Two-electron electrochemical oxidation of quercetin and kaempferol changes only the flavonoid C-ring

Bulk electrolysis of the antioxidant flavonoids quercetin and kaempferol in acetonitrile both yield a single oxidation product in two-electron processes. The oxidation products are more polar than their parent compounds, with an increased molecular weight of 16 g/mol, and were identified as 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone and 2-(4-hydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone for quercetin and kaempferol, respectively. Two-electron oxidation of the parent flavonoid is suggested to yield a 3,4-flavandione with unchanged substitution pattern in the A- and B-ring, which may rearrange to form the substituted 3(2H)-benzofuranone through the chalcan-trione ring-chain tautomer. The acidity of the 3-OH group is suggested to determine the fate of the flavonoid phenoxyl radical, originally formed by one-electron oxidation, as no well-defined oxidation product of luteolin (lacking the 3-OH group) could be isolated despite rather similar half-peak potentials: E_P/2 = 0.97 V, 0.98 V and 1.17 V vs. NHE for quercetin, kaempferol and luteolin, respectively, as measured by cyclic voltammetry in acetonitrile.     

Enzymatic activity of cell-free extracts from Burkholderia oxyphila OX-01 bio-converts (+)-catechin and (−)-epicatechin to (+)-taxifolin

This study characterized the enzymatic ability of a cell-free extract from an acidophilic (+)-catechin degrader Burkholderia oxyphila (OX-01). The crude OX-01 extracts were able to transform (+)-catechin and (?)-epicatechin into (+)-taxifolin via a leucocyanidin intermediate in a two-step oxidation. Enzymatic oxidation at the C-4 position was carried out anaerobically using H₂O as an oxygen donor. The C-4 oxidation occurred only in the presence of the 2R-catechin stereoisomer, with the C-3 stereoisomer not affecting the reaction. These results suggest that the OX-01 may have evolved to target both (+)-catechin and (?)-epicatechin, which are major structural units in plants.     

Zn-polyphenol chelation: complexes with quercetin, (+)-catechin, and derivatives: II Electrochemical and EPR studies

In this work, we have studied the formation of complexes between flavonols, (quercetin, rutin, quercitrin, kaempferol, luteolin, tamarixetin) and flavanols ((+)-catechin, (−)-epicatechin), flavanonol, (+)-taxifolin, and Zn acetate in two hydro-organic media at neutral pH in the absence of oxygen. The complexation was first studied by cyclic voltammetry. Then preparative electrolysis have been attempted followed by coulometry, UV-Vis optical absorption and circular dicroism spectroscopies for characterizing the oxidized compounds. Spectroelectrochemistries monitored either in the UV-Vis or in the EPR spectrometers at room temperature have been also used and we have identified o-semi-quinone intermediates in some cases. Different behaviour vis-à-vis the complexation by Zn²⁺ according to the molecular structures of these different families of polyphenols have been found. Some of them are more easily oxidizible.     

Relevance of apple polyphenols as antioxidants in human plasma: contrasting in vitro and in vivo effects

Apples are a major source of flavonoids in the Western diet, and flavonoid-rich foods may help protect against chronic diseases by antioxidant mechanisms. In the present study we investigated: (1) the antioxidant capacity of representative apple polyphenols and their contribution to the total antioxidant capacity of apple extracts; (2) the effects of adding apple extract to human plasma in vitro on oxidation of endogenous antioxidants and lipids; and (3) the effects of apple consumption by humans on ex vivo oxidation of plasma antioxidants and lipids. We found that the apple-contained flavonols and flavanols, quercetin, rutin, (-)-epicatechin, and (+)-catechin, had a higher antioxidant capacity than the dihydrochalcones, phloridzin and phloretin, and the hydroxycinnamate, chlorogenic acid. However, together these apple polyphenols contributed less than 20% to the total antioxidant capacity of aqueous apple extracts. When human plasma was exposed to a constant flux of aqueous peroxyl radicals, endogenous ascorbate (70.0 +/- 10.3 microM) was oxidized within 45 min of incubation, while endogenous urate (375 +/- 40 microM) and alpha-tocopherol (24.7 +/- 1.2 microM) were oxidized after ascorbate. Addition of 7.1 or 14.3 micrograms/ml total phenols of apple extract did not protect ascorbate from oxidation, but increased the half-life (t1/2) of urate from 136 +/- 15 to 192 +/- 16 and 208 +/- 23 min, respectively (p < 0.05 each), and t1/2 of alpha-tocopherol from 141 +/- 18 to 164 +/- 8 min (p = ns) and 188 +/- 8 min (p < 0.05). Lipid peroxidation started after ascorbate depletion, and addition of apple extract increased the lag time preceding detectable lipid peroxidation from 36.3 +/- 3.7 to 50.9 +/- 2.7 min (p < 0.05) and 70.4 +/- 4.2 min (p < 0.001). However, when six healthy volunteers ate five apples and plasma was obtained up to 4 h after apple consumption, no significant increases in the resistance to oxidation of endogenous urate, alpha-tocopherol, and lipids were found. Thus, despite the high antioxidant capacity of individual apple polyphenols and apple extracts and the significant antioxidant effects of apple extract added to human plasma in vitro, ingestion of large amounts of apples by humans does not appear to result in equivalent in vivo antioxidant effects of apple polyphenols.     

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.     

Polyphenols and red wine as antioxidants against peroxynitrite and other oxidants

The antioxidant capacity of polyphenols (+)-catechin, (-)-epicatechin and myricetin, and of different types of red wines (Cabernet Sauvignon, Malbec and blended wine) was evaluated by three assays. (a) NADH oxidation by peroxynitrite (ONOO-): the ONOO- scavenging activity was higher for myricetin (IC50=35 microM) than for (+)-catechin (IC50=275 microM) and (-)-epicatechin (IC50=313 microM). (b) Peroxynitrite initiated chemiluminescence in rat liver homogenate: (-)-epicatechin (IC50=7.0 microM) and (+)-catechin (IC50=13 microM) were more potent than myricetin (IC50=20 microM) in inhibiting the chemiluminescence signal. (c) Lucigenin chemiluminescence in aortic rings: (-)-epicatechin (IC50=15 microM) and (+)-catechin (IC50=18 microM) showed higher antioxidant capacity than myricetin (IC50=32 microM). All the assayed red wines were able to scavenge the oxidants and free radical species that generate the signal in each assay. Cabernet Sauvignon was the red wine with the highest antioxidant capacity in comparison with Malbec and blended wine. It is concluded that the use of sensitive biological systems (as the aortic ring chemiluminescence) provides important information in addition to the results from chemical (NADH oxidation by peroxynitrite) and biochemical (homogenate chemiluminescence) assays and offers advances in the physiological role of polyphenols.     

Inhibitory effects of dietary flavonoids on purified hepatic NADH-cytochrome b5 reductase: structure-activity relationships

The structure-activity relationships of flavonoids with regard to their inhibitory effects on NADH-cytochrome b5 reductase (E.C. 1.6.2.2), a clinically and toxicologically important enzyme, are not known. In the present study, the inhibitory effects of fourteen selected flavonoids of variable structure on the activity of purified bovine liver cytochrome b5 reductase, which shares a high degree of homology with the human counterpart, were investigated and the relationship between structure and inhibition was examined. Of all the compounds tested, the flavone luteolin was the most potent in inhibiting b5 reductase with an IC50 value of 0.11 μM, whereas naringenin, naringin and chrysin were inactive within the concentration range tested. Most of the remaining flavonoids (morin, quercetin, quercitrin, myricetin, luteolin-7-O-glucoside, (-)-epicatechin, and (+)-catechin) produced a considerable inhibition of enzyme activity with IC50 values ranging from 0.81 to 4.5 μM except apigenin (36 μM), rutin (57 μM) and (+)-taxifolin (IC50 not determined). The magnitude of inhibition was found to be closely related to the chemical structures of flavonoids. Analysis of structure-activity data revealed that flavonoids containing two hydroxyl groups in ring B and a carbonyl group at C-4 in combination with a double bond between C-2 and C-3 produced a much stronger inhibition, whereas substitution of a hydroxyl group at C-3 was associated with a less inhibitory effect. The physiologically relevant IC50 values for most of the flavonoids tested regarding b5 reductase inhibition indicate a potential for significant flavonoid-drug and/or flavonoid-xenobiotic interactions which may have important therapeutic and toxicological outcomes for certain drugs and/or xenobiotics.     

(+)-Catechin is more bioavailable than (-)-catechin: relevance to the bioavailability of catechin from cocoa

Catechin is a flavonoid present in fruits, wine and cocoa products. Most foods contain the (+)-enantiomer of catechin but chocolate mainly contains ( - )-catechin, in addition to its major flavanol, ( - )-epicatechin. Previous studies have shown poor bioavailability of catechin when consumed in chocolate. We compared the absorption of ( - ) and (+)-catechin after in situ perfusion of 10, 30 or 50 micromol/l of each catechin enantiomer in the jejunum and ileum in the rat. We also assayed 23 samples of chocolate for (+) and ( - )-catechin. Samples were analyzed using HPLC with a Cyclobond I-2000 RSP chiral column. At all concentrations studied, the intestinal absorption of ( - )-catechin was lower than the intestinal absorption of (+)-catechin (p < 0.01). Plasma concentrations of ( - )-catechin were significantly reduced compared to (+)-catechin (p < 0.05). The mean concentration of ( - )-catechin in chocolate was 218 +/- 126 mg/kg compared to 25 +/- 15 mg/kg (+)-catechin. Our findings provide an explanation for the poor bioavailability of catechin when consumed in chocolate or other cocoa containing products.     

Anthocyanidin synthase from Gerbera hybrida catalyzes the conversion of (+)-catechin to cyanidin and a novel procyanidin

Anthocyanidins were proposed to derive from (+)-naringenin via (2R,3R)-dihydroflavonol(s) and (2R,3S,4S)-leucocyanidin(s) which are eventually oxidized by anthocyanidin synthase (ANS). Recently, the role of ANS has been put into question, because the recombinant enzyme from Arabidopsis exhibited primarily flavonol synthase (FLS) activity with negligible ANS activity. This and other studies led to the proposal that ANS as well as FLS may select for dihydroflavonoid substrates carrying a "beta-face" C-3 hydroxyl group and initially form the 3-geminal diol by "alpha-face" hydroxylation. Assays with recombinant ANS from Gerbera hybrida fully supported the proposal and were extended to catechin and epicatechin isomers as potential substrates to delineate the enzyme specificity. Gerbera ANS converted (+)-catechin to two major and one minor product, whereas ent(-)-catechin (2S,3R-trans-catechin), (-)-epicatechin, ent(+)-epicatechin (2S,3S-cis-epicatechin) and (-)-gallocatechin were not accepted. The K(m) value for (+)-catechin was determined at 175 microM, and the products were identified by LC-MS(n) and NMR as the 4,4-dimer of oxidized (+)-catechin (93%), cyanidin (7%) and quercetin (trace). When these incubations were repeated in the presence of UDP-glucose:flavonoid 3-O-glucosyltransferase from Fragariaxananassa (FaGT1), the product ratio shifted to cyanidin 3-O-glucoside (60%), cyanidin (14%) and dimeric oxidized (+)-catechin (26%) at an overall equivalent rate of conversion. The data appear to identify (+)-catechin as another substrate of ANS in vivo and shed new light on the mechanism of its catalysis. Moreover, the enzymatic dimerization of catechin monomers is reported for the first time suggesting a role for ANS beyond the oxidation of leucocyanidins.     

Reaction of vanadium(V) with thiols generates vanadium (IV) and thiyl radicals

The in vivo toxicity of vanadium(V) has been found to correlate with the depletion of cellular glutathione and related non-protein thiols. With a view to understanding the mechanism for this observation, we have investigated the oxidation of glutathione, cysteine N-acetylcysteine and penicillamine by vanadium(V), using electron spin resonance (ESR) and ESR spin trapping methodology. The spin trap used was 5,5-dimethyl-1-pyrroline 1-oxide (DMPO). It is found that the oxidation of these thiols by vanadium(V) generates the corresponding thiyl radicals and vanadium- (IV) complexes. The results suggest that free radical reactions play a significant role in the depletion of cellular thiols by vanadium(V) and hence in vanadium(V) toxicity.     

Quercetin but not luteolin suppresses the induction of lethal shock upon infection of mice with Salmonella typhimurium

Tumor necrosis factor-alpha (TNF-alpha) is important for the induction of systemic inflammatory responses that lead to lethal shock. Quercetin and luteolin, which differ by one hydroxyl group, are known to suppress the lipopolysaccharide-induced production of TNF-alpha in vitro. We show differing inhibitory effects of quercetin and luteolin on the induction of lethal shock in Salmonella typhimurium aroA-infected mice. In a time- and dose-dependent manner, quercetin reduced the plasma levels of TNF-alpha, lowered bacterial titers in livers, prevented liver damage and prolonged survival, while luteolin had little or no effect. Compared with luteolin, quercetin increased the infiltration of Gr-1(+)CD69(+) neutrophils into the peritoneal cavity and lowered heat shock protein 70 expression. Obviously, the additional hydroxyl group in quercetin is important for suppressing infection-induced lethal shock in mice.     

Antioxidant aryl-prenylcoumarin, flavan-3-ols and flavonoids from Eysenhardtia subcoriacea

Antioxidant activity (AOA) assay-guided chemical analysis, using a rat pancreas homogenate model, of aerial parts from Eysenhardtia subcoriacea, led to isolation of the new compound subcoriacin (3-(2'-hydroxy-4',5'-methylendioxyphenyl)-6-(3'-hydroxymethyl-4'-hydroxybut-2'-enyl)-7-hydroxycoumarin) together with the known substances: (+)-catechin, (-)-epicatechin, (+)-afzelechin, eriodictyol, (+)-catechin 3-O-beta-D-galactopyranoside and quercetin 3-O-beta-D-galactopyranoside as bioactive constituents. The structure of the compound was determined from 1D and 2D NMR spectroscopic analyses. Additional known constituents were characterized. The bioactive compounds showed also moderate to strong radical scavenging properties against diphenylpicrylhydrazyl radical (DPPH). In addition, subcoriacin, (+)-catechin, (-)-epicatechin and (+)-afzelechin improved the reduced glutathione levels in rat pancreatic homogenate.     

Chemical facilitation and induced pathogen resistance mediated by a root-secreted phytotoxin

The flavonol (+/-)-catechin is an allelochemical produced by the invasive weed Centaurea maculosa (spotted knapweed). The full effects of (+/-)-catechin on plant communities in both the native and the introduced ranges of C. maculosa remain uncertain. Here, by supplementing plant growth media with (+/-)-catechin, we showed that low (+/-)-catechin concentrations may induce growth and defense responses in neighboring plants. Doses of the allelochemical lower than the minimum inhibitory concentration (MIC) induced growth in Arabidopsis thaliana; plants treated with 25 microg ml(-1) (+/-)-catechin accumulated more than twice the biomass of untreated control plants. Further, pretreatment of A. thaliana roots with low concentrations of (+/-)-catechin induced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 in A. thaliana leaves. Low doses of (+/-)-catechin resulted in moderate increases in reactive oxygen species (ROS) in the meristems of treated plants, which may have loosened the cell walls and thus increased growth. Experiments with A. thaliana mutants indicated that (+/-)-catechin induces pathogen resistance by up-regulating defense genes via the salicylic acid (SA)/nonexpressor of pathogenesis related protein 1 (NPR1)-dependent pathway. Our results suggest that the growth and defense-inducing effects of (+/-)-catechin are concentration dependent, as (+/-)-catechin at higher concentrations is phytotoxic, thus suggesting the potential for hormesis to occur in nature.     

Inhibition of β-Glucosidase (Amygdalae dulces) by (+)-Catechin Oxidation Products and Procyanidin Dimers

The sensitivity and specificity of the inhibition of β-glucosidase (Amygdalae dulces) by (+ )-catechin, an oxidized (+)-catechin solution, three dimeric procyanidins, and five (+)-catechin dimers obtained by enzymatic oxidation were evaluated by using a chromatographic method. All the polyphenols tested presented a significant inhibitory effect. Non-competitive inhibition was observed for the oxidized (+)-catechin solution. Some oxidation products were at least as powerful inhibitors as procyanidins which are known for their tanning effect. Yellow oxidation products were among the strongest inhibitors. No marked role of the number of hydroxyl and o-diphenol groups nor of the nature or position of the interflavanic linkage in the inhibitory effect was apparent.     

Antioxidant components of Laetiporus sulphureus (Bull.: Fr.) Murr. fruit bodies

Antioxidant activity of fruit bodies of Laetiporus sulphureus (Bull.: Fr.) Murr. (Polyporales) obtained by the natural plantation growing method in Pribaikal'e (Irkutsk region) has been studied. It was determined that the ethyl acetate fraction of L. sulphureus, which was chromatographically separated into seven compounds identified as quercetin, kaempferol, (+)-catechin, p-coumaric, gallic, caffeic, and chlorogenic acids was characterized with more expressed antioxidant activity. All compounds were extracted from this basidiomycete species for the first time. The quantitative amount of the substances isolated from L. sulphureus was determined by HPLC. It was found that antioxidant activity of preparations obtained from L. sulphureus is conditioned by phenolic compounds.     

ESR study on the structure-antioxidant activity relationship of tea catechins and their epimers

The purpose of this study is to examine the relationship between the free radical scavenging activities and the chemical structures of tea catechins ((-)-epigallocatechin gallate (EGCG), (-)-epigallocatechin (EGC) and (-)-epicatechin (EC)) and their corresponding epimers ((-)-gallocatechin gallate (GCG), (-)-gallocatechin (GC) and (+)-catechin ((+)-C)). With electron spin resonance (ESR) we investigated their scavenging effects on superoxide anions (O-.2) generated in the irradiated riboflavin system, singlet oxygen(1O2) generated in the photoradiation-hemoporphyrin system, the free radicals generated from 2,2'-azobis(2-amidinopropane)hydrochloride (AAPH) and 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical. The results showed that the scavenging effects of galloylated catechins (EGCG and GCG) on the four free radicals were stronger than those of nongalloylated catechins (EGC, GC, EC, (+)-C), and the scavenging effects of EGC and GC were stronger than those of EC and (+)-C. Thus, it is suggested that the presence of the gallate group at the 3 position plays the most important role in their free radical-scavenging abilities and an additional insertion of the hydroxyl group at the 5' position in the B ring also contributes to their scavenging activities. Moreover, the corresponding phenoxyl radicals formed after the reaction with O-.2 were trapped by DMPO and the ESR spectra of DMPO/phenoxyl radical adducts were observed (aN=15.6 G and aHbeta=21.5 G). No significant differences were found between the scavenging effects of the catechins and their epimers when their concentrations were high. However, significant differences were observed at relatively low concentrations, and the lower their concentrations, the higher the differences. The scavenging abilities of GCG, GC and (+)-C were stronger than those of their corresponding epimers (EGCG, EGC and EC). The differences between their sterical structures played a more important role in their abilities to scavenge large free radicals, such as the free radicals generated from AAPH and the DPPH radical, than to scavenge small free radicals, such as O-.2 and 1O2, especially in the case with EGCG and GCG with more bulky steric hindrance.     

Influence of Hypericum perforatum extract and its single compounds on amyloid-beta mediated toxicity in microglial cells

As immunocompetent cells of the brain, microglia are able to counteract the damaging effects of amyloid-beta in Alzheimer's disease by phagocytosis-mediated clearance of protein aggregates. The survival and health of microglia are therefore critical for attenuating and preventing neurodegenerative diseases. In a microglial cell line pretreated with St. John's wort (Hypericum perforatum L.) extract (HPE), the cell death evoked by treatment with amyloid-beta (25-35) and (1-40) was attenuated significantly in a dose-dependent manner. Investigation of the single compounds in the extract revealed that the flavanols (+)-catechin and (-)-epicatechin increase cell viability slightly, whereas the flavonol quercetin and its glycosides rutin, hyperosid and quercitrin showed no effect on cell viability. In contrast, at the same concentration, the flavonoids reduced the formation of amyloid-induced reactive oxygen species in microglia, indicating that improvement of cell viability by the catechins is not correlated to the antioxidant activity. No influence of HPE on the capacity of microglia to phagocytose sub-toxic concentrations of fibrillar amyloid-beta (1-40) was observed. Other experiments showed that HPE, (+)-catechin and (-)-epicatechin can alter cellular membrane fluidity and thereby may have a beneficial effect on cell health. Our findings provide in vitro evidence that treatment especially with the complex plant extract HPE may restore or improve microglial viability and thereby attenuate amyloid-beta mediated toxicity in Alzheimer's disease.     

Potent anti-amyloidogenic and fibril-destabilizing effects of polyphenols in vitro: implications for the prevention and therapeutics of Alzheimer's disease

Cerebral deposition of amyloid beta-peptide (Abeta) in the brain is an invariant feature of Alzheimer's disease (AD). A consistent protective effect of wine consumption on AD has been documented by epidemiological studies. In the present study, we used fluorescence spectroscopy with thioflavin T and electron microscopy to examine the effects of wine-related polyphenols (myricetin, morin, quercetin, kaempferol (+)-catechin and (-)-epicatechin) on the formation, extension, and destabilization of beta-amyloid fibrils (fAbeta) at pH 7.5 at 37 degrees C in vitro. All examined polyphenols dose-dependently inhibited formation of fAbeta from fresh Abeta(1-40) and Abeta(1-42), as well as their extension. Moreover, these polyphenols dose-dependently destabilized preformed fAbetas. The overall activity of the molecules examined was in the order of: myricetin = morin = quercetin > kaempferol > (+)-catechin = (-)-epicatechin. The effective concentrations (EC50) of myricetin, morin and quercetin for the formation, extension and destabilization of fAbetas were in the order of 0.1-1 micro m. In cell culture experiments, myricetin-treated fAbeta were suggested to be less toxic than intact fAbeta, as demonstrated by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay. Although the mechanisms by which these polyphenols inhibit fAbeta formation from Abeta, and destabilize pre-formed fAbetain vitro are still unclear, polyphenols could be a key molecule for the development of preventives and therapeutics for AD.     

Oxidation of phenolic compounds by the bifunctional catalase–phenol oxidase (CATPO) from Scytalidium thermophilum

The thermophilic fungus Scytalidium thermophilum produces a novel bifunctional catalase with an additional phenol oxidase activity (CATPO); however, its phenol oxidation spectrum is not known. Here, 14 phenolic compounds were selected as substrates, among which (+)-catechin, catechol, caffeic acid, and chlorogenic acid yielded distinct oxidation products examined by reversed-phase HPLC chromatography method. Characterization of the products by LC-ESI/MS and UV–vis spectroscopy suggests the formation of dimers of dehydrocatechin type B (hydrophilic) and type A (hydrophobic), as well as oligomers, namely, a trimer and tetramer from (+)-catechin, the formation of a dimer and oligomer of catechol, a dimer from caffeic acid with a caffeicin-like structure, as well as trimeric and tetrameric derivatives, and a single major product from chlorogenic acid suggested to be a dimer. Based on the results, CATPO oxidizes phenolic compounds ranging from simple phenols to polyphenols but all having an ortho-diphenolic structure in common. The enzyme also appears to have stereoselectivity due to the oxidation of (+)-catechin, but not that of epicatechin. It is suggested that CATPO may contribute to the antioxidant mechanism of the fungus and may be of value for future food and biotechnology applications where such a bifunctional activity would be desirable.     

The antimutagenic activity of the major flavonoids of rooibos (Aspalathus linearis): some dose-response effects on mutagen activation-flavonoid interactions

The antimutagenic properties of the most prevalent flavonoids in rooibos (Aspalathus linearis) were compared in the Salmonella typhimurium mutagenicity assay using tester strains TA98 and TA100 with, respectively, 2-acetamido-fluorene (2-AAF) and aflatoxin B(1) (AFB(1)) as mutagens in the presence of metabolic activation. The flavonoids included the dihydrochalcones aspalathin and nothofagin and their flavone analogues, orientin and isoorientin, and vitexin and isovitexin, respectively, as well as luteolin, chrysoeriol, (+)-catechin, quercetin, isoquercitrin, hyperoside and rutin. Flavonoid-mutagen interactions ranged from antimutagenic, comutagenic and promutagenic to mutagenic, while dose-response effects were mutagen-specific and ranged from typical to atypical including biphasic and threshold effects. Aspalathin and nothofagin and their structural flavonoid analogues displayed moderate antimutagenic properties while luteolin and to some extent, chrysoeriol, showed activities comparable to those of the green tea flavonoid (-) epigallocatechin gallate (EGCG). Apart from their mutagenic and promutagenic properties, quercetin and isoquercitrin exhibited concentration-dependent comutagenic and/or antimutagenic effects against 2-AAF- and AFB(1)-induced mutagenesis. Different structural parameters known to affect the antimutagenic properties of flavonoids include their hydrophilic or lipophilic nature due to the extent of hydroxylation and O-methylation, glycosylation on the A and B rings, the C4-keto group and the C2-C3 double bond. The C ring does not appear to be a prerequisite when comparing for the antimutagenic activity of the dihydrochalcones when compared of the dihydrochalcones with the structural flavone analogues.