Bioactive flavonoids in medicinal plants: Structure, activity and biological fate

Tian-yang Wang,Qing Li,Kai-shun Bi*

School of Pharmacy,Shenyang Pharmaceutical University,103 Wenhua Road,Shenyang 110016,China

1. Introduction

Flavonoids are a class of compounds presented broadly in nature.Concerns about their extensive profitable bioactive benefits,including anti-viral/bacterial,anti-inf l ammatory,cardioprotective,anti-diabetic,anti-cancer,anti-aging,have long been received great attention and well supported by numerous studies[1–4].Till now,more than 9000 f l avonoids have been reported[5],and their daily intake varies between 20 mg and 500 mg,mainly from dietary supplements including tea,red wine,apples,onions and tomatoes[6,7].Flavonoids are frequently found as glycosylated or esterified forms,consisting of C6—C3—C6rings,namely rings A and B linked by threecarbon-ring C(Fig.1)[8].According to substitution pattern variations,f l avonoids can thus be classified into different subclasses,providing an extremely diverse range of derivatives[8].Although wide distribution and broad benefits,bioavailability of f l avonoids is poor which may significantly inf l uence the impact of nutritional effects,besides,information about pharmacokinetics in detail is limited.How to improve the issue is far from settled.This review attempts to bring some order into structure,activity as well as biological fate of f l avonoids with particular emphasis on their relationships involved.Moreover,detailed information on structure-based drug design is crucial and required.

Fig.1–Basic skeleton or structure of f l avonoids.

2. Chemical structure and classi fi cation of fl avonoids

Flavonoids are a group of low molecular weight substances based on 2-phenyl-chromone nucleus(Fig.2).They are biosynthesized from derivatives of acetic acids/phenylalanine by means of shikimic acid pathway.Traditionally,f l avonoids are classified by oxidation degree,annularity of ring C,and connection position of ring B(Fig.3).Flavones and f l avonols contain the largest number of compounds,representing the narrowsense f l avonoids,namely 2-benzo-γ-pyrone category.Quercetin belongs to f l avonol class,for example,has been studied most commonly.Flavanones and f l avanonols possess saturated C2=C3bonds,and often coexist with relevant f l avones and f l avonols in plants.Isof l avones,such as daidzein,are 3-phenyl-chromone substances.As key precursors of f l avonoid biosynthesis,chalcones are ring C-opening isomers of dihydrof l avones,responsible for color appearance of plants.Lacking typical structure of f l avonoids,aurones are five-membered ring C benzofuran derivatives.Anthocyanidins are a group of important chromene pigments for characteristic color of plants,existing in the form of ions.Flavanols are reduction products of dihydrof l avonols,especially with f l avan-3-ols widely distribution in plant kingdom,also known as catechins.However,there are still other f l avonoids without C6—C3—C6skeleton,for instance,bif l avones,furan chromones and xanthones.Glycosides,with different category,number and connecting pattern,are predominate existing forms of f l avonoids.Preferred glycosylation sites are associated with the structure of aglycones.

Fig.2–Chemical structures of the f l avonoid classes.

3. Structure activity relationship(SAR)

A myriad of epidemiological studies have suggested a negativecorrelationbetweenmedicinalf l avonoidsconsumptionand developmentofvariousdiseases[9–11],thereinto,f l avonoidswith typical structures can interact with enzyme systems involved incrucialpathways,showingeffectivepolypharmacologicalbehaviors[1,6,7].Thus,it is not surprising that the relationships between chemical structures and activities have been extensively studied.

3.1. SAR for anti-viral/bacterial activity

Nowadays,bioactive f l avonoids have been investigated for potent anti-viral/bacterial activity.For instance,therapeutic activities against inf l uenza virus[6],canine distemper virus[12],hepatitis C virus[9],and Escherichia coli[13],have been attributed,largely,to chemical structures in particular patterns of methoxylation,glycosylation and hydroxylation[12,14].Over years,related SAR researches have been characterized in diverse aspects.The C2=C3double bond has been documented in most cases as a basic favorable element,which has been illustrated via the human fibroblast collagenase catalytic domain expression inhibitory activity loss of ampelopsin in comparison to quercetin[13].

In the case of hydroxylation,substitution style takes an important role.With regard to ring A hydroxylation,the positive role of 5-/7-hydroxyl derivatives has been suggested by six potential anti-H5N1 inf l uenza A virus 5,7-diOH f l avonoid candidates[15],and less potent anti-human fibroblast collagenase catalytic domain(MMP1ca)effects of daidzein than quercetin[13].Additionally,better MMP1ca inhibitory activity of 3′-OH ampelopsin/5′-OH gallocatechin gallate compared to daidzein/epicatechin gallate implys the contribution of hydroxylation in ring B[13].Amongst others,a catechol group is the most common functional moiety.For example,better inhibitory activity of quercetin than morin in canine distemper virus inhibition[12],has provided a prominent therapeutic thought for novel drug synthesis.In the aspect of ring C,significant contribution of 3-OH has been observed(quercetin vs.luteolin)[16].Apart from the site,the number of hydroxyl groups is another inf l uencing factor.More hydroxyl groups results in lower hydrophobicity,which is obstructive for f l avonoids to partition into biological membranes.Interestingly,sometimes certain hydroxyl group-rich-f l avonoids do possess higher activity.The impact of hydrophobicity and electronic delocalization on the strength of hydroxylation assignment should be considered together,however.Additive hydroxyl groups might confer reduced hydrophobicity but higher C3charges which is a direct indicator for pharmacological activity[16].

As for methoxylation,its inf l uence on membrane f l uidity increase is correlated a large extent to the pathopoiesia of some viruses/bacteria,decreasing activity is therefore obtained.On this occasion,two polymethoxy f l avonoids(PMFs)have been observed to exhibit decreasing anti-E.coli activity compared with related aglycones[16].The study of Amorpha fruticose L.f l avanones corroborates the previous experiment that bacterial neuraminidase inhibition of compound 2 is 70-fold stronger than unmethylated compound 3[14].

Fig.3–Chem ical structures and classification of f l avonoid s.

For f l avonoid glycosides,greater anti-viral effects have been described and exem plified by puerarin and rutin/hesperidin w ith resp ect to d aid zein and quercetin,w h ich further p rovides favorable evid ences for saccharides linkage w ith higher biological activity[13].How ever,one deficiency about aforem entioned structural inf l uencing factors is the particular level of increase or reduction is not recorded in detail.The decip herm ent of SAR exerted by selected f l avonoid s in the context of anti-viral/bacterial effects m ay lead to screening of op tim al com p ound s for d ietetic th erapy an d/or m ed ical treatment.

3.2. SAR for anti-cancer activity

By far,different m echanism s have highlighted the role of f l avonoids in cancer-therapy,including induction of apoptosis[1],proteasome inhibition,nuclear factor signaling inhibition[17],differentiation induction[1],cell cycle arrest induction[9],recep tor interaction[18],or interaction w ith carcin ogen ic associated enzym es[18].Moreover,f l avonoids could exhibit specific cytotoxicity on cancer cells,causing large attention focused on f l avonoid-based cytostatics as anti-cancer agents[19].

The significant role of C2=C3double bond contributes to molecular planarity and conjugation between rings Cand A/B,which is essential for potent tum or inhibition(apigenin vs.naringenin)[1,18].In order to explore interactions betw een C2=C3double bond and anti-cancer effects,tum or cell lines such as colon adenocarcinoma cells[20],and MDA-MB-231 breast cancer cells[17],have been utilized for in-depth analysis involving gene expression.Com parative enhancing tum or inhibitory effects about 2,3-dihydrochrysoeriol and dihydroisorham netin w ith respect to unsaturated counterparts are further elucidated in detail by 65%and 82%,respectively[17].Moreover,greater inhibition would occur w ith co-existence of C2=C3unsaturation and tw o ring B hydroxyl groups[18].

Notably,m any reports have provided evidences about infl uences of hyd roxylation on tum or m od ulation.Sp eci fi c hydroxylated f l avonoids possess stronger inhibitory activity on cancer cells than permethoxylated counterparts.The contributive role of 6-OH and 5,7-diOH has been disclosed[20,21].Ring B substitution such as a catechol moiety with vital inf l uences has been proposed,and additional hydroxyl group substitution in ring B does not alter the activity[1].In the case of ring C,3-hydroxylation has been considered as a highly decisive moiety for improving biological effects(quercetin vs.kaempferol)[17].Apigenin,for another example,lacking 3-OH possesses significantly lower antiproliferative activity than kaempferol[20].Higher affinity between binding site and 3-OH has further been proposed[21].

Flavonoid derivatives of O-methylation are contributed to enhanced biological activity,which is frequently associated with ring A polymethoxylation.Out of several Ougan f l avonoids tested in cell morphology change study,two A-ring PMFs,nobiletin and tangeretin,exhibit the highest proliferative inhibition,also suggesting the significance of C-8 position in anti-proliferative activity of f l avonoids.Additionally,the antiproliferative promoting effect of 3′-methoxy group has been disclosed from the higher inhibition of nobiletin than tangeretin[19].In accordance with the previous study,glycosylation does not contribute to cell differentiation induction[1].The antiproliferative weakening effects of f l avonoid goycosides might be derived from steric blocking involved in cell entry and receptor binding[19].Further explanation should be given,however.

3.3. SAR for anti-age-dependent-neuropathology activity

With respect to brain,f l avonoid extracts or monomers have been effectively utilized,thus preventing neuro-degeneration[22].For example,cholinesterase inhibition of fourteen Salvia species f l avonoid extracts on Alzheimer’s disease are explored[23].Historically bioactivity of f l avonoids against neurodegenerationisattributedtoclassicalantioxidanteffects,however,emerging evidences now have been attaching importance to interactions on acetylcholinesterace(AChE)/butyrylcholinesterase(BChE)[23],GABA-receptor[24],mitochondrialdysfunction[25],criticalneuronalsignalingpathwaysincontrollingneuronalresistancetoneurotoxicaloxidants and inf l ammatory mediators[26],or through chelation of transition metal ions[22].

In order to delineate anti-age-dependent neuropathology more in-depth,structure-dependent stimulating manner of f l avonoids has been investigated.As an example of this,beneficial role of ring B hydroxylation has been suggested at the side of galangin,kaempferol and myricetin,since only the latter two could significantly improve learning capability[27].Besides,in light of differences in neuroprotective activity of 10 Rhus vernicif l ua Stokes f l avonoids[28],the positive contribution of 5-dehydroxylation and 3′,4′-ortho-dihydroxylation in ring B have been proposed.As above-mentioned,it appears that hydroxylation takes an important role.Furthermore,a study of fl avonoid-PI3-kinase interaction has further con fi rmed the pivotal role of ring B hydroxylation[29].Pattern of hydroxylation in ring B and unsaturation degree of C2=C3double bond account for a great deal.In light of this,it appears that interactions of f l avonoids with some other receptors,downstream kinases or signaling pathways may be structure-dependent,since highly sensitive allosteric modulation has been proposed[24].Meanwhile,a 12 times higher BChE binding ability of galangin than AChE,has supported the standpoint more concretely[30].The researchers finally selected galangin as a promising potent therapeutic agent.In vivo,in a D-galactoseinduced cognitive impairment model of mice,cognitive therapeutic mechanism of galangin has been demonstrated in the perspective of oxidative stress amelioration,Na+,K+-ATPase enhancement and regulation of ERK-CREB pathway expression[27].Detailed SAR need further investigation,however.

3.4. SAR for cardioprotective activity

The pivotal role of f l avonoids from apple peel[11],cranberry[31],onion[32],and herbs[3],in cardiovascular disease prevention,has been reported.In addition,evidences from studies in human,animal and cell model further suggest the contribution of f l avonoid intake.In a recent cross-sectional study,higher dietary f l avonoid intake is associated with improving lipid profile in a cohort of 1393 subjects in China[31].

In recent years,several SAR evidences such as specific alteration degree have been gathered from evaluation of f l avonoid effects on eNOS transcription factor Krüpple like factor-2 expression[33].In the light of the results,gene dependent effects have been provided that the presence of C2=C3double bond results in a double efficient structure in terms of eNOS and ET-1 expression(apigenin vs.naringenin),substitution in position 3 including hydroxylation or glycosylation could decrease eNOS/ET-1 expression by 2 times or so(quercetin/rutin vs.luteolin),and a 4-carbonyl moiety leads to an approximately 1.35 fold gene expression(quercetin vs.epicatechin/catechin).Meanwhile,electronic distribution modification effects by those characteristic functional groups have been suggested in the case of SAR.More detailed,results from a SAR study of 12 f l avonoids with paraoxonase1(rePON1)has emphasized the‘proteinbinding’mechanism which was due,moreover,at least partly,to different hydroxylation substitution,C2=C3double bond and 4-carbonyl group in ring C[34].Owing to C2=C3double bond in ring C,PON1 interactions are higher for f l avones and f l avonols because of molecular planarity,which may contribute to electron delocalization between rings A and B,therefore 3-hydroxyl group and 4-carbonyl oxygen atom are coplanar.Accordingly,SAR is investigated into deeper levels.Apparently,to establish a putative relevance of f l avonoids for cardiovascular protection,it is necessary to study different sub-class f l avonoids with typical substitution on the top of pharmacological mechanism investigation.

3.5. SAR for anti-inf l ammatory activity

Generally,foremost role of f l avonoids on inf l ammation involved diseases such as leukemia,sepsis,asthma,sclerosis,atherosclerosis,psoriasis,allergic rhinitis,ileitis/colitis,rheumatoid arthritis,etc.has been proposed[8,35–41].Thereinto,diverse inf l ammatory mediators have been studied including plasma proteases,prostaglandins,leukotrienes,interleukin,interleukins,nitric oxide,proinf l ammatory cytokines,chemokines[42–46],as well as relevance signaling pathways[47].Taken together,anti-inf l ammation activity of f l avonoids has been widely investigated,and specific mechanisms involved might not be united,which adds the urgency to explore SAR in-depth.

Generally,preferred structural aspects for anti-inf l ammatory effects of f l avonoids are summarized as following:(a)The C2=C3double bond might attribute to molecular planarity.Its absence results in a larger volume/surface ratio,since diosmetin shows stronger effects than hesperetin[48].(b)Hydroxylation pattern,such as 3′-hydroxylation(since fisetin shows maximal effect),5-hydroxylation in the case of isof l avones,especially ring B catechol moiety,provides effects for inducing cell differentiation(apigenin vs.chrysin)[48].(c)Methoxylation greatly enhances anti-inf l ammatory property,probably through ionization of hydroxyl groups and more pronounced NF-kB signaling pathway inhibition(O-methylation of chrysin)[47].(d)Glycosides with lower lipophilicity,showed lower anti-inf l ammatory property,which may be due to lower hydrophobicity as well as sterical hindrance,decreasing membrane permeability[8].(e)Additionally,bulky substitution has been investigated.The presence of C7=C8double bond,C-butyrolactone moiety and 5-acetic acid/lactone group have been recognized as possible taxonomic markers in an anti-inf l ammatory study of f l avonoids from Cryptocarya chingii[49].Of note,more compounds need evaluation in order to draw definitive conclusions about SAR in regard to f l avonoids.

3.6. SAR for anti-diabetic activity

Diabetes mellitus(DM)is a multifactorial chronic hyperglycemic disease.Bene fi cial role of fl avonoids in DM treatment is obvious on account of preeminent ef fi cacy in terms of complications and decreased side reactions.Numerous cell,animal and epidemiological studies support the hypoglycemic activity of fl avonoids[4,50].For bayberry fl avonoids,anti-diabetic effects on glucose consumption have been well investigated in HepG2 cells[4].In the 2007–2009 Korean National Health and Nutrition Examination Survey,4186 Korean participants have been administrated with various subclass fl avonoids,underlying relationships between DM risk factors and fl avonoids intake[50].

As DM is concerned,anti-diabetic mechanism of fl avonoids has been well known.Moreover,effects on various enzymes and molecular targets/signaling pathways,have been mentioned.It is therefore pertinent to point out a de fi nite SAR to explain different actions of various fl avonoids.Accordingly,it is of special interest that chalcones are found to be potential inhibitorsofa-glucosidase,whichisaneffectivetargetonglucose homeostasis.Phlorizin has been used as a classical SGLT-1 inhibitor clinically,which may link sugar to glucose site and bind the aglycone,thus affecting inhibitor binding[51].Interestingly,hydroxylation and planarity in position 7 in several fl avonoids provide capacity for PPAR activation[52].The exploration of anti-diabetic ability of 44 fl avonoids on adipogenesis of 3T3-L1 cells has shown positive contribution of methoxylation and inverse relationship between hydroxylation and anti-diabetic activity[53].Unlike common SAR regarding hydroxylation and methoxylation,substitution of glycosylation especially glucosylation in position 3,has been demonstrated a lot.In a HepG2 cell model,signi fi cant hypoglycemic behavior of fl avonoid-3-glucose has been observed instead of rhamnose[4].Recently,effects of regulating blood glucose level and improving pancreatic β cell function of quercetin-3-glucoside have been observed in a diabetic KK-Aymice model[54].Detailed mechanism of C-3-Glu/Gly in regulating glucose consumption deserve further convincing support,however.Importantly,among synthesized novel anti-diabetic hybrids of 6-/7-OH f l avones,compound 64 and 65 are evaluated as the most potent with bulky hydrophobic substitution in ring B as well as smaller functional groups like tertbutyl/isopropylamine at nitrogen atom.Collectively,relevant SAR studies of f l avonoids remain elusive,requiring to precisely depict interactions of f l avonoids with molecular targets.

3.7. SAR for anti-oxidant activity

Numerous studies have attributed abroad nutritional effects of fl avonoids to anti-oxidant activity,and most anti-oxidant chemical assays are owing to free radical scavenging mechanisms[19].However,those results are often ambiguous and incomparable based on different oxidant species or analytical methods applied.Generally,mechanisms underlying their antioxidant property are free radical scanvenging and transition metal ion chelating activity.Due to reducing activities of phenolic hydroxyl groups, fl avonoids are able to donate hydrogen.Along with delocalization of phenoxy radical products,fl avonoids can protect against various disease damage from ROS[55].On the other hand, fl avonoids can chelate transition metals which are able to promote hydroxyl radicals formation in reduced forms by virtue of Fenton reaction under abnormal conditions.Furthermore,considerable attention has been focused on SAR for antioxidant activity of fl avonoids.In a zebra fi sh larvae organism,15 commercially available fl avonoids have been used to screen optimum radical oxygen scavenging compounds with lower toxicity as well as higher antioxidant activity[56].In another example,with iso fl avan showing the highest antioxidant capacity among the tested fl avonoids,contribution of resorcinol moiety in ring A has been highlighted[34].Those obtained results are clear evidences of basic structural elements for available anti-oxidant activity.

3.7.1. The C2=C3double bond and 4-carbonyl group in ring C

A characteristic structural feature among fl avonoids subclasses is the existence of a C2=C3double bond in conjugation with a 4-carbonyl group in ring C,whose contribution to SAR has been investigated.However,several authors believe that there is no direct relationship between these moieties with antioxidant activity while other structural criteria are ful fi lled.For example,although potent electron donating capacity has been obtained for certain selected fl avonols,no signi fi cant deviation is observed between cellular ROS inhibition and characteristic structural moieties[55].On the other hand,with other structural criteria ful fi lled as the premise,the presence of a C2=C3double bond in conjugation with a 4-carbonyl group plays an assisting role in anti-oxidant activity.For example,apigenin could bind rePON1 more effectively than naringenin with C2=C3saturation,providing favorable evidence[34].The presence of 4-carbonyl group is able to induce electron shifts via resonance effects,therefore in fl uencing the dissociation constant of phenolic hydroxyl groups and phenoxy radical stability in ring B.Unsaturation of C2=C3double bond provides planarity and electron coupling to the molecule so that conjugation between ring C and ring A/B could be obtained.Similarly,association with 5-OH often provides a hydrogen bond.Taken together,combination of 4-carbonyl group with C2=C3double bond or other electron donating groups efficiently delocalizes ring B electron,thus significantly enhancing antioxidant activity.

3.7.2. Hydroxyl groups

Generally,position and number of hydroxylation correlate reasonably to anti-oxidation of fl avonoids.Hydrogens and electrons are donated by ring B hydroxyl groups to hydroxyl,peroxyl,and peroxynitrite radicals,forming relatively stable fl avonoid radicals.On the other side, fl avonoids could scavenge the resulting radicals to neutralize the prior effect.The premise of at least two hydroxyl groups in ring B for anti-oxidant capacity is suggested on the basis of signi fi cantly improved anti-oxidant effects[48].Amongst others,3′,4′-catechol group is recognized as the most signi fi cant responsible pharmacophore,producing extremely stable ortho–semiquinone radical via electron delocalization to confer high activity through intra-molecular hydrogen bonding between catechol hydroxyl groups.Apart from two hydroxyl groups in ring B,in fl uence of only one substitution does make sense.In this perspective,apigenin with 4′-OH has been deduced to increase erythroid differentiation activity[48].Besides, fl avonoids with ortho-dihydroxyl group in ring B all possess stronger inhibitory effects than those with 4′-hydroxylation,which has been shown by mean Imaxvalues as 36.2%and 22.5%,respectively.RingA hydroxylation may contribute less to anti-oxidant activity than ring B,since orthodihydroxyl group in ring B is more easily oxidizable than ring A meta-dihydroxylation[57].Anyway,5,7-diOH in ring A does interfere with anti-oxidant effects.Strong activities of luteolin,quercetin,kaempferol and apigenin emphasize the contribution of 5-and 7-OH combination as 2,4-substituted resorcinol substructure[34].Evidence has been proposed about ring C hydroxylation represented by 3-hydroxylation which is impaired by electron donating substitution in position 5 and position 7 in ring A.In the light of a comparison of antioxidant property of luteolin and quercetin,the presence of 3-OH clearly contributes to suppression of anti-oxidant activity[58].Hypothesis of activity diminishment from 3-hydroxylation has been supplemented in the study of Haydar Çelik et al.[48].Taken together,although individual in fl uence of hydroxylation has been demonstrated,overall modulation on the molecule is more than just a collection.As for quercetin,3-hydroxyl blocking in ring C and catechol moiety retaining in ring B do not promote antioxidant ability in brief[59].Generally,electron transfer within the resonance system and total number of hydroxyl groups are usually considered while taking into account overall hydroxylation system.Since hydrophilicity is enhanced with hydroxyl number increasing,insertion of fl avonoid nucleus with more hydroxyl groups is held up in the hydrophobic cavity which might constitute connection to the active site of relevance enzyme[58].

3.7.3. O-methylation

The inf l uences of O-methylation include molecular hydrophobicity,electron donation and planarity.O-methyl substitution may cause steric hindrance,therefore decreasing anti-oxidant activity.Ring B is particularly sensitive to substitution position.Varying methylation on free hydroxyl groups in ring B alleviates anti-oxidant ability by altering coplanarity[60].In the in vitro ferric reducing antioxidant power tests,inactivation of anti-oxidant property induced by ring B O-methylation has been proposed,however,increasing antioxidant property is obtained in methoxyl fl avonoid derivates with fl avonoidfl avonoid interaction under consideration[59].It is rational to postulate that multiple methoxylation substitution in ring A would counterbalance the contribution of catechol moiety in ring B.Given the fact that radicals used may not always participate into hydrophobic membrane where polymethoxylated fl avonoids accumulate,it is reasonable to suggest that the infl uence of O-methylation depends on many factors including substitution and lipophilicity of related substrates.

3.7.4. Glycosylation

Anti-oxidant ability of fl avonoid glycosides in different forms such as O-or C-glycosides has been investigated.In the case of C-glycosides,whose antioxidant activities have been confi rmed by chemical assays and elucidated with higher abilities in comparison to O-glycosides,moreover[2].Almost total radical scavenging ability has been attributed to C-glycosyl fl avonoids rather than O-glycosides in the study of Davide Barreca et al.[60].Similar results have been obtained in another assay with C-glycosides responsible for nearly 50%antioxidant activity[60].It is worth noting that aforementioned experiments about C-glycosides are carried out by virtue of chemical assays,in vivo data and in-depth interpretation are still required.Flavonoid glycosides occur in diet generally in ring A or C as O-glycosides[61],and corresponding substitution in ring A has a far greater impact on activity.Like methylation,coplanarity and electron delocalization are in fl uenced by glycosylation,which confers decreasing activity.On the basis of anti-oxidant effects of fourteen structural different fl avonoids,the signi fi cant role of 3-glycosylation has been pointed out in the case of quercetrin and rutin[48].According to the author,those attenuating effects may be the result of enhancing polarity or increasing steric hindrance due to sugar moiety in position 3.Moreover,anti-oxidantpropertyenhancingeffectof 6-glucosylation and attenuating effect of 8-glucosylation in ring A,which are attributed to torsion angle and coplanarity broken,have been disclosed[57].In spite of those results,infl uence of the number of glycosylation which is associated with lipophilicity has been considered.In addition,to the number of glycosylation,position and structure of saccharides are of great signi fi cance.Interestingly,different antioxidant inhibitory activities of eight Epimedium elatum fl avonoids has been determined by 3-or 7-glycosylation with different number,position and structure,which may mainly stem from saccharide itself[61].Although anti-oxidant activity of glycosides is weaker than corresponding aglycones,bioavailability is plausibly increased on account of cleavage of glycosidic bonds often occurred in vivo,thus raising anti-oxidant activity.

3.7.5. Summary of SAR for favonoid-induced anti-oxidant activity

Fig.4–Summary of SAR of f l avonoids.

Taken together,to f l avonoids,the existence of a C2=C3double bond in conjugation with a C4-carbonyl group,certain hydroxylation pattern especially a catechol moiety in ring B,methoxyl groups,and less saccharides connection confer higher anti-oxidant properties.Of which,the mechanism might involve planarity that is contributed to electron shifts across the molecule further inf l uencing dissociation constants of phenolic hydroxyl groups,so that the whole molecule could bind to relevance molecular targets like enzymes in a more efficient pattern.Hydrophobicity is another consideration that relates to absorption across bio-membrane.As previously mentioned,however,there are discrepancies in SAR for f l avonoidinduced anti-oxidant activity,which may probably stem from different mechanisms as well as various methods of detection/measurement of oxidative processes.Apart from those factors,inf l uences among each functional moieties on final antioxidant property of the molecule cannot be ignored.In a Ceric Reducing/Antioxidant Capacity antioxidant test,the order of eight studied f l avonoids has been established,and related antioxidant hierarchy of individual functional moiety is summarized owing to mutual correlations:2′,4′-diOH,4′-OH ≈ 3′,4′-diOH ＞ 2,3-double bond in conjugation with 4-carbonyl substitution,3,5-diOH in conjugation with 4-carbonyl substitution,3-OH in conjugation with 4-carbonyl substitution,5-OH in conjugation with 4-carbonyl substitution,and 3,5-diOH[62].It can be concluded,therefore,that contributions of each structural moiety are different,and their synergistic/antagonistic interactions might further inf l uence interactions between f l avonoids.Substantiating antioxidant activities of f l avonoids with respect to structural characteristics is very challenging,nevertheless,which may definitely require further investigation and eventually lead to developing nutraceuticals for relieving oxidative stress in human.

3.8. Summary of SAR

Cumulative findings concerning SAR derived from pharmacological studies have provided beneficial evidence of the role of various functional groups on nutritional utilities.Based on the foregoing,it’s rational to conclude that a C2=C3double bond,a 4-carbonyl group,and hydroxylation patterns especially 3-OH and a ring B catechol moiety,are major recognized beneficial determinants for various beneficial effects of f l avonoids(Fig.4).Apart from disagreements about SAR of certain biological activity,several inter-pharmacological crosslights may exist and originate in different mechanisms of action,diverse analytical methods and different subjective opinions.For example,O-methylation is beneficial to anti-viral/bacterial,anti-diabetic but adverse for anti-cancer and anti-inf l ammatory activities.Positive effects of hydroxylation have been delivered in the aspect of anti-viral/bacterial,anti-cancer,cardioprotective,ruling out anti-diabetic activity.Generally,glycosylation may decrease corresponding activity of anti-age-depentdent,but antiviral/bacterial on the contrary.

Centered on existing discussion about SAR of fl avonoids,research status is as follows.First,the majority of studies have focused on characteristic functional groups that would alter related pharmacological activity,offering favorable reference for therapeutic substance screening.Nevertheless,speci fi c in fl uencing degree is rarely mentioned.Second,deeper investigation on interactions among various functional moieties is de fi cient.Last but not least,opinions about concrete mechanism of certain functional moiety,which undoubtedly are consequences of interactions of multiple factors,have been confi ned to surface phenomena.There are many changes resulted from the transformation of functional groups,such as the alteration of steric con fi guration,polarity of whole molecule,and physico-chemical property.In detail,the alteration of steric confi guration induced by different functional groups,is a critical factor for evaluating suitability with target sites of action,thereinto,certain molecular size resulted from particular substitution is required for matching with the gap of target;the alteration of polarity of whole molecule is one of the decisive factors about electron distribution as well as interaction forces such as hydrogen bonding,which is the critical step for curative effects expression;physico-chemical property alteration could lead to variation of solubility and modi fi cation of in vivo absorption that active metabolites may be produced by given compounds with speci fi c functional groups,which thus expands another research area for systematic elucidation of SAR of fl avonoids thoroughly.

4. Absorption and metabolism

4.1. SAR for pharmacokinetics

Fig.5–Flavonoid biotransformation.

Owing to broad-spectrum nutritional effects and wide-spread in diet,f l avonoids have been administrated orally in the majority situation.As mentioned,their poor bioavailability has been the primary limitation for successful utilization.It is therefore necessary to study the pharmacokinetic profile and related inf l uencing factors(Fig.5).On this issue,structural inf l uence is recognized as potential inf l uencing factor.Intestinal mucosa and liver are major sites for biotransformation.Aglycones are absorbed into enterocytes in passive transport form.After oral administration,however,most f l avonoids exist as glycosides.The first step to enter into systemic circulation may be deglycosylation through active uptake by sodium-dependent glucose transporter(SGLT1)with following deglycosylation by cytosolic β-glycosidase,or primarily undergo luminal hydrolysis by lactase phlorizin hydrolase(LPH)with subsequent passive absorption of released aglycones[63].It is noteworthy and reasonable to point out that deglycosylation pattern may depend on the nature of aglycone and connected sugar.Absorption trend of f l avonoid glycosides has been investigated and ascribed to absorption promotion effect of SGLT upon glucosides[64],however,SGLT may not the only explanation for better absorption,since glucosides are more hydrophilic,diffusion through unstirred water layer to LPH located brush border membrane is much easier.Furthermore,incorporation in polar-nonpolar interface of membrane,which is achieved by electrostatic and hydrophobic interactions with phospholipids,could be obtained by evaluating characteristic structures of f l avonoids,so that bioavailability may be predicted.Aglycones are thus considered with higher bioavailability and earlier absorption than glycosides by virtue of better membrane interactions.In contrast to O-glycosides,C-glycosides are more resistant to hydrolysis.The pharmacokinetic profile of vicein-2(first and zero order absorption constant as 0.274/min and 16.3%/min respectively,and bioavailability as up to 40.2±2.5%),which is a C-glycoside from Lychnophora ericoides leaf,has indicated relatively stable metabolic process[65].In parallel,the findings of a comparative absorption study of apigenin and correlative glycosides,wherein apigenin 8-C-glucoside-2-O-xyloside is almost unchanged while major metabolites of apigenin/O-glycosides are related aglycone as well as glucuronides in portal blood,has further confirmed the metabolic stability of C-glycosides[66].Without doubt,those studies have left room for detailed data about causal SAR for glycoside absorption.

Followed by deglycosylation,phase 2 metabolism often occurs continuously in epithelial cells of small intestine.Then,further biliary excretion or enterohepatic cycling in colon occured.Corresponding aglycones are thus released and absorbed in large intestine,or undergo further degradation.As one of favorable phase 2 metabolic enzymes,catechol-O-methyltransferase catalyzes f l avonoids with catechol moiety in ring B.Generally,methylated f l avonoids exist predominantly in the form of 3′-rather than 4′-methyl metabolites.According to comparative pharmacokinetic behaviors of fisetin and three of its metabolites,geraldol as 3′-methylated product possess faster elimination process(t1/2=0.45 h)[67].Flavonoids are able to act as CYP1 inhibitors of procarcinogens or substrates[68,69].Hydroxylation is attributed to inhibition,while methoxylation to metabolization[69,70].Therefore,it is reasonable to figure out bioavailability increasing effect of certain f l avonoids as CYP substrates while co-administration with other f l avonoids as CYP inhibitors,based on first-pass metabolism weakening effect.Flavonoids with hydroxyl groups are vulnerable to conjugation.In contrast,both absorption and excretion of O-methylated f l avonoids are relatively slow thus facilitating better bioavailability,owing to delayed hepatic metabolism protective effects,increasing permeability across bio-membrane and more accumulation exerted by O-methylation[70,71].With comparison to hydroxyl f l avonoids,100-fold higher plasma concentration has been obtained on account of methoxylated ones[70].In a comparative pharmacokinetic study of quercetin,kaempferol and isorhamnetin,the slowest process and the highest degree of absorption have been observed for isorhamnetin(tmax=7.21 h,Cmax=195.96 ng/ml),which is a methylated f l avonoid[72].It can be concluded that,due to extensive metabolism,no matter where f l avonoids undergo absorption,intact f l avonoids enter into systemic circulation rarely.In a pharmacokinetic/excreted model of rats,0.81%and0.05%ofuncoveredformof5,7,3′,4′-tetramethoxyf l avone have been excreted in feces and urine,respectively,indicating metabolites are the main form of excretion[71].Information from other experiments has further supported the thesis that nearly 74%luteolin formed glucuronide conjugates with hydroxyl groups substituted in 3′-(51%),4′-(44%)and 7-(5%)position[73].

4.2. Pharmacological activities of aglycones and related metabolites

Not only biological fates but also nutritional effects of fl avonoids may be in fl uenced by metabolism,which may be related to structural moieties altering.Some authors believe that demethylation often confers more potent pharmacological activity.Identi fi ed as the major metabolite of nobiletin in mice urine,3′,4′-didemethyl-nobiletin,has been recognized with stronger anti-tumor property[70].Similarly,O-methylation of 7-hydroxy fl avone confers lower activity[74].Moreover,the same research group has idtenti fi ed galangin with additional 3-hydroxylation as the most potent derivatives,suggesting the potential impact of hydroxylation on hydrogen bond and hydrophobic interaction.In addition,conjugated fl avonoid metabolites could serve as storages of relevant aglycones,and the extent of which might be the result of therapeutic degree of active aglycones in certain tissues.Since hydroxyl groups are commonreactionsites,severalglucuronidatedandsulfatedconjugates of fi setin on 4-hydroxyl moiety have been detected at relative high levels in mice plasma,displaying anti-angiogenic effects in vivo[67].Moreover,additional pharmacological properties may be provided by metabolites.In breast cancer cells,one CYP1 metabolite of nobiletin produces not only antiinvasive but also cytostatic effects[70].In another model of humanaorticendothelialcells,differentanti-in fl ammatoryand anti-oxidant effects of several major metabolites of quercetin and(-)-epigallocatechin-3-O-gallateinhumanversustheirparent fl avonoids have been obtained[75].Above all,it is of interest,therefore,toattributethenutritionalbehaviorof fl avonoidspartly to bioactive metabolites in vivo,in the face of low bioavailability.On the other hand,relative low physiologic levels of certain fl avonoids may also meet therapeutic requirements.For instance,a pharmacokinetic/pharmacodynamic study of Da-Cheng-Qi Decoctionhas suggestedthedirectacutepancreatitis therapeutic effects of prototypes of four major fl avonoids detected in serum,in the light of the same time between maximal pharmacodynamic effects and maximum serum concentrations in rats[76].

Compared to various pharmacodynamic/pharmacokinetic studies,the reason for bioef fi cacy of fl avonoids in vivo,although signi fi cant and discussed a lot,still remains detailed elucidation.Meanwhile,species differences about pharmacokinetics must be taken into account.Take quercetin,which is one of the most prevalent and documented fl avonoids,as an example,its pro fi le of absorption and metabolism varies signi fi cantly between different species.The bioavailability of quercetin in human is known as values ranging from 0.001 to 0.04%[77].Bioavailability studies of quercetin aglycone and related glycosides in neonatal calves,cows,and rats,have provided illustrative information on discrepancy of bioavailability[77–79].As a physiologically based kinetic model suggested,different metabolic systems(enzymatic metabolic rate and regioselectivity)may be responsible for observed inconsistent in vivo behaviors of various species[77].Since major metabolites of quercetin in human are monoglucuronides(96%)and predominantly quercetin-3′-O-glucuronides,while di-and tri-glucuronic acid/sulfate/methyl conjugates in rats.A two timehigher rate of glucuronidation reported in rats(mainly at 4′-OH)rather than human(mainly at 3′-OH),has supported the point of view that animal models may not necessary be an adequate substitute to elucidate f l avonoid behaviors in human,since significant species differences in pharmacokinetics of certain f l avonoids do exist.

5. Conclusion

Concerning f l avonoids,the significant role of pharmacokinetic behaviors in pharmacodynamics effects and utilization as nutritional supplements in the area of therapy,has been highlighted.However,pharmacokinetic profile of f l avonoids with certain functional groups remains sysmetically elusive,which is important for screening out more common/easily synthetic f l avonoids with better absorption and higher nutritional/therapeutic or less side effects.It is therefore important to elucidate biological fates and cellular metabolism of different subclassf l avonoids,andinvestigateactionsofmechanismatthe molecularlevel aswell asstructure-activity-pharmacokineticsrelationships.Further investigation is required to elucidate biological fates and activities of f l avonoids,to characterize their metaboliteswithspecialfunctionalgroups,thusmeetingtherapeutic and nutritional requirements.

Declaration of interest

The authors have declare that there is no conf l icts of interest.

Acknowledgements

This study was supported by the National Natural Science Foundation of China(NO.81473324),National Key Scientific Project for China New Drug Discovery and Development during the Twelfth Five-Year Plan-preclinical study on suanzaoren granules(2014ZX09304306-007),National Key Scientific Project for China New Drug Discovery and Development during theTwelfth Five-Year Plan-Technology re-innovation of generic and superstar drugs(2014ZX09201-002),KeyTechnologies of Common Quality Evaluation of New Drugs(Grant No.2015010201)and Liaoning Province Science and Technology Research Project(Grant No.201610163L02).

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