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At least 19 recordsLinked to original sources
PubMed1995

Flavonol 3-O-glycosyltransferases associated with petunia pollen produce gametophyte-specific flavonol diglycosides.

Wild-type petunia pollen accumulates high levels of flavonol 3-O-glycosides. Pollen from conditionally male-fertile petunia has no flavonols and is unable to germinate. Pollen function is restored both in vivo and in vitro by providing flavonol aglycones, but not flavonol glycosides, to the pollen. In the present study, incubation of an in vitro suspension of conditionally male-fertile pollen with kaempferol or quercetin resulted in the accumulation of kaempferol and quercetin 3-O-glycosides in the pollen. We identified two glycosyltransferase activities associated with the intact pollen grain that catalyze the formation of a gametophyte-specific class of flavonol glycosides. Feeding studies showed that product formation was highly specific for flavonols with an unsubstituted 3-hydroxyl group and was not dependent on an external source of UDP-hexose. Ultraviolet spectral analysis, fast atom bombardment mass spectrometry, 1H-nuclear magnetic resonance, and 13C-nuclear magnetic resonance identified the products as kaempferol and quercetin 3-O-(2"- O-beta-D-glucopyranosyl)-beta-D-galactopyranoside, identical with the flavonol 3-O-glycosides present in wild-type pollen. The sugars are linked in a 1-->2 configuration that results in a pollen-specific class of compounds. To retain both glycosyltransferase activities in a cell-free extract, it was necessary to add triton X-100, suggesting that one or both of the proteins may be associated with a pollen membrane. A model for flavonol glycoside biosynthesis and uptake into the pollen is discussed in terms of the germination requirement for flavonols.

Carbohydrate Conformation↗
PubMed1979

Mutagenicity of plant flavonols in the Salmonella/mammalian microsome test: activation of flavonol glycosides by mixed glycosidases from rat cecal bacteria and other sources.

Over 70 naturally occurring and synthetic flavonoids were screened for mutagenicity with 5 tester strains in the Salmonella/mammalian microsome assay: TA1535, TA100, TA1537, TA1538 and TA98. Frameshift mutagenicity was confined to the flavonols (flavon-3-ols) in strain TA98, TA1537 and TA100. The two most mutagenic falvonols, namely, quercetin (3,3',4',5,7-pentahydroxyflavone) and kaempferol (3,4',5,7-tetrahydroxyflavone), exhibiting 12 and 7 revertants/nmol in TA98 respectively, are also the most common flavonols occurring in plants. Other flavonols exhibited less activity (revertants/nmol): galangin (2.0), rhamnetin (0.45), kaempferide (0.24), fisetin (0.14), myricetin (0.12), robinetin (0.06) and morin (0.05). All of these flavonols apparently exhibited significant activation by Aroclor 1254 induced rat-liver microsome preparations (S9). However, subsequent study revealed that only those flavonols either lacking or possessing one B ring hydroxyl group had an absolute requirement for microsomal activation. Alternatively, quercetin with two B-ring OH groups is not activated by microsomal enzymes, but by soluble (S100) enzymes from liver which are apparently constitutive and not subject to the usual chemical induction. 3 flavonol glycosides, namely, quercetrin (quercetin-3-O-rhamnoside), rutin (quercetin-3-O-rutinoside) and robinin (kaempferol-3-O-galactosido-rhamnoside-7-O-rhamnoside), were found to be nonmutagenic. They could, however, be activated by a variety of mixed glycosidases incorporated in the usual pour plate procedure. The most effective enzyme mixtures were obtained from rat cecal bacteria and from the snail Helix pomatia.

Animals↗
PubMed1976

[Flavonols and flavones of vegetables. VI. On the changes of the flavonols of onions (authors transl)].

The epidermis of onion scales exclusively contains glucosides of quercetin as flavonols, while the dry outer skins contain quercetin in the free state mainly. First spiraeoside (quercetin-4'-glucoside) is built; the formation of diglucosides follows during storage and increases continously. The mesophyll appears to be free of flavonols. The flavonol concentration decreases from the outer to the inner scales, with higher levels in the outer than in the inner epidermis. In the green leaves the flavonol synthesis is light-dependent and also kaempferol glucosides are formed beside quercetin glucosides, but not spiraeoside and the known diglucosides of the scales. During drying on the field an accumulation of flavonols takes place in the drying leaves. This production is associated with the formation of free quercetin and spiraeoside, but not of free kaempferol.

Flavonoids↗
PubMed2000

HPLC separation of flavonols, flavones and oxidized flavonols with UV-, DAD-, electrochemical and ESI-ion trap MS detection.

The cation-induced or electrochemical oxidation of flavonols has been reported to yield 2-(hydroxybenzoyl)-2-hydroxy-3(2H)-benzofuranone. Two new gradient reversed phase HPLC methods are presented which allow the determination of those oxidized flavonols simultaneously with flavonols and flavones. UV and electrochemical detection are used because of their high sensitivity. Qualitative detection together with quantification of all compounds is achieved with photodiode-array detection. An electrospray ionization ion trap mass spectrometric method is presented for unique identification of the benzofuranones after HPLC separation.

Chromatography, High Pressure Liquid↗
PubMed1985

Enzymatic synthesis of polymethylated flavonols in Chrysosplenium americanum. I. Partial purification and some properties of S-adenosyl-L-methionine:flavonol 3-, 6-, 7-, and 4'-O-methyltransferases.

Four novel flavonol O-methyltransferases (OMTs) were partially purified from Chrysosplenium americanum by precipitation with ammonium sulfate, successive chromatography on Sephacryl S-200 and hydroxylapatite, and chromatofocusing on Polybuffer ion exchanger. They exhibited strict position specificity for positions 3 of quercetin, 7 of 3-methylquercetin, 4' of 3,7-dimethylquercetin, and 6 of 3,7,3'-trimethylquercetagetin. None of these enzymes reacted with phenylpropanoids, flavones, dihydroflavonols, or any of their glucosides. Except for the 7-OMT whose activity was lost during chromatofocusing, the other enzymes had apparent pI values of 4.8, 5.4, and 5.7 for the 3-, 4'-, and 6-OMTs, respectively. They had similar molecular weights (Mr 57,000) and their pH optima varied between 7.8 and 9.0, with a shift in optimal activity from lower to higher pH with increasing level of substrate methylation. Unlike the 3 and 4' enzymes, the 6-OMT showed an absolute requirement for Mg2+ whose activation was saturable and was inhibited by EDTA. The in vitro stepwise O-methylation of quercetin----3-methylquercetin----3,7-dimethylquercetin----3 ,7, 4'-trimethylquercetin tends to suggest a coordinated sequence of methyl transfers in this tissue.

Cations, Divalent↗
PubMed1975

[Flavonols and flavones of vegetables. V. Flavonols and flavones of root vegetables (author's transl)].

Root vegetables contain flavon(ol) glycosides in tracers up to small amounts, while the level of their leaves are in part considerable (to more than 1 g/kg, calculated as aglycon). Radish, rutabagas, scorzoneras, and beets contain less than 1 mg/kg kaempferol and/or quercetin; carrots less than 1 mg/kg apigenin and luteolin; celery roots ca. 75 mg apigenin/kg and 14 mg luteolin/kg; horseradish about 20 mg kaempferol/kg and small radish 1-10 mg kaempferol/kg, whereby all these flavones and flavonols occur as glycosides in the vegetables. In leaves of small radish, variety "Eiszapfen", we found besides isoquercitrin (quercetin-3-glucoside) a quercetin-3-0-diglycoside and a kaempferol-0-diglycoside, both with the sugars rhamnose and arabinose, by tlc.

Arabinose↗
PubMed1985

Enzymatic synthesis of polymethylated flavonols in Chrysosplenium americanum. II. Substrate interaction and product inhibition studies of flavonol 3-, 6-, and 4'-O-methyltransferases.

The steady-state kinetic behavior of three position-specific O-methyltransferases (3-, 4'-, and 6-OMTs) was compared with reference to substrate inhibition patterns in Chrysosplenium americanum. The 6-OMT was severely inhibited by the flavonoid substrate at concentrations close to Km, whereas the other two enzymes were less affected by their respective flavonoid substrates. Substrate interaction kinetics for the 6-OMT gave converging lines consistent with a sequential binding mechanism, whereas the data generated for the 3- and 4'-OMTs could be fitted to the equation for a ping-pong mechanism or to that of a sequential binding mechanism where Kia was much smaller Ka. More information on the mechanism of reaction was obtained from product inhibition studies. The three enzymes exhibited competitive inhibition patterns between S-adenosyl-L-methionine (SAM) and S-adenosyl-L-homocysteine (SAH), whereas other patterns were either noncompetitive or uncompetitive. The steady-state kinetic properties of the 3-, 4'-, and 6-OMTs were consistent with a sequential ordered reaction mechanism, in which SAM and SAH were leading reaction partners and included an abortive EQB complex. Product inhibition constants were sufficiently low to suggest that these may be important in regulating the pathway of polymethylated flavonoid synthesis. It was suggested that due to their greater sensitivity to inhibition by SAH, the OMTs involved in earlier steps of the methylation sequence may regulate the rate of synthesis of final products in Chrysosplenium.

Flavonoids↗
PubMed1976

[Flavonols and flavones of vegetables. VII. Flavonols of leek, chive and garlic (author's transl)].

Green leaves of leek and chive mainly contain kaempferol glycosides, with mono- and di-glycosides dominating in leek and di- and tri-glycosides in chive. In leek glucose is dominant as sugar component compared to xylose; in chive we found glucose and galactose. Kaempferol-3-beta-D-glucoside and kaempferol-3-xylosyl-beta-D-glucoside were isolated from leek and the 3-beta-D-glucosides of kaempferol, quercetin and isorhamnetin as by-glycosides from chive. In leek traces of quercetin-3-glucoside were identified by tlc, but no spiraeoside (quercetin-4'-glucoside) could be detected in the two species. The bulbs of garlic and leek contain only few milligram of glycosides of kaempferol and quercetin per kg fresh weight.

Flavonoids↗
PubMed1998

[Flavonols, flavone and anthocyanins as natural antioxidants of food and their possible role in the prevention of chronic diseases].

Flavonoids are non-nutritive compounds of plants that have been intensively investigated during the past years due to their possible protective effects against chronic diseases. In vitro studies were able to demonstrate for flavonols, flavones, and most recently also for anthocyanins a considerable antioxidative activity, mainly based on scavenging of oxygen radicals. Flavonols and anthocyanins are commonly found in European fruits and vegetables. In addition, black tea and red wine may have a high content of these compounds. Those food items are the main sources of flavonol consumption each contributing to a different degree to the overall intake. The absorption of aglycones has been established before. However, only recently could the absorption of flavonolglycosides be demonstrated. The mean intake of flavonols of the German population was calculated using data from the National German Food Consumption Survey. According to this analysis, the daily per capita intake was about 11.5 mg flavonols, mainly derived from fruits and vegetables, but also from black tea and red wine. Epidemiological studies have been directed to investigate the association between flavonol consumption and diseae risk. An inverse association between flavonol intake and mortality from myocardial infarction was observed. According to one of three studies, the flavonoid intake can be inversely correlated with cancer risk. This review summarizes the current knowledge on the occurrence, intake, bioavailability, and antioxidative properties of flavonols, flavones, and anthocyanins as well as the associations between flavonol intake and disease risks. Possible health related effects especially of flavonols are critically reflected, and the necessity of further research in outlined.

Anthocyanins↗
PubMed1997

Regulation of flavonol biosynthesis during anther and pistil development, and during pollen tube growth in Solanum tuberosum.

The regulation of flavonol biosynthesis was studied in anthers and pistils of Solanum tuberosum. Flavonols are essential for functional pollen tube growth in a number of species. Flavonol accumulation in whole anthers started at the unicellular stage of pollen development and continued until pollen maturity. A cDNA clone encoding flavonol synthase (FLS) was isolated. Fls gene expression was detected in pistils, anthers, petals and ovaries, the organs in which flavonols are accumulating. Fls transcripts were present in unicellular and bicellular pollen, but not in mature pollen. The expression patterns of three genes encoding enzymes in the flavonoid biosynthetic pathway, chalcone synthase (chs), flavanone-3-hydroxylase and fls were analysed in developing anthers and pistils. Only chs transcripts accumulated concomitantly with the flavonols in anthers. In pistils of potato, pollen tube growth induced an increase in fls gene expression that, unlike the situation in pollinated pistils of petunia, did not result in an increased flavonol content. Flavonol biosynthesis in anthers is probably initiated by the expression of the chs gene, and flavonol accumulation in pistils upon pollen tube growth is not an universal phenomenon.

Acyltransferases↗
PubMed2001

Overexpression of petunia chalcone isomerase in tomato results in fruit containing increased levels of flavonols.

Tomatoes are an excellent source of the carotenoid lycopene, a compound that is thought to be protective against prostate cancer. They also contain small amounts of flavonoids in their peel ( approximately 5-10 mg/kg fresh weight), mainly naringenin chalcone and the flavonol rutin, a quercetin glycoside. Flavonols are very potent antioxidants, and an increasing body of epidemiological data suggests that high flavonoid intake is correlated with a decreased risk for cardiovascular disease. We have upregulated flavonol biosynthesis in the tomato in order to generate fruit with increased antioxidant capacity and a wider range of potential health benefit properties. This involved transformation of tomato with the Petunia chi-a gene encoding chalcone isomerase. Resulting transgenic tomato lines produced an increase of up to 78 fold in fruit peel flavonols, mainly due to an accumulation of rutin. No gross phenotypical differences were observed between high-flavonol transgenic and control lines. The phenotype segregated with the transgene and demonstrated a stable inheritance pattern over four subsequent generations tested thus far. Whole-fruit flavonol levels in the best of these lines are similar to those found in onions, a crop with naturally high levels of flavonol compounds. Processing of high-flavonol tomatoes demonstrated that 65% of flavonols present in the fresh fruit were retained in the processed paste, supporting their potential as raw materials for tomato-based functional food products.

Chalcone↗
PubMed1999

Purification, cloning, and heterologous expression of a catalytically efficient flavonol 3-O-galactosyltransferase expressed in the male gametophyte of Petunia hybrida.

Flavonols are plant-specific molecules that are required for pollen germination in maize and petunia. They exist in planta as both the aglycone and glycosyl conjugates. We identified a flavonol 3-O-galactosyltransferase (F3GalTase) that is expressed exclusively in the male gametophyte and controls the formation of a pollen-specific class of glycosylated flavonols. Thus an essential step to understanding flavonol-induced germination is the characterization of F3GalTase. Amino acid sequences of three peptide fragments of F3GalTase purified from petunia pollen were used to isolate a full-length cDNA clone. RNA gel blot analysis and enzyme assays confirmed that F3GalTase expression is restricted to pollen. Heterologous expression of the F3GalTase cDNA in Escherichia coli yielded active recombinant enzyme (rF3GalTase) which had the identical substrate specificity as the native enzyme. Unlike the relatively nonspecific substrate usage of flavonoid glycosyltransferases from sporophytic tissues, F3GalTase uses only UDP-galactose and flavonols to catalyze the formation of flavonol 3-O-galactosides. Kinetic analysis showed that the k(cat)/K(m) values of rF3GalTase, using kaempferol and quercetin as substrates, approaches that of a catalytically perfect enzyme. rF3GalTase catalyzes the reverse reaction, generation of flavonols from UDP and flavonol 3-O-galactosides, almost as efficiently as the forward reaction. The biochemical characteristics of F3GalTase are discussed in the context of a role in flavonol-induced pollen germination.

Amino Acid Sequence↗
PubMed1998

The role of glycosylation in flavonol-induced pollen germination.

Flavonols are small (C15) plant-specific molecules that are required for petunia and maize pollen to germinate. They exist in two chemical forms: the aglycone or glycosyl conjugates. Flavonol-deficient pollen is biochemically complemented by flavonol aglycones but not by the glycosylated forms that accumulate in wild type (WT) pollen. Coincident with the biochemical induction of germination, the added flavonol aglycone is rapidly converted to a galactoside and then to a glucosyl galactoside (diglycoside) that is identical to the compound present in WT pollen. A flavonol 3-O-galactosyltransferase (F3GalTase) activity has been identified that controls the formation of glycosylated flavonols in pollen. Importantly, this enzyme also catalyzes the reverse reaction, i.e. the production of the flavonol aglycone from the galactoside and UDP (Fig. 1). F3GalTase/RevGalTase therefore has the potential to control the level of the bioactive flavonol species and as a result, pollen germination.

Flavonoids↗
PubMed2000

Prediction of dietary flavonol consumption from fasting plasma concentration or urinary excretion.

OBJECTIVES: to predict flavonols content of the habitual diets of free-living subjects from urine and plasma concentrations of flavonols. DESIGN: Ten type 2 diabetic patients (five male, five female), mean age 60 (s.e.m. 7) y and BMI 30.2 (s.e.m. 3.5) kg/m2 were treated in a random crossover design for a 2 week period on either a low flavonoid diet or on the same diet supplemented at one of two high flavonols levels (total 77.3 or 110.4 mg/day) provided by supplements of 1500 ml tea daily and 400 g fried white onion in olive oil with and without tomato ketchup and herbs. SETTING: Glasgow Royal Infirmary, University of Glasgow, Scotland. MAIN OUTCOME MEASURES: Fasting plasma concentration, urine concentration and 24 h excretion of quercetin, isorhamnetin, kaempferol and myricetin. RESULTS: Plasma flavonol concentration (r=0.750, P=0.001), 24 h urine concentration (r=0.847, P=0.001) and 24 h urine excretion (r=0.728, P=<0.001) were all highly significantly related to dietary intake and gave similar estimates of intakes. Fasting plasma flavonols concentrations on habitual diets ranged from 0 to 43.7 ng/ml mean. Regression equations were constricted: total flavonols intake r=0.74, P<0.001 and quercetin intake r=0.744, P<0. 001. From these equations, flavonol intakes from habitual diets were estimated at 17-50, mean 35 mg/day. Of this, 91% was from quercetin. CONCLUSIONS: Dietary flavonols are absorbed and appear in plasma and urine as potential biomarkers in concentrations related quantitatively to intake. Estimation of dietary intake from plasma or urine concentrations appears possible. SPONSORSHIP: Rank Prize Funds and Rank Foundation of the Department of Human Nutrition; Ministry of Health and Medical Education, IR Iran. European Journal of Clinical Nutrition (2000) 54, 143-149

Aged↗
PubMed2002

Functional expression and mutational analysis of flavonol synthase from Citrus unshiu.

Flavonols are produced by the desaturation of flavanols catalyzed by flavonol synthase. The enzyme belongs to the class of intermolecular dioxygenases which depend on molecular oxygen and FeII/2-oxoglutarate for activity, and have been in focus of structural studies recently. Flavonol synthase cDNAs were cloned from six plant species, but none of the enzymes had been studied in detail. Therefore, a cDNA from Citrus unshiu (Satsuma mandarin) designated as flavonol synthase was expressed in Escherichia coli, and the purified recombinant enzyme was subjected to kinetic and mutational chacterizations. The integrity of the recombinant synthase was revealed by a molecular ion from MALDI-TOF mass spectrometry at m/z 37888 +/- 40 (as compared to 37899 Da calculated for the translated polypeptide), and by partial N-terminal sequencing. Maximal flavonol synthase activity was observed in the range of pH 5-6 with dihydroquercetin as substrate and a temperature optimum at about 37 degrees C. Km values of 272, 11 and 36 micro m were determined for dihydroquercetin, FeII and 2-oxoglutarate, respectively, with a sixfold higher affinity to dihydrokaempferol (Km 45 micro m). Flavonol synthase polypeptides share an overall sequence similarity of 85% (47% identity), whereas only 30-60% similarity were apparent with other dioxygenases. Like the other dioxygenases of this class, Citrus flavonol synthase cDNA encodes eight strictly conserved amino-acid residues which include two histidines (His221, His277) and one acidic amino acid (Asp223) residue for FeII-coordination, an arginine (Arg287) proposed to bind 2-oxoglutarate, and four amino acids (Gly68, His75, Gly261, Pro207) with no obvious functionality. Replacements of Gly68 and Gly261 by alanine reduced the catalytic activity by 95%, while the exchange of these Gly residues for proline completely abolished the enzyme activity. Alternatively, the substitution of Pro207 by glycine hardly affected the activity. The data suggest that Gly68 and Gly261, at least, are required for proper folding of the flavonol synthase polypeptide.

Amino Acid Sequence↗
PubMed1999

Dietary flavonols protect diabetic human lymphocytes against oxidative damage to DNA.

Diabetic patients have reduced antioxidant defenses and suffer from an increased risk of free radical-mediated diseases such as coronary heart disease. Epidemiological evidence has suggested that antioxidant dietary flavonoids may protect against heart disease, but a biological effect has yet to be demonstrated directly in humans. In this study, 10 stable type 2 diabetic patients were treated for 2 weeks on a low-flavonol diet and for 2 weeks on the same diet supplemented with 76-110 mg of flavonols (mostly quercetin) provided by 400 g of onions (and tomato sauce) and six cups of tea daily. Freshly collected lymphocytes were subjected to standard oxidative challenge with hydrogen peroxide, and DNA damage was measured by single-cell gel electrophoresis. Fasting plasma flavonol concentrations (measured by high-performance liquid chromatography) were 5.6 +/- 2.9 ng/ml on the low-flavonol diet and increased 12-fold to 72.1 +/- 15.8 ng/ml on the high-flavonol diet (P < 0.001). Oxidative damage to lymphocyte DNA was 220 +/- 12 on an arbitrary scale of 0-400 U on the low-flavonol diet and 192 +/- 14 on the high-flavonol diet (P = 0.037). This decrease was not accounted for by any change in the measurements of diabetic control (fasting plasma glucose or fructosamine) or by any change in the plasma levels of known antioxidants, including vitamin C, carotenoids, alpha-tocopherol, urate, albumin, and bilirubin. In conclusion, we have shown a biological effect of potential medical importance that appears to be associated with the absorption of dietary flavonols.

Aged↗