Oxidation products of quercetin catalyzed by mushroom tyrosinase

Quercetin was oxidized as a substrate catalyzed by mushroom tyrosinase to the corresponding o-quinone and subsequent isomerization to p-quinone methide type intermediate; followed by the addition of water on C-2 yielding a relatively stable intermediate, 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. In the presence of a catalytic amount of l-DOPA as a cofactor, the rate of this oxidation was enhanced. Fisetin, which lacks the C-5 hydroxyl group, was also oxidized but the rate of oxidation was faster than that of quercetin, indicating that the C-5 hydroxyl group is not essential but is associated with the activity.     

Peroxidative metabolism of apigenin and naringenin versus luteolin and quercetin: glutathione oxidation and conjugation

GSH was readily depleted by a flavonoid, H(2)O(2), and peroxidase mixture but the products formed were dependent on the redox potential of the flavonoid. Catalytic amounts of apigenin and naringenin but not kaempferol (flavonoids that contain a phenol B ring) when oxidized by H(2)O(2) and peroxidase co-oxidized GSH to GSSG via a thiyl radical which could be trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) to form a DMPO-glutathionyl radical adduct detected by ESR spectroscopy. On the other hand, quercetin and luteolin (flavonoids that contain a catechol B ring) or kaempferol depleted GSH stoichiometrically without forming a thiyl radical or GSSG. Quercetin, luteolin, and kaempferol formed mono-GSH and bis-GSH conjugates, whereas apigenin and naringenin did not form GSH conjugates. MS/MS electrospray spectroscopy showed that mono-GSH conjugates for quercetin and luteolin had peaks at m/z 608 [M + H](+) and m/z 592 [M + H](+) in the positive-ion mode, respectively. (1)H NMR spectroscopy showed that the GSH was bound to the quercetin A ring. Spectral studies indicated that at a physiological pH the luteolin-SG conjugate was formed from a product with a UV maximum absorbance at 260 nm that was reducible by potassium borohydride. The quercetin-SG conjugate or kaempferol-SG conjugate on the other hand was formed from a product with a UV maximum absorbance at 335 nm that was not reducible by potassium borohydride. These results suggest that GSH was oxidized by apigenin/naringenin phenoxyl radicals, whereas GSH conjugate formation involved the o-quinone metabolite of luteolin or the quinoid (quinone methide) product of quercetin/kaempferol.     

Characterisation of new oligoglycosidic compounds in two Chinese medicinal herbs

A series of caffeic acid derivatives (3,5-dicaffeoyl-quinic acid, 3,4-dicaffeoyl-quinic acid, and 4,5-dicaffeoyl-quinic acid), and the new compound beta,3,4-trihydroxyphenethyl-O-[beta-apiofuranosyl-(1-->4)-alpha- rhamnopyranosyl-(1-->3)]-(4-O-caffeoyl)-beta-glucopyranoside (wedelosin), as well as three known flavonoid glycosides (quercetin 3-O-beta-glucoside, kaempferol 3-O-beta-apiosyl-(1-2)-beta-glucoside, and astragalin or kaempferol 3-O-beta-glucoside) were isolated from the Chinese medicinal herb Wedelia chinensis. Wedelosin showed an inhibitory activity on both the classical and the alternative activation pathway of the complement system. Another Chinese medicinal herb, Kyllinga brevifolia, yielded two known flavonoid glycosides [kaempferol 3-O-beta-apiosyl-(1-2)-beta-glucoside and isorhamnetin 3-O-beta-apiosyl-(1-2)-beta-glucoside], and a new quercetin triglycoside [quercetin 3-O-beta-apiofuranosyl-(1-->2)-beta-glucopyranoside 7-O-alpha-rhamnopyranoside]. The latter compound showed a moderate anti-viral activity.     

Are the biological properties of kaempferol determined by its oxidation products?

Although flavonoid molecules have attracted considerable interest in recent years because of their antioxidant effect, there are considerable differences in their chemical properties. Electron paramagnetic resonance (EPR) spectroscopy was used to compare the oxidative free radical chemistry of two such molecules, kaempferol and luteolin, which have the same empirical formula but differ in the position of one OH group. Whereas the basic flavonoid structure remains intact in luteolin, structural changes occur in kaempferol after one-electron oxidation. Autoxidation of kaempferol in alkaline solution and oxidation by at pH 7 led to rapid fragmentation. In contrast, oxidation by horseradish peroxidase/hydrogen peroxide, xanthine/xanthine oxidase (X/XO) or a Fenton reaction system produced a radical whose structure appeared to be based on dimerisation of either the original or a fragment of the flavonoid. Hence, the biological properties of kaempferol are likely to be determined by the chemistry of its oxidation products.     

Cytotoxic and new tetralone derivatives from Berchemia floribunda (Wall.) Brongn

Two new alpha-tetralone (=3,4-dihydronaphthalen-1(2H)-one) derivatives, berchemiaside A and B (1 and 2, resp.), and one new flavonoid, quercetin-3-O-(2-acetyl-alpha-L-arabinofuranoside (3), together with ten known flavonoids compounds, eriodictyol (4), aromadendrin (5), trans-dihydroquercetin (6), cis-dihydroquercetin (7), kaempferol (8), kaempferol-3-O-alpha-L-arabinofuranoside (9), quercetin (10), quercetin-3-O-alpha-L-arabinofuranoside or avicularin (11), quercetin 3'-methyl ether, 3-O-alpha-L-arabinofuranoside (12), and maesopsin (13), were isolated from the bark of Berchemia floribunda. Their structures were determined by various NMR techniques and chemical studies. Compounds 3-13 were tested for their cytotoxic activity against human leukemia cells. Among them, kaempferol (8) and maesopsin (13) showed significant inhibitory activities against human leukemia cells CCRF-CEM and its multidrug-resistant sub-line, CEM/ADR5000, with IC(50) values of 14.0, 5.3, 10.2, and 12.3 microM, respectively.     

Structure-activity relationships of flavonoids for vascular relaxation in porcine coronary artery

Flavonoids are polyphenolic compounds that are widespread in the plant kingdom, and structure-activity relationships (SAR) for vascular relaxation effects were examined for 17 of them using porcine coronary arteries. Density functional theory was employed to calculate the chemical parameters of these compounds. The order of potency for vascular relaxation was as follows: flavones (apigenin and luteolin) >or= flavonols (kaempferol and quercetin)>isoflavones (genistein and daidzein)>flavanon(ol)es (naringenin)>chalcones (phloretin)>anthocyanidins (pelargonidin)>flavan(ol)es ((+)-catechin and (-)-epicatechin). SAR analysis revealed that for good relaxation activity, the 5-OH, 7-OH, 4'-OH, C2=C3 and C4=O functionalities were essential. Comparison of rutin with quercetin, genistin with genistein, and puerarin with daidzein demonstrated that the presence of a glycosylation group greatly reduced relaxation effect. Total energy and molecular volume were also predictive of their relaxation activities. Our findings indicated that the most effective relaxing agents are apigenin, luteolin, kaempferol and genistein. These flavonoids possess the key chemical structures demonstrated in our SAR analysis.     

Flavonoids and their oxidation products protect efficiently albumin-bound linoleic acid in a model of plasma oxidation

Although LDL esterified polyunsaturated fatty acids (PUFA) contribute largely to the pool of oxidizable lipids in plasma, they coexist with a non-negligible content of free PUFA. In some pathological conditions, the free PUFA/albumin ratio becomes abnormally elevated. Modeling was performed in a system constituted of linoleic acid bound to human serum albumin (HSA) in which oxidation was initiated by hydrophilic AAPH. Inhibition of lipid peroxidation was evaluated for various flavonoids. The accumulations of hydroperoxyoctadecadienoic acids (HPODE), hydroxyoctadecadienoic acids (HODE) and ketooctadecadienoic acids (KODE) were similarly inhibited: isoquercitrin>quercetin>catechin=isorhamnetin>kaempferol>quercetin-4'-beta-D-glucoside=quercetin-3,4'-di-beta-D-glucoside. Surprisingly, quercetin and isorhamnetin afforded a protection to linoleic acid long after their consumption. Elucidation by mass spectrometry and NMR of the quercetin oxidation products and assessment of their antioxidant capacity pointed out that 3,4-dihydroxybenzoic acid and 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxybenzofuran-3(2H)-one are major contributors to the apparent quercetin antioxidant capacity.     

The chemosystematics of local members of the subtribe gnaphaliinae (Compositae)

Eight species belonging to the genera Filago, Ifloga, Achyrocline, Helichrysum and Pseudognaphalium, subtribe Gnaphaliinae, tribe Inuleae (Compositae), were investigated for their flavonoid glycosides. All glycosides were found to belong to the aglycones kaempferol, quercetin, apigenin and luteolin. Calycopterin and its 3'-methyl ether were present in the free form.     

Flavonoid <Emphasis Type="Italic">O</Emphasis>-Diglucosyltransferase from Rice: Molecular Cloning and Characterization

Among more than 100 rice uridine diphosphate glycosyltransferases (UGTs), OsUGT-3 was selected as a candidate for producing flavonoid O-diglycosyltransferases based on phylogenetic analysis and molecular docking. This gene was functionally expressed in Escherichia coli. Analysis of kaempferol, luteolin, quercetin, and tricin reaction products using liquid chromatography-mass spectrometry revealed that these were diglucosylated. The glucosylation positions of kaempferol, which was the best substrate, were determined to be the 3- and 7-hydroxyl groups. This is the first flavonoid O-diglucosyltransferase described from rice.     

Flavonoids of Helichrysum chasmolycicum and its antioxidant and antimicrobial activities

From the aerial parts of Helichrysum chasmolycicum P.H Davis, which is an endemic species in Turkey, the flavonoids apigenin, luteolin, kaempferol, 3,5-dihydroxy-6,7,8-trimethoxyflavone, 3,5-dihydroxy-6,7,8,4′-tetramethoxyflavone, apigenin 7-O-glucoside, apigenin 4′-O-glucoside, luteolin 4′-O-glucoside, luteolin 4′,7-O-diglucoside, kaempferol 3-O-glucoside, kaempferol 7-O-glucoside and quercetin 3-O-glucoside were isolated. The methanol extract of the aerial parts of H. chasmolycicum showed antioxidant activity by DPPH method (IC₅₀ 0.92 mg/mL). Antimicrobial activity test was performed on the B, D, E extracts and also 3,5-dihydroxy-6,7,8-trimethoxyflavone and kaempferol 3-O-glucoside which were the major flavonoid compounds obtained from aerial parts of H. chasmolycicum by microbroth dilutions technique. The E (ethanol-ethyl acetate) extract showed moderate antimicrobial activity against Pseudomonas aeruginosa, B (petroleum ether-60% ethanol-chloroform) extract and 3,5-dihydroxy-6,7,8-trimethoxyflavone showed moderate antifungal activity against Candida albicans.     

Synthesis of flavonol 3-O-glycoside by UGT78D1

Glycosylation is an important method for the structural modification of various flavonols, resulting in the glycosides with increased solubility, stability and bioavailability compared with the corresponding aglycone. From the physiological point of view, glycosylation of plant flavonoids is of importance and interest. However, it is notoriously complicated that flavonols such as quercetin, kaempferol and myricetin, are glucosylated regioselectively at the specific position by chemical method. Compared to the chemical method, enzymatic synthesis present several advantages, such as mild reaction condition, high stereo or region selectivity, no protection/deprotection and high yield. UGT78D1 is a flavonol-specific glycosyltransferase, responsible for transferring rhamnose or glucose to the 3-OH position in vitro. In this study, the activity of UGT78D1 was tested against 28 flavonoids acceptors using UDP-glucose as donor nucleoside in vitro, and 5 acceptors, quercetin, myricetin, kaempferol, fisetin and isorhamnetin, were discovered to be glucosylated at 3-OH position. Herein, the small-scale 3-O-glucosylated quercetin, kaempferol and myricetin were synthesized by UGT78D1 and their chemical structures were confirmed by (1)H and (13)C nuclear magnetic resonance (NMR) and high resolution mass spectrometry (HRMS).     

Flavonoid chemistry of Fallopia section Fallopia (Polygonaceae)

Five controversial species of Fallopia sect. Fallopia sensu Holub were examined for leaf flavonoid constituents. Twenty-one flavonoid compounds were isolated and identified; they were glycosylated derivatives of the flavonols kaempferol, quercetin, and myricetin, and of the flavones apigenin and luteolin. Among them, quercetin 3-O-galactoside and quercetin 3-O-glucoside were major flavonoid constituents and present in all species. Although the flavonoid data for some species are lacking, those available appear to be useful for species delimitation and for recognizing species relationships in the section. The flavonoid data, in conjunction with morphological evidence, strongly suggest that F. scandens, F. dentatoalata, F. dumetorum, and F. convolvulus are closely allied but distinct species. In addition, the flavonoid data for F. cilinodis lend additional support to the segregation of sect. Parogonum from sect. Fallopia.     

The systematic significance of flavonoids in Persea of the Southeastern United States

The systematic significance of flavonoid distribution in the southeastern United States taxa of Persea is discussed. The compounds thus far elucidated are glycosides based on quercetin, kaempferol, apigenin or luteolin.     

Inhibition Mode of Soybean Lipoxygenase‐1 by Quercetin

Quercetin noncompetitively inhibited the peroxidation of linoleic acid catalyzed by soybean lipoxygenase‐1 (EC 1.13.11.12, Type 1) with an IC₅₀ value of 4.8 μM (1.45 μg/ml). This inhibition is considered to proceed in sequential order, by initially reducing the ferric form of the enzyme to an inactive ferrous form and then, by chelating the iron of the active site of the enzyme. In the pseudoperoxidase assay, quercetin was slowly oxidized by hydroperoxides to a rather stable intermediate, 2‐(3,4‐dihydroxybenzoyl)‐2,4,6‐trihydroxybenzofuran‐3(2H)‐one, and this oxidized intermediate still inhibited the enzymatic oxidation, probably as a chelator. Rutin and kaempferol also exhibited lipoxygenase‐1 inhibitory activity, but to a much lesser extent than quercetin.     

Flavonoid glycosides and the chemosystematics of Eucalyptus camaldulensisis

Leaf flavonoid glycosides of Eucalyptus camaldulensis were identified as kaempferol 3-glucoside and 3-glucuronide; quercetin 3-glucoside, 3-glucuronide, 3-rhamnoside, 3-rutinoside and 7-glucoside, apigenin 7-glucuronide and luteolin 7-glucoside and 7-glucuronide. Two chemical races were observed based on the flavonoid glycosides. These races correspond to the northern and southern populations of species growing in Australia. The Middle Eastern species examined were found to belong to the southern Australian chemical race. The major glycosides of E. occidentalis proved to be quercetin and myricetin 3-glucuronide.     

Regioselectivity of 7-O-methyltransferase of poplar to flavones

POMT-7, an O-methyltransferase from poplar (Populus deltoids) was used to modify a variety of flavonoid compounds. POMT-7 was able to transfer a methyl group to several flavonoids containing a C-7 hydroxyl group. However, POMT-7 showed a higher affinity toward flavonol and flavone such as apigenin, kaempferol, luteolin, and quercetin than flavanone and isoflavone. Based on comparison of HPLC retention times with authentic compounds and corresponding nuclear magnetic resonance spectroscopy data, the methylation position of the reaction products was determined to be at the hydroxyl group of C-7. Biotransformation kinetics indicated that the enzyme converted more than 80% of the apigenin, kaempferol, luteolin and quercetin substrates, which were added at concentration of 70 microM, into corresponding 7-methoxy compounds within 24 h.     

Regeneration of phenolic antioxidants from phenoxyl radicals: an ESR and electrochemical study of antioxidant hierarchy

Radicals from the flavonoids quercetin, (+)-catechin, (+/-)-taxifolin and luteolin, and from all-rac-alpha-tocopherol have been generated electrochemically by one-electron oxidation in deaerated dimethylformamide (DMF), and characterised by electron spin resonance spectroscopy (ESR) after spin-trapping by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Simulations of the ESR spectrum based on estimated coupling constants of the spin-trapped quercetin radical, confirmed that this antioxidant radical is oxygen-centered. The complex mixture of radicals, quinoid intermediates and stable two-electron oxidation products, were for each antioxidant allowed to react with each of the four other antioxidants, and the progression of reaction followed by ESR after addition of DMPO, and the product solution further analysed by HPLC. All-rac-alpha-tocopherol was found to be most efficient in regenerating each of the other antioxidants from their oxidation products with a regeneration index (defined as moles regenerated of the oxidised phenolic antioxidant divided with moles of all-rac-alpha-tocopherol consumed) of 0.90+/-0.16 for quercetin, 0.48+/-0.11 for (+)-catechin, 0.48+/-0.06 for (+/-)-taxifolin and 0.50+/-0.10 for luteolin in equimolar 1.00 mM solution. Quercetin was found to have the highest regeneration index among the flavonoids: 0.88+/-0.13 for (+/-)-catechin, 0.41+/-0.03 for (+/-)-taxifolin and 0.41+/-0.02 for luteolin. The antioxidant hierarchy based on the reduction potentials determined by cyclic voltammetry under similar conditions (0.93 V for all-rac-alpha-tocopherol, 1.07 V for quercetin, 1.15 V for luteolin, 1.16V for (+)-catechin and 1.20 V for (+/-)-taxifolin) is compared with the observed over-all regeneration (34% for quercetin, 34% for (+)-catechin, 52% for (+/-)-taxifolin and 43% for luteolin by all-rac-alpha-tocopherol).     

Flavonoid pattern and systematics of the genus Leucocyclus

The flavonoid pattern of the monotypic Turkish genus Leucocyclus consists of C-glycosylflavones (isovitexin; isoorientin and derivatives; several di-C-glycosylapigenins; schaftoside, isoschaftoside and vicenin-3; lucenin-2), of flavonol 3-O-glycosides (quercetin and kaempferol 3-O-rhamnoglucoside) and trace amounts of luteolin 7-O-rhamnoglucoside. The systematic significance of the flavonoid diversification within Leucocyclus as well as possible relationships to other genera of the Anthemideae are discussed.     

Regioselective synthesis of plant (iso)flavone glycosides in <Emphasis Type="Italic">Escherichia coli</Emphasis>

The flavonoids genistein, biochanin A, luteolin, quercetin, and kaempferol are plant natural products with potentially useful pharmacological and nutraceutical activities. These natural products usually exist in plants as glycosides, and their glycosylation has a remarkable influence on their pharmacokinetic properties. The glycosyltransferases UGT71G1 and UGT73C8 from Medicago truncatula are excellent reagents for the regioselective glycosylation of (iso)flavonoids in Escherichia coli grown in Terrific broth. Ten to 20 mg/L of either genistein or biochanin A 7-O-glucoside was produced after feeding genistein or biochanin A to E. coli expressing UGT71G1, and similar levels of luteolin 4'-O- and 7-O-glucosides were produced after feeding luteolin to cultures expressing UGT73C8. For the production of kaempferol 3-O-glucoside or quercetin 3-O-glucoside, the Phe148Val or Tyr202Ala mutants of UGT71G1 were employed. Ten to 16 mg/L of either kaempferol 3-O- or quercetin 3-O-glucosides were produced on feeding kaempferol or quercetin to E. coli expressing these enzymes. More than 90% of the glucoside products were released to the medium, facilitating their isolation.     

Flavonoids in leaves and inflorescences of australian Cyperus species

A survey of the flavonoids of some 92 species of Australian Cyperus, mainly of subtropical or tropical origin, has confirmed a correlation previously reported in this family between flavonoid pattern and plant geography. The pattern found was similar to that of African and South American Cyperaceae, particularly in the occurrence of the rare marker substance, luteolin 5-methyl ether. Tricin and luteolin are relatively common, in glycosidic form, in the leaves while the flavonol quercetin is infrequent. When present, quercetin occurs either in glycosidic form or free as a methyl ether. The 3-monomethyl and 3, 7-dimethyl ethers of kaempferol and quercetin and the 3, 7, ?-trimethyl ether of quercetin are reported for the first time from the Cyperaceae. The flavonoid pattern of inflorescences is distinct from that of the leaves in that tricin is not detectable and that luteolin 5-methyl ether appears to be replaced by 7, 3′, 4′-trihydroxyflavone. In addition, the aurone aureusidin is more commonly present than in the leaves and is occasionally accompanied by two further aurones. The glycoxanthones mangiferin and isomangiferin occur rarely in all three species examined in the section Haspani, i.e. in C. haspan, C. prolifer and C. tenuispica. In general, however, the flavonoid data do not offer any markers which separate off different sections within the genus; there are, however, some significant correlations between the frequency of the flavonoid classes and subgeneric groupings.