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At least 217 records · Page 12Linked to original sources
PubMed1994

Catechol-O-methyltransferase-catalyzed rapid O-methylation of mutagenic flavonoids. Metabolic inactivation as a possible reason for their lack of carcinogenicity in vivo.

Quercetin is highly mutagenic in vitro, yet is not carcinogenic when administered chronically at large doses to rodents for 12 months. We hypothesized that catechol-O-methyltransferase-catalyzed O-methylation of quercetin and other mutagenic catechol-containing flavonoids may provide an efficient inactivation in vivo and may therefore prevent tumor induction by these flavonoids. After one intraperitoneal administration of 50 mg/kg quercetin to hamsters, a urinary ether extract contained 2% quercetin and 97% 3'-O-methylquercetin. When the urine was treated first with beta-glucuronidase and sulfatase, 13% quercetin and 87% 3'-O-methylquercetin were recovered. Quercetin was rapidly O-methylated by either porcine liver or hamster kidney catechol-O-methyltransferase, with Km values of 6.1 and 6.9 microM and Vmax values of 14,870 and 200 pmol/mg of protein/min, respectively. S-Adenosyl-L-homocysteine exhibited a potent feedback inhibition of the catechol-O-methyltransferase-catalyzed O-methylation of quercetin by a competitive mechanism with respect to S-adenosyl-L-methionine and by a competitive plus noncompetitive mechanism with respect to the substrate. A comparison of the O-methylation rates and kinetic characteristics (Km, Vmax, and Vmax/Km) demonstrated that rates of O-methylation of quercetin and fisetin were up to three orders of magnitude higher than those of catechol estrogens and catecholamines. In conclusion, the rapid metabolic inactivation of mutagenic flavonoids catalyzed by catechol-O-methyltransferase may be a major reason for the lack of their carcinogenic activities in vivo.

Animals↗
PubMed2006

Directed evolution of a non-heme-iron-dependent extradiol catechol dioxygenase: identification of mutants with intradiol oxidative cleavage activity.

The non-heme-iron(II)-dependent extradiol catechol dioxygenases catalyse the oxidative cleavage of substituted catechols found on bacterial aromatic degradation pathways. The reaction mechanism of the extradiol dioxygenases is believed to proceed through the same proximal hydroperoxide intermediate as the iron(III)-dependent intradiol catechol dioxygenases. Directed evolution was carried out on members of the class III extradiol catechol dioxygenases, by using 1) error-prone polymerase chain reaction, 2) a primer-based cross-over method; the mutant dioxygenases were then screened for their ability to process a range of substituted catechols. Several mutant enzymes were found to show higher activity towards certain substituted catechols, including 4-chlorocatechol, and higher affinity for the iron(II) cofactor. Two mutants isolated from error-prone PCR of Escherichia coli MhpB (mutants R215W and K273R) were found to produce a mixture of extradiol and intradiol cleavage products, as detected by GC-MS and 1H NMR spectroscopy. The residue corresponding to K273 in protocatechuate 4,5-dioxygenase (LigAB), Val244, is located approximately 12 A from the iron(II) centre, but close to the putative dioxygen channel; R215 is found on a sequence loop not present in LigB.

Dioxygenases↗
PubMed2005

Electronic structure and spectra of catechol and alizarin in the gas phase and attached to titanium.

Ab initio electronic structure calculations elucidate the dramatic differences observed in the electronic spectra of the catechol and alizarin molecules upon binding to titanium. Catechol and alizarin are similar chromophores with analogous electronic spectra in the free state. Binding alizarin to titanium red-shifts the spectrum. The binding of catechol to titanium produces a new optically active transition, at the same time preserving the features of the free catechol spectrum. By examining the details of the calculations, we can rationalize the spectral differences in the catechol and alizarin systems by the positioning of the catechol and alizarin pi molecular orbitals relative to the conduction band of TiO(2).

Journal Article↗
PubMed1997

Catechol changes in the rat rostral ventrolateral medulla following changes in systemic CO2.

A catechol signal recorded with in vivo voltammetry within the rat rostral ventrolateral medulla (RVLM) was taken as an index of the activity of RVLM adrenergic neurons and related to the level of arterial PCO2, under halothane anesthesia. Reversible increases in catechol signal were observed during reversible increases in arterial partial CO2 pressure (PaCO2) from 20 to 60 mmHg after alteration of tidal volume (n = 5 intact rats, n = 5 after carotid sinus deafferentation). A reversible increase in inspiratory CO2 combined with constant tidal volume led to changes in PaCO2 from 40 mmHg to 50 or 60 or 70 mmHg for 60 min (n = 5 in each group) and to a reversible increase in catechol signal (r = 0.76). These changes were also observed after carotid sinus deafferentation (PaCO2 = 40 to 60 to 40 mmHg, n = 5). Lowering the PaCO2 from 40 to 20 mmHg led to a minor, nonsignificant reduction in catechol signal (n = 5). Changes in arterial pressure were minimal, although they reached statistical significance in some groups of experiments. The level of catechol metabolism in the RVLM 1) is continuously related to the level of arterial CO2, 2) functions close to its resting level under baseline nonstimulated condition with respect to CO2, and 3) is reversibly modified on changes in capnia. Sensitivity of the catechol signal, recorded in the rostral ventrolateral medulla, to CO2 appears primarily to be centrally mediated. Thus adrenergic RVLM neurons may relay inputs from the central respiratory generator to the sympathetic chemoreflex or act as chemosensors for CO2, next to the ventrolateral medulla surface.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1990

Differential distribution of phenol and catechol sulphotransferases in human liver and intestinal mucosa.

Phenol and catechol sulphotransferases were studied with p-nitrophenol and dopamine as substrates in the mucosa of the ileum and colon obtained from 6 subjects and also in the liver from 6 subjects. The ileum and colon were from the same donor. The kinetics of phenol and catechol sulphotransferases were studied in each tissue specimen. The maximum velocity of reaction (Vmax) for phenol sulphotransferase (in pmol X min-1 X mg-1; mean +/- SD) was 165 +/- 28 (ileum), 79 +/- 42 (colon) and 1,361 +/- 370 (liver), whereas Vmax for catechol sulphotransferase was 489 +/- 75 (ileum), 198 +/- 93 (colon) and 39 +/- 23 (liver). Phenol sulphotransferase is the predominant pathway in the liver, whereas catechol sulphotransferase is the predominant pathway in the intestine. The ileum catalysed the sulphation of p-nitrophenol and dopamine at a higher rate than the colon. The Michaelis-Menten constant (Km) for phenol sulphotransferase (in mumol/l; mean +/- SD) was 0.96 +/- 0.11 (ileum), 1.00 +/- 0.19 (colon) and 0.84 +/- 0.07 (liver), whereas Km for catechol sulphotransferase was 17.8 +/- 2.8 (ileum), 18.2 +/- 3.4 (colon) and 21.4 +/- 1.2 (liver). Km values of hepatic phenol or catechol sulphotransferases are not different from those of intestinal enzymes. Previous work has shown that 2-naphthol sulphotransferase obeys non-Michaelis-Menten kinetics in the human intestinal mucosa [Pharmacology, 1988;43:411]. Here, we show that 2-naphthol is sulphated by at least two enzymes in human intestine.

Aged↗
PubMed1980

A sensitive radioenzymatic assay for catechol drugs.

This assay measures picogram quantities of catechol drugs and endogenous catecholamines in body tissues and fluids. The catechols are converted to their 3H-O-methyl metabolites during incubation with 3H-S-adenosylmethionine then separated by solvent extraction and thin-layer chromatography. Most drugs containing the catechol structure can be radiolabeled and separated from norepinephrine and epinephrine by this technique to provide simultaneous measurement of endogenous and exogenously administered catechols. The disposition of isoproterenol in tissues and fluids of man and experimental animals is measured to illustrate the utility of this assay. The reactivity of several commonly administered catechol drugs with COMT is described and the possible implications discussed.

Adult↗
PubMed1981

Conjoint radioenzymatic measurement of catecholamines, their catechol metabolites and DOPA in biological samples.

An assay is described for the simultaneous determination of dopamine, noradrenaline, adrenaline, dopa, 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenylethanol, 3,4-dihydroxymandelic acid and 3,4-dihydroxy-phenylglycol, capable of detecting amounts in the femtomol range. The assay is based on the O-methylation of the catechol moiety utilizing S-[3H-methyl]-adenosyl-L-methionine and a partially purified catechol-O-methyl transferase to form the various O-[3H-methyl]-catechol derivatives. The O-[3H-methyl]-catechol derivatives are purified by thin layer chromatography, solvent partitions and/or ion exchange chromatography. The assay was successfully applied to biological sample. It was possible for the first time, to detect free 3,4-dihydroxyphenylglycol, free 3,4-dihydroxyphenylethanol and 3,4-dihydroxymandelic acid in a small volume (25 microliter) of blood plasma of man, rat, dog and rabbit. The conjoint measurement of catecholamines and their catechol metabolites in minute amounts of biological samples may contribute to a more detailed understanding of catecholamine metabolism in the peripheral and central nervous system.

Animals↗
PubMed1980

2-hydroxyestradiol-17 alpha and 4-hydroxyestradiol-17 alpha, catechol estrogen analogs with reduced estrogen receptor affinity.

2-Hydroxyest radiol-17 alpha and 4-hydroxyestradiol-17 alpha, the catechol derivatives of estradiol-17 alpha, have reduced affinity for hypothalamic, pituitary, and uterine estrogen receptors, but retain a potency for interaction with catechol-0-methyltransferase equal to that of the natural, 17 beta-hydroxy catechols. This dissociation of receptor binding and catecholamine interactions may allow the use of the 17 alpha catechols as a probe for the mechanism of action of the catechol estrogens.

Amygdala↗
PubMed1998

No change of brain extracellular catecholamine levels after acute catechol-O-methyltransferase inhibition: a microdialysis study in anaesthetized rats.

Catechol-O-methyltransferase inhibitors have been newly introduced as adjunct drugs to the levodopa/dopa decarboxylase inhibitor therapy in Parkinson's disease. When given alone, catechol-O-methyltransferase inhibitors seem to affect behaviour. We wanted to determine whether the concentrations of free amine would be increased by catechol-O-methyltransferase inhibition with tolcapone and underpin the positive behavioural effects. To this end, dopamine and noradrenaline levels were analyzed in the microdialysis perfusion fluid collected from several brain regions in chloral hydrate anaesthetized rats. We also analyzed the turnover rate of catecholamines in the brain after single doses of tolcapone and entacapone using the alpha-methyl-p-tyrosine method. On their own, tolcapone (at 10 or 30 mg/kg) did not elevate dopamine or noradrenaline levels in any brain region studied although the formation of catechol-O-methyltransferase-dependent metabolites was strongly reduced. Neither tolcapone nor entacapone (at 30 mg/kg) affected the turnover rate of catecholamines. It seems that catechol-O-methyltransferase inhibitors do not alter behaviour by elevating extracellular levels of free catecholamines levels but other explanations are needed.

3,4-Dihydroxyphenylacetic Acid↗
PubMed2001

Effect of intracerebral 6-nitronoradrenaline, an endogenous catechol-O-methyltransferase (COMT) inhibitor, on striatal dopamine metabolism in anaesthetised rats.

6-Nitronoradrenaline, a bioactive compound recently identified in the brain, is known to inhibit catechol-O-methyltransferase. To study its effect on dopamine metabolism, it was administered into rat striatum via a microdialysis probe. Other nitrated catechols (6-nitrodopamine, 6-nitro-DOPAC and 5-nitro-HVA) were studied for comparison. Tolcapone, a selective catechol-O-methyltransferase inhibitor, was used as a positive reference compound. Both 6-nitronoradrenaline and tolcapone increased striatal extracellular dopamine levels during the perfusion (at 100 microM concentration but not at 10 microM) and decreased the efflux of homovanillic acid. Tolcapone, but not other nitrated catechols, increased 3,4-dihydroxyphenylacetic acid efflux. None of the compounds inhibited MAO-B activity at 100 microM or lower. At 1 mM, 6-nitrodopamine inhibited MAO-B by 60%. Compared to tolcapone, other nitrated catechols were very weak COMT inhibitors in vitro. Neither tolcapone nor 6-nitronoradrenaline modified the metabolism of L-dopa which was given peripherally. In binding studies, both 6-nitronoradrenaline and other nitrocatechols failed to affect the dopamine transporter even at high micromolar concentrations. In conclusion, exogenous 6-nitronoradrenaline can act as a COMT inhibitor in the striatum and elevate striatal dopamine levels without inhibiting dopamine reuptake. Whether endogenous 6-nitronoradrenaline can be formed also in vivo in the striatum and act as a regulator of dopaminergic tone remains to be determined.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1998

Characterization of catechol- and chlorocatechol-degrading activity in the ortho-chlorinated benzoic acid-degrading Pseudomonas sp. CPE2 strain.

Pyrocatechase activity was studied in the Pseudomonas sp. CPE2 strain, which is capable of growing on 2-chlorobenzoic and 2,5-dichlorobenzoic acid, giving rise to catechol and 4-chlorocatechol, respectively, as intermediate metabolites. The CPE2 crude extract was found to metabolize both catechol and 4-chlorocatechol. Enzymatic as well as phenotypic studies performed both on this strain and on a mutant strain lacking the chlorocatechol-degrading genes were consistent with the presence of two catechol-cleaving enzymes, one active mainly against catechol (pyrocatechase I) and the other with broader substrate specificity (pyrocatechase II). The latter enzyme also appeared to be induced when CPE2 cells were grown on 2-chlorobenzoic acid, thus contributing to catechol metabolism, in addition to pyrocatechase I. Despite the presence of a large plasmid in CPE2 cells, the chlorocatechol-degrading genes, highly homologous to the clc operon, were located on the chromosome. The selection at relatively high frequency of mutant strains with altered growth capabilities and which lacked the chlorocatechol-degrading genes suggests a transposon-like character for these catabolic genes in the CPE2 strain.

Biodegradation, Environmental↗
PubMed2002

Catechol ortho-quinones: the electrophilic compounds that form depurinating DNA adducts and could initiate cancer and other diseases.

Catechol estrogens and catecholamines are metabolized to quinones, and the metabolite catechol (1,2-dihydroxybenzene) of the leukemogenic benzene can also be oxidized to its quinone. We report here that quinones obtained by enzymatic oxidation of catechol and dopamine with horseradish peroxidase, tyrosinase or phenobarbital-induced rat liver microsomes react with DNA by 1,4-Michael addition to form predominantly depurinating adducts at the N-7 of guanine and the N-3 of adenine. These adducts are analogous to the ones formed with DNA by enzymatically oxidized 4-catechol estrogens (Cavalieri,E.L., et al. (1997) PROC: Natl Acad. Sci., 94, 10937). The adducts were identified by comparison with standard adducts synthesized by reaction of catechol quinone or dopamine quinone with deoxyguanosine or adenine. We hypothesize that mutations induced by apurinic sites, generated by the depurinating adducts, may initiate cancer by benzene and estrogens, and some neurodegenerative diseases (e.g. Parkinson's disease) by dopamine. These data suggest that there is a unifying molecular mechanism, namely, formation of specific depurinating DNA adducts at the N-7 of guanine and N-3 of adenine, that could initiate many cancers and neurodegenerative diseases.

Apurinic Acid↗
PubMed1992

Expression of functional membrane-bound and soluble catechol-O-methyltransferase in Escherichia coli and a mammalian cell line.

Human catechol-O-methyltransferase (hCOMT) cDNA was used to express the recombinant hCOMT enzyme in sufficient quantities in prokaryotic as well as in eukaryotic cells to allow kinetic studies. When human membrane-bound catechol-O-methyltransferase (MB-COMT; amino acids 1-271) and the soluble catechol-O-methyltransferase COMT (S-COMT; delta membrane anchor hCOMT; amino acids 27-271), with the latter lacking the first 26 hydrophobic amino acids, were expressed in Escherichia coli, a relatively high-level synthesis of catalytically active enzymes was obtained. Insertion of the human MB-COMT-coding sequence into an eukaryotic expression vector under transcriptional control of the cytomegalovirus (CMV) promoter and enhancer yielded large quantities of hCOMT in human kidney 293 cells. Subcellular fractionation of 293 cells transfected with pBC12/CMV-hCOMT showed hCOMT to be located predominantly in the membrane fraction. The catechol-O-methyltransferase (COMT) activity was measured in cytosolic and membrane fractions at 37 degrees C, giving values of 33 and 114 units/mg of protein, respectively (1 unit produces 1 nmol of guaiacol/h). Km values were 10 microM for MB-COMT and 108 microM for S-COMT, indicating that recombinant MB-COMT exhibits a higher affinity for catechol as the substrate than the soluble form. RNA blot analysis of human hepatome cells (Hep G2), kidney, liver, and fetal brain revealed only one species of hCOMT mRNA of approximately 1.4 kb. Its level in these various tissues was similar to those of COMT protein in each tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1999

Conversion of 3-chlorocatechol by various catechol 2,3-dioxygenases and sequence analysis of the chlorocatechol dioxygenase region of Pseudomonas putida GJ31.

Pseudomonas putida GJ31 contains an unusual catechol 2,3-dioxygenase that converts 3-chlorocatechol and 3-methylcatechol, which enables the organism to use both chloroaromatics and methylaromatics for growth. A 3.1-kb region of genomic DNA of strain GJ31 containing the gene for this chlorocatechol 2,3-dioxygenase (cbzE) was cloned and sequenced. The cbzE gene appeared to be plasmid localized and was found in a region that also harbors genes encoding a transposase, a ferredoxin that was homologous to XylT, an open reading frame with similarity to a protein of a meta-cleavage pathway with unknown function, and a 2-hydroxymuconic semialdehyde dehydrogenase. CbzE was most similar to catechol 2,3-dioxygenases of the 2.C subfamily of type 1 extradiol dioxygenases (L. D. Eltis and J. T. Bolin, J. Bacteriol. 178:5930-5937, 1996). The substrate range and turnover capacity with 3-chlorocatechol were determined for CbzE and four related catechol 2,3-dioxygenases. The results showed that CbzE was the only enzyme that could productively convert 3-chlorocatechol. Besides, CbzE was less susceptible to inactivation by methylated catechols. Hybrid enzymes that were made of CzbE and the catechol 2, 3-dioxygenase of P. putida UCC2 (TdnC) showed that the resistance of CbzE to suicide inactivation and its substrate specificity were mainly determined by the C-terminal region of the protein.

Amino Acid Sequence↗
PubMed2001

The effects of catechol-O-methyltransferase inhibition on estrogen metabolite and oxidative DNA damage levels in estradiol-treated MCF-7 cells.

Many of the major identified risk factors for breast cancer are associated with exposure to endogenous estrogen. In addition to the effects of estrogen as a growth factor, experimental and epidemiological evidence suggest that catechol metabolites of estrogen also contribute to estrogen carcinogenesis by both direct and indirect genotoxic mechanisms. O-Methylation catalyzed by catechol-O-methyltransferase (COMT) is a Phase II metabolic inactivation pathway for catechol estrogens. We and others have found that a polymorphism in the COMT gene, which codes for a low activity variant of the COMT enzyme, is associated with an increased risk of developing breast cancer; therefore, the goal of the current study was to investigate the role of decreased COMT activity on estrogen catechol levels and on oxidative DNA damage, as measured by 8-hydroxy-2'-deoxyguanosine (8-oxo-dG) levels. MCF-7 cells were pretreated with dioxin as a means to increase estrogen metabolism to catechol estrogens, then treated with estradiol (E2) +/- Ro 41-0960, a COMT-specific inhibitor. After extraction from culture medium, estrogen metabolites were separated using an high-performance liquid chromatography-electrochemical detection method. As expected, dioxin dramatically increased E2 oxidative metabolism, primarily to its 2-OH and 2-methoxy metabolites. The COMT inhibitor blocked 2-methoxy E2 formation. This was associated with increased 2-hydroxy E2 (2-OH E2) and 8-oxo-dG levels. In the presence of COMT inhibition, increased oxidative DNA damage was detected in MCF-7 cells exposed to as low as 0.1 microM E2, whereas in the absence of COMT inhibition, no increase in 8-oxo-dG was detected at E2 concentrations < or =10 microM. This study is the first to show that O-methylation of 2-OH E2 by COMT is protective against oxidative DNA damage caused by 2-OH E2, a major oxidative metabolite of E2.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed1981

Catechol estrogen formation in placental and fetal tissues of humans, macaques, rats and rabbits.

Placental and fetal tissues obtained from humans, monkeys, rats and rabbits contained monooxygenases capable of catalyzing the formation of catechol estrogens. Treatments of pregnant rats with phenobarbital, Aroclor 1254, or 3-methylcholanthrene each increased measured rates of catechol estrogen formation in placentas, fetal brains and fetal livers, while other rat tissues exhibited either increased or decreased activity. Treatment of pregnant rabbits with Arocolor 1254 produced an increase in catechol estrogen formation in all fetal tissues studied and in the maternal liver and kidney. Catechol estrogen formation in placental microsomes of macaque monkeys (Macaca arctoides) was generally less than that observed in human placental microsomes. Human placentas obtained from smokers exhibited enhanced catechol estrogen formation and this activity appeared to be highly correlated with placental aryl hydrocarbon hydroxylase activity. The data suggested [a]pyrene in a similar fashion in placental tissues and that the enzyme systems involved may be under similar regulatory control.

Animals↗
PubMed1992

Increased catechol estrogen metabolism as a risk factor for nonfamilial breast cancer.

The metabolism of estrone (E1) or estradiol-17 beta (E2) to catechols seldom has been investigated in biochemical studies related to the risk of development of human breast cancer, as a result of the extreme lability and reactivity of these hormones. A method of indirect calculation was developed in which estimated catechol estrogen excretion (ECE) from urinary excretion of E1, E2, and estriol (E3) was used, based on the obligate reciprocal relation between 16 alpha-hydroxylase activity (r3) and estrogen 2/4 hydroxylase function (r2). This relationship is expressed by r2 x r3 = K, the estrogen oxidative constant. From published data relating chiefly to 2-OH estrone excretion, K = 12.4 +/- 0.8 (standard error of the mean). Urinary E1 + E2 excretion rates reflect nonprotein-bound plasma ovarian estrogen concentrations available for cell metabolism, which influence the value of K. The equation: r2 = [E1 + E2] K/[E3 + 16 alpha OH E1] = ECE gives a median correlation coefficient between actual catechol estrogen excretion and ECE in micrograms/24 hours of +0.88 (range, 0.61 to 0.97). When tested against the best product isolation analysis of catechol estrogen excretion, ECE was 95% accurate. Using this method a metaanalysis was conducted of published fractional estrogen excretion collected from 2846 healthy women worldwide aged 15 to 59 years, with a risk of breast cancer varying fivefold. Overall ECE was 78% to 97% higher in high-risk women of all ages and menstrual cycle phases (P less than 0.001, by Wilcoxon test). With increasing cancer risk (as estimated by the authors), ECE rose linearly exponentially with a slope of 0.149 (follicular phase) and 0.136 (luteal phase). The correlation coefficient (R2) between the two variables was 0.77 and 0.57, respectively (P less than 0.05). These data derived from calculations of ECE in healthy women confirmed recent analytic results of a twofold increase in the ratio of 2-OH E1/4-OH E1 in healthy Finnish women compared with recent Japanese migrants to Hawaii. In Finnish women with breast cancer, this ratio increased further (almost twofold). Metaanalysis supported the conclusion that increased rates of oxidation of estradiol 17-beta to 2-OH catechols supply the principal proximal human mammary carcinogens active after menarche.

Adolescent↗
PubMed2003

Substituent effects on O--H bond dissociation enthalpies and ionization potentials of catechols: a DFT study and its implications in the rational design of phenolic antioxidants and elucidation of structure-activity relationships for flavonoid antioxidants.

Density functional theory (DFT) on B3LYP/6-31G(d,p) level was employed to investigate the substituent effects on O--H bond dissociation enthalpies (BDEs) and ionization potentials (IPs) of catechols. It was revealed that the ortho hydroxyl of catechol was effective for the reduction of the O--H BDE; however, the group had little influence on the IP. The para substituent effects upon O--H BDEs and IPs for catechols were roughly the same as those for monophenols, and this gave the catechol moiety more potential than monophenol to be used as a lead compound in rational design of phenolic antioxidants. In addition, the 1,4-pyrone effects on O--H BDEs of catecholic rings A or B of flavonoids were also investigated. Although 1,4-pyrone extended the conjugation system of flavonoids, it was not beneficial to reduce the O--H BDE as a result of its electron-withdrawing property. Thus, 1,4-pyrone was unlikely to be favorable to enhance the H-abstraction activity of flavonoids.

Antioxidants↗