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PubMed2003

Identification of flavone aglycones and glycosides in soybean pods by liquid chromatography-tandem mass spectrometry.

High-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry was used to identify flavone aglycones and glycosides in soybean pods. Tandem mass spectrometry (MS-MS and MS-MS-MS) and photodiode array detection were also utilized in flavone characterization. A total of seven flavone aglycones and glycosides were identified. Among them three flavone aglycones--apigenin, 7,4'-dihydroxyflavone, and luteolin--and two flavone glycosides--apigenin-7-O-beta-D-glucoside, and luteolin-7-O-beta-D-glucoside--were unambiguously identified based on their abundant (M+H)+ ions, UV spectra, retention time, and tandem mass spectrometric analysis compared with authentic standards. The tentative identification of two flavone glycosides as 7,4'-dihydroxyflavone-7-O-beta-D-glucoside and apigenin-7-O-beta-D-glucoside-6"-O-malonate was based on UV spectra, (M+H)+ ions, and tandem mass spectrometry. This is the first report identifying flavone aglycones and glycosides in soybean pods.

Apigenin↗
PubMed2004

[Effect of osteopractic total flavone on bone mineral density and bone histomorphometry in ovariectomized rats].

OBJECTIVE: To observe the effect of osteopractic total flavone on bone histomorphometry of osteoporosis model in rath with ovariectomized female rat models. METHOD: The ovariectomy-induced model, bone sliceswith calcium, pigmentation, and Leica Qwin image analysis system were adopted on bone histomorphometry. RESULT: As compared with the model group, the effect of small-dose group and middle-dose group of osteopractic total flavone on TBV% of shankbone increased significantly; the effect of small-dose group and middle-dose group of osteopractic total flavone on TRS% of shankbone reduced significantly and TFS%, AFS%, MAR, BFR of shankbone reduced obviously; the effect of middle-dose group of osteopractic total flavone on OSW and mAR reduced obviously, the effect of small-dose group of osteopractic total flavone on them had the tendency of reducing, but there was no statistical significance. CONCLUSION: The ovariectomized rats having been fed with osteopractic total flavone for 6 months, TBV% increased significantly while TRS%, AFS%, MAR, BFR, OSW, and mAR reduced obviously. It indicates that the therapeutical effect of osteopractic total flavone on ovariectomy-induced osteoporosis model is significant. And the ovariectomy-induced osteoporosis model is a high transformative type of osteoporosis model in which bone absorption is higher than bone formation.

Animals↗
PubMed2006

Tissue distribution in mice and metabolism in murine and human liver of apigenin and tricin, flavones with putative cancer chemopreventive properties.

PURPOSE: The flavones apigenin and tricin, which occur in leafy vegetables and rice bran, respectively, possess cancer chemopreventive properties in preclinical rodent models. Their pharmacology is only poorly understood. We compared their tissue levels in mice in vivo and their metabolism in liver fractions in vitro. METHODS: Mice received apigenin or tricin (0.2%) with their diet for 5-7 days, and flavone levels were compared in the plasma, liver and gastrointestinal mucosa using HPLC-UV. Flavone metabolism was investigated in murine and human liver microsomes or cytosol in vitro co-incubated with uridine 5'-diphosphoglucuronic acid or 3'-phosphoadenosine-5' phosphosulfate. Flavone metabolites were characterized by on-line HPLC-mass spectrometry. RESULTS: After dietary administration of flavones for 7 days, levels of tricin in plasma, liver and mucosa exceeded those of apigenin by 350, 33 and 100%, respectively. Apigenin was more rapidly glucuronidated than tricin in liver microsomes, whilst tricin underwent swifter sulfonation than apigenin in liver cytosol. For either flavone the rate of glucuronidation was much faster than that of sulfonation. Flavone monoglucuronides and monosulfates were identified as metabolites in microsomal and cytosolic incubations, respectively. CONCLUSIONS: When consumed with the diet in mice tricin seems to be more available than apigenin in blood and tissues. Differences in their glucuronidation may account for their differential availability. Thus tricin may have a pharmacokinetic advantage over apigenin. This type of information may help decide which flavonoids to select for clinical development.

Animals↗
PubMed1994

Inhibition of HIV-1 integrase by flavones, caffeic acid phenethyl ester (CAPE) and related compounds.

The inhibition of HIV-1 integrase by flavones and related compounds was investigated biochemically and by means of structure-activity relationships. Purified enzyme and synthetic oligonucleotides were used to assay for three reactions catalysed by integrase: (1) processing of 3' termini by cleavage of the terminal dinucleotide; (2) strand transfer, which models the integration step; and (3) "disintegration," which models the reversal of the strand transfer reaction. Inhibitions of all three reactions by flavones generally occurred in parallel, but caffeic acid phenethyl ester (CAPE) appeared to inhibit reaction 2 selectively. CAPE, however, inhibited reactions 1 and 3 effectively when preincubated with the enzyme, suggesting that this compound differs from the flavones primarily in requiring more time to block the enzyme. The core integrase fragment consisting of amino acids 50-212 retained the ability to catalyse reaction 3, and flavones and CAPE retained the ability to inhibit. Hence, the putative zinc-finger region that is deleted in this fragment is probably not the target of inhibition. Inhibition by flavones usually required the presence of at least one ortho pair of phenolic hydroxyl groups and at least one or two additional hydroxyl groups. Potency was enhanced by the presence of additional hydroxyl groups, especially when present in ortho pairs or in adjacent groups of three. Inhibitory activity was reduced or eliminated by methoxy or glycosidic substitutions or by saturation of the 2,3 double bond. These structure-activity findings for flavones were generally concordant with those previously reported for reverse transcriptase and topoisomerase II. These findings are discussed in the context of a review of the effects of flavones on various enzymes, the possible mechanisms of inhibition, and the potential for building upon a general pharmacophore to generate target specificity.

Base Sequence↗
PubMed1996

Substituted flavones as aryl hydrocarbon (Ah) receptor agonists and antagonists.

The structure-dependent aryl hydrocarbon (Ah) receptor agonist and antagonist activities of the following substituted flavones were investigated: flavone, 4'-methoxy-, 4'-amino-, 4'-chloro-, 4'-bromo-, 4'-nitro-, 4'-chloro-3'-nitro-, 3'-amino-4'-hydroxy-, 3',4'-dichloro-, and 4'-iodoflavone. The halogenated flavones exhibited competitive Ah receptor binding affinities (IC50 = 0.79 to 2.28 nM) that were comparable to that observed for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (1.78 nM). The compounds also induced transformation of the rat cytosolic Ah receptor and induced CYP1A1 gene expression in MCF-7 human breast cancer cells. However, despite the high Ah receptor binding affinities for these responses, the halogenated flavones were > 1000 times less active than TCDD for the other responses. Moreover, for other substituted flavones, there was no correlation between Ah receptor binding affinities and their activities as Ah receptor agonists. For example, 4'-aminoflavone induced CYP1A1 mRNA levels in MCF-7 cells but exhibited relatively low Ah receptor binding affinity (IC50 = 362 nM) and did not induce transformation of the rat cytosolic Ah receptor. All of the substituted flavones inhibited TCDD-induced transformation of the Ah receptor, and 4'-iodoflavone, an Ah receptor agonist at high concentrations (1-50 microM), inhibited the transformation at concentrations as low as 0.05 and 0.5 microM. Subsequent interaction studies with TCDD and 4'-iodoflavone confirmed that the latter compound inhibits induction of CYP1A1 gene expression by TCDD in MCF-7 cells. The results obtained for the substituted flavones suggest that within this structural class of compounds, various substituent groups can affect markedly the activity of each individual congener as an Ah receptor agonist or antagonist. These substituent-dependent differences in activity may be related to ligand-induced conformational changes in the Ah receptor complex and/or support the proposed existence of more than one form of the Ah receptor.

Animals↗
PubMed1995

Synthesis and biological evaluation of substituted flavones as gastroprotective agents.

Flavone (1) was found to protect against ethanol-induced gastric damage in rats; however, it is known that certain compounds in the flavone class, including flavone itself, are inducers of hepatic drug metabolizing enzymes. With the hope of identifying gastroprotective flavones that have minimal effects on drug metabolizing enzymes, we have synthesized and evaluated selected flavone analogs. Gastroprotective potency in the ethanol model was retained by methoxy substitution in the 5-position (4) and by methoxy (12) or methyl (14) substitution in the 7-position. A number of substituted analogs of the potent molecule 5-methoxyflavone (4) were also synthesized, and in many cases, these substitutions provided gastroprotective molecules. In order to assess liver enzyme induction potential, two of the gastroprotective flavones, 7-methoxyflavone (12) and 5-methoxy-4'-fluoroflavone (26), were examined for their effect on liver microsomal cytochrome P450 and 7-ethoxyresorufin O-dealkylase (CYP1A) activity. These two compounds caused minimal changes in the cytochrome P450 concentration and were considerably less potent than beta-naphthoflavone as inducers of CYP1A enzyme activity. Furthermore, following oral administration to rats, 5-methoxy-4'-fluoroflavone (26) was found to protect against indomethacin-induced gastric damage. These results indicate that, through appropriate substitution, flavones can be obtained that are gastroprotective but have minimal effects on drug-metabolizing enzymes.

Animals↗
PubMed2003

Cis-2', 3'-dihydrodiol production on flavone B-ring by biphenyl dioxygenase from Pseudomonas pseudoalcaligenes KF707 expressed in Escherichia coli.

Escherichia coli JM109 strains expressing either toluene dioxygenase from Pseudomonas putida F1 or biphenyl dioxygenase from Pseudomonas pseudoalcaligenes KF707 were examined for their ability to catalyze flavones. Biphenyl dioxygenase produced metabolites from flavone and 5,7-dihydroxyflavone which were not found in the control experiments. The absorption maxima of UV-visible spectra for the metabolites from flavone and 5,7-dihydroxyflavone were found at 337 and 348 nm respectively by using a photodiode array detector in the HPLC. Liquid chromatography/mass spectroscopy (LC/MS) showed molecular weights 256 and 288 for the metabolites, respectively. The metabolite of flavone, which was isolated and purified from the bacterial culture, was further subjected to analysis by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. Based on the LC/MS and NMR results, biphenyl dioxygenase inserted oxygen at C2' and C3' on the B-ring of flavone, resulting in the formation of flavone cis-2', 3'-dihydrodiol (2-[3,4-dihydroxy-1.5-cyclohexadienyl]-4H-chromen-4-one). Since this product is not found in Chemical Abstracts, this compound is considered a novel one. In addition, biotransformation of flavones by biphenyl dioxygenase suggested a potential role of bacterial dioxygenase to synthesize novel compounds from plant secondary metabolites.

Bacterial Proteins↗
PubMed2004

Pleiotropic molecular effects of the pro-apoptotic dietary constituent flavone in human colon cancer cells identified by protein and mRNA expression profiling.

The flavonoid flavone contained in a variety of fruits and vegetables was identified as a very potent apoptosis inducer in human colonic cancer cells. In search of the molecular targets of flavone action in HT-29 cells we analyzed changes in mRNA and protein expression levels by proteomics and oligonucleotide array technologies. Proteome analysis identified several heat-shock proteins, annexins, and cytoskeletal caspase substrates as regulated by flavone and these protein classes are known to play a role in apoptosis induction and execution. Protein kinase C-beta, which serves as an ultimate marker for colon cancer development was no longer detectable in HT-29 cells exposed to flavone. Besides proteins involved in gene regulation or detoxification pathways, proteins involved in intermediary metabolism were altered by flavone exposure and this was associated with changes in the flux of energetic substrates. Oligonucleotide arrays, using chips with around 10 000 oligonucleotides spotted, revealed numerous changes in transcript levels of genes related to signaling, transcription, cancer development but also to metabolism. In conclusion, flavone has a surprisingly broad spectrum of effects on mRNA and protein expression in a human colonic cancer cell line with clusters of targets related to its apoptosis-inducing activity and to cellular metabolism.

Adenosine Triphosphate↗
PubMed2006

Influence of sea buckthorn (Hippophae rhamnoides L.) flavone on dermal wound healing in rats.

The present investigation was undertaken to determine the efficacy of topical administration of flavone of sea buckthorn (Hippophae rhamnoides L.) on cutaneous wound healing in rats. Four full-thickness excision wounds were created on the back of rat and 1.0% w/v flavone prepared in propylene glycol was applied topically. Control animals received the vehicle alone in an identical manner. The healing of the wound was assessed by the rate of wound contraction, period of epithelialization, hydroxyproline, hexosamine, antioxidants estimation and histopathology of the granulation tissue. The sea buckthorn flavone promoted the wound healing activity as indicated by improved rate of wound contraction, decreased time taken for epithelialization (16.3 days versus 24.8 days in controls) and significant increase in hydroxyproline (26.0%) and hexosamine (30.0%) content. These findings were also confirmed by histopathological examinations. In addition, it was observed that sea buckthorn flavone possesses potent antioxidant properties as evidenced by significant increase in reduced glutathione (55.0%), vitamin C (70.0%) and catalase (20.0%) activities in wound granulation tissue. The flavone treatment also resulted in significant decrease in lipid peroxide levels (39.0%). The results suggest that the sea buckthorn flavone promotes wound healing activity.

Animals↗
PubMed2005

The comparison of the relaxant effects of two methoxylated flavones in rat aortic rings.

The vascular effect of salvigenin (6-hydroxyapigenin 6,7,4' -trimethyl ether), a natural flavone, was investigated in comparison with another flavone, 6-hydroxyluteolin 6,7,3',4' -tetramethyl ether in rat aortic rings. Cumulative addition of their increasing concentrations (10(-9)-10(-4)M) produced graded relaxations on rings precontracted with noradrenaline (10(-6) M) and KCl (40 mM). The maximal relaxations induced by flavones were similar, however, based on their pEC50 values salvigenin displayed a higher potency than 6-hydroxyluteolin 6,7,3',4'-tetramethyl ether. Endothelium removal markedly reduced the relaxations to salvigenin while the responses to 6-hydroxyluteolin 6,7,3',4'-tetramethyl ether were partially affected. In addition, a significant decrease was observed in maximal responsiveness and sensitivity to flavones in the presence of L-NOARG, a NO synthase inhibitor. The cyclooxygenase inhibitor indomethacin significantly inhibited the relaxations to salvigenin, but not altered the responses to 6-hydroxyluteolin 6,7,3',4'-tetramethyl ether. Our results provide evidence that salvigenin is an effective flavone in causing vasorelaxation which appears to be mediated by endothelium derived NO and prostacyclin. Whereas, the other flavone, 6-hydroxyluteolin 6,7,3',4'-tetramethyl ether induced relaxant responses are partially endothelium, presumably NO mediated.

Acetylcholine↗
PubMedDate not supplied

A study on the antipicornavirus activity of flavonoid compounds (flavones) by using quantum chemical and chemometric methods.

The AM1 semiempirical method is employed to calculate a set of molecular properties (variables) of 45 flavone compounds with antipicornavirus activity, and 9 new flavone molecules are used for an activity prediction study. Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), Stepwise Discriminant Analysis (SDA), and K-Nearest Neighbor (KNN) are employed in order to reduce dimensionality and investigate which subset of variables should be more effective for classifying the flavone compounds according to their degree of antipicornavirus activity. The PCA, HCA, SDA, and KNN methods showed that the variables MR (molar refractivity), B(9) (bond order between C(9) and C(10) atoms), and B(25) (bond order between C(11) and R(7) atoms) are important properties for the separation between active and inactive flavone compounds, and this fact reveals that electronic and steric effects are relevant when one is trying to understand the interaction between flavone compounds with antipicornavirus activity and the biological receptor. In the activity prediction study, using the PCA, HCA, SDA, and KNN methodologies, three of the 9 new flavone compounds studied were classified as potentially active against picornaviruses.

Antiviral Agents↗
PubMed1985

Flavone modulators of rat hepatic aryl hydrocarbon hydroxylase.

The cytochrome P-450-dependent aryl hydrocarbon hydroxylase (AHH) metabolizes a wide variety of endogenous and exogenous compounds to nontoxic metabolites and/or toxic products. We have utilized a series of 18 flavone modulators of AHH to distinguish and probe for different cytochrome P-450 isozymes in liver microsomes from control and 3-methylcholanthrene (MC)-injected rats. some flavones (maackiain acetate, flavanone, mollisacacidin, embinin, sciadopitysin) activated, while most of the tested compounds inhibited the MC-induced type of AHH. Although all flavones either inhibited or had little effect on the constitutive AHH in microsomes from control rats, the degree of inhibition varied greatly: some flavones (chrysin, chrysoeriol, baicalein, maackiain acetate, isoliquiritigenin, sciadopitysin) inhibited over 75% of the AHH. The various flavones we screened may prove useful in defining the cytochrome P-450 content of tissues and for probing the active sites of individual isozymes. The modulatory effects of the naturally occurring flavones assume additional importance in that they may be factors in animal and human responsiveness to cytochrome P-450 substrates.

Animals↗
PubMed2005

[Effects of flavones extracted from Portulaca oleracea on ability of hypoxia tolerance in mice and its mechanism].

OBJECTIVE: To identify anti-hypoxia ingredients extracted from Portulaca oleracea and to find out the possible mechanism of its anti-hypoxia actions. METHODS: Seventy mice were randomly divided into seven groups which were untreated (normal saline), ginsenosides-treated, polysaccharide-treated, acidic components-treated, basic components-treated, alkaloids-treated and flavones-treated groups, and the ingredients of polysaccharide, acidic components, basic components, alkaloids and flavones were extracted from Portulaca olerace. The mice in each group were fed with corresponding ingredients for one week respectively. Then the survival time of mice in hypoxic conditions was observed. Another 90 mice were divided into 3 groups: untreated (normal saline), ginsenosides-treated and flavones-treated groups. The mice in each of these 3 groups were divided into 3 subgroups according to 12-, 24- and 36-hour exposure to hypoxia (10% oxygen and 90% nitrogen), respectively. After exposure to hypoxia, the red blood cell count (RBC), hemoglobin (Hb) concentration and hematocrit (HCT) in mice were determined. The plasma erythropoietin (EPO) levels of mice were detected by enzyme-linked immunosorbent assay (ELISA) and the relative values of EPO mRNA in renal tissue and pallium of mice were determined by reverse transcriptase-polymerase chain reaction (RT-PCR). RESULTS: The survival time of mice in hypoxic conditions in flavones-treated group was significantly longer than that in the untreated group. The RBC, Hb concentration, HCT, plasma EPO level and the relative values of EPO mRNA in renal tissue and pallium of mice were significantly higher in the flavones-treated group than those in the untreated group. CONCLUSION: The anti-hypoxia ingredients extracted from Portulaca oleracea are flavones and the anti-hypoxia effects may be obtained by improving the expression level of EPO and accelerating the generations of erythrocyte and Hb.

Animals↗
PubMed2003

[Effects of flavone from leaves of Diospyros kaki on adventitial fibroblast proliferation induced by advanced glycation end-products in vitro].

OBJECTIVE: To observe the effects of flavone from the leaves of Diospyros kaki on proliferation of adventitial fibroblasts induced by advanced glycation end-products (AGEs) in vitro. METHODS: NIH/3T3 cells cultured in vitro were treated with both AGEs and flavone from the leaves of Diospyros Kaki for observation in comparison with the cells that received treatments with either AGEs or flavone, or neither. The ratio of cell proliferation was determined by non-radioactive MTS/PES assay. RESULTS: The ratio of cell proliferation was 0.840+/-0.061 in the untreated control group, and was 1.330+/-0.055, 1.210+/-0.119, 1.029+/-0.076 and 0.792+/-0.060 in AGEs groups corresponding to AGE concentrations of 100, 50, 10 and 1 microg/ml respectively. AGEs significantly induced fibroblast proliferation in a dose-dependent manner when the concentration was above 10 microg/ml (P<0.05), as compared with the untreated control group. (P<0.05). The ratio of cell proliferation was 0.829+/-0.056 in cells treated with flavone at the concentration of 50 microg/ml, which alone failed to affect fibroblast proliferation (P>0.05). With AGEs stimulation, however, flavone from the leaves of Diospyros kaki significantly inhibited the proliferation of the fibroblasts (P<0.05). CONCLUSION: Flavone from the leaves of Diospyros kaki can significantly inhibit the proliferation of adventitial fibroblasts stimulated by AGEs in vitro.

Animals↗
PubMed2004

[Effects of flavone from leaves of Diospyros kaki on adventitial fibroblasts proliferation by advanced oxidation protein products in vitro].

OBJECTIVE: To observe the effects of flavone from leaves of Diospyros kaki on adventitial fibroblasts proliferation by advanced oxidation protein products (AOPP) in vitro. METHODS: NIH-3T3 cells were cultured in vitro and treated with AOPP and flavone from leaves of Diospyros kaki, respectively, and observed in comparison with the control group. The ratio of cell proliferation was determined by non-radioactive MTS/PES assay. RESULTS: The ratio of cell proliferation was 1.789 +/- 0.299 in the control group, and 2.064 +/- 0.141, 2.149 +/- 0.218, 2.108 +/- 0.165, 2.124 +/- 0.131 and 2.087 +/- 0.125 in AOPP groups corresponding to AOPP concentrations of 100, 50, 10, 1 and 0.1 microg/ml, respectively. It showed that AOPP significantly induced the fibroblasts proliferation when the concentration was above 100 ng/ml (P < 0.05). The ratio of cell proliferation was 1.714 +/- 0.179 in flavone from leaves of Diospyros kaki group corresponding to concentration of 50 microg/ml. It also showed that flavone from leaves of Diospyros kaki alone had no effect on fibroblasts proliferation (P > 0.05). With AOPP stimulation, flavone from leaves of Diospyros kaki significantly inhibited fibroblasts proliferation (P < 0.05). CONCLUSIONS: Flavone from leaves of Diospyros kaki can significantly inhibit the adventitial fibroblasts proliferation stimulated by AOPP in vitro.

Animals↗
PubMed1993

Inhibition of NAD(P)H:quinone acceptor oxidoreductase by flavones: a structure-activity study.

A structure-activity study was carried out to determine the important regions of baicalein and oroxylin A, two flavones isolated from the Chinese herb Scutellariae radix, in inhibiting NAD(P)H:quinone acceptor oxidoreductase (EC 1.6.99.2; DT-diaphorase). This quinone reductase is a vitamin K reductase. It is a target for and has been used as a model enzyme to investigate the mode of action of oral anticoagulants. The two flavones were found to inhibit this quinone reductase in nanomolar ranges. The 5-hydroxyl, 7-hydroxyl, 8-hydroxyl, and 2-phenyl groups of these flavones were found to be important for their inhibition of the enzyme. The inhibition profiles of the flavones on the NADH-menadione reductase activity, the NADH-potassium ferricyanide reductase activity, and the NADH-methyl red reductase activity of this enzyme were different. Therefore, even though the flavones were found to be competitive inhibitors with respect to NADH, they probably did not inhibit the enzyme by binding to the nicotinamide nucleotide binding site. Inhibition kinetic studies which indicated that these compounds bound to different sites than those for dicoumarol and phenindone were performed. These results indicate that these flavones are a new type of inhibitor of NAD(P)H:quinone acceptor oxidoreductase and potentially useful as anticoagulant drugs.

Azo Compounds↗
PubMed1998

Inhibition of human cytochrome P450 1A2 by flavones: a molecular modeling study.

Cytochrome P450 1A2 metabolizes a number of important drugs, procarcinogens, and endogenous compounds. Several flavones, a class of phytochemicals consumed in the human diet, have been shown to differentially inhibit human P450 1A2-mediated methoxyresorufin demethylase. A molecular model of this P450 was constructed in order to elucidate the molecular basis of the P450-flavone interaction. Flavone and its 3,5,7-trihydroxy and 3,5,7-trimethoxy derivatives were docked into the active site to assess their mode of binding. The site is hydrophobic and includes several residues that hydrogen bond with substituents on the flavone nucleus. The binding interactions of these flavones in the modeled active side are consistent with their relative inhibitory potentials, namely 3,5,7-trihydroxylflavone > flavone > 3,5,7-trimethoxylflavone, toward P450 1A2-mediated methoxyresorufin demethylation.

Amino Acid Sequence↗
PubMed1990

Purification and characterization of flavone synthase I, a 2-oxoglutarate-dependent desaturase.

Soluble flavone synthase I from illuminated parsley cells was purified to near homogeneity by a six-step procedure. A molecular mass of 48 +/- 2 kDa was determined by gel permeation chromatography and denaturing polyacrylamide gel electrophoresis. A single protein with an isoelectric point at pH 4.8 +/- 0.1 was detected on isoelectric focusing gels, which catalyzed the overall conversion of 2S-flavanones into the corresponding flavones in the presence of molecular oxygen, 2-oxoglutarate, ferrous ion, and ascorbate. Apparent Michaelis constants for 2S-naringenin, 2S-eriodictyol, and 2-oxoglutarate were determined as 5, 8, and 16 microM, respectively. (+)-Dihydrokaempferol and 2R-naringenin were not accepted as substrates. The enzyme was strongly inhibited by Cu2+ and Zn2+. Potent competitive inhibition with respect to 2-oxoglutarate was observed with 2,4-pyridinedicarboxylate (Ki = 1.8 microM). With crude extracts as well as with the purified enzyme neither the hypothetical intermediate 2-hydroxyflavanone nor a dehydratase activity capable of converting the chemically synthesized compound to flavone could be observed. Moreover, the introduction of the double bond into the substrate naringenin was not altered by addition of chemically synthesized 2-hydroxynaringenin into the reaction mixture. Therefore, 2-hydroxyflavanones are apparently not freely dissociable intermediates in the biosynthesis of flavones in parsley and are not capable of entering the active site of the enzyme to compete with the flavanone. It is postulated that flavone synthase I catalyzes double-bond formation by direct abstraction of vicinal hydrogen atoms at C-2 and C-3 of the substrate. Thus, flavone synthase I is a member of a novel subgroup within the 2-oxoglutarate-dependent dioxygenases that can be referred to as 2-oxoglutarate-dependent desaturases.

Carbon Radioisotopes↗