The regioselectivity of glutathione adduct formation with flavonoid quinone/quinone methides is pH-dependent

Generation of Hydrogen Peroxide in Cancer Cells: Advancing Therapeutic Approaches for Cancer Treatment

Cancer cells show altered antioxidant defense systems, dysregulated redox signaling, and increased generation of reactive oxygen species (ROS). Targeting cancer cells through ROS-mediated mechanisms has emerged as a significant therapeutic strategy due to its implications in cancer progression, survival, and resistance. Extensive research has focused on selective generation of H2O2 in cancer cells for selective cancer cell killing by employing various strategies such as metal-based prodrugs, photodynamic therapy, enzyme-based systems, nano-particle mediated approaches, chemical modulators, and combination therapies. Many of these H2O2-amplifying approaches have demonstrated promising anticancer effects and selectivity in preclinical investigations. They selectively induce cytotoxicity in cancer cells while sparing normal cells, sensitize resistant cells, and modulate the tumor microenvironment. However, challenges remain in achieving selectivity, addressing tumor heterogeneity, ensuring efficient delivery, and managing safety and toxicity. To address those issues, H2O2-generating agents have been combined with other treatments leading to optimized combination therapies. This review focuses on various chemical agents/approaches that kill cancer cells via H2O2-mediated mechanisms. Different categories of compounds that selectively generate H2O2 in cancer cells are summarized, their underlying mechanisms and function are elucidated, preclinical and clinical studies as well as recent advancements are discussed, and their prospects as targeted therapeutic agents and their therapeutic utility in combination with other treatments are explored. By understanding the potential of these compounds, researchers can pave the way for the development of effective and personalized cancer treatments.

Mechanisms of polyphenols on quality control of aquatic products in storage: A review

Aquatic products are easily spoiled during storage due to oxidation, endogenous enzymes, and bacteria. At the same time, compared with synthetic antioxidants, based on the antibacterial and antioxidant mechanism of biological agents, the development of natural, nontoxic, low-temperature, better-effect green biological preservatives is more acceptable to consumers. The type and molecular structure of polyphenols affect their antioxidant and antibacterial effectiveness. This review will describe how they achieve their antioxidant and antibacterial effects. And the recent literature on the mechanism and application of polyphenols in the preservation of aquatic products was updated and summarized. The conclusion is that in aquatic products, polyphenols alleviate lipid oxidation, protein degradation and inhibit the growth and reproduction of microorganisms, so as to achieve the effect of storage quality control. And put forward suggestions on the application of the research results in aquatic products. We hope to provide theoretical support for better exploration of the application of polyphenols and aquatic product storage.

Plant-Mediated Synthesis of Magnetite Nanoparticles with Matricaria chamomilla Aqueous Extract

Magnetite nanoparticles (NPs) possess properties that make them suitable for a wide range of applications. In recent years, interest in the synthesis of magnetite NPs and their surface functionalization has increased significantly, especially regarding their application in biomedicine such as for controlled and targeted drug delivery. There are several conventional methods for preparing magnetite NPs, all of which mostly utilize Fe(iii) and Fe(ii) salt precursors. In this study, we present a microwave hydrothermal synthesis for the precipitation of magnetite NPs at temperatures of 200 °C for 20 min and 260 °C for 5 min, with only iron(iii) as a precursor utilizing chamomile flower extract as a stabilizing, capping, and reducing agent. Products were characterized using FTIR, PXRD, SEM, and magnetometry. Our analysis revealed significant differences in the properties of magnetite NPs prepared with this approach, and the conventional two-precursor hydrothermal microwave method (sample MagH). FTIR and PXRD analyses confirmed coated magnetite particles. The temperature and magnetic-field dependence of magnetization indicate their superparamagnetic behavior. Importantly, the results of our study show the noticeable cytotoxicity of coated magnetite NPs-toxic to carcinoma cells but harmless to healthy cells-further emphasizing the potential of these NPs for biomedical applications.

On the Role of ROS and Glutathione in the Mode of Action Underlying Nrf2 Activation by the Hydroxyanthraquinone Purpurin

Purpurin is a major anthraquinone present in the roots of Rubia cordifolia (madder). Purpurin is known to activate Nrf2 (Nuclear transcription factor erythroid 2-related factor 2) EpRE (electrophile responsive element) mediated gene expression as a potential beneficial effect. This study aimed to elucidate the balance between the electrophilicity or pro-oxidant activity of purpurin underlying the Nrf2 induction. For this, Nrf2 activation with modified intracellular glutathione (GSH) levels was measured in an Nrf2 CALUX reporter gene assay. In addition, both cell-free and intracellular ROS formation of purpurin with modified (intracellular) GSH levels at different pH were quantified using the DCF-DA assay. GSH adduct formation was evaluated by UPLC and LC-TOF-MS analysis. GSH and GSSG levels following purpurin incubations were quantified by LC-MS/MS. We show that Nrf2 induction by purpurin was significantly increased in cells with buthionine sulfoximine depleted GSH levels, while Nrf2 induction was decreased upon incubation of the cells with N-acetylcysteine being a precursor of GSH. In cell-free incubations, ROS formation increased with increasing pH pointing at a role for the deprotonated form of purpurin. Upon incubations of purpurin with GSH at physiological pH, GSH adduct formation appeared negligible (<1.5% of the added purpurin). The addition of GSH resulted in conversion of GSH to GSSG and significantly reduced the ROS formation. Together these results demonstrate that Nrf2 induction by purpurin originates from intracellular ROS formation and not from its electrophilicity, which becomes especially relevant when intracellular GSH levels can no longer scavenge the ROS. The present study demonstrated that the efficiency of intracellular Nrf2 activation by purpurin and related anthraquinones will depend on (i) their pKa and level of deprotonation at the intracellular pH, (ii) the oxidation potential of their deprotonated form and (iii) the intracellular GSH levels. Thus, the Nrf2 induction by purpurin depends on its pro-oxidant activity and not on its electrophilicity.

New opportunities and advances in quercetin-added functional packaging films for sustainable packaging applications: a mini-review

Recently, research on functional packaging films and their application to food preservation has been actively conducted. This review discusses recent advances and opportunities for using quercetin in developing bio-based packaging films for active food packaging. Quercetin is a plant-based yellow pigment flavonoid with many useful biological properties. Quercetin is also a GRAS food additive approved by the US FDA. Adding quercetin to the packaging system improves the physical performance as well as the functional properties of the film. Therefore, this review focused on quercetin's effect on the various packaging film properties, such as mechanical, barrier, thermal, optical, antioxidant, antimicrobial, and so on. The properties of films containing quercetin depend on the type of polymer and the interaction between the polymer and quercetin. Films functionalized with quercetin are useful in extending shelf life and maintaining the quality of fresh foods. Quercetin-added packaging systems can be very promising for sustainable active packaging applications.

Biological Activities Underlying the Therapeutic Effect of Quercetin on Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic autoimmune disorder stemming from unrestrained immune activation and subsequent destruction of colon tissue. Genetic susceptibility, microbiota remodeling, and environmental cues are involved in IBD pathogenesis. Up to now, there are limited treatment options for IBD, so better therapies for IBD are eagerly needed. The therapeutic effects of naturally occurring compounds have been extensively investigated, among which quercetin becomes an attractive candidate owing to its unique biochemical properties. To facilitate the clinical translation of quercetin, we aimed to get a comprehensive understanding of the cellular and molecular mechanisms underlying the anti-IBD role of quercetin. We summarized that quercetin exerts the anti-IBD effect through consolidating the intestinal mucosal barrier, enhancing the diversity of colonic microbiota, restoring local immune homeostasis, and restraining the oxidative stress response. We also delineated the effect of quercetin on gut microbiome and discussed the potential side effects of quercetin administration. Besides, quercetin could serve as a prodrug, and the bioavailability of quercetin is improved through chemical modifications or the utilization of effective drug delivery systems. Altogether, these lines of evidence hint the feasibility of quercetin as a candidate compound for IBD treatment.

Revisiting the Oxidation of Flavonoids: Loss, Conservation or Enhancement of Their Antioxidant Properties

Flavonoids display a broad range of health-promoting bioactivities. Among these, their capacity to act as antioxidants has remained most prominent. The canonical reactive oxygen species (ROS)-scavenging mode of the antioxidant action of flavonoids relies on the high susceptibility of their phenolic moieties to undergo oxidation. As a consequence, upon reaction with ROS, the antioxidant capacity of flavonoids is severely compromised. Other phenol-compromising reactions, such as those involved in the biotransformation of flavonoids, can also markedly affect their antioxidant properties. In recent years, however, increasing evidence has indicated that, at least for some flavonoids, the oxidation of such residues can in fact markedly enhance their original antioxidant properties. In such apparent paradoxical cases, the antioxidant activity arises from the pro-oxidant and/or electrophilic character of some of their oxidation-derived metabolites and is exerted by activating the Nrf2–Keap1 pathway, which upregulates the cell’s endogenous antioxidant capacity, and/or, by preventing the activation of the pro-oxidant and pro-inflammatory NF-κB pathway. This review focuses on the effects that the oxidative and/or non-oxidative modification of the phenolic groups of flavonoids may have on the ability of the resulting metabolites to promote direct and/or indirect antioxidant actions. Considering the case of a metabolite resulting from the oxidation of quercetin, we offer a comprehensive description of the evidence that increasingly supports the concept that, in the case of certain flavonoids, the oxidation of phenolics emerges as a mechanism that markedly amplifies their original antioxidant properties. An overlooked topic of great phytomedicine potential is thus unraveled.

Nonalcoholic fatty liver disease: The role of quercetin and its therapeutic implications

Nonalcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease, affecting almost one-third of the general population and 75% of obese patients with type 2 diabetes. The aim of this article is to review the current evidence concerning the role of quercetin, a natural compound and flavonoid, and its possible therapeutic effects on this modern-day disease. Despite the fact that the exact pathophysiological mechanisms through which quercetin has a hepatoprotective effect on NAFLD are still not fully elucidated, this review clearly demonstrates that this flavonoid has potent antioxidative stress action and inhibitory effects on hepatocyte apoptosis, inflammation, and generation of reactive oxygen species, factors which are linked to the development of the disease. NAFLD is closely associated with increased dietary fat consumption, especially in Western countries. The hepatoprotective effect of quercetin against NAFLD merits serious consideration and further validation by future studies.

Design and optimization of quercetin-based functional foods

Quercetin is a flavonoid present in a wide variety of plant resources. Over the years, extensive efforts have been devoted to examining the potential biological effects of quercetin and to manipulating the chemical and physical properties of the flavonoid. However, limited studies have reviewed the opportunities and challenges of using quercetin in the development of functional foods. To address this necessity, in this review; we foremost present an overview of the chemical properties and stability of quercetin in food products followed by a detailed discussion of various strategies that enhance its oral bioavailability. We further highlight the areas to be practically considered during development of quercetin-based functional foods. By revisiting the current status of applied research on quercetin, it is anticipated that useful insights enabling research on quercetin can be potentially translated into practical applications in food product development.

Glutathione-Mediated Cu(I)/Cu(II) Complexes: Valence-Dependent Effects on Clearance and In Vivo Imaging Application

Contrast imaging agents need to be cleared in a reasonable time (less than 72 h), so it is quite urgent to understand the structure, biocompatibility, and metabolism features of imaging agents. In this work, luminescent Cu(I)-GSH complex and their derivative oxidized Cu(II)-GSSG complex have been easily synthesized. Through systematically probing the renal clearance and biodistribution of the as-prepared copper complexes, we found that Cu(I)-GSH complex revealed much more efficient renal clearance and remarkably lower liver accumulation than that of their oxidation states, which could be due to strong protein binding of partial forms of Cu(II)-GSSG complex. Besides, we also attempted to incorporate radioactive copper-64 into Cu(I)-GSH complex for the synthesis of radioactive contrast agent. Indeed, the as-prepared radioactive Cu(I)-GSH complex also showed consistent high efficiency renal excretion, allowing them to be potential PET imaging agents in clinical translation.

Effect of Constituent Units, Type of Interflavan Bond, and Conformation on the Antioxidant Properties of Procyanidin Dimers: A Computational Outlook

Procyanidin (PC) dimers are powerful antioxidants, abundant in plant tissues, and also bioavailable. However, the role of the molecular structure of PCs on their antioxidant properties is still a controversial and not fully understood issue that needs to be addressed in a more specific way. The objective of this study was to analyze the effect of the constituent units, type of interflavan bond, and conformation on the antioxidant properties of PC dimers including PB3, PB4, PB5, PB6, PB7, and PB8, using the density functional theory (DFT) computational method. The analysis was performed in function of parameters that allow determining the ability of the molecules to transfer or to capture electrons, among which the chemical potential, bond dissociation enthalpy (BDE), gap energy, Fukui indices, and charge distribution of HOMO-LUMO orbitals. The factors that showed the most notable effects on the antioxidant properties of the PC dimers were the type of interflavan bond and the conformation. The antioxidant ability of the dimers PB3 and PB4 containing the interflavan bond C4–C8, in their Compact conformation, was very similar to each other but greater than those of dimers PB5, PB6, PB7, and PB8 containing the C4–C6 interflavan bond. PB8 showed the lowest antioxidant ability.

Chemical Constituents of the Leaves of Tussilago farfara and their Aldose Reductase Inhibitory Activity

The inhibitory activity of 37 medicinal plant extracts against aldose reductase (AR) activity was evaluated. The most potent AR inhibitory activity was found in the MeOH extract of the leaves of Tussilago farfara (Compositae). Enzyme assay-guided fractionation of the extract led to the isolation of a new flavonoid glycoside, kaempferol 3- O-[3,4- O-(isopropylidene)-α-L-arabinopyranoside] (1), along with 15 known compounds (2–16), of which 3, 5, 13, 15, and 16 were isolated from T. farfara for the first time. The structures of 1–16 were elucidated based on MS and NMR data. Dicaffeoylquinic acid derivatives (7–12) showed potent AR inhibitory activity with IC50 values ranging from 0.58 to 5.38 μM, whereas flavonoid glycosides 1, 3, 5, and 6 showed weak inhibitory activity with IC50 values of 13.9, 15.1, 13.3, and 14.1 μM, respectively.

Formation and Biological Targets of Quinones: Cytotoxic versus Cytoprotective Effects

Quinones represent a class of toxicological intermediates, which can create a variety of hazardous effects in vivo including, acute cytotoxicity, immunotoxicity, and carcinogenesis. In contrast, quinones can induce cytoprotection through the induction of detoxification enzymes, anti-inflammatory activities, and modification of redox status. The mechanisms by which quinones cause these effects can be quite complex. The various biological targets of quinones depend on their rate and site of formation and their reactivity. Quinones are formed through a variety of mechanisms from simple oxidation of catechols/hydroquinones catalyzed by a variety of oxidative enzymes and metal ions to more complex mechanisms involving initial P450-catalyzed hydroxylation reactions followed by two-electron oxidation. Quinones are Michael acceptors, and modification of cellular processes could occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radical anions leading to the formation of reactive oxygen species (ROS) including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can alter redox balance within cells through the formation of oxidized cellular macromolecules including lipids, proteins, and DNA. This perspective explores the varied biological targets of quinones including GSH, NADPH, protein sulfhydryls [heat shock proteins, P450s, cyclooxygenase-2 (COX-2), glutathione S-transferase (GST), NAD(P)H:quinone oxidoreductase 1, (NQO1), kelch-like ECH-associated protein 1 (Keap1), IκB kinase (IKK), and arylhydrocarbon receptor (AhR)], and DNA. The evidence strongly suggests that the numerous mechanisms of quinone modulations (i.e., alkylation versus oxidative stress) can be correlated with the known pathology/cytoprotection of the parent compound(s) that is best described by an inverse U-shaped dose-response curve.

Silymarin as a Natural Antioxidant: An Overview of the Current Evidence and Perspectives

Silymarin (SM), an extract from the Silybum marianum (milk thistle) plant containing various flavonolignans (with silybin being the major one), has received a tremendous amount of attention over the last decade as a herbal remedy for liver treatment. In many cases, the antioxidant properties of SM are considered to be responsible for its protective actions. Possible antioxidant mechanisms of SM are evaluated in this review. (1) Direct scavenging free radicals and chelating free Fe and Cu are mainly effective in the gut. (2) Preventing free radical formation by inhibiting specific ROS-producing enzymes, or improving an integrity of mitochondria in stress conditions, are of great importance. (3) Maintaining an optimal redox balance in the cell by activating a range of antioxidant enzymes and non-enzymatic antioxidants, mainly via Nrf2 activation is probably the main driving force of antioxidant (AO)  action of SM. (4) Decreasing inflammatory responses by inhibiting NF-κB pathways is an emerging mechanism of SM protective effects in liver toxicity and various liver diseases. (5) Activating vitagenes, responsible for synthesis of protective molecules, including heat shock proteins (HSPs), thioredoxin and sirtuins and providing additional protection in stress conditions deserves more attention. (6) Affecting the microenvironment of the gut, including SM-bacteria interactions, awaits future investigations. (7) In animal nutrition and disease prevention strategy, SM alone, or in combination with other hepatho-active compounds (carnitine, betaine, vitamin B₁₂, etc.), might have similar hepatoprotective effects as described in human nutrition.

Biotransformations and biological activities of hop flavonoids

Female hop cones are used extensively in the brewing industry, but there is now increasing interest in possible uses of hops for non-brewing purposes, especially in the pharmaceutical industry. Among pharmaceutically important compounds from hops are flavonoids, having proven anticarcinogenic, antioxidant, antimicrobial, anti-inflammatory and estrogenic effects. In this review we aim to present current knowledge on the biotransformation of flavonoids from hop cones with respect to products, catalysis and conversion. A list of microbial enzymatic reactions associated with gastrointestinal microbiota is presented. A comparative analysis of the biological activities of hop flavonoids and their biotransformation products is described, indicating where further research has potential for applications in the pharmaceutical industry.

Glutathione and the antioxidant potential of binary mixtures with flavonoids: synergisms and antagonisms

Polyphenols are able to trap free radicals, which contributes to their known antioxidant capacity. In plant extracts, these secondary metabolites may act in concert, in a way that their combined activities will be superior to their individual effects (synergistic interaction). Several polyphenols have demonstrated clear antioxidant properties in vitro, and many of their biological actions have been attributed to their intrinsic reducing capabilities. As so, the intake of these compounds at certain concentrations in the diet and/or supplementation may potentiate the activity of reduced form glutathione (GSH), thus better fighting oxidative stress. The aim of this work was to predict a structure-antioxidant activity relationship using different classes of flavonoids and to assess, for the first time, possible synergisms and antagonisms with GSH. For these purposes a screening microassay involving the scavenging of DPPH^• was applied. In general, among the tested compounds, those lacking the catechol group in B ring showed antagonistic behaviour with GSH. Myricetin displayed additive effect, while quercetin, fisetin, luteolin, luteolin-7-O-glucoside, taxifolin and (+)-catechin demonstrated synergistic actions. Furthermore, adducts formed at C2′ and C5′ of the B ring seem to be more important for the antioxidant capacity than adducts formed at C6 and C8 of the A ring.

UHPLC-(ESI)QTOF MS/MS profiling of quercetin metabolites in human plasma postconsumption of applesauce enriched with apple peel and onion

An ultrahigh pressure liquid chromatography accurate mass quadrupole time-of-flight mass spectrometry with electrospray ionization (UHPLC-(ESI)QTOF MS/MS) method was developed for measuring individual quercetin metabolites in human plasma with high sensitivity and high selectivity. Quercetin (3,3',4',5,7-pentahydroxyflavone) occurs as glycosides in foods. The composition of glycosides is species and cultivar specific. In humans, quercetin undergoes extensive biotransformation, resulting in a range of metabolites. The bioactivity of quercetin metabolites will depend on the type and position of the conjugates. Herein, individual quercetin metabolites (i.e., sulfate, glucuronide or methyl conjugates) were identified by accurate mass MS in human plasma (females = 8 and males = 8) over 24 h after consumption of applesauce enriched with either micronized apple peel (AP) or onion powder (OP). The AP and OP contained ~180 μmol of quercetin glycosides. The relative amounts of quercetin metabolites were quantified in plasma. The complement of identified quercetin metabolites was similar after consumption of AP and OP. Primary metabolites included the following: quercetin sulfate, quercetin glucuronide, and quercetin diglucuronide. A quercetin glutathione adduct was identified in negative ion mode but not apparent in positive ion mode. The pharmacokinetic parameters for AUC0-24 h and Cmax were significantly different for AP and OP. For example, consumption of the AP resulted in Cmax of quercetin sulfate, 4.6 ng/mL; quercetin glucuronide, 15.5 ng/mL; quercetin diglucuronide, 9.3 ng/mL; quercetin glucuronide sulfate, 1.3 ng/mL; methyl quercetin glucuronide, 7.5 ng/mL; and methyl quercetin diglucuronide, 3.6 ng/mL, whereas the OP resulted in Cmax of quercetin sulfate, 37.3 ng/mL; quercetin glucuronide, 212.8 ng/mL; quercetin diglucuronide, 168.8 ng/mL; quercetin glucuronide sulfate, 43.0 ng/mL; methyl quercetin glucuronide, 90.1 ng/mL; methyl quercetin diglucuronide, 65.4 ng/mL. Gender-related differences in the AUC0-24 h for quercetin sulfate and quercetin sulfate glucuronide metabolites were also observed.

Structural determinant of chemical reactivity and potential health effects of quinones from natural products

Although many phenols and catechols found as polyphenol natural products are antioxidants and have putative disease-preventive properties, others have deleterious health effects. One possible route to toxicity is the bioactivation of the phenolic function to quinones that are electrophilic, redox-agents capable of modifying DNA and proteins. The structure-property relationships of biologically important quinones and their precursors may help understand the balance between their health benefits and risks. We describe a mass-spectrometry-based study of four quinones produced by oxidizing flavanones and flavones. Those with a C2-C3 double bond on ring C of the flavonoid stabilize by delocalization of an incipient positive charge from protonation and render the protonated quinone particularly susceptible to nucleophilic attack. We hypothesize that the absence of this double bond is one specific structural determinant that is responsible for the ability of quinones to modify biological macromolecules. Those quinones containing a C2-C3 single bond have relatively higher aqueous stability and longer half-lives than those with a double bond at the same position; the latter have short half-lives at or below ∼1 s. Quinones with a C2-C3 double bond show little ability to depurinate DNA because they are rapidly hydrated to unreactive species. Molecular-orbital calculations support that quinone hydration by a highly structure-dependent mechanism accounts for their chemical properties. The evidence taken together support a hypothesis that those flavonoids and related natural products that undergo oxidation to quinones and are then rapidly hydrated are unlikely to damage important biological macromolecules.

Herbal bioactivation, molecular targets and the toxicity relevance

There have been increasing reports on the adverse reactions associated with herbal consumption. For many of these adverse reactions, the underlying biochemical mechanisms are unknown, but bioactivation of herbal compounds to generate reactive intermediates have been implicated. This minireview updates our knowledge on metabolic activation of herbal compounds, molecular targets and the toxicity relevance. A number of studies have documented that some herbal compounds can be converted to toxic or even carcinogenic metabolites by Phase I [e.g. cytochrome P450s (CYPs)] and less frequently by Phase II enzymes. For example, aristolochic acids (AAs) in Aristolochia spp, which undergo reduction of the nitro group by hepatic CYP1A1/2 or peroxidases in extrahepatic tissues to generate highly reactive cyclic nitrenium ions. The latter can react with macromolecules (DNA and protein), resulting in activation of H-ras and myc oncogenes and gene mutation in renal cells and finally carcinogenesis of the kidneys. Teucrin A and teuchamaedryn A, two diterpenoids found in germander (Teuchrium chamaedrys) used as an adjuvant to slimming herbal supplements that caused severe hepatotoxicity, are converted by CYP3A4 to reactive epoxide which reacts with proteins such as CYP3A and epoxide hydrolase and inactivate them. Some naturally occurring alkenylbenzenes (e.g. safrole, methyleugenol and estragole) and flavonoids (e.g. quercetin) can undergo bioactivation by sequential 1-hydroxylation and sulfation, resulting in reactive intermediates capable of forming DNA adducts. Extensive pulegone metabolism generated p-cresol that is a glutathione depletory. The hepatotoxicity of kava is possibly due to intracellular glutathione depletion and/or quinone formation. Moreover, several herbal compounds including capsaicin from chili peppers, dially sulfone in garlic, methysticin and dihydromethysticin in kava, oleuropein in olive oil, and resveratrol found in grape seeds are mechanism-based (suicide) inhibitors of various CYPs. Together with advances of proteomics, metabolomics and toxicogenomics, an integrated systems toxicological approach may provide deep insights into mechanistic aspects of herb-induced toxicities, and contribute to bridging the relationships between herbal bioactivation, protein/DNA adduct formation and the toxicological consequences.

Antioxidant and prooxidant properties of flavonoids

The interest in possible health benefits of flavonoids has increased owing to their potent antioxidant and free radical scavenging activities observed in vitro. Nevertheless, the antioxidant efficacy of flavonoids in vivo is less documented and their prooxidant properties have been actually described in vivo. Due to their prooxidant properties, they are able to cause oxidative damage by reacting with various biomolecules, such as lipids, proteins and DNA. Hence, the aim of this review is to discuss both the antioxidant and prooxidant effects of flavonoids.

Roles for stress-inducible lambda glutathione transferases in flavonoid metabolism in plants as identified by ligand fishing

The glutathione transferases (GSTs) of plants are a superfamily of abundant enzymes whose roles in endogenous metabolism are largely unknown. For example, the lambda class of GSTs (GSTLs) have members that are selectively induced by chemical stress treatments and based on their enzyme chemistry are predicted to have roles in redox homeostasis. However, using conventional approaches these functions have yet to be determined. To address this, recombinant GSTLs from wheat and Arabidopsis were tagged with a Strep tag and after affinity-immobilization, incubated with extracts from Arabidopsis, tobacco, and wheat. Bound ligands were then recovered by solvent extraction and identified by mass spectrometry (MS). With the wheat enzyme TaGSTL1, the ligand profiles obtained with in vitro extracts from tobacco closely matched those observed after the protein had been expressed in planta, demonstrating that these associations were physiologically representative. The stress-inducible TaGSTL1 was found to selectively recognize flavonols (e.g. taxifolin; K(d) = 25 nM), with this binding being dependent upon S-glutathionylation of an active site cysteine. In the case of the wheat extracts, this selectivity in ligand recognitions lead to the detection of flavonols that had not been previously described in this cereal. Subsequent in vitro assays showed that the co-binding of flavonols, such as quercetin, to the thiolated TaGSTL1 represented an intermediate step in the reduction of the respective S-glutathionylated quinone derivatives to yield free flavonols. These results suggest a novel role for GSTLs in maintaining the flavonoid pool under stress conditions.

Prooxidant activity of fisetin: effects on energy metabolism in the rat liver

Flavonols, which possess the B-catechol ring, as quercetin, are capable of producing o-hemiquinones and to oxidize NADH in a variety of mammalian cells. The purpose of this study was to investigate whether fisetin affects the liver energy metabolism and the mitochondrial NADH to NAD+ ratio. The action of fisetin on hepatic energy metabolism was investigated in the perfused rat liver and isolated mitochondria. In isolated mitochondria, fisetin decreased the respiratory control and ADP/O ratios with the substrates α-ketoglutarate and succinate. In the presence of ADP, respiration of isolated mitochondria was inhibited with both substrates, indicating an inhibitory action on the ATP-synthase. The stimulation of the ATPase activity of coupled mitochondria and the inhibition of NADH-oxidase activity pointed toward a possible uncoupling action and the interference of fisetin with mitochondrial energy transduction mechanisms. In livers from fasted rats, fisetin inhibited ketogenesis from endogenous sources. The β-hydroxybutyrate/ acetoacetate ratio, which reflects the mitochondrial NADH/NAD+ redox ratio, was also decreased. In addition, fisetin (200 μM) increased the production of (14)CO2 from exogenous oleate. The results of this investigation suggest that fisetin causes a shift in the mitochondrial redox potential toward a more oxidized state with a clear predominance of its prooxidant activity.

Flavonoids and alkenylbenzenes: New concepts in bioactivation studies

The present paper focuses on the biological reactive intermediates formed from two categories of botanical ingredients: flavonoids and alkenylbenzenes. The paper especially presents an overview of three concepts in bioactivation studies on flavonoids and alkenylbenzenes elucidated by our recent studies. These new concepts include (i) the fact that reactive electrophilic quinone/quinone methide type metabolites of flavonoids may be the intermediates required for the induction of the beneficial gene expression through electrophile responsive element (EpRE)-mediated pathways, pointing at a possible beneficial effect of a reactive intermediate, (ii) the development of physiologically based kinetic (PBK) and physiologically based dynamic (PBD) models providing a new way to obtain insight in levels of formation of biologically reactive and unstable intermediates in vivo at high but also more realistic low dose levels, and (iii) the concept of the matrix effect that should be taken into account when studying the bioactivation of food-borne genotoxic carcinogens including the alkenylbenzenes, the bioactivation of which was shown to be inhibited by flavonoids. Together the results presented reveal that by studying the mode of action (MOA) new concepts in bioactivation studies of importance for future risk assessment and/or risk-benefit assessment of the flavonoids and alkenylbenzenes are obtained.

In vitro catabolism of rutin by human fecal bacteria and the antioxidant capacity of its catabolites

The role of colonic microflora in the breakdown of quercetin-3-O-rutinoside (rutin) was investigated. An in vitro fermentation model was used and (i) 28 micromol of rutin and (ii) 55 micromol of quercetin plus 18 x 10(6) dpm of [4-(14)C]quercetin (60 nmol) were incubated with fresh fecal samples from three human volunteers, in the presence and absence of glucose. The accumulation of quercetin during in vitro fermentation demonstrated that deglycosylation is the initial step in the breakdown of rutin. The subsequent degradation of quercetin was dependent upon the interindividual composition of the bacterial microflora and was directed predominantly toward the production of either hydroxyphenylacetic acid derivatives or hydroxybenzoic acids. Possible catabolic pathways for these conversions are proposed. The presence of glucose as a carbon source stimulated the growth and production of bacterial microflora responsible for both the deglycosylation of rutin and the catabolism of quercetin. 3,4-Dihydroxyphenylacetic acid accumulated in large amounts in the fecal samples and was found to possess significant reducing power and free radical scavenging activity. This catabolite may play a key role in the overall antioxidant capacity of the colonic lumen after the ingestion of quercetin-rich foods.

Role of catechin quinones in the induction of EpRE-mediated gene expression

In the present study, the ability of green tea catechins to induce electrophile-responsive element (EpRE)-mediated gene expression and the role of their quinones in the mechanism of this induction were investigated. To this end, Hepa1c1c7 mouse hepatoma cells were used, stably transfected with a luciferase reporter gene under the expression regulation of an EpRE from the human NAD(P)H:quinone oxidoreductase 1 (NQO1) gene. The results obtained show that several, but not all, catechins tested are able to induce EpRE-mediated gene transcription, with epigallocatechin gallate (EGCG) and gallocatechin gallate (GCG), both containing a pyrogallol and a galloyl moiety, being the most powerful inducers. Moreover, it was demonstrated that the EpRE-mediated response to catechins was increased in cells with reduced cellular glutathione (GSH) levels and decreased in cells with increased levels of GSH, corroborating a role for catechin quinones. The intrinsic capacity of catechins to form quinone type metabolites upon their oxidation was demonstrated using incubations of epigallocatechin (EGC) and EGCG with tyrosinase and the GSH-trapping method. Glutathione conjugates formed in these incubations were identified as 2'-glutathionyl-EGC, 2',6'-diglutathionyl-EGC, 2'-glutathionyl-EGCG, and 2',6'-diglutathionyl-EGCG, supporting the formation of quinone type metabolites involving especially the pyrogallol moiety of these catechins. Formation of the EGCG-quinone-glutathionyl adducts was also observed in the EpRE-LUX cellular system. This further supports the importance of the pyrogallol moiety for the quinone chemistry of the catechins. Finally, the presence of the pyrogallol moiety in the catechins also results in a relatively lower half-wave oxidation potential (E1/2) and calculated heat of formation (DHF) for conversion of the catechins to their corresponding quinones, pointing at an increased ability to become oxidized. Altogether, our studies reveal that catechins, especially those containing a pyrogallol moiety, induce EpRE-mediated detoxifying gene expression and that this induction is likely to be the result of their quinone chemistry.

Plant catechols and their S-glutathionyl conjugates as antinitrosating agents: expedient synthesis and remarkable potency of 5-S-glutathionylpiceatannol

With a view to elucidating the structural requisites for effective antinitrosating properties in plant polyphenolics and their metabolites, we have undertaken a comparative investigation of the nitrite scavenging effects of representative catechol derivatives of dietary relevance in the 2,3-diaminonaphthalene (DAN) nitrosation and tyrosine nitration assays. Compounds tested included caffeic acid (1), chlorogenic acid (2), piceatannol (3), hydroxytyrosol (4), and the corresponding S-glutathionyl conjugates 5-8, which were prepared using either tyrosinase (5 and 6) or a novel, o-iodoxybenzoic acid (IBX)-based oxygenation/ conjugation methodology (7b and 8). In the DAN nitrosation assay at pH 4.0, the rank order of inhibitory activities was found to be 5-S-glutathionylpiceatannol (7b) > 3 > 1 > 2 > 2-S-glutathionylcaffeic acid (5) > 2-S-glutathionylchlorogenic acid (6) > 4 approximately 5-S-glutathionylhydroxytyrosol (8). Quite unexpectedly, in the tyrosine nitration assay in 0.5 M HCl, 2 was the most efficient inhibitor followed by 1 > 4 > 3 > 7b approximately 5 > 8 > 6. Under the assay conditions, the glutathionyl conjugates were usually consumed at faster rates than the parent catechols (decomposition rates: 3 > 1 > 4 > 2). The 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) assay indicated that the most effective hydrogen donors were 4 > 7b > 1 approximately 3. Overall, these results indicated that catechol compounds and their glutathionyl conjugates may exhibit profoundly different inhibitory properties depending on the specific conditions of the assay, including especially pH, and that their antinitrosating properties do not correlate tout-court with their hydrogen donor capacity. The glutathionyl-piceatannol conjugate 7b was found to be one of the most potent inhibitors in the physiologically relevant DAN assay and may provide a new structural lead for the design of effective antinitrosating agents based on dietary polyphenolic compounds.

How relevant are flavonoids as antioxidants in plants?

Flavonoids are a large family of plant secondary metabolites, principally recognized for their health-promoting properties in human diets. Most flavonoids outperform well-known antioxidants, such as ascorbate (vitamin C) and alpha-tocopherol (vitamin E), in in vitro antioxidant assays because of their strong capacity to donate electrons or hydrogen atoms. However, experimental evidence for an antioxidant function in plants is limited to a few individual flavonoids under very specific experimental and developmental conditions. As we discuss here, although flavonoids have been demonstrated to accumulate with oxidative stress during abiotic and biotic environmental assaults, a convincing spatio-temporal correlation with the flavonoid oxidation products is not yet available. Thereby, the widely accepted antioxidant function of flavonoids in plants is still a matter of debate.

Enzymatic hemisynthesis of metabolites and conjugates of anthocyanins

This work aims to study the phase II metabolization of anthocyanins that is likely to occur in vivo. Anthocyanins (delphinidin, cyanidin, and malvidin-3-glucosides) were incubated with phase II enzymes in the presence of activated cofactors in order to obtain glutathionyl conjugates, methylated and glucuronydated compounds. Overall, the three anthocyanins tested were metabolized in vitro. Two compounds were detected by HPLC after incubation of human liver cytosolic fraction with cyanidin-3-glucoside and one compound with delphinidin-3-glucoside. These compounds were identified as monomethylated products. LC-MS analysis yielded mass data that fit with the anthocyanin structures bearing an additional methyl group in ring B. Several compounds were detected by HPLC after incubation of human liver microsomes with malvidin, cyanidin, and delphinidin-3-glucosides. These compounds were identified as monoglucuronides products after HPLC analysis. Conjugation with glutathione also occurred as proved by the mass data obtained. However, in this case, two anthocyanin equilibrium forms (flavylium and chalcone or water adducts) conjugated with glutathione were detected. Overall, the data of the present work shows the feasibility of the in vitro enzymatic hemisynthesis of metabolites and glutathione conjugates of anthocyanins. This first experimental approach may further allow the achievement of new purified forms of anthocyanins, some of which do not occur in nature, and also the determination of whether these compounds are the bioactive forms responsible for some of the biological activities reported for anthocyanins.

Nutritional benefit of olive oil: the biological effects of hydroxytyrosol and its arylating quinone adducts

Olive oil is the essential component of the Mediterranean diet, a nutritional regimen gaining ever-increasing renown for its beneficial effects on inflammation, cardiovascular disease, and cancer. A unique characteristic of olive oil is its enrichment in oleuropein, a member of the secoiridoid family, which hydrolyzes to the catechol hydroxytyrosol and functions as a hydrophilic phenolic antioxidant that is oxidized to its catechol quinone during redox cycling. Little effort has been spent on exploring the biological properties of the catechol hydroxytyrosol quinone, a strong arylating electrophile that forms Michael adducts with thiol nucleophiles in glutathione and proteins. This study compares the chemical and biological characteristics of hydroxytyrosol with those of the tocopherol family in which Michael adducts of arylating desmethyltocopherol quinones have been identified and correlated with biologic properties including cytotoxicity and induction of endoplasmic reticulum stress. It is noted that hydroxytyrosol and desmethyltocopherols share many similarities, suggesting that Michael adduct formation by an arylating quinone electrophile may contribute to the biological properties of both families, including the unique nutritional benefit of olive oil.

Health effects of quercetin: from antioxidant to nutraceutical

Quercetin, a member of the flavonoids family, is one of the most prominent dietary antioxidants. It is ubiquitously present in foods including vegetables, fruit, tea and wine as well as countless food supplements and is claimed to exert beneficial health effects. This includes protection against various diseases such as osteoporosis, certain forms of cancer, pulmonary and cardiovascular diseases but also against aging. Especially the ability of quercetin to scavenge highly reactive species such as peroxynitrite and the hydroxyl radical is suggested to be involved in these possible beneficial health effects. Consequently, numerous studies have been performed to gather scientific evidence for these beneficial health claims as well as data regarding the exact mechanism of action and possible toxicological aspects of this flavonoid. The purpose of this review is to evaluate these studies in order to elucidate the possible health-beneficial effects of the antioxidant quercetin. Firstly, the definitions as well as the most important aspects regarding free radicals, antioxidants and oxidative stress will be discussed as background information. Subsequently, the mechanism by which quercetin may operate as an antioxidant (tested in vitro) as well as the potential use of this antioxidant as a nutraceutical (tested both ex vivo and in vivo) will be discussed.

Study of the complexation of fisetin with cyclodextrins

In this work, the interaction between fisetin (3,3',4',7-tetrahydroxyflavone) (Fis) and cyclodextrins (CDs) (alpha and beta) was studied through UV-vis absorption, steady-state fluorescence, induced circular dichroism, and (1)H NMR experiments with dependence on temperature and pH. Some experimental data were compared with quantum-mechanics studies based on the SAM1 (AMPAC) semiempirical model, as well as with the B3LYP and MPW1PW91 functional models from density functional theory using the 6-311G and 3-21G basis sets. The spectroscopic measurements show that Fis does not form stable complexes with alpha-CD. On the other hand, at pH 4.0 and 6.5, the complex Fis-beta-CD is formed in a Fis:beta-CD 1:1 stoichiometry and an equilibrium constant (K) of 900 +/- 100 M(-1). In basic medium (pH 11.5), K decreases to 240 +/- 90 M(-1) because Fis deprotonation leads to its better solubilization in water. Molecular modeling points out that Fis is not totally inserted into the inner cavity of beta-CD. The formation of the inclusion complex renders an environment that enhances intramolecular excited state proton transfer. The inclusion complex is formed preferentially via entry of the Fis phenyl group into beta-CD.

Phenolic constituents of Malus doumeri var. formosana in the field of skin care

Plant phenolic compounds isolated from a 70% aqueous acetone extract of the leaves of Malus doumeri A. CHEV. var. formosana (KAWAK. & KOIDZ.) S. S. YING, a type of Taiwanese indigenous plant, were evaluated for potential application in the field of skin care. A phytochemical investigation of the active fractions resulted in the isolation of seven compounds of which the structures were identified by spectroscopic characterization. In the present study, the isolated phenolic compounds were evaluated for their free radical-scavenging activities against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and the superoxide radicals, anti-elastase, and for their anti-matrix metalloproteinase-1 (MMP-1) activity in human skin fibroblast cells. Of these compounds, 3-hydroxyphloridzin (2), 3-hydroxyphloretin (6), and quercetin (7) exhibited the strongest DPPH and superoxide radical-scavenging activities. The IC50 values of these compounds were 9.2, 7.7, and 15.4 microM, respectively, for the DPPH radical, and 25.0, 19.6, and 42.6 microM, respectively, for the superoxide radical. 3-Hydroxyphloridzin (2) and 3-hydroxyphloretin (6) also showed xanthine oxidase inhibitory activity, with IC(50) values of 52.1 and 22.4 muM, respectively. In the test for elastase inhibitory activity, phloretin (5) and 3-hydroxyphloretin (6) were the most potent compounds. Phloretin (5), 3-hydroxyphloretin (6), and quercetin (7) showed better inhibition of MMP-1 production in fibroblast cells. To the best of our knowledge, this is the first time that the active phenolic compounds from M. doumeri var. formosana have been isolated, reported, and described. The above results suggest that the extract of M. doumeri var. formosana containing phenolic compounds could be suitable naturally occurring active constituents for use in anti-aging or cosmetic products.

No role of DT-diaphorase (NQO1) in the protection against oxidized quercetin

Quercetin is one of the most studied alimentary antioxidants. During its antioxidant activity, quercetin becomes oxidized into its ortho-quinone/quinone methide, denoted as QQ. QQ is toxic since it is highly reactive towards thiols. DT-diaphorase (NQO1) might protect against QQ toxicity by reducing QQ to quercetin. However, conflicting data have been reported. The aim of the present study is to elucidate the role of DT-diaphorase in the protection against QQ-mediated thiol reactivity. It was found that QQ is indeed a substrate for DT-diaphorase. However, QQ reacted much faster with glutathione or protein thiols than with DT-diaphorase in experiments with isolated compounds as well as with human liver cytosol or blood plasma. This indicates that DT-diaphorase has no role in the protection against QQ.