Advance in Dietary Polyphenols as α-Glucosidases Inhibitors: A Review on Structure-Activity Relationship Aspect

The dietary polyphenols as α-glucosidases inhibitors have attracted great interest among researchers. The aim of this review is to give an overview of the research reports on the structure-activity relationship of dietary polyphenols inhibiting α-glucosidases. The molecular structures that influence the inhibition are the following: (1) The hydroxylation and galloylation of flavonoids including catechins improve the inhibitory activity. (2) The glycosylation of hyroxyl group and hydrogenation of the C2=C3 double bond on flavonoids weaken the inhibition. (3) However, cyaniding glycosides show higher inhibition against than cyanidin. Proanthocyanidins oligomers exhibit a stronger inhibitory activity than their polymers. (4) The hydroxylation on B ring and the glycosylation of stilbenes reduce the inhibitory activity. (5) Caffeoylquinic acids display strong inhibition against α-glucosidases. However, hydroxycinnamic acid, ferulic acid, and gallic acid hardly inhibited α-glucosidases. (6) The coupled galloyl structures attached to C-3 and C-6 of the ₄C¹ glucose core of ellagitanin gave basic inhibitory activity. (7) The mono-glycosylation of chalcones slightly lowers the inhibition. However, the diglycosylation of chalcones significantly decreased the activity.     

Interaction of dietary polyphenols with bovine milk proteins: molecular structure-affinity relationship and influencing bioactivity aspects

Scope : Dietary flavonoids and stilbenes are important polyphenols in foods, such as, e.g. fruits, vegetables, nuts, and tea as they are of great interest for their bioactivities, which are related to the anti‐oxidative property. Methods and results : The relationship between the structural properties of dietary polyphenols and their affinities for milk proteins (MP) was investigated. Methylation and methoxylation of flavonoids decreased (or hardly affected) the affinities for MP. Hydroxylation on the rings A and B of flavones and flavonols enhanced the interaction slightly. The hydroxylation on the ring A of flavanones significantly improved the affinities. Glycosylation of flavonoids weakened the affinities by 1–2 orders of magnitude. The hydrogenation of the C2?C3 double bond of flavonoids decreased the binding affinities by 7.24–75.86‐fold. Galloylation of catechins significantly improved the binding affinities by about 100–1000‐fold. Glycosylation of resveratrol decreased its affinity for MP. Esterification of gallic acid increased its binding affinity. MP significantly weakened the DPPH radical scavenging activity of polyphenols. The decreasing DPPH scavenging percentages of polyphenols increased with increasing affinities of MP–polyphenol complexes. Conclusion : The binding affinities with MP were strongly influenced by the structural differences of dietary polyphenols. The MP–polyphenol interaction weakened with the DPPH free radical scavenging potential of polyphenols.     

A Review on Structure–Activity Relationship of Dietary Polyphenols Inhibiting α-Amylase

The inhibitory effects of dietary polyphenols against α-amylase have attracted great interest among researchers. The aim of this review is to give an overview of the research reports on the structure–activity relationship of polyphenols inhibiting α-amylase. The molecular structures that influence the inhibition are the following: (1) The hydroxylation of flavonoids improved the inhibitory effect on α-amylase; (2) Presence of an unsaturated 2,3-bond in conjugation with a 4-carbonyl group has been associated with stronger inhibition; (3) The glycosylation of flavonoids decreased the inhibitory effect on α-amylase depending on the conjugation site and the class of sugar moiety; (4) The methylation and methoxylation of flavonoids obviously weakened the inhibitory effect; (5) The galloylated catechins have higher inhibition than nongalloylated catechins; the catechol-type catechins were stronger than the pyrogallol-type catechins; the inhibition activities of the catechins with 2,3-trans structure were higher than those of the catechins with 2,3-cis structure; (6) Cyanidin-3-glucoside showed higher inhibition against than cyanidin and cyanidin-3-galactoside and cyanidin-3,5-diglucoside had no inhibitory activity; (7) Ellagitannins with β-galloyl groups at glucose C-1 positions have higher inhibitory effect than the α-galloyl and nongalloyl compounds and the molecular weight of ellagitannins is not an important element.     

Structure–activity relationships of acetylated flavone glycosides from Galeopsis ladanum L. (Lamiaceae)

Antioxidant activity, neuroprotective effect and acetylcholinesterase inhibitor activity were studied in acetylated flavones from Galeopsis ladanum L. (Lamiaceae), previously isolated and identified by UV-spectra, UPLC–MS/MS and ¹H-NMR spectroscopy. Structure–activity relationships of flavonoids have been determined in many antioxidant assays, generally focused on hydroxyl groups. In this study we have detected new interesting structure–activity relationships for the isolated flavonoids due to the acetylation of sugar moiety of these flavonoids. Methylation at 4′-OH and monoacetylation is beneficial to inhibiting AChE and shows a neouroprotective effect.     

Investigating Flavonoids as Molecular Templates for the Design of Small‐Molecule Inhibitors of Cell Signaling

Epidemiological and clinical trials reveal compelling evidence for the ability of dietary flavonoids to lower cardiovascular disease risk. The mechanisms of action of these polyphenolic compounds are diverse, and of particular interest is their ability to function as protein and lipid kinase inhibitors. We have previously described structure–activity studies that reinforce the possibility for using flavonoid structures as templates for drug design. In the present study, we aim to begin constructing rational screening strategies for exploiting these compounds as templates for the design of clinically relevant, antiplatelet agents. We used the platelet as a model system to dissect the structural influence of flavonoids, stilbenes, anthocyanidins, and phenolic acids on inhibition of cell signaling and function. Functional groups identified as relevant for potent inhibition of platelet function included at least 2 benzene rings, a hydroxylated B ring, a planar C ring, a C ring ketone group, and a C‐2 positioned B ring. Hydroxylation of the B ring with either a catechol group or a single C‐4′ hydroxyl may be required for efficient inhibition of collagen‐stimulated tyrosine phosphorylated proteins of 125 to 130 kDa, but may not be necessary for that of phosphotyrosine proteins at approximately 29 kDa. The removal of the C ring C‐3 hydroxyl together with a hydroxylated B ring (apigenin) may confer selectivity for 37 to 38 kDa phosphotyrosine proteins. We conclude that this study may form the basis for construction of maps of flavonoid inhibitory activity on kinase targets that may allow a multitargeted therapeutic approach with analogue counterparts and parent compounds.     

INHIBITION OF β-GALACTOSIDASE OF APPLE BY FLAVONOIDS AND OTHER POLYPHENOLS

The inhibition of a purified β-galactosidase from "Spartan" apples by flavonoids and related compounds has been studied with a fluorimetric assay using the substrate 4-methylumbelliferyl β-D-galactoside. Certain flavonoids, flavonoid glycosides, phenolic acids, and polyphenols (quercetin, fustin, rutin, p-coumaric acid, and catechol) gave classical noncompetitive inhibition at moderate substrate concentrations. Others (kaempferol, chlorogenic acid) gave nonlinear Dixon plots. The best inhibitors were flavones. In general, the degree of inhibition increased with the degree of hydroxylation and glycosylation of the phenol. Correlation of the structure of these substances with the observed inhibition suggests that the trans-3-(4-hydroxyphenyl)-propenoyl group, common to 4'-hydroxyflavones and p-hydroxyphenolic acids, is important for effective inhibition.     

Flavonoids as Potent Scavengers of Hydroxyl Radicals

Oxidative stress is a fundamental principle in the pathophysiology of many diseases. It occurs when the production of reactive oxygen species exceeds the capacity of the cell defense system. The hydroxyl radical is a reactive oxygen species that is commonly formed in vivo and can cause serious damage to biomolecules, such as lipids, proteins, and nucleic acids. It plays a role in inflammation‐related diseases, like chronic inflammation, neurodegeneration, and cancer. To overcome excessive oxidative stress and thus to prevent or stop the progression of diseases connected to it, scientists try to combat oxidative stress and to find antioxidant molecules, including those that scavenge hydroxyl radical or diminish its production in inflamed tissues. This article reviews various methods of hydroxyl radical production and scavenging. Further, flavonoids, as natural plant antioxidants and essential component of the human diet, are reviewed as compounds interacting with the production of hydroxyl radicals. The relationship between hydroxyl radical scavenging and the structure of the flavonoids is discussed. The structural elements of the flavonoid molecule most important for hydroxyl radical scavenging are hydroxylation of ring B and a C2–C3 double bond connected with a C‐3 hydroxyl group and a C‐4 carbonyl group. Hydroxylation of ring A also enhances the activity, as does the presence of gallate and galactouronate moieties as substituents on the flavonoid skeleton.     

Natural antioxidant polyphenols on inflammation management: Anti-glycation activity vs metalloproteinases inhibition

The diet polyphenols are a secondary metabolites of plants able to act on inflammation process. Their anti-inflammatory activity is articulated through several mechanisms that are related to their antioxidative and radical scavengers properties. Our work is focused on a novel approach to inflammatory disease management, based on anti-glycative and matrix metalloproteinases (MMPs) inhibition effects, as a connected phenomena. To better understand these correlation, polyphenols Structure–Activity Relationship (SAR) studies were also reported. The antioxidant polyphenols inhibit the AGEs at different levels of the glycation process in the following ways: (1) prevention of Amadori adduct oxidation; (2) trapping reactive dycarbonyl compounds; (3) attenuation of receptor for AGEs (RAGE) expression. Moreover, several flavonoids with radical scavenging property showed also MMPs inhibition interact directly with MMPs or indirectly via radical scavengers and AGEs reduction. The essential polyphenols features involved in these mechanisms are C2-C3 double bond and number and position of hydroxyl, glycosyl and O-methyl groups. These factors induce a change in molecular planarity interfering with the hydrogen bond formation, electron delocalization and metal ion chelation. In particular, C2-C3 double bond improve the antioxidant and MMPs inhibition, while the hydroxylation, glycosylation and methylation induce a positive and negative correlation, respectively.     

Antioxidant and hepatoprotective activity of ethanolic extract of leaves of Solidago microglossa containing polyphenolic compounds

The antioxidative and hepatoprotective potential of Solidago microglossa D.C, a widely used medicinal plant from Brazil was investigated. The leaf extract showed inhibition against thiobarbituric acid reactive species (TBARS) induced by different prooxidants (10 μM FeSO₄ and 5 μM sodium nitroprusside SNP) in rat liver, brain and phospholipid homogenates from egg yolk. Moreover, the free radical scavenging activities of the extract was evaluated by the scavenging of 2,2-diphenyl-1-picrylhydrazyl (DPPH) (IC_50, 3.8 ± 0.5 μg/ml) and hydroxyl radical on benzoic acid hydroxylation (IC_50, 32.3 ± 1.3 μg/ml) and deoxyribose (IC_50, 39.1 ± 2.4 μg/ml) assays. The ethanolic extract showed significant hepatoprotective activity against paracetamol (250 mg/kg) induced liver damage in mice in a dose dependent manner. The phenolic composition and their quantification by high performance liquid chromatography (HPLC) resulted in the identification of gallic acid and flavonoids: quercetrin (quercetin-3-O-rhamnoside), rutin (quercetin-3-O-rutinoside) and quercetin.     

Iron binding characteristics of phenolic compounds: some tentative structure–activity relations

Two mechanisms are commonly proposed to explain the antioxidant role of phenolic compounds; these are metal chelation and/or free radical scavenging. However, the structural requirements for each may not be the same. This paper describes the determination of iron-binding efficiencies (in vitro) for a series of pure phenolics of known structure [gallic acid, tannic acid, catechin, epicatechin (EC), epicatechingallate (ECG), epigallocatechin (EGC), epigallocatechingallate (EGCG) and quercetin] and of tea beverages of known flavonoid composition. Iron-binding efficiency was measured as catechin equivalents or tannic acid equivalents using the two wavelength assay. High catechin equivalents require the presence of a 3′,4′ dihydroxy (catechol ) group on flavanoid ring B. In contrast, the presence of a 3′,4′,5′-trihydroxy (galloyl) group of ring B (epigallocatechin) or ring C (epicatechin gallate) was associated with reduced Fe-binding. The extent of Fe-binding was also found to be lower for quercetin than catechin, which is probably due to the presence of conjugation extending from the C4-keto group, via C2-3 to the 3′-OH group (rings B and C). For the polyphenols examined, catechin equivalents were inversely correlated with tannic acid equivalents and with antioxidant activity of flavonoids, as measured by the TEAC assay. Requirements for efficient iron-binding are discussed.     

Development of a reversed-phase high performance liquid chromatography (RP-HPLC) procedure for the simultaneous determination of phenolic compounds in peanut skin extracts

A reversed-phase high performance liquid chromatography (RP-HPLC) procedure was developed for simultaneous determination of five phenolic acids, two stilbenes and eight flavonoids in peanut skin extract. A C₁₈ column fitted with diode array detection at 250 and 320, 280 and 370, and 306 nm for phenolic acids, flavonoids and stilbenes, respectively, with mobile phase consisting of 0.1% formic acid in water and 0.1% formic acid in 100% acetonitrile. Phenolic compounds were eluted with good resolution (Rs > 1.5) within 95 min as follows: gallic, protocatechuic, epigallocatechin, catechin, β-resorcylic (internal standard), caffeic, procyanidin B₂, epicatechin, epigallocatechin gallate, p-coumaric, ferulic, piceid, epicatechin gallate, catechin gallate, resveratrol and quercetin. The variation in recovery and reproducibility in peak area was <11 and <2.5%, respectively. The correlation coefficients, r, of calibration curves of the 15 compounds were >0.999. The method was used to quantify phenolic compounds in peanut skin extracts from Runner, Virginia and Spanish peanuts.     

The influence of the storage conditions heat and humidity on conformation,state transitions and degradation behaviour of dried pectins

For testing the influence of long-term storage on pectins, differently prepared samples were incubated under defined temperature and relative humidity in a climate chamber. This caused pectin demethoxylation, depolymerisation and non-enzymatic browning and influenced the dissolution behaviour and pH. The extent of chemical changes and the reaction mechanisms depended on incubation conditions, type and “history” of the pectins and on their conformation (⁴C₁ or ¹C₄ chair). Higher temperature and higher humidity increased demethoxylation and browning. Depolymerisation was high even at low degree of methoxylation. Therefore, an additional mechanism besides β-elimination and glycosidic linkage cleavage was suggested.     

The Nitrite‐Scavenging Properties of Catechol,Resorcinol, and Hydroquinone: A Comparative Study on Their Nitration and Nitrosation Reactions

The nitration and nitrosation reactions of catechol, resorcinol, and hydroquinone (0.05 mmol/L) with sodium nitrite (0.05 mmol/L) at pH 3 and 37 °C were studied by using liquid chromatography and mass spectrometry (LC‐MS) and atom charge analysis, which was aimed to provide chemical insight into the nitrite‐scavenging behavior of polyphenols. The 3 benzenediols showed different mechanisms to scavenge nitrite due to their differences in hydroxyl position. Catechol was nitrated with 1 NO₂ group at the hydroxyl oxygen, and resorcinol was nitrosated with 2 NO groups at the C₂ and C₄ (or C₆) positions of the benzene ring. Hydroquinone could scavenge nitrite through both nitration and nitrosation mechanisms. The nitrated hydroquinone had 1 NO₂ group at the hydroxyl oxygen in the molecule, while the nitrosated 1 containing 2 NO groups at the benzene ring might have 3 structure probabilities. The results may provide a structure‐activity understanding on the nitrite‐scavenging property of polyphenols, so as to promote their application in the food industry for the removal of possibly toxic nitrites found in many vegetables and often in processed meat products.     

Antimicrobial activity of 5-hydroxy-1,4-naphthoquinone isolated from Caesalpinia sappan toward intestinal bacteria

The methanol extract of Caesalpinia sappan heartwoods was tested for the growth effects toward five intestinal microorganisms. The biologically active constituent of the C. sappan extract was characterized as 5-hydroxy-1,4-naphthoquinone (C₁₀H₆O₃). The growth responses varied depending on the bacterial species and dose tested. In the test with Clostridium perfringens, 5-hydroxy-1,4-naphthoquinone produced the strong (+++) inhibition at 5 and 2 mg/disk and moderate (++) inhibition at 1, 0.5, and 0.25 mg/disk. Furthermore, this isolate revealed a weak (+) growth inhibition against Lactobacillus casei at 5 and 2 mg/disk. In comparison of naphthoquinone derivatives, 5-hydroxy-2-methyl-1,4-naphthoquinone had moderate growth inhibition against C. perfringens at 5 and 2 mg/disk, whereas 1,4-naphthoquinone at 5 mg/disk significantly inhibited the growth of all bacteria tested. 1,2-Naphthoquinone had growth inhibition against all bacteria tested at 1 mg/disk. The structure-activity relationship revealed that 5-hydroxy-1,4-naphthoquinone had selective growth inhibition against C. perfringens. These results indicate that hydroxyl functional group of naphthoquinone seems to be required for selective growth-inhibiting activity against C. perfringens. Accordingly, as naturally occurring antimicrobial agents, C. sappan heartwood-derived material could be useful as a preventive agent against diseases caused by C. perfringens.     

Identification of flavonoids and flavonoid rhamnosides from Rhododendron mucronulatum for. albiflorum and their inhibitory activities against aldose reductase

To investigate the therapeutic potential of compounds from natural sources, Rhododendron mucronulatum for. albiflorum flowers (RMAF) and R. mucronulatum flowers (RMF) were tested for inhibition of aldose reductase (AR). The methanol extracts of RMAF and RMF exhibited AR inhibitory activities (IC₅₀ values 1.07 and 1.29 μg/mL, respectively). The stepwise polarity fractions of RMAF were tested for in vitro inhibition of AR from rat lenses. Of these, the ethyl acetate (EtOAc) fraction exhibited AR inhibitory activity (IC₅₀ 0.15 μg/mL). A chromatography of the active EtOAc fraction of RMAF led to the isolation of six flavonoids, which were identified by spectroscopic analysis as kaempferol (1), afzelin (2), quercetin (3), quercitrin (4), myricetin (5) and myricitrin (6). Compounds 1–6 exhibited high AR inhibitory activity, with IC₅₀ values of 0.79, 0.31, 0.48, 0.13, 11.92 and 2.67 μg/mL, respectively. HPLC/UV analysis revealed that the major flavonoids of RMAF and RMF are quercitrin (4) and myricitrin (6). Our results suggest that RMAF containing these six flavonoids could be a useful natural source in the development of a novel AR inhibitory agent against diabetic complications.     

Dietary flavonoids fisetin,luteolin and their derived compounds inhibit arginase,a central enzyme in Leishmania (Leishmania) amazonensis infection

Fisetin, quercetin, luteolin and 7,8-hydroxyflavone show high activity in Leishmania cultures and present low toxicity to mammalian cells. In this work, the structural aspects of 13 flavonoids were analyzed for their inhibition of the arginase enzyme from Leishmania (Leishmania) amazonensis  . A higher potency of arginase inhibition was observed with fisetin, which was four and ten times greater than that of quercetin and luteolin, respectively. These data show that the hydroxyl group at position 3 contributed significantly to the inhibitory activity of arginase, while the hydroxyl group at position 5 did not. The absence of the catechol group on apigenin drastically decreased arginase inhibition. Additionally, the docking of compounds showed that the inhibitors interact with amino acids involved in the Mn⁺²-Mn⁺²${\text{Mn}}^{+2}-{\text{Mn}}^{+2}$ metal bridge formation at the catalytic site. Due to the low IC₅₀ values of these flavonoids, they may be used as a food supplement in leishmaniasis treatment.     

Structure-affinity relationship of bovine serum albumin with dietary flavonoids with different C-ring substituents in the presence of Fe ion

The relationship of four dietary flavonoids, quercein (QU), luteolin (LU), taxifolin (TA) and (+)-catechin (CA), with different C-ring substituents binding with bovine serum albumin (BSA) have been investigated in the absence and presence of Fe³⁺ by means of spectroscopic methods. The quenching constant (K_SV), binding constant (K_a), binding site number (n) and spatial-distance (r) of flavonoids with BSA were calculated. The results indicated that hydroxyl group at 3-position and C2–C3 double bond affected the binding affinities between flavonoids and BSA. The values of the binding affinities were in the order: QU > CA > TA > LU. The presence of Fe³⁺ affected the interactions between flavonoids and BSA significantly, and the influence was distinct for the flavonoids with different C-ring structures. The binding affinities of QU, LU and CA for BSA were increased about 0.85%, 41.3% and 56.8% in the presence of Fe³⁺, probably because of the formation of QU–Fe³⁺–BSA complex, Fe³⁺–LU complex and the conformational change of BSA induced by Fe³⁺, respectively. The binding affinities of TA for BSA were decreased about 20.7%, mainly because of the existence of competitive binding between TA and Fe³⁺ with BSA. However, the quenching mechanism for QU, LU, TA and CA to BSA were based on static quenching combined with non-radiative energy transfer irrespective of the absence or presence of Fe³⁺ ion.     

A comparison of the in vitro biotransformation of (–)‐epicatechin and procyanidin B2 by human faecal microbiota

The catabolism by human faecal microbiota of (–)‐epicatechin ( 1 ) (2, 3‐cis stereochemistry) and its dimer pure procyanidin B2 ( 2 ), has been compared using a static in vitro culture model. The catabolites were characterised by LC‐MSⁿ, UV absorption and relative retention time, and quantified relative to standards. No more than ～10% of procyanidin B2 ( 2 ) was converted to epicatechin ( 1 ) by scission of the interflavan bond. Five phenolic acid catabolites (M_r<290) were unique to 2 , and ten phenolic acid catabolites (M_r<290) were common to both substrates. The dominant catabolites (≥24 h incubation) were 5‐(3′‐hydroxy phenyl) valeric acid ( 9 ), 3‐(3′‐hydroxyphenyl) propionic acid ( 10 ) and phenyl acetic acid ( 12 ) (maximum yields 27.4±4.2, 38.2±4.2, 22.7±2.9%, respectively, from 1 and 9.4±1.2, 52.8±2.1, 28.8±1.6%, respectively, from 2 ). Substrate 2 was degraded twice as rapidly as 1 . Evidence is presented for the production of previously unreported catabolites of 2 that retain the flavanol A‐ring and the C4→C8 interflavan bond. It was confirmed that catabolism favoured removal of the 4′‐hydroxyl rather than the 3′‐hydroxyl group and that both β‐oxidation and α‐oxidation occurred.     

Preparative separation of flavonoids in Adinandranitida leaves by high-speed counter-current chromatography and their effects on human epidermal carcinoma cancer cells

Preparative separation of camellianins A and B in Adinandra nitida leaves, which is a flavonoid-rich plant source with great practical prospects, was conducted by high-speed counter-current chromatography (HSCCC) with a solvent system composed of ethyl acetate–ethanol–water (5:1:5, v/v). Apigenin (their aglycone) was prepared by hydrolysing a water extract and recrystallising. The purity and identity of these flavonoids was confirmed by HPLC-ESI/MS, and ¹H and ¹³C NMR. These flavones were used in cultures of the A431 cell line. Their inhibitory effect was evaluated by using the MTT assay. The results showed that three flavones could significantly inhibit the growth of human epidermal carcinoma cancer cells in a concentration-dependent manner. After 48 h of treatment, IC₅₀ values of camellianin A, camellianin B and apigenin against A431 tumour cells were 9.09 μM, 12.5 μM, 21.0 μM, respectively.