Screening of the influence of flavonoids on lipoxygenase and cyclooxygenase activity, as well as on nonenzymic lipid oxidation

Thirty nine flavonoids, isolated from plants, were tested in respect of their influence on soybean lipoxygenase activity, cyclooxygenase activity and inhibition of ascorbic acid-stimulated malonaldehyde formation in liver lipids. Almost all of the tested compounds were antioxidants and stimulated cyclooxygenase when arachidonic acid was used as a substrate at a concentration of 100 microM. Eleven flavonoids were inhibitors of soybean lipoxygenase. A good correlation between the chemical structure and the tested activity was observed. The most active compounds in all tests were luteolin, 6-hydroxyluteolin, nepetin, quercetagetin, patuletin and myricetin.     

Inhibition of cyclooxygenase/lipoxygenase from human platelets by polyhydroxylated/methoxylated flavonoids isolated from medicinal plants

Various flavonoid derivatives were previously reported to possess the inhibitory activity on cyclooxygenase/lipoxygenase. And these properties of flavonoids might contribute to their anti-inflammatory activity in vivo. In this study, several polyhydroxylated/methoxylated flavonoid derivatives such as oroxylin A, wogonin, skullcapflavone II, tectorigenin and iristectorigenin A were isolated from the medicinal plants. These compounds were evaluated for their inhibitory effects on cyclooxygenase/lipoxygenase from the homogenate of human platelets in vitro. It was found that isoflavones including daidzein and tectorigenin possessed the inhibitory activity on cyclooxygenase, although the potency of inhibition was far less than that of indomethacin. In addition, oroxylin A, baicalein and wogonin inhibited 12-lipoxygenase activity without affecting cyclooxygenase, which suggested that 5,6,7- or 5,7,8-trisubstitutions of A-ring of flavone gave favorable results. The IC50 values of oroxylin A and NDGA against 12-lipoxygenase were found to be 100 and 1.5 microM, respectively.     

The influence of sulfonated bioflavonoids on enzymatic oxidation of arachidonic acid and on non-enzymatic lipid oxidation

Sulfonic acids of quercetin and morin as well as their ferrous and cupric complexes were synthetized and investigated. Sulfonic derivatives of quercetin were much weaker inhibitors of soybean lipoxygenase than quercetin itself. Morin and its derivatives were inactive. Antioxidant properties of quercetin derivatives were in the same range as for quercetin. Most of the investigated compounds stimulate cyclooxygenase when 100 microM of arachidonic acid is used as a substrate. Ferrous complex of quercetin 5'-sulfonic acid was an inhibitor of this enzyme.     

Effects of prenylated flavonoids and biflavonoids on lipopolysaccharide-induced nitric oxide production from the mouse macrophage cell line RAW 264.7

Certain flavonoid derivatives possess anti-inflammatory activity in vitro and in vivo. Besides their antioxidative properties and effects on the arachidonic acid metabolism including cyclooxygenase/lipoxygenase inhibition, some flavones and flavonols were previously found to show inhibitory activity on nitric oxide production by inducible nitric oxide synthase (iNOS; NOS type 2) through suppression of iNOS induction. As part of our continuing investigations, the effects of unique and minor flavonoids (prenylated flavonoids and biflavonoids) on nitric oxide production from lipopolysaccharide-induced macrophage cell line (RAW 264.7) were evaluated in order to establish their inhibitory activity on NO production and correlate this action with their in vivo anti-inflammatory potential. Among the derivatives tested, prenylated compounds including morusin, kuwanon C, and sanggenon D and biflavonoids such as bilobetin and ginkgetin were found to inhibit NO production from lipopolysaccharide (LPS)-induced RAW 264.7 cells at > 10 microM. Inhibition of nitric oxide production was mediated by suppression of iNOS enzyme induction but not by direct inhibition of iNOS enzyme activity. An exception was echinoisoflavanone that inhibited iNOS enzyme activity (IC50 = 83 microM) and suppressed iNOS enzyme induction as well. While most prenylated derivatives showed cytotoxicity to RAW cells at 10-100 microM, all biflavonoids tested were not cytotoxic. Since nitric oxide (NO) produced by inducible NO synthase (iNOS) plays an important role in inflammatory disorders, inhibition of NO production by these flavonoids may contribute, at least in part, to their anti-inflammatory and immunoregulating potential in vivo.     

In silico studies reveal the mechanisms behind the antioxidant and anti-inflammatory activities of hydroxytyrosol

Hydroxytyrosol is one of the most important phytochemical constituents of olive oil and possesses a very interesting pharmacological profile ranging from antioxidant, antimicrobial, and anti-inflammatory to low density lipoproteins oxidation inhibition. The present study aims to shed light on the mechanisms behind the antioxidant and the anti-inflammatory activities of hydroxytyrosol. Towards the first aim, quantum mechanical calculations of several properties including bond dissociation energy, proton dissociation enthalpy, adiabatic ionization potential, proton affinity, and electron transfer enthalpy has been performed in vacuum and water environments in order to propose an antioxidant activity mechanism. Results support the hydrogen atom transfer mechanism as the most prevalent in gas phase, while in aqueous environment SPLET mechanism can explain the antioxidant mechanism of hydroxytyrosol. Towards the second aim, the potential of hydroxytyrosol to act as cyclooxygenase and lipoxygenase inhibitor against inflammation was examined through molecular docking studies. Cyclooxygenase is an enzyme, responsible for formation of prostanoids and its inhibition can provide relief from the symptoms of inflammation and pain. Lipoxygenases are a family of iron-containing enzymes involved in the metabolism of eicosanoids and their role in the regulation of pro-inflammatory responses has been well documented. Results indicate the binding mode in which hydroxytyrosol may exert its activity through lipoxygenase enzyme. Moreover, hydroxytyrosol can act as a core structure for bio-inspired inhibitors and may provide information for the design of dual cyclooxygenase/lipoxygenase inhibitors against inflammation.     

The effects of natural flavonoids on lipoxygenase-mediated oxidation of compounds with a benzene ring structure--a new possible mechanism of flavonoid anti-chemical carcinogenesis and other toxicities

Numerous studies have strongly suggested that flavonoids exhibit antimutagenic, anticarcinogenic, antiallergic, and anti-inflammatory properties, but the mechanism is still far from clear. In this study, the effect of natural flavonoid compounds, such as green tea polyphenol, epigallocatechin gallate, quercetin, and rutin on lipoxygenase-mediated co-oxidation of guaiacol, benzidine, paraphenylenediamine, and dimethoxybenzidine was investigated. Green tea polyphenol, epigallocatechin gallate, quercetin, and rutin can reduce the co-oxidation reaction speed of tested compounds mediated by soybean lipoxygenase and the production of oxidative products and free radical intermediates. Their median inhibition concentrations on guaiacol oxidation mediated by soybean lipoxygenase were 8.22 mg.L-1, 17.8 micromol.L-1, 41.5 micromol.L-1, and 46.3 micromol.L-1, respectively. These were all significantly lower than glutathione, dithiothreitol, butylated hydroxyanisole and gossypol. The data collected in this study suggest that flavonoids may have an anticarcinogenicity and antitoxicity effect through inhibition of oxidative activation.     

Structure activity relationship of antioxidative property of flavonoids and inhibitory effect on matrix metalloproteinase activity in UVA-irradiated human dermal fibroblast

Collagenase, a matrix metalloproteinases (MMPs), is a key regulator in the photoaging process of skin due to the reactive oxygen species generated after exposure to ultraviolet A (UVA). Flavonoid compounds have been demonstrated to possess antioxidant properties, and could be useful in the prevention of photoaging. In this study, to investigate the structure-activity relationship of flavonoid compounds on their antioxidant property and inhibitory effects against the MMP activity, the effects of several flavonoids; myricetin, quercetin, kaempferol, luteolin, apigenin and chrysin, on the reactive oxygen species scavengering activity and inhibitory effect against the MMP activity were examined in vitro and in human dermal fibroblasts induced by UVA. The relative order of antioxidative efficacy, as determined using the 1, 1-diphenyl-2-picrylhydrazyl (DPPH) method and the xanthine/xanthine oxidase system, was as follows; flavones: luteolin > apigenin > chrysin, flavonols: myricetin > quercetin > kaempferol, and correlated with the respective number of OH group on their B-ring. In good correlation with the antioxidant properties, the flavonoids inhibited the collagenase activities, in a dose-dependent manner, and the MMP expression. These results suggested the UVA induced antioxidative activity and inhibitory effects of flavonoids on the collagenase in human dermal fibroblasts depends on the number of OH group in the flavonoid structure, and those with a higher number of OH group may be more useful in the prevention of UV stressed skin aging.     

Modification of platelet function and arachidonic acid metabolism by bioflavonoids. Structure-activity relations

The mechanism of the antiaggregating activity of flavonoids was studied in vitro. The activity of fifteen different compounds was tested on platelet aggregation and arachidonic acid metabolism. The effect of flavonoids on platelet adenosine 3',5'-cyclic monophosphate (cyclic AMP) levels under basal conditions, as well as after stimulation by prostacyclin (PGI2), was also measured. The glycons of flavonoids in general and the flavanone derivatives that we tested did not affect platelet function. On the other hand, flavone, chrysin , apigenin and phloretin inhibited platelet aggregation by depressing the cyclooxygenase pathway. In addition, flavone, chrysin and apigenin reduced the platelet cyclic AMP response to PGI2. This effect was probably mediated by an inhibition of adenylate cyclase. Myricetin and quercetin, however, increased the PGI2-stimulated rise of platelet cyclic AMP. Both of these flavonoids inhibited primarily lipoxygenase activity. Modification of platelet cyclic AMP metabolism through inhibition of phosphodiesterase activity was found to be the probable mechanism of their antiaggregating effect.     

Metabolic transformation has a profound effect on anti-inflammatory activity of flavonoids such as quercetin: lack of association between antioxidant and lipoxygenase inhibitory activity

Dietary flavonoids are thought to have health benefits possibly due to antioxidant and anti-inflammatory properties. Many previous in vitro studies examining the bioactivity of flavonoids have failed to consider the effects of metabolic transformation on flavonoid activity. In this study we examined the effect of quercetin and its major metabolites on the production of pro-inflammatory eicosanoids by human leukocytes. Studies comparing free radical scavenging, antioxidant activity and eicosanoid production demonstrate that there are different structural requirements for antioxidant and anti-inflammatory activity. We also investigated the effect of metabolic transformation on flavonoid bioactivity by comparing the activity of quercetin and its major metabolites to inhibit inflammatory eicosanoid production from human leukocytes. Quercetin was a potent inhibitor of leukotriene B4 formation in leukocytes (IC50 approximately 2 microM), and its activity was dependent on specific structural features, particularly the 2,3-double bond of the C-ring. Functionalisation of the 3'-OH group with either methyl or sulfate reduced inhibitory activity up to 50% while a glucuronide substituent at the 3-OH effectively removed the LTB4 inhibitory activity. The major quercetin metabolite quercetin-3'-O-sulfate retained considerable lipoxygenase inhibitory activity (IC50 approximately 7 microM) while quercetin-3-O-glucuronide maintained antioxidant activity but had no lipoxygenase inhibitory activity at physiological concentrations. In conclusion, we have found that structural modification of quercetin due to metabolic transformation had a profound effect on bioactivity, and that the structural features required for antioxidant activity of quercetin and related flavonoids were unrelated to those required for inhibition of inflammatory eicosanoids.     

Myricetin inhibits panel of kinases implicated in tumorigenesis

Myricetin is a flavonoid with several biological properties, including antioxidant and anti‐inflammatory features. Its protective effect in chronic diseases may occur through the inhibition of protein kinases that trigger inflammation and carcinogenesis pathways. Considering the influence of kinases on such pathological disorders, it is crucial to study compounds that inhibit these proteins. This study aims to evaluate the inhibitory potential of 14 flavonoids on TNF‐α release in human whole blood as well as the inhibitory potential of myricetin towards kinases involved in tumorigenesis. Our results showed that, out of all flavonoids, myricetin had the highest inhibitory effect on TNF‐α level. In addition, myricetin showed potential as a multi‐anti‐kinase compound, reducing the activity of 7 kinases by >70% and of 9 kinases by >90%. Together these data demonstrate the great inhibitory activity of myricetin on tumorigenic kinases and potential for the development of new therapeutics.     

In vitro effects of selected flavonoids on the 5'-nucleotidase activity.

A series of structurally related flavonoids and related compounds were evaluated whether they have inhibitory properties on the 5'-nucleotidase (5'-ribonucleotide phosphohydrolase; EC 3.1.3.5, 5'-NT) activity. Some of the flavonoids tested inhibit the enzyme such as quercetin, morin, apigenin, chrysin, myricetin, luteolin, diosmetin, (+/-)naringenin and diosmin. Rutin, naringin, hyperosid, (+/-)catechin, caffeic acid and rosmarinic acid had no inhibitory effect on the 5'-NT activity. Myricetin and quercetin were the most potent inhibitors for 5'-NT with IC50 values of 1.1 and 1.4 microM, respectively. Kinetic analysis showed a mixed type of inhibitor for both myricetin (Ki = 1.5 microM at pH 7.45), and quercetin (Ki = 0.6 microM at pH 7.45). The K(m) value for 5'-adenosine monophosphate (5-AMP) was determined with 77 microM at pH 7.45. The differential inhibitory potencies of flavonoids seem to be structurally related (hydroxylation pattern). The results demonstrate that some flavonoids are strong inhibitors of 5'-NT activity which can be correlated to their pharmacological effects.     

Interaction of flavonoids with 1,1-diphenyl-2-picrylhydrazyl free radical, liposomal membranes and soybean lipoxygenase-1

The interaction of the antiperoxidative flavonoids namely, quercetin, quercetrin, rutin, myricetin, phloretin, phloridzin, catechin, morin and taxifolin with the 1,1,-diphenyl-2-picrylhydrazyl (DPPH) free radical was demonstrated. Flavonoid-DPPH interaction was looked at in the absence and presence of liposomes so as to reveal some information on bilayers. Perturbations in the lipid bilayers were monitored with the fluorescent probe, dansylhexadecylamine (DSHA). It was observed that the interaction of the flavonoids on the lipid bilayer occurred in the polar zone of the lipid bilayers. The flavonoids were able to scavenge free radicals and could do so in biomembranes. It is suggested that the DPPH free radical abstracts the phenolic hydrogen of the flavonoid molecule and that this could be the general mechanism of the scavenging action of the antiperoxidative flavonoids. The effects of the flavonoids on soybean lipoxygenase-1 were investigated both in buffer and also in liposomal suspension. All the flavonoids studied showed inhibition of the enzyme in both systems but the inhibition was greater in the liposomal suspension. Quercetin was the most potent and it inhibited the lipoxygenase in the liposomal suspension by about 42% while the other flavonoids inhibited the enzyme by about 14-23%. We observed that the effect of myricetin and quercetin on the enzyme was pH dependent.     

Effect of myricetin and other flavonoids on the liver plasma membrane Ca2+ pump. Kinetics and structure-function relationships

Thirty-three different flavonoids were screened for their ability to influence ATP-dependent Ca2+ uptake by rat liver plasma membrane vesicles. Nine of the flavonoids, at a concentration of 100 microM inhibited Ca2+ uptake by more than 20%. The remaining 24 flavonoids exhibited little or no effect. The relative order of potency of the more biologically active flavonoids was myricetin greater than butein greater than phloretin = luteolin greater than eriodictyol = silybin. Myricitrin and phloridzin, the glycosides of myricetin and phloretin, respectively, had no effect. The degree of inhibition caused by myricetin was concentration dependent and was also affected by the preincubation time. After 10 min of preincubation, 52 microM myricetin lowered the initial rate of 45Ca uptake by 50%. The inhibition by myricetin was non-competitive with respect to Mg-ATP and of a mixed type with respect to Ca2+. At a concentration of 100 microM, myricetin had no effect on several plasma membrane enzymes such as 5'-nucleotidase, alkaline phosphatase and a Ca2(+)-activated ATPase but inhibited K(+)-dependent p-nitrophenyl phosphatase by 83%. The ATP-dependent Ca2+ transport systems located on the plasma membrane or endoplasmic reticulum derived from other tissues were also inhibited by myricetin. Analysis of the structure-activity relationship revealed that lipid solubility and polyhydroxylation particularly at positions 5,7,3' and 4' of the flavonoid ring structure enhanced the ability of the flavonoid to inhibit Ca2+ uptake. The results suggest that inhibition of Ca2+ transport activity probably involves the interaction of the phenolic groups of the flavonoid with the Ca2+ transporting protein.     

Flavonoids inhibit histamine release and expression of proinflammatory cytokines in mast cells

Mast cells participate in allergy and inflammation by secreting inflammatory mediators such as histamine and proinflammatory cytokines. Flavonoids are naturally occurring molecules with antioxidant, cytoprotective, and antiinflammatory actions. However, effect of flavonoids on the release of histamine and proinflammatory mediator, and their comparative mechanism of action in mast cells were not well defined. Here, we compared the effect of six flavonoids (astragalin, fisetin, kaempferol, myricetin, quercetin, and rutin) on the mast cell-mediated allergic inflammation. Fisetin, kaempferol, myricetin, quercetin, and rutin inhibited IgE or phorbol-12-myristate 13-acetate and calcium ionophore A23187 (PMACI)-mediated histamine release in RBL-2H3 cells. These five flavonoids also inhibited elevation of intracellular calcium. Gene expressions and secretion of proinflammatory cytokines such as tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6, and IL-8 were assessed in PMACI-stimulated human mast cells (HMC-1). Fisetin, quercetin, and rutin decreased gene expression and production of all the proinflammatory cytokines after PMACI stimulation. Myricetin attenuated TNF-α and IL-6 but not IL-1β and IL-8. Fisetin, myricetin, and rutin suppressed activation of NF-κB indicated by inhibition of nuclear translocation of NF-κB, NF-κB/DNA binding, and NF-κB-dependent gene reporter assay. The pharmacological actions of these flavonoids suggest their potential activity for treatment of allergic inflammatory diseases through the down-regulation of mast cell activation.     

Flavonoid effects on DNA oxidation at low concentrations relevant to physiological levels.

Flavonoids, which are abundant in fruits and vegetables, are known to have many beneficial health effects. Antioxidant activity is likely to be a main function but has been mostly studied at high flavonoid concentrations which are not feasible at the intracellular level. In this experiment, several flavonoids (e.g., catechin, quercetin, myricetin, luteolin, morin and cyanidin) were examined at low physiologically relevant concentrations. Calf thymus DNA was treated with different flavonoids at concentrations of 0.1, 1, 10 and 100 microM using Fenton conditions to induce oxidation and several oxidative adducts including 8-hydroxy guanine (7,8-dihydro-8-oxo-2'-deoxyguanosine; 8-OH guanine) were analyzed using gas chromatography-mass spectrometry-selective ion monitoring (GC-MS-SIM). Catechin, quercetin and cyanidin inhibited 8-OH guanine formation by 92%, 33% and 45%, respectively, at low concentrations (0.1 microM). In addition catechin and quercetin showed antioxidant activities on 8-OH guanine formation over all concentrations. When the oxidative DNA adduct 4,6-diamino-5-formamidopyrimidine (fapy-adenine) was measured, however, the highest concentrations of catechin and quercetin actually increased adduct formation. These results indicate that flavonoids can act as antioxidants at low concentrations relevant to physiological levels. However measuring only one oxidative DNA adduct as a biomarker may result in misleading conclusions regarding antioxidant activities of natural products.     

The activity of flavonoids extracted from Tanacetum microphyllum DC. (Compositae) on soybean lipoxygenase and prostaglandin synthetase

• 1.1. The in vitro effects of centaureidin and 5,3′-dihydroxy-4′-methoxy-7-carbomethoxyflavonol (Fig. 1), two anti-inflammatory flavonoids extracted from Tanacetum microphyllum DC., have been examined on both cyclooxygenase and lipoxygenase activity.
• 2.2. These flavonoids produced an inhibition of soybean lipoxygenase activity in a dose-dependent manner, with IC₅₀ values (20 and 29 μM respectively) similar to the reference drug.
• 3.3. The IC₅₀ values for the in vitro inhibition of cyclooxygenase activity by these flavonoids, were higher than those that produced lipoxygenase activity (318 and 60μM respectively).
• 4.4. These results suggest that the anti-inflammatory activity of our flavonoids may, at least in part, be due to the inhibition of leukotriene synthesis.
• 5.5. This is the first report of the biological activity in vitro of these compounds.

     

Structure-activity relationship of flavonoids derived from medicinal plants in preventing methylmercury-induced mitochondrial dysfunction

In the present study, we investigated the potential protective effects of three flavonoids (myricetin, myricitrin and rutin) derived from medicinal plants against methyl mercury (MeHg)-induced mitochondrial dysfunction in vitro. Incubation of mouse brain mitochondria with MeHg induced a significant decrease in mitochondrial function, which was correlated with decreased glutathione (GSH) levels and increased generation of reactive oxygen species (ROS) and lipid peroxidation. The co-incubation of mouse brain mitochondria with myricetin or myricitrin caused a concentration-dependent decrease of MeHg-induced mitochondrial dysfunction and oxidative stress. The flavonoid rutin was ineffective in counteracting MeHg toxicity. Among the three tested flavonoids, myricetin was the most efficient in protecting against MeHg-induced mitochondrial dysfunction. Moreover, myricetin completely blocked MeHg-induced ROS formation and lipid peroxidation and partially prevented MeHg-induced GSH depletion. The ability of myricetin to attenuate MeHg-induced mitochondrial dysfunction and oxidative stress appears to be related to its higher scavenging capability when compared to myricitrin and rutin. Overall, the results suggest that MeHg-induced mitotoxicity is associated with oxidative stress. The ability of myricetin to prevent MeHg-induced oxidative damage in brain mitochondria renders this flavonoid a promising molecule for further in vivo studies in the search for potential antidotes to counteract MeHg-induced neurotoxicity.     

Structure–activity relationship of flavonoids derived from medicinal plants in preventing methylmercury-induced mitochondrial dysfunction

In the present study, we investigated the potential protective effects of three flavonoids (myricetin, myricitrin and rutin) derived from medicinal plants against methyl mercury (MeHg)-induced mitochondrial dysfunction in vitro. Incubation of mouse brain mitochondria with MeHg induced a significant decrease in mitochondrial function, which was correlated with decreased glutathione (GSH) levels and increased generation of reactive oxygen species (ROS) and lipid peroxidation. The co-incubation of mouse brain mitochondria with myricetin or myricitrin caused a concentration-dependent decrease of MeHg-induced mitochondrial dysfunction and oxidative stress. The flavonoid rutin was ineffective in counteracting MeHg toxicity. Among the three tested flavonoids, myricetin was the most efficient in protecting against MeHg-induced mitochondrial dysfunction. Moreover, myricetin completely blocked MeHg-induced ROS formation and lipid peroxidation and partially prevented MeHg-induced GSH depletion. The ability of myricetin to attenuate MeHg-induced mitochondrial dysfunction and oxidative stress appears to be related to its higher scavenging capability when compared to myricitrin and rutin. Overall, the results suggest that MeHg-induced mitotoxicity is associated with oxidative stress. The ability of myricetin to prevent MeHg-induced oxidative damage in brain mitochondria renders this flavonoid a promising molecule for further in vivo studies in the search for potential antidotes to counteract MeHg-induced neurotoxicity.     

Prevention of cellular ROS damage by isovitexin and related flavonoids

The antioxidant properties of isovitexin and related flavonoids were studied. Isovitexin inhibited xanthine oxidase with an IC50 value of = 15.2 microM. The flavonoid analogues, apigenin, kaempferol, quercetin, myricetin, and genistein also inhibited xanthine oxidase with IC50 values of 0.58, 2.18, 1.09, 9.90, and 4.83 microM, respectively. Isovitexin protected DNA from the Fenton reaction-induced breakage in a dose-dependent manner with an IC50 value of 9.52 microM. Isovitexin also protected HL-60 cells from the ROS damage induced by the xanthine/xanthine oxidase reaction. Isovitexin exhibited the lowest cytotoxicity toward HL-60 cells (LD50 >400 microM) compared to the other flavonoids examined. In addition, excess hydrogen peroxide induced by cadmium in A2780 ovarian cells was significantly suppressed by isovitexin. These results suggest that isovitexin in rice may protect cells from oxidative stress.     

Cytoprotective activity against peroxide-induced oxidative damage and cytotoxicity of flavonoids in C6 rat glioma cells

The aim of this study was to investigate the relationship between cytoprotective and cytotoxic activities of selected plant flavonoids in C6 glioma cells. Apigenin, kaempferol, luteolin, and quercetin were cytotoxic at low μM concentrations (LOECs: 5-20 μM), whereas myricetin was less toxic (LOEC > 20 μM). Cytotoxicity was not due to H₂O₂ generation from flavonoids in culture medium. Quercetin, luteolin, and kaempferol protected the cells from peroxide-induced cytotoxicity. Concentration-effect curves for cytoprotection had a biphasic shape. In contrast, apigenin and myricetin did not exhibit any cytoprotective activity. The first three compounds also inhibited cellular lipid peroxidation induced by CHP, while the latter were ineffective. Importantly, concentrations of luteolin and kaempferol protecting cells under oxidative stress were identical to those causing cell damage under normal conditions. Only in case of quercetin there was a narrow range of concentrations protecting cells without being cytotoxic to non-stressed cells. Thus, even for flavonoids with a high antioxidant capacity in cell-free systems the cytoprotective selectivity (LOEC_cytotox/LOEC_cytoprot) was very low or even absent. These results should be taken into account when the prophylactic or therapeutic application of flavonoids as antioxidants is discussed.