Structure-antioxidant activity relationships of flavonoids and phenolic acids

The recent explosion of interest in the bioactivity of the flavonoids of higher plants is due, at least in part, to the potential health benefits of these polyphenolic components of major dietary constituents. This review article discusses the biological properties of the flavonoids and focuses on the relationship between their antioxidant activity, as hydrogen donating free radical scavengers, and their chemical structures. This culminates in a proposed hierarchy of antioxidant activity in the aqueous phase. The cumulative findings concerning structure-antioxidant activity relationships in the lipophilic phase derive from studies on fatty acids, liposomes, and low-density lipoproteins; the factors underlying the influence of the different classes of polyphenols in enhancing their resistance to oxidation are discussed and support the contention that the partition coefficients of the flavonoids as well as their rates of reaction with the relevant radicals define the antioxidant activities in the lipophilic phase.

Polyphenols: food sources and bioavailability

Polyphenols are abundant micronutrients in our diet, and evidence for their role in the prevention of degenerative diseases such as cancer and cardiovascular diseases is emerging. The health effects of polyphenols depend on the amount consumed and on their bioavailability. In this article, the nature and contents of the various polyphenols present in food sources and the influence of agricultural practices and industrial processes are reviewed. Estimates of dietary intakes are given for each class of polyphenols. The bioavailability of polyphenols is also reviewed, with particular focus on intestinal absorption and the influence of chemical structure (eg, glycosylation, esterification, and polymerization), food matrix, and excretion back into the intestinal lumen. Information on the role of microflora in the catabolism of polyphenols and the production of some active metabolites is presented. Mechanisms of intestinal and hepatic conjugation (methylation, glucuronidation, sulfation), plasma transport, and elimination in bile and urine are also described. Pharmacokinetic data for the various polyphenols are compared. Studies on the identification of circulating metabolites, cellular uptake, intracellular metabolism with possible deconjugation, biological properties of the conjugated metabolites, and specific accumulation in some target tissues are discussed. Finally, bioavailability appears to differ greatly between the various polyphenols, and the most abundant polyphenols in our diet are not necessarily those that have the best bioavailability profile. A thorough knowledge of the bioavailability of the hundreds of dietary polyphenols will help us to identify those that are most likely to exert protective health effects.

Flavonoids as antioxidants

Flavonoids are phenolic substances isolated from a wide range of vascular plants, with over 8000 individual compounds known. They act in plants as antioxidants, antimicrobials, photoreceptors, visual attractors, feeding repellants, and for light screening. Many studies have suggested that flavonoids exhibit biological activities, including antiallergenic, antiviral, antiinflammatory, and vasodilating actions. However, most interest has been devoted to the antioxidant activity of flavonoids, which is due to their ability to reduce free radical formation and to scavenge free radicals. The capacity of flavonoids to act as antioxidants in vitro has been the subject of several studies in the past years, and important structure-activity relationships of the antioxidant activity have been established. The antioxidant efficacy of flavonoids in vivo is less documented, presumably because of the limited knowledge on their uptake in humans. Most ingested flavonoids are extensively degraded to various phenolic acids, some of which still possess a radical-scavenging ability. Both the absorbed flavonoids and their metabolites may display an in vivo antioxidant activity, which is evidenced experimentally by the increase of the plasma antioxidant status, the sparing effect on vitamin E of erythrocyte membranes and low-density lipoproteins, and the preservation of erythrocyte membrane polyunsaturated fatty acids. This review presents the current knowledge on structural aspects and in vitro antioxidant capacity of most common flavonoids as well as in vivo antioxidant activity and effects on endogenous antioxidants.

Antimicrobial activity of flavonoids

Flavonoids are ubiquitous in photosynthesising cells and are commonly found in fruit, vegetables, nuts, seeds, stems, flowers, tea, wine, propolis and honey. For centuries, preparations containing these compounds as the principal physiologically active constituents have been used to treat human diseases. Increasingly, this class of natural products is becoming the subject of anti-infective research, and many groups have isolated and identified the structures of flavonoids possessing antifungal, antiviral and antibacterial activity. Moreover, several groups have demonstrated synergy between active flavonoids as well as between flavonoids and existing chemotherapeutics. Reports of activity in the field of antibacterial flavonoid research are widely conflicting, probably owing to inter- and intra-assay variation in susceptibility testing. However, several high-quality investigations have examined the relationship between flavonoid structure and antibacterial activity and these are in close agreement. In addition, numerous research groups have sought to elucidate the antibacterial mechanisms of action of selected flavonoids. The activity of quercetin, for example, has been at least partially attributed to inhibition of DNA gyrase. It has also been proposed that sophoraflavone G and (-)-epigallocatechin gallate inhibit cytoplasmic membrane function, and that licochalcones A and C inhibit energy metabolism. Other flavonoids whose mechanisms of action have been investigated include robinetin, myricetin, apigenin, rutin, galangin, 2,4,2'-trihydroxy-5'-methylchalcone and lonchocarpol A. These compounds represent novel leads, and future studies may allow the development of a pharmacologically acceptable antimicrobial agent or class of agents.

Chemistry and biological activities of flavonoids: an overview

There has been increasing interest in the research on flavonoids from plant sources because of their versatile health benefits reported in various epidemiological studies. Since flavonoids are directly associated with human dietary ingredients and health, there is need to evaluate structure and function relationship. The bioavailability, metabolism, and biological activity of flavonoids depend upon the configuration, total number of hydroxyl groups, and substitution of functional groups about their nuclear structure. Fruits and vegetables are the main dietary sources of flavonoids for humans, along with tea and wine. Most recent researches have focused on the health aspects of flavonoids for humans. Many flavonoids are shown to have antioxidative activity, free radical scavenging capacity, coronary heart disease prevention, hepatoprotective, anti-inflammatory, and anticancer activities, while some flavonoids exhibit potential antiviral activities. In plant systems, flavonoids help in combating oxidative stress and act as growth regulators. For pharmaceutical purposes cost-effective bulk production of different types of flavonoids has been made possible with the help of microbial biotechnology. This review highlights the structural features of flavonoids, their beneficial roles in human health, and significance in plants as well as their microbial production.

The biochemistry and medical significance of the flavonoids

Flavonoids are plant pigments that are synthesised from phenylalanine, generally display marvelous colors known from flower petals, mostly emit brilliant fluorescence when they are excited by UV light, and are ubiquitous to green plant cells. The flavonoids are used by botanists for taxonomical classification. They regulate plant growth by inhibition of the exocytosis of the auxin indolyl acetic acid, as well as by induction of gene expression, and they influence other biological cells in numerous ways. Flavonoids inhibit or kill many bacterial strains, inhibit important viral enzymes, such as reverse transcriptase and protease, and destroy some pathogenic protozoans. Yet, their toxicity to animal cells is low. Flavonoids are major functional components of many herbal and insect preparations for medical use, e.g., propolis (bee's glue) and honey, which have been used since ancient times. The daily intake of flavonoids with normal food, especially fruit and vegetables, is 1-2 g. Modern authorised physicians are increasing their use of pure flavonoids to treat many important common diseases, due to their proven ability to inhibit specific enzymes, to simulate some hormones and neurotransmitters, and to scavenge free radicals.

Antioxidant and prooxidant behavior of flavonoids: structure-activity relationships

The antioxidant and prooxidant behavior of flavonoids and the related activity-structure relationships were investigated in this study using the oxygen radical absorbance capacity assay. Three different reactive species were used in the assay: 2,2'-azobis(2-amidino-propane) dihydrochloride, a peroxyl radical generator; Cu(2+)-H2O2, mainly a hydroxyl radical generator; and Cu2+, a transition metal. Flavonoids including flavones, isoflavones, and flavanones acted as antioxidants against peroxyl and hydroxyl radicals and served as prooxidants in the presence of Cu2+. Both the antioxidant and the copper-initiated prooxidant activities of a flavonoid depend upon the number of hydroxyl substitutions in its backbone structure, which has neither antioxidant nor prooxidant action. In general, the more hydroxyl substitutions, the stronger the antioxidant and prooxidant activities. The flavonoids that contain multiple hydroxyl substitutions showed antiperoxyl radical activities several times stronger than Trolox, an alpha-to copherol analogue. The single hydroxyl substitution at position 5 provides no activity, whereas the di-OH substitution at 3' and 4' is particularly important to the peroxyl radical absorbing activity of a flavonoid. The conjugation between rings A and B does not affect the antioxidant activity but is very important for the copper-initiated prooxidant action of a flavonoid. The O-methylation of the hydroxyl substitutions inactivates both the antioxidant and the prooxidant activities of the flavonoids.

Novel total antioxidant capacity index for dietary polyphenols and vitamins C and E, using their cupric ion reducing capability in the presence of neocuproine: CUPRAC method

The chemical diversity of antioxidants makes it difficult to separate and quantify antioxidants from the vegetable matrix. Therefore, it is desirable to establish a method that can measure the total antioxidant activity level directly from vegetable extracts. The current literature clearly states that there is no "total antioxidant" as a nutritional index available for food labeling because of the lack of standard quantitation methods. Thus, this work reports the development of a simple, widely applicable antioxidant capacity index for dietary polyphenols and vitamins C and E, utilizing the copper(II)-neocuproine [Cu(II)-Nc] reagent as the chromogenic oxidizing agent. Because the copper(II) (or cupric) ion reducing ability of polyphenols is measured, the method is named by our research group "cupric reducing antioxidant capacity" abbreviated as the CUPRAC method. This method should be advantageous over the ferric reducing antioxidant power (FRAP) method because the redox chemistry of copper(II)-as opposed to that of ferric ion-involves faster kinetics. The method comprises mixing of the antioxidant solution (directly or after acid hydrolysis) with a copper(II) chloride solution, a neocuproine alcoholic solution, and an ammonium acetate aqueous buffer at pH 7 and subsequent measurement of the developed absorbance at 450 nm after 30 min. Because the color development is fast for compounds such as ascorbic acid, gallic acid, and quercetin but slow for naringin and naringenin, the latter compounds were assayed after incubation at 50 degrees C on a water bath for 20 min [after Cu(II)-Nc reagent addition] so as to force the oxidation reaction to reach completion. The flavonoid glycosides were hydrolyzed to their corresponding aglycons by refluxing in 1.2 M HCl-containing 50% MeOH so as to exert maximal reducing power toward Cu(II)-Nc. Certain compounds also needed incubation after acid hydrolysis to fully exhibit their reducing capability. The CUPRAC antioxidant capacities of synthetic mixtures of antioxidants were experimentally measured as Trolox equivalents and compared to those theoretically found by making use of the principle of additivity of absorbances assuming no chemical interaction between the mixture constituents. Because ascorbic acid is not resistant to elevated temperature incubation, it should be assayed initially by measuring the absorbance (at 450 nm) difference of original and ascorbate oxidase-added mixture solutions at the end of 1 min of Cu(II)-Nc reagent addition. Thus, the total CUPRAC antioxidant capacity of a mixture containing various antioxidants should be that finally measured after a suitable combination of hydrolysis and incubation procedures, added to the initially measured capacity due to ascorbate. The antioxidant polyphenolic compounds tested demonstrate that the highest capacities in the CUPRAC method were observed for epicatechin gallate, epigallocatechin gallate, quercetin, fisetin, epigallocatechin, catechin, and caffeic acid in this order, in accordance with theoretical expectations, because the number and position of the hydroxyl groups as well as the degree of conjugation of the whole molecule are important. The antioxidant potency of flavonoids is nearly proportional to the total number of -OH groups and is positively affected by the presence of an o-dihydroxy moiety in the B-ring. beta-Carotene, which did not react with the CUPRAC reagent in alcoholic aqueous medium, could be assayed in dichloromethane solvent. Linear calibration curves for ascorbic acid and flavonoids were redrawn in synthetic solutions containing a mixture of antioxidants, and also in real matrices such as grape and orange juices, green tea, and blackberry tea, showing an initial nonzero absorbance with the CUPRAC reagent. The parallellism of the linear calibration curves of pure compounds in a given complex matrix effectively demonstrated that there were no interferent chemical interactions among the solution constituents and that the antioxidant capacities of the tested antioxidants were additive. The CUPRAC reagent is reasonably selective, stable, easily accessible, and sensitive toward thiol-type oxidants, unlike the FRAP method. The reaction is carried out at nearly physiological pH as opposed to the unrealistic acidic pH of FRAP.

Flavonoids: antioxidants or signalling molecules?

Many studies are accumulating that report the neuroprotective, cardioprotective, and chemopreventive actions of dietary flavonoids. While there has been a major focus on the antioxidant properties, there is an emerging view that flavonoids, and their in vivo metabolites, do not act as conventional hydrogen-donating antioxidants but may exert modulatory actions in cells through actions at protein kinase and lipid kinase signalling pathways. Flavonoids, and more recently their metabolites, have been reported to act at phosphoinositide 3-kinase (PI 3-kinase), Akt/protein kinase B (Akt/PKB), tyrosine kinases, protein kinase C (PKC), and mitogen activated protein kinase (MAP kinase) signalling cascades. Inhibitory or stimulatory actions at these pathways are likely to affect cellular function profoundly by altering the phosphorylation state of target molecules and by modulating gene expression. A clear understanding of the mechanisms of action of flavonoids, either as antioxidants or modulators of cell signalling, and the influence of their metabolism on these properties are key to the evaluation of these potent biomolecules as anticancer agents, cardioprotectants, and inhibitors of neurodegeneration

Biosynthesis of plant pigments: anthocyanins, betalains and carotenoids

Plant compounds that are perceived by humans to have color are generally referred to as 'pigments'. Their varied structures and colors have long fascinated chemists and biologists, who have examined their chemical and physical properties, their mode of synthesis, and their physiological and ecological roles. Plant pigments also have a long history of use by humans. The major classes of plant pigments, with the exception of the chlorophylls, are reviewed here. Anthocyanins, a class of flavonoids derived ultimately from phenylalanine, are water-soluble, synthesized in the cytosol, and localized in vacuoles. They provide a wide range of colors ranging from orange/red to violet/blue. In addition to various modifications to their structures, their specific color also depends on co-pigments, metal ions and pH. They are widely distributed in the plant kingdom. The lipid-soluble, yellow-to-red carotenoids, a subclass of terpenoids, are also distributed ubiquitously in plants. They are synthesized in chloroplasts and are essential to the integrity of the photosynthetic apparatus. Betalains, also conferring yellow-to-red colors, are nitrogen-containing water-soluble compounds derived from tyrosine that are found only in a limited number of plant lineages. In contrast to anthocyanins and carotenoids, the biosynthetic pathway of betalains is only partially understood. All three classes of pigments act as visible signals to attract insects, birds and animals for pollination and seed dispersal. They also protect plants from damage caused by UV and visible light.

The relative antioxidant activities of plant-derived polyphenolic flavonoids

The relative antioxidant activities, against radicals generated in the aqueous phase, of a range of plant-derived polyphenolic flavonoids, constituents of fruit, vegetables, tea and wine, have been assessed. The results show that compounds such as quercetin and cyanidin, with 3',4' dihydroxy substituents in the B ring and conjugation between the A and B rings, have antioxidant potentials four times that of Trolox, the vitamin E analogue. Removing the ortho-dihydroxy substitution, as in kaempferol, or the potential for electron delocalisation by reducing the 2,3 double bond in the C ring, as in catechin and epicatechin, decreases the antioxidant activity by more than 50%, but these structures are still more effective than alpha-tocopherol or ascorbate. The relative significance of the positions and extents of hydroxylation of the A and B rings to the total antioxidant activity of these plant polyphenolics is demonstrated.

Review of the biology of Quercetin and related bioflavonoids

The French paradox is a dietary anomaly which has focused attention on the Mediterranean diet. Epidemiological studies revealed that this diet, replete in flavonoid-rich foods (Allium and Brassica vegetables, and red wine), correlated with the increased longevity and decreased incidence of cardiovascular disease seen in these populations. The most frequently studied flavonoid, quercetin, has been shown to have biological properties consistent with its sparing effect on the cardiovascular system. Quercetin and other flavonoids have been shown to modify eicosanoid biosynthesis (antiprostanoid and anti-inflammatory responses), protect low-density lipoprotein from oxidation (prevent atherosclerotic plaque formation), prevent platelet aggregation (antithrombic effects), and promote relaxation of cardiovascular smooth muscle (antihypertensive, antiarrhythmic effects). In addition, flavonoids have been shown to have antiviral and carcinostatic properties. However, flavonoids are poorly absorbed from the gut and are subject to degradation by intestinal micro-organisms. The amount of quercetin that remains biologically available may not be of sufficient concentration, theoretically, to explain the beneficial effects seen with the Mediterranean diet. The role of flavonoids may transcend their presence in food. The activity of flavonoids as inhibitors of reverse transcriptase suggests a place for these compounds in the control of retrovirus infections, such as acquired immunodeficiency syndrome (AIDS). In addition to specific effects, the broad-modulating effects of flavonoids as antioxidants, inhibitors of ubiquitous enzymes (ornithine carboxylase, protein kinase, calmodulin), and promoters of vasodilatation and platelet disaggregation can serve as starting material for drug development programmes.

Flavonoids as antioxidants in plants: location and functional significance

Stress-responsive dihydroxy B-ring-substituted flavonoids have great potential to inhibit the generation of reactive oxygen species (ROS) and reduce the levels of ROS once they are formed, i.e., to perform antioxidant functions. These flavonoids are located within or in the proximity of centers of ROS generation in severely stressed plants. Efficient mechanisms have been recently identified for the transport of flavonoids from the endoplasmic reticulum, the site of their biosynthesis, to different cellular compartments. The mechanism underlying flavonoid-mediated ROS reduction in plants is still unclear. 'Antioxidant' flavonoids are found in the chloroplast, which suggests a role as scavengers of singlet oxygen and stabilizers of the chloroplast outer envelope membrane. Dihydroxy B-ring substituted flavonoids are present in the nucleus of mesophyll cells and may inhibit ROS-generation making complexes with Fe and Cu ions. The genes that govern the biosynthesis of antioxidant flavonoids are present in liverworts and mosses and are mostly up-regulated as a consequence of severe stress. This suggests that the antioxidant flavonoid metabolism is a robust trait of terrestrial plants. Vacuolar dihydroxy B-ring flavonoids have been reported to serve as co-substrates for vacuolar peroxidases to reduce H(2)O(2) escape from the chloroplast, following the depletion of ascorbate peroxidase activity. Antioxidant flavonoids may effectively control key steps of cell growth and differentiation, thus acting regulating the development of the whole plant and individual organs.

Biosynthesis of flavonoids and effects of stress

The accumulation of red or purple flavonoids is a hallmark of plant stress. Mounting evidence points to diverse physiological functions for these compounds in the stress response. Advances are also being made toward understanding how plants control the types and amounts of flavonoids that are produced in response to different cues.

Structural determinants of phosphoinositide 3-kinase inhibition by wortmannin, LY294002, quercetin, myricetin, and staurosporine

The specific phosphoinositide 3-kinase (PI3K) inhibitors wortmannin and LY294002 have been invaluable tools for elucidating the roles of these enzymes in signal transduction pathways. The X-ray crystallographic structures of PI3Kgamma bound to these lipid kinase inhibitors and to the broad-spectrum protein kinase inhibitors quercetin, myricetin, and staurosporine reveal how these compounds fit into the ATP binding pocket. With a nanomolar IC50, wortmannin most closely fits and fills the active site and induces a conformational change in the catalytic domain. Surprisingly, LY294002 and the lead compound on which it was designed, quercetin, as well as the closely related flavonoid myricetin bind PI3K in remarkably different orientations that are related to each other by 180 degrees rotations. Staurosporine/PI3K interactions are reminiscent of low-affinity protein kinase/staurosporine complexes. These results provide a rich basis for development of isoform-specific PI3K inhibitors with therapeutic potential.

Flavonoids in food and their health benefits

There has been increasing interest in the research of flavonoids from dietary sources, due to growing evidence of the versatile health benefits of flavonoids through epidemiological studies. As occurrence of flavonoids is directly associated with human daily dietary intake of antioxidants, it is important to evaluate flavonoid sources in food. Fruits and vegetables are the main dietary sources of flavonoids for humans, along with tea and wine. However, there is still difficulty in accurately measuring the daily intake of flavonoids because of the complexity of existence of flavonoids from various food sources, the diversity of dietary culture, and the occurrence of a large amount of flavonoids itself in nature. Nevertheless, research on the health aspects of flavonoids for humans is expanding rapidly. Many flavonoids are shown to have antioxidative activity, free-radical scavenging capacity, coronary heart disease prevention, and anticancer activity, while some flavonoids exhibit potential for anti-human immunodeficiency virus functions. As such research progresses. further achievements will undoubtedly lead to a new era of flavonoids in either foods or pharmaceutical supplements. Accordingly, an appropriate model for a precise assessment of intake of flavonoids needs to be developed. Most recent research has focused on the health aspects of flavonoids from food sources for humans. This paper reviews the current advances in flavonoids in food, with emphasis on health aspects on the basis of the published literature, which may provide some guidance for researchers in further investigations and for industries in developing practical health agents.

Structural aspects of antioxidant activity of flavonoids

Flavonoids, a group of naturally occurring antioxidants and iron chelators, might be used as cardioprotective agents in doxorubicin-induced cardiotoxicity, which is believed to be caused by the formation of oxygen free radicals. To investigate the underlying molecular mechanism, we tested a large group of flavonoids from all major structural subclasses on their ability to inhibit doxorubicin (enzymatically)-induced and Fe2+/ascorbate (nonenzymatically)-induced microsomal lipid peroxidation (LPO) and to chelate Fe2+. In addition, we measured half peak oxidation potentials (Ep/2). LPO inhibition data gave a good qualitative correlation with the oxidation potentials. Most flavonoids tested chelated Fe2+, but there were large differences in the chelating capacity. For good scavenging activity, a catechol moiety on ring B is required. The 3-OH moiety can function as a chelation site and can also be oxidized. The 3-OH group in combination with a C2 C3 double bond, increases the scavenging activity. Fe2+ chelation only plays a role in the LPO inhibition by less active scavengers. Chelation can then raise the activity to the level of the most active scavengers, possibly by site-specific scavenging. It can be concluded that Ep/2 values and iron chelating activity can almost completely describe the LPO inhibiting behaviour of the flavonoids.

Inhibition of amyloid fibril formation by polyphenols: structural similarity and aromatic interactions as a common inhibition mechanism

The formation of well-ordered fibrillar protein deposits is common to a large group of amyloid-associated disorders. This group consists of several major human diseases such as Alzheimer's disease, Parkinson's disease, prion diseases, and type II diabetes. Currently, there is no approved therapeutic agent directed towards the formation of fibrillar assemblies, which have been recently shown to have a key role in the cytotoxic nature of amyloidogenic proteins. One important approach in the development of therapeutic agents is the use of small molecules that specifically and efficiently inhibit the aggregation process. Several small polyphenol molecules have been demonstrated to remarkably inhibit the formation of fibrillar assemblies in vitro and their associated cytotoxicity. Yet, the inhibition mechanism was mostly attributed to the antioxidative properties of these polyphenol compounds. Based on several observations demonstrating that polyphenols are capable of inhibiting amyloid fibril formation in vitro, regardless of oxidative conditions, and in view of their structural similarities we suggest an additional mechanism of action. This mechanism is assuming structural constraints and specific aromatic interactions, which direct polyphenol inhibitors to the amyloidogenic core. This proposed mechanism is highly relevant for future de novo inhibitors' design as therapeutic agents for the treatment of amyloid-associated diseases.

Plant flavonoids: Classification, distribution, biosynthesis, and antioxidant activity

Flavonoids are a group of natural polyphenol substances abundant in vegetables, fruits, grains, and tea. As plant secondary metabolites, flavonoids play essential roles in many biological processes and responses to environmental factors in plants. Flavonoids are common in human diets and have antioxidant effects as well as other bioactivities (e.g., antimicrobial and anti-inflammatory properties), which reduce the risk of disease. Flavonoid bioactivity depends on structural substitution patterns in their C6-C3-C6 rings. However, reviews of plant flavonoid distribution and biosynthesis, as well as the health benefits of its bioactivity, remain scarce. Therefore, in the present review, we systematically summarize recent progress in the research of plant flavonoids, focusing on their biosynthesis (pathway and transcription factors) and bioactive mechanisms based on epidemic evidence, in vitro and in vivo research, and bioavailability in the human body. We also discuss future opportunities in flavonoid research, including biotechnology, therapeutic phytoproducts, and dietary flavonoids.

Review of pharmacological effects of Glycyrrhiza sp. and its bioactive compounds

The roots and rhizomes of licorice (Glycyrrhiza) species have long been used worldwide as a herbal medicine and natural sweetener. Licorice root is a traditional medicine used mainly for the treatment of peptic ulcer, hepatitis C, and pulmonary and skin diseases, although clinical and experimental studies suggest that it has several other useful pharmacological properties such as antiinflammatory, antiviral, antimicrobial, antioxidative, anticancer activities, immunomodulatory, hepatoprotective and cardioprotective effects. A large number of components have been isolated from licorice, including triterpene saponins, flavonoids, isoflavonoids and chalcones, with glycyrrhizic acid normally being considered to be the main biologically active component. This review summarizes the phytochemical, pharmacological and pharmacokinetics data, together with the clinical and adverse effects of licorice and its bioactive components.

Mass spectrometry in the structural analysis of flavonoids

Flavonoids are very common and widespread secondary plant metabolites. They have a wide range of biological and physiological activities and serve as chemotaxonomic marker compounds. Therefore, they have been extensively investigated both in the past and during recent years. The interest in them is still increasing. In the search for new compounds, and also in quality control, there is a need to have reliable methodology for the analysis of flavonoids. Mass spectrometry can make an invaluable contribution because of its high sensitivity, possibilities of coupling with liquid chromatography and the availability of powerful tandem mass spectrometric techniques. A review of currently available mass spectrometric methodology used in the structure elucidation of flavonoids is presented. Sample preparation, liquid chromatographic/mass spectrometric analysis and tandem mass spectrometric procedures for the characterization of flavonoid aglycones, O-glycosides, C-glycosides and acylated glycosides are considered.

Flavonoids: old and new aspects of a class of natural therapeutic drugs

Flavonoids are natural products widely distributed in the vegetable kingdom and currently consumed in large amounts in the daily diet. Flavonoids are capable of modulating the activity of enzymes and affect the behaviour of many cell systems, suggesting that the compounds may possess significant antihepatotoxic, antiallergic, anti-inflammatory, antiosteoporotic and even antitumor activities. This review summarizes available data on these beneficial effects of flavonoids.

Enhancement of oxidative and drought tolerance in Arabidopsis by overaccumulation of antioxidant flavonoids

The notion that plants use specialized metabolism to protect against environmental stresses needs to be experimentally proven by addressing the question of whether stress tolerance by specialized metabolism is directly due to metabolites such as flavonoids. We report that flavonoids with radical scavenging activity mitigate against oxidative and drought stress in Arabidopsis thaliana. Metabolome and transcriptome profiling and experiments with oxidative and drought stress in wild-type, single overexpressors of MYB12/PFG1 (PRODUCTION OF FLAVONOL GLYCOSIDES1) or MYB75/PAP1 (PRODUCTION OF ANTHOCYANIN PIGMENT1), double overexpressors of MYB12 and PAP1, transparent testa4 (tt4) as a flavonoid-deficient mutant, and flavonoid-deficient MYB12 or PAP1 overexpressing lines (obtained by crossing tt4 and the individual MYB overexpressor) demonstrated that flavonoid overaccumulation was key to enhanced tolerance to such stresses. Antioxidative activity assays using 2,2-diphenyl-1-picrylhydrazyl, methyl viologen, and 3,3'-diaminobenzidine clearly showed that anthocyanin overaccumulation with strong in vitro antioxidative activity mitigated the accumulation of reactive oxygen species in vivo under oxidative and drought stress. These data confirm the usefulness of flavonoids for enhancing both biotic and abiotic stress tolerance in crops.