetin-3'-O-sulfate or 20-100 μM EGCG, 4'-O-methyl-EGCG, and 4',4'-di-O-methyl-EGCG, prior to coincubation with 100 U/ml of TNFα. 3'-O-Methyl-quercetin, 4'-O-methyl-quercetin, and their parent aglycone compound, quercetin, all effectively inhibited expression of intercellular adhesion molecule-1 (ICAM-1) with IC(50) values (concentration required for 50% inhibition) of 8.0, 5.0, and 4.4 μM, respectively; E-selectin expression was suppressed to a somewhat lesser but still significant degree by all three compounds, whereas vascular cell adhesion molecule-1 (VCAM-1) was not affected. In contrast, quercetin-3-O-glucuronide (20-100 μM), quercetin-3'-O-sulfate (10-30 μM), and phenolic acid metabolites of quercetin (20-100 μM) did not inhibit adhesion molecule expression. 4',4'-Di-O-methyl-EGCG selectively inhibited ICAM-1 expression with an IC(50) value of 94 μM, whereas EGCG (20-60 μM) and 4'-O-methyl-EGCG (20-100 μM) had no effect. The inhibitory effects of 3'-O-methyl-quercetin and 4',4'-di-O-methyl-EGCG on adhesion molecule expression were not related either to inhibition of NF-κB activation or to their antioxidant reducing capacity. Our data indicate that flavonoid metabolites have different biological and antioxidant properties than their parent compounds, and suggest that data from in vitro studies using nonmetabolites of flavonoids are of limited relevance in vivo.     

Effect of quercetin and its glucuronide metabolite upon 6-hydorxydopamine-induced oxidative damage in Neuro-2a cells

Abstract

Quercetin is ubiquitously distributed in plant foods. This antioxidative polyphenol is mostly converted to conjugated metabolites in the body. Parkinson disease (PD) has been suggested to be related to oxidative stress derived from abnormal dopaminergic activity. We evaluated if dietary quercetin contributes to the antioxidant network in the central nervous system from the viewpoint of PD prevention. A neurotoxin, 6-hydroxydopamine (6-OHDA), was used as a model of PD. 6-OHDA-induced H₂O₂ production and cell death in mouse neuroblastoma, Neuro-2a. Quercetin aglycone suppressed 6-OHDA-induced H₂O₂ production and cell death, although aglycone itself reduced cell viability at higher concentration. Quercetin 3-O-β-d-glucuronide (Q3GA), which is an antioxidative metabolite of dietary quercetin, was little incorporated into the cell resulting in neither suppression of 6-OHDA-induced cell death nor reduction of cell viability. Q3GA was found to be deconjugated to quercetin by microglial MG-6 cells. These results indicate that quercetin metabolites should be converted to their aglycone to exert preventive effect on damage to neuronal cells.     

Lipid peroxidation and antioxidants in hyperlipidemia and hypertension

Lipid peroxidation and lipid-derived oxidized products have been implicated in the pathogenesis of a variety of human diseases. To clarify the role of oxidative stress in essential hypertension and hypercholesterolemia the in vitro oxidative susceptibility of LDL, the antioxidant status and the lipid peroxide content of blood plasma were examined in hypercholesterolemic (HC), hypertensive (H), hypercholesterolemic/hypertensive (HH) and normolipidemic/normotensive subjects (N). Plasma ascorbate and lipid-soluble antioxidants were lower, while LDL oxidizability, CE-OOH and TL-OOH were higher in H, HC, and HH groups than in the N group. No difference was observed among groups for PL-OOH and isoprostanes. In summary, the results show that: 1) lipid- and water-soluble antioxidants are lower in hypercholesterolemic and hypertensive patients as compared to normal subjects, whereas the lipid peroxide content and the LDL susceptibility to oxidation were higher; 2) total cholesterol, LDL-cholesterol, apoB and CE-OOH were negatively correlated with the content of a-tocopherol; 3) there was a positive correlation between the content of lipid-soluble antioxidants and the resistance of LDL to oxidation; and 4) CE-OOH and TL-OOH were positively correlated with total cholesterol and LDL-cholesterol.     

Efficiency of absorption and metabolic conversion of quercetin and its glucosides in human intestinal cell line Caco-2

The efficiency of intestinal absorption and metabolic conversion of quercetin aglycone and its glucosides, quercetin-4'-O-beta-D-glucoside (Q4'G), quercetin-3-O-beta-D-glucoside (Q3G), and quercetin-3,4'-di-O-beta-D-glucoside (Q3,4'G), was estimated by using Caco-2 cell monolayers as an intestinal epithelial cell model. Aglycone was significantly lost from the apical side, resulting in the appearance of free and conjugated forms of quercetin and those of isorhamnetin in the cellular extracts. In the basolateral solution, the conjugated form of quercetin was predominant and increased with the elapse of incubation. As compared with quercetin aglycone, none of the quercetin glucosides were absorbed efficiently from apical side. However, Q4'G yielded conjugated quercetin and isorhamnetin in basolateral solution at higher amounts than Q3G or Q3,4'G. Lipophilicity of Q4'G was found to be higher than that of Q3G or Q3,4'G. This suggests that lipophilicity contributes to the relatively efficient absorption of Q4'G. It is likely that the occurrence of hydrolysis enhances the efficiency of intestinal absorption and metabolic conversion of dietary quercetin glucosides.