Quenching of quercetin quinone/quinone methides by different thiolate scavengers: stability and reversibility of conjugate formation

Discovery of the covalent SARS-CoV-2 Mpro inhibitors from antiviral herbs via integrating target-based high-throughput screening and chemoproteomic approaches

The main proteases (Mpro ) are highly conserved cysteine-rich proteins that can be covalently modified by numerous natural and synthetic compounds. Herein, we constructed an integrative approach to efficiently discover covalent inhibitors of Mpro from complex herbal matrices. This work begins with biological screening of 60 clinically used antiviral herbal medicines, among which Lonicera japonica Flos (LJF) demonstrated the strongest anti-Mpro effect (IC50  = 37.82 μg/mL). Mass spectrometry (MS)-based chemical analysis and chemoproteomic profiling revealed that LJF extract contains at least 50 constituents, of which 22 exhibited the capability to covalently modify Mpro . We subsequently verified the anti-Mpro effects of these covalent binders. Gallic acid and quercetin were found to potently inhibit severe acute respiratory syndrome coronavirus 2 Mpro in dose- and time- dependent manners, with the IC50 values below 10 µM. The inactivation kinetics, binding affinity and binding mode of gallic acid and quercetin were further characterized by fluorescence resonance energy transfer, surface plasmon resonance, and covalent docking simulations. Overall, this study established a practical approach for efficiently discovering the covalent inhibitors of Mpro from herbal medicines by integrating target-based high-throughput screening and MS-based assays, which would greatly facilitate the discovery of key antiviral constituents from medicinal plants.

Divergent Proteome Reactivity Influences Arm-Selective Activation of Pharmacological Endoplasmic Reticulum Proteostasis Regulators

Pharmacological activation of the activating transcription factor 6 (ATF6) arm of the Unfolded Protein Response (UPR) has proven useful for ameliorating proteostasis deficiencies in a variety of etiologically diverse diseases. Previous high-throughput screening efforts identified the small molecule AA147 as a potent and selective ATF6 activating compound that operates through a mechanism involving metabolic activation of its 2-amino- p -cresol substructure affording a quinone methide, which then covalently modifies a subset of ER protein disulfide isomerases (PDIs). Intriguingly, another compound identified in this screen, AA132, also contains a 2-amino- p -cresol moiety; however, this compound showed less transcriptional selectivity, instead globally activating all three arms of the UPR. Here, we show that AA132 activates global UPR signaling through a mechanism analogous to that of AA147, involving metabolic activation and covalent PDI modification. Chemoproteomic-enabled analyses show that AA132 covalently modifies PDIs to a greater extent than AA147. Paradoxically, activated AA132 reacts slower with PDIs, indicating it is less reactive than activated AA147. This suggests that the higher labeling of PDIs observed with activated AA132 can be attributed to its lower reactivity, which allows this activated compound to persist longer in the cellular environment prior to quenching by endogenous nucleophiles. Collectively, these results suggest that AA132 globally activates the UPR through increased engagement of ER PDIs. Consistent with this, reducing the cellular concentration of AA132 decreases PDI modifications and allows for selective ATF6 activation. Our results highlight the relationship between metabolically activatable-electrophile stability, ER proteome reactivity, and the transcriptional response observed with the enaminone chemotype of ER proteostasis regulators, enabling continued development of next-generation ATF6 activating compounds.

Disentangling the Puzzling Regiochemistry of Thiol Addition to o-Quinones

The addition of thiol compounds to o-quinones, as exemplified by the biologically relevant conjugation of cysteine to dopaquinone, displays an anomalous 1,6-type regiochemistry compared to the usual 1,4-nucleophilic addition, for example, by amines, which has so far eluded intensive investigations. By means of an integrated experimental and computational approach, herein, we provide evidence that the addition of glutathione, cysteine, or benzenethiol to 4-methyl-o-benzoquinone, modeling dopaquinone, proceeds by a free radical chain mechanism triggered by the addition of thiyl radicals to the o-quinone. In support of this conclusion, DFT calculations consistently predicted the correct regiochemistry only for the proposed thiyl radical-quinone addition pathway. These results would prompt a revision of the commonly accepted mechanisms for thiol-o-quinone conjugation and stimulate further work aimed at assessing the impact of the free radical processes in biologically relevant thiol–quinone interactions.

Thymus quinquecostatus Celak. ameliorates cerebral ischemia-reperfusion injury via dual antioxidant actions: Activating Keap1/Nrf2/HO-1 signaling pathway and directly scavenging ROS

BACKGROUND

Thymus quinquecostatus Celak. has been widely used as a spice and a folk medicine for relieving exterior syndrome and alleviating pain in China.

PURPOSE

To explore the protective effects and the underlying mechanism against cerebral ischemia-reperfusion injury (CIRI) of the T. quinquecostatus combining with its chemical composition.

STUDY DESIGN AND METHODS

High-polar extract (HPE) was extracted from T. quinquecostatus and polyphenols in HPE were enriched to obtain polyphenol-rich fraction (PRF) using Macroporous resin. The free radicals and zebrafish embryos were used to compare the antioxidant activities of HPE and PRF in vitro and in vivo. Then, the transient middle cerebral artery occlusion (tMCAO) model was established in rats. Neurological deficit score, infarction rate, morphology and apoptosis of neurons were examined to investigate the protective effects of PRF on CIRI. The mRNA and protein levels of nuclear factor erythroid 2-related factor 2 (Nrf2) and hemeoxygenase-1 (HO-1) and the activities of downstream antioxidant enzymes in ischemia tissues were determined to clarify the underlying mechanisms. Also, reactive oxygen species (ROS) level in zebrafish embryos were detected after incubation with PRF for a short time (2 h) to investigate whether PRF could directly eliminate free radicals. Finally, chemical composition of PRF were analyzed to investigate the material basis for antioxidant activity and anti-CIRI effect.

RESULTS

Compared with HPE, PRF showed stronger antioxidant activities. PRF exhibited obvious protective effects including ameliorating neurological deficit, lowering infarction rate, and improving the cellular morphology in hippocampus CA1 and cortex after tMCAO. TUNEL staining suggested PRF dose-dependently improved the apoptosis of the neurons in ischemic cortex. RT-qPCR and Western Blot results suggested that PRF regulated oxidative stress (OS) via activating the Keap1/Nrf2/HO-1 signaling pathway. Also, PRF could directly scavenge excessive ROS in zebrafish embryos after a short-time PRF incubation. The anti-CIRI effect might be primarily attributed to the abundant polyphenols in PRF, including flavonoids, polymethoxylated flavonoids, flavonoid glycosides, and phenolic acids.

CONCLUSION

T. quinquecostatus contains abundant polyphenols and exhibited a good protective effect against CIRI via dual antioxidant mechanisms, providing a reference for further research and application for this plant.

Engineering Escherichia coli towards de novo production of gatekeeper (2S)-flavanones: naringenin, pinocembrin, eriodictyol and homoeriodictyol

Natural plant-based flavonoids have drawn significant attention as dietary supplements due to their potential health benefits, including anti-cancer, anti-oxidant and anti-asthmatic activities. Naringenin, pinocembrin, eriodictyol and homoeriodictyol are classified as (2S)-flavanones, an important sub-group of naturally occurring flavonoids, with wide-reaching applications in human health and nutrition. These four compounds occupy a central position as branch point intermediates towards a broad spectrum of naturally occurring flavonoids. Here, we report the development of Escherichia coli production chassis for each of these key gatekeeper flavonoids. Selection of key enzymes, genetic construct design and the optimization of process conditions resulted in the highest reported titers for naringenin (484 mg/l), improved production of pinocembrin (198 mg/l) and eriodictyol (55 mg/l from caffeic acid), and provided the first example of in vivo production of homoeriodictyol directly from glycerol (17 mg/l). This work provides a springboard for future production of diverse downstream natural and non-natural flavonoid targets.

Quercetin and Vitamin C: An Experimental, Synergistic Therapy for the Prevention and Treatment of SARS-CoV-2 Related Disease (COVID-19)

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) represents an emergent global threat which is straining worldwide healthcare capacity. As of May 27th, the disease caused by SARS-CoV-2 (COVID-19) has resulted in more than 340,000 deaths worldwide, with 100,000 deaths in the US alone. It is imperative to study and develop pharmacological treatments suitable for the prevention and treatment of COVID-19. Ascorbic acid is a crucial vitamin necessary for the correct functioning of the immune system. It plays a role in stress response and has shown promising results when administered to the critically ill. Quercetin is a well-known flavonoid whose antiviral properties have been investigated in numerous studies. There is evidence that vitamin C and quercetin co-administration exerts a synergistic antiviral action due to overlapping antiviral and immunomodulatory properties and the capacity of ascorbate to recycle quercetin, increasing its efficacy. Safe, cheap interventions which have a sound biological rationale should be prioritized for experimental use in the current context of a global health pandemic. We present the current evidence for the use of vitamin C and quercetin both for prophylaxis in high-risk populations and for the treatment of COVID-19 patients as an adjunct to promising pharmacological agents such as Remdesivir or convalescent plasma.

Molecular and cellular basis of the anticancer activity of the prenylated flavonoid icaritin in hepatocellular carcinoma

The prenylated flavonoid icaritin (ICT) is currently undergoing phase 3 clinical trial for the treatment of advanced hepatocellular carcinoma (HCC), based on a solid array of preclinical and clinical data. The antitumor activity originates from the capacity of the drug to modulate several signaling effectors in cancer cells, mainly the estrogen receptor splice variant ERα36, the transcription factors STAT3 and NFκB, and the chemokine receptor CXCR4. Recent studies have implicated additional components, including different microRNAs, the generation of reactive oxygen species and the targeting of sphingosine kinase-1. ICT also engages the RAGE-HMGB1 signaling route and modulates the apoptosis/autophagy crosstalk to promote its anticancer activity. In addition, ICT exerts profound changes on the tumor microenvironment to favor an immune-response. Collectively, these multiple biochemical and cellular characteristics confer to ICT a robust activity profile which can be exploited to treat HCC, as well as other cancers, including glioblastoma and onco-hematological diseases such as chronic myeloid leukemia. This review provides an update of the pharmacological properties of ICT and its metabolic characteristics. It also addresses the design of derivatives, including both natural products and synthetic molecules, such as SNG1153 also in clinical trial. The prenylated flavonoid ICT deserves attention as a multifunctional natural product potentially useful to improve the treatment of advanced hepatocellular carcinoma.

A phytotoxic impact of phenolic compounds in olive oil mill wastewater on fenugreek "Trigonella foenum-graecum"

The objective of this study is the determination of the chemical structure of nine phenolic molecules responsible for the phytotoxic action on the germination of the plant species "Trigonella foenum-graecum". The phytotoxic action was evaluated by calculating the germination index of the plant species for a period of 5 days of incubation. The analysis of the physicochemical properties of phenolic molecules shows that hydrophobicity is a key factor in phytotoxicity. The sublethal concentration varies as follows: hydroquinone (0.91 mM), 4-aminophenol (0.85 mM), phenol (0.75 mM), gallic acid (0.59 mM), caffeic acid (0.56 mM), 3,5-di-tert-butylcatechol (0,45 mM), quercetin (0.33 mM), oleuropein (0.3 mM), and catechol (0.13 mM). Phytotoxicity varies depending on the nature and position of the substituents on the aromatic ring. The reactivity of this type of molecule is partly linked to the presence of catechol function that can play the main role in phytotoxicity of the Fenugreek.

Metabolism and growth inhibitory activity of cranberry derived flavonoids in bladder cancer cells

In the present study, anti-proliferative activities of cranberry derived flavonoids and some of their in vivo metabolites were evaluated using a panel of human bladder tumor cell lines (RT4, SCABER, and SW-780) and non-tumorigenic immortalized human uroepithelial cells (SV-HUC). Among the compounds tested, quercetin 3-O-glucoside, isorhamnetin (3'-O-methylquercetin), myricetin and quercetin showed strong concentration-dependent cell growth inhibitory activities in bladder cancer cells with IC₅₀ values in a range of 8-92 μM. Furthermore, isorhamnetin and myricetin had very low inhibitory activity against SV-HUC even at very high concentrations (>200 μM) compared to bladder cancer cells, indicating that their cytotoxicity is selective for cancer cells. To determine whether the differential cell growth inhibitory effects of isomeric flavonoids quercetin 3-O-glucoside (active) and hyperoside (quercetin 3-O-galactoside) (inactive) are related to their metabolism by the cancer cells, SW-780 cells were incubated with these compounds and their metabolism was examined by LC-MS/MS. Compared to quercetin 3-O-glucoside, hyperoside undergoes relatively less metabolic biotransformation (methylation, glucuronidation and quinone formation). These data suggest that isorhamnetin and quercetin 3-O-glucoside may be the active forms of quercetin in prevention of bladder cancer in vivo and emphasize the importance of metabolism for the prevention of bladder cancer by diets rich in cranberries.

Quercetin Oxidation Paradoxically Enhances its Antioxidant and Cytoprotective Properties

Quercetin oxidation is generally believed to ultimately result in the loss of its antioxidant properties. To test this assertion, quercetin oxidation was induced, and after each of its major metabolites was identified and isolated by HPLC-DAD-ESI-MS/MS, the antioxidant (dichlorodihydrofluorescein oxidation-inhibiting) and cytoprotective (LDH leakage-preventing) properties were evaluated in Hs68 and Caco2 cells exposed to indomethacin. Compared to quercetin, the whole mixture of metabolites (Q_OX) displayed a 20-fold greater potency. After resolution of Q_OX into 12 major peaks, only one (peak 8), identified as 2,5,7,3',4'-pentahydroxy-3,4-flavandione or its 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone tautomer, could account for the antioxidant and cytoprotective effects afforded Q_OX. Peak 8 exerted such effects at a 50 nM concentration, revealing a potency 200-fold higher than that of quercetin. The effects of peak 8 were seen regardless of whether it was added to the cells 40 min before or simultaneously with the oxygen-reactive species-generating agent, suggesting an intracellular ability to trigger early antioxidant responses. Thus, the present study is the first to reveal that in regard to the intracellular actions of quercetin, attention should be extended toward some of its oxidation products.

Oxidative metabolism of curcumin-glucuronide by peroxidases and isolated human leukocytes

Conjugation with glucuronic acid is a prevalent metabolic pathway of orally administrated curcumin, the bioactive diphenol of the spice turmeric. The major in vitro degradation reaction of curcumin is autoxidative transformation resulting in oxygenation and cyclization of the heptadienedione chain to form cyclopentadione derivatives. Here we show that curcumin-glucuronide is much more stable than curcumin, degrading about two orders of magnitude slower. Horseradish peroxidase-catalyzed oxidation of curcumin-glucuronide occurred at about 80% of the rate with curcumin, achieving efficient transformation. Using LC-MS and NMR analyses the major products of oxidative transformation were identified as glucuronidated bicyclopentadione diastereomers. Cleavage into vanillin-glucuronide accounted for about 10% of the products. Myeloperoxidase and lactoperoxidase oxidized curcumin-glucuronide whereas tyrosinase and xanthine oxidase were not active. Phorbol ester-activated primary human leukocytes showed increased oxidative transformation of curcumin-glucuronide which was inhibited by the peroxidase inhibitor sodium azide. These studies provide evidence that the glucuronide of curcumin is not an inert product and may undergo further enzymatic and non-enzymatic metabolism. Oxidative transformation by leukocyte myeloperoxidase may represent a novel metabolic pathway of curcumin and its glucuronide conjugate.

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.

Spectrophotometric Determination of Phenolic Antioxidants in the Presence of Thiols and Proteins

Development of easy, practical, and low-cost spectrophotometric methods is required for the selective determination of phenolic antioxidants in the presence of other similar substances. As electron transfer (ET)-based total antioxidant capacity (TAC) assays generally measure the reducing ability of antioxidant compounds, thiols and phenols cannot be differentiated since they are both responsive to the probe reagent. In this study, three of the most common TAC determination methods, namely cupric ion reducing antioxidant capacity (CUPRAC), 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt/trolox equivalent antioxidant capacity (ABTS/TEAC), and ferric reducing antioxidant power (FRAP), were tested for the assay of phenolics in the presence of selected thiol and protein compounds. Although the FRAP method is almost non-responsive to thiol compounds individually, surprising overoxidations with large positive deviations from additivity were observed when using this method for (phenols + thiols) mixtures. Among the tested TAC methods, CUPRAC gave the most additive results for all studied (phenol + thiol) and (phenol + protein) mixtures with minimal relative error. As ABTS/TEAC and FRAP methods gave small and large deviations, respectively, from additivity of absorbances arising from these components in mixtures, mercury(II) compounds were added to stabilize the thiol components in the form of Hg(II)-thiol complexes so as to enable selective spectrophotometric determination of phenolic components. This error compensation was most efficient for the FRAP method in testing (thiols + phenols) mixtures.

Degradation of Curcumin: From Mechanism to Biological Implications

Curcumin is the main bioactive ingredient in turmeric extract and widely consumed as part of the spice mix curry or as a dietary supplement. Turmeric has a long history of therapeutic application in traditional Asian medicine. Biomedical studies conducted in the past two decades have identified a large number of cellular targets and effects of curcumin. In vitro curcumin rapidly degrades in an autoxidative transformation to diverse chemical species, the formation of which has only recently been appreciated. This paper discusses how the degradation and metabolism of curcumin, through products and their mechanism of formation, provide a basis for the interpretation of preclinical data and clinical studies. It is suggested that the previously unrecognized diversity of its degradation products could be an important factor in explaining the polypharmacology of curcumin.

Identification and quantification of adducts between oxidized rosmarinic acid and thiol compounds by UHPLC-LTQ-Orbitrap and MALDI-TOF/TOF tandem mass spectrometry

LTQ Orbitrap MS/MS was used to identify the adducts between quinones derived from rosmarinic acid (RosA) and thiol compounds, including cysteine (Cys), glutathione (GSH), and peptides digested from myosin. Two adducts of quinone-RosA/Cys and quinone-RosA/2Cys, one quinone-RosA/GSH adduct, and three quinone-RosA/peptide adducts were identified by extracted ion and MS(2) fragment ion chromatograms. By using MALDI-TOF/TOF MS, the adduction reaction between RosA and myosin in myofibrillar protein isolates was determined, demonstrating that the accurate reaction site was at Cys949 of myosin. The effect of reaction conditions, including stirring time, temperature, and oxidative stress, on the formation of adducts was further investigated. The formation of quinone-RosA/Cys and quinone-RosA/GSH increased with stirring time. Both adducts increased with temperature, whereas the reactivity of the addition reaction of GSH was higher than that of Cys. With increasing oxidation stress, the formation of quinone-RosA/GSH adduct increased and that of quinone-RosA/Cys adduct decreased.

Chemical and biological mechanisms of phytochemical activation of Nrf2 and importance in disease prevention

Plants are an incredibly rich source of compounds that activate the Nrf2 transcription factor, leading to upregulation of a battery of cytoprotective genes. This perspective surveys established and proposed molecular mechanisms of Nrf2 activation by phytochemicals with a special emphasis on a common chemical property of Nrf2 activators: the ability as "soft" electrophiles to modify cellular thiols, either directly or as oxidized biotransformants. In addition, the role of reactive oxygen/nitrogen species as secondary messengers in Nrf2 activation is discussed. While the uniquely reactive C151 of Keap1, an Nrf2 repressor protein, is highlighted as a key target of cytoprotective phytochemicals, also reviewed are other stress-responsive proteins, including kinases, which play non-redundant roles in the activation of Nrf2 by plant-derived agents. Finally, the perspective presents two key factors accounting for the enhanced therapeutic windows of effective phytochemical activators of the Keap1-Nrf2 axis: enhanced selectivity toward sensor cysteines and reversibility of addition to thiolate molecules.

Facilitated uptake of a bioactive metabolite of maritime pine bark extract (pycnogenol) into human erythrocytes

Many plant secondary metabolites exhibit some degree of biological activity in humans. It is a common observation that individual plant-derived compounds in vivo are present in the nanomolar concentration range at which they usually fail to display measurable activity in vitro. While it is debatable that compounds detected in plasma are not the key effectors of bioactivity, an alternative hypothesis may take into consideration that measurable concentrations also reside in compartments other than plasma. We analysed the binding of constituents and the metabolite δ-(3,4-dihydroxy-phenyl)-γ-valerolactone (M1), that had been previously detected in plasma samples of human consumers of pine bark extract Pycnogenol, to human erythrocytes. We found that caffeic acid, taxifolin, and ferulic acid passively bind to red blood cells, but only the bioactive metabolite M1 revealed pronounced accumulation. The partitioning of M1 into erythrocytes was significantly diminished at higher concentrations of M1 and in the presence of glucose, suggesting a facilitated transport of M1 via GLUT-1 transporter. This concept was further supported by structural similarities between the natural substrate α-D-glucose and the S-isomer of M1. After cellular uptake, M1 underwent further metabolism by conjugation with glutathione. We present strong indication for a transporter-mediated accumulation of a flavonoid metabolite in human erythrocytes and subsequent formation of a novel glutathione adduct. The physiologic role of the adduct remains to be elucidated.

Catalase is inhibited by flavonoids

Catalases, heme enzymes, which catalyze decomposition of hydrogen peroxide to water and molecular oxygen, belong to the antioxidant defense system of the cell. In this work we have shown that catalase from bovine liver is inhibited by flavonoids. The inhibition is, at least partially, due to the formation of hydrogen bonds between catalase and flavonoids. In the presence of some flavonoids the formation of unreactive catalase compound II has been detected. The most potent catalase inhibitors among the tested flavonoids have appeared myricetin, epicatechin gallate and epigallocatechin gallate. The relationship between the degree of enzyme inhibition and molecular structure of flavonoids has been analyzed.

Oxidative stability of (-)-epigallocatechin gallate in the presence of thiols

Polyphenols are attractive ingredients due to their purported health benefits, but their addition to foods is limited by their chemical instability, as they are rapidly oxidized under many conditions. This oxidation not only compromises the potential biological activity of the phenolic compound, but can also affect the chemical stability of the surrounding food matrix. Polyphenols bearing catechol or gallate groups, when oxidized to their benzoquinone forms, are strong electrophiles capable of reacting with nucleophilic thiols via 1,4-Michael addition reactions. These reactions are known to proceed in foods during processing and storage, and can profoundly affect the quality and biological efficacy of polyphenols when they are added as functional food ingredients. The stability of (-)-epigallocatechin gallate (EGCG) in the presence of three thiol-containing species [cysteine (Cys), glutathione (GSH), 3-mercaptohexan-1-ol (3SH)] was followed under both neutral and acidic conditions. Both Cys and GSH increased the rate of EGCG oxidation at pH 4. At pH 7, only Cys was found to increase the rate of EGCG oxidation. On the basis of these results, the reactivity of thiols toward EGCG follows the trend: Cys > GSH > 3SH, which is consistent with observed thiol-quinone adduct formation rates. Contrary to the results observed for Cys and GSH, 3SH was observed to inhibit EGCG oxidation.

Chemical interaction between polyphenols and a cysteinyl thiol under radical oxidation conditions

Chemical interaction between polyphenols and thiols was investigated under radical oxidation conditions using a model cysteinyl thiol derivative, N-benzoylcysteine methyl ester. The radical oxidation was carried out with a stoichiometric amount of 2,2-diphenyl-1-picrylhydrazyl (DPPH), and the decreases in the amounts of polyphenols and the thiol were measured by HPLC analysis. Cross-coupling products between various polyphenols and the thiol were examined by LC-MS in reactions that showed decreases in both the polyphenols and the thiol. The LC-MS results indicated that three phenolic acid esters (methyl caffeate, methyl dihydrocaffeate, and methyl protocatechuate) and six flavonoids (kaempferol, myricetin, luteolin, morin, taxifolin, and catechin) gave corresponding thiol adducts, whereas three polyphenols (methyl ferulate, methyl sinapate, and quercetin) gave only dimers or simple oxidation products without thiol substituents. Thiol adducts of the structurally related compounds methyl caffeate and methyl dihydrocaffeate were isolated, and their chemical structures were determined by NMR analysis. The mechanism for the thiol addition was discussed on the basis of the structures of the products.

(-)-Epigallocatechin-3-gallate suppresses growth of AZ521 human gastric cancer cells by targeting the DEAD-box RNA helicase p68

(-)-Epigallocatechin-3-gallate (EGCG), the most abundant and biologically active polyphenol in green tea, induces apoptosis and suppresses proliferation of cancer cells by modulating multiple signal transduction pathways. However, the fundamental mechanisms responsible for these cancer-preventive effects have not been clearly elucidated. Recently, we found that EGCG can covalently bind to cysteine residues in proteins through autoxidation and subsequently modulate protein function. In this study, we demonstrate the direct binding of EGCG to cellular proteins in AZ521 human gastric cancer cells by redox-cycle staining. We comprehensively explored the binding targets of EGCG from EGCG-treated AZ521 cells by proteomics techniques combined with the boronate-affinity pull-down method. The DEAD-box RNA helicase p68, which is overexpressed in a variety of tumor cells and plays an important role in cancer development and progression, was identified as a novel EGCG-binding target. Exposure of AZ521 cells to EGCG lowered the p68 level dose dependently. The present findings show that EGCG inhibits AZ521 cell proliferation by preventing β-catenin oncogenic signaling through proteasomal degradation of p68 and provide a new perspective on the molecular mechanism of EGCG action.

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.

Few constraints limit the design of quinone methide-oligonucleotide self-adducts for directing DNA alkylation

Nucleotide sequences minimally containing adenosine, cytosine or guanosine are sufficient to form intrastrand oligonucleotide quinone methide self-adducts reversibly for subsequent alkylation of complementary DNA. The general lack of sequence restrictions should now allow for alkylation of most any target of interest although reaction is most efficient when the self-adducts contain guanine residues and do not form hairpin structures.

Human serum albumin as an antioxidant in the oxidation of (-)-epigallocatechin gallate: participation of reversible covalent binding for interaction and stabilization

Human serum albumin (HSA) contributes to the stabilization of (-)-epigallocatechin gallate (EGCg) in serum. We characterize in the present study the mechanisms for preventing EGCg oxidation by HSA. EGCg was stable in human serum or buffers with HSA, but (-)-epigallocatechin (EGC) was unstable. We show by comparing EGCg and EGC in a neutral buffer that EGCg had a higher binding affinity than EGC. This indicates that the galloyl moiety participated in the interaction of EGCg with HSA and that this interaction was of critical importance in preventing EGCg oxidation. The binding affinity of EGCg for HSA and protein carbonyl formation in HSA were enhanced in an alkaline buffer. These results suggest the reversible covalent modification of EGCg via Schiff-base formation, and that the immobilization of EGCg to HSA, through the formation of a stable complex, prevented the polymerization and decomposition of EGCg in human serum.

Green tea polyphenol epigallocatechin-3-gallate (EGCG) induced intermolecular cross-linking of membrane proteins

Increasing evidence has demonstrated that EGCG possesses prooxidant potential in biological systems, including modifying proteins, breaking DNA strands and inducing the generation of reactive oxygen species. In the present study, the prooxidant effect of EGCG on erythrocyte membranes was investigated. SDS-PAGE and NBT-staining assay were utilized to detect the catechol-protein adducts that generated upon treating the membranes with EGCG. The results indicated that EGCG was able to bind covalently to sulfhydryl groups of membrane proteins, leading to the formation of protein aggregates with intermolecular cross-linking. We suggested that the catechol-quinone originated from the oxidation of EGCG acted as a cross-linker on which peptide chains were combined through thiol-S-alkylation at the C2- and C6-sites of the gallyl ring. EGC showed similar effects as EGCG on the ghost membranes, whereas ECG and EC did not, suggesting that a structure with a gallyl moiety is a prerequisite for a catechin to induce the aggregation of membrane proteins and to deplete membrane sulfhydryls. EDTA and ascorbic acid inhibited the EGCG-induced aggregation of membrane proteins by blocking the formation of catechol-quinone. The information of the present study may provide a fresh insight into the prooxidant effect and cytotoxicity of tea catechins.

Structural characteristics of green tea catechins for formation of protein carbonyl in human serum albumin

Catechins are polyphenolic antioxidants found in green tea leaves. Recent studies have reported that various polyphenolic compounds, including catechins, cause protein carbonyl formation in proteins via their pro-oxidant actions. In this study, we evaluate the formation of protein carbonyl in human serum albumin (HSA) by tea catechins and investigate the relationship between catechin chemical structure and its pro-oxidant property. To assess the formation of protein carbonyl in HSA, HSA was incubated with four individual catechins under physiological conditions to generate biotin-LC-hydrazide labeled protein carbonyls. Comparison of catechins using Western blotting revealed that the formation of protein carbonyl in HSA was higher for pyrogallol-type catechins than the corresponding catechol-type catechins. In addition, the formation of protein carbonyl was also found to be higher for the catechins having a galloyl group than the corresponding catechins lacking a galloyl group. The importance of the pyrogallol structural motif in the B-ring and the galloyl group was confirmed using methylated catechins and phenolic acids. These results indicate that the most important structural element contributing to the formation of protein carbonyl in HSA by tea catechins is the pyrogallol structural motif in the B-ring, followed by the galloyl group. The oxidation stability and binding affinity of tea catechins with proteins are responsible for the formation of protein carbonyl, and consequently the difference in these properties of each catechin may contribute to the magnitude of their biological activities.

Methylation of dietary flavones increases their metabolic stability and chemopreventive effects

Dietary flavones have promising chemoprotective properties, in particular with regard to cancer, but problems with low oral bioavailability and sometimes unacceptable toxicity have made their use as protective additives to normal diets questionable. However, methylation of free phenolic hydroxyl groups leads to derivatives not susceptible to glucuronic acid or sulfate conjugation, resulting in increased metabolic stability. Methylation also leads to greatly improved transport through biological membranes, such as in intestinal absorption, and much increased oral bioavailability. Recent studies also indicate that methylation results in derivatives with increasing potency to kill cancer cells. They also show high potency towards inhibition of hormone-regulating enzymes, e.g., aromatase, important in the causation of breast cancer. Methylation of the flavones may also result in derivatives with diminished toxic side-effects and improved aqueous solubility. In conclusion, it appears that methylation of dietary flavones as well as of other food products may produce derivatives with much improved health effects.

Cytotoxic effects of the dietary flavones chrysin and apigenin in a normal trout liver cell line

Many flavonoids have been shown to possess prooxidant properties, capable of causing oxidative stress, especially at larger doses. Here, we examined the potential cell toxicity caused by exposure to the hydroxylated flavones chrysin, apigenin, luteolin and quercetin in comparison to the methylated flavones 5,7-dimethoxyflavone and 3',4'-dimethoxyflavone in normal Rainbow trout hepatocytes. The hydroxylated flavones, especially chrysin, demonstrated cell toxicity and inhibition of DNA synthesis at very low (2 microM) concentrations. The cytotoxicity of chrysin may partially be due to its metabolism by myeloperoxidase, which was shown to be present in these normal trout liver cells (164pmol/(min mg protein)). In contrast, methylated flavones showed no significant metabolism by myeloperoxidase and no signs of toxicity, even at much higher concentrations. These results may be useful for further investigations of cytotoxicity of dietary flavonoids.

In Vitro Bioactivation of 3-(N-Phenylamino)propane-1,2-diol by Human and Rat Liver Microsomes and Recombinant P450 Enzymes. Implications for Toxic Oil Syndrome

Toxic oil syndrome (TOS) was a massive food-borne intoxication that occurred in Spain in 1981. Epidemiological studies imputed 3-( N-phenylamino)propane-1,2-diol (PAP) derivatives as the toxic agents. The in vitro bioactivation of PAP by rat and human liver microsomes was studied. In both cases, 3-[ N-(4'-hydroxyphenyl)amino]propane-1,2-diol ( 1) was detected as the main metabolite. Inhibition studies with pooled human liver microsomes in the presence and absence of P450-specific inhibitors suggest that 2C8 and 2E1 are the main enzymes involved in PAP bioactivation, followed by 3A4/5, 1A1/2, and 2C9. Incubations of PAP with 10 different recombinant P450 enzymes showed that 2C8, 2C9, 2C18, 2D6, and 2E1 catalyzed PAP 4'-hydroxylation. Incubations of phenol 1 with rat and human liver microsomes in the presence of GSH resulted in the formation of a glutathione conjugate of a quinoneimine metabolite derived from 1. In rat liver microsomes, P450 enzymes play a key role in the bioactivation of 1, whereas in human liver microsomes, autoxidation appears to be the major mechanism. The implications of these results for toxic oil syndrome are discussed.

Induction of apoptosis by quercetin: different response of human chronic myeloid (K562) and acute lymphoblastic (HSB-2) leukemia cells

This work shows that 25 microM quercetin caused a marked inhibition of K562 cells growth together with a mild cytotoxicity, while HSB-2 cells were practically unaffected. Moreover, quercetin induced caspase-3 and cytochrome c-dependent apoptosis almost exclusively in the former cell line. Exposure of K562 cells to quercetin caused also a significant increase of cells in G(2)/M phase that reached the maximum peak at 24 h (4-fold with respect to the basal value). The major sensitivity exhibited by K562 cells was only in part imputable to their higher glutathione content, as compared to HSB-2 cells, thus confirming previous reports describing the formation of intracellular quercetin-thiol toxic adducts in cells exposed to the flavonoid. In fact, after induction of intracellular glutathione increase we detected in both cell lines a significant rise of apoptotic cells, again more marked in K562 cells. By contrast, glutathione-depleted cells, failed to show a decrease of apoptosis in both cell lines, thus contradicting our previous findings and literature data. Since the yet unresolved question about the anti-oxidant or the pro-oxidant capacity of quercetin, we investigated which of these two properties worked in our experimental model. Interestingly, not only quercetin did not produce reactive oxygen species but also prevented their formation, as observed in cells exposed to the oxidizing agent ter-butylhydroperoxide, acting as an efficient oxygen radicals scavenger. This result indicates that quercetin exhibited, in these cell lines, anti-oxidant more than pro-oxidant ability.

Effects of isocoumarins isolated from Paepalanthus bromelioides on mitochondria: Uncoupling, and induction/inhibition of mitochondrial permeability transition

Isolated mitochondria may undergo uncoupling, and in presence of Ca(2+) at different conditions, a mitochondrial permeability transition (MPT) linked to protein thiol oxidation, and demonstrated by CsA-sensitive mitochondrial swelling; these processes may cause cell death either by necrosis or by apoptosis. Isocoumarins isolated from the Brazilian plant Paepalanthus bromelioides (Eriocaulaceae) paepalantine (9,10-dihydroxy-5,7-dimethoxy-1H-naptho(2,3c)pyran-1-one), 8,8'-paepalantine dimer, and vioxanthin were assayed at 1-50 microM on isolated rat liver mitochondria, for respiration, MPT, protein thiol oxidation, and interaction with the mitochondrial membrane using 1,6-diphenyl-1,3,5-hexatriene (DPH). The isocoumarins did not significantly affect state 3 respiration of succinate-energized mitochondria; they did however, stimulate 4 respiration, indicating mitochondrial uncoupling. Induction of MPT and protein thiol oxidation were assessed in succinate-energized mitochondria exposed to 10 microM Ca(2+); inhibition of these processes was assessed in non-energized organelles in the presence of 300 microM t-butyl hydroperoxide plus 500 microM Ca(2+). Only paepalantine was an effective MPT/protein thiol oxidation inducer, also releasing cytochrome c from mitochondria; the protein thiol oxidation, unlike mitochondrial swelling, was neither inhibited by CsA nor dependent on the presence of Ca(2+). Vioxanthin was an effective inhibitor of MPT/protein thiol oxidation. All isocoumarins inserted deeply into the mitochondrial membrane, but only paepalantine dimer and vioxantin decreased the membrane's fluidity. A direct reaction with mitochondrial membrane protein thiols, involving an oxidation of these groups, is proposed to account for MPT induction by paepalantine, while a restriction of oxidation of these same thiol groups imposed by the decrease of membrane fluidity, is proposed to account for MPT inhibition by vioxanthin.

Inhibition of Mammalian Thioredoxin Reductase by Some Flavonoids: Implications for Myricetin and Quercetin Anticancer Activity

The thioredoxin system, composed of thioredoxin reductase (TrxR), thioredoxin (Trx), and NADPH, exerts a wide range of activities in cellular redox control, antioxidant function, cell viability, and proliferation. Recently, the selenocysteine (Sec)-containing mammalian TrxR has emerged as a new target for anticancer drug development because TrxR and Trx are overexpressed in many aggressive tumors and the tumor cells seem to be more dependent on Trx system than normal cells. Here we have investigated the inhibition of mammalian TrxR by flavonoids which have been presumed to be cancer chemoprevention agents because of their antioxidant activities. Myricetin and quercetin were found to have strong inhibitory effects on mammalian TrxRs with IC50 values of 0.62 and 0.97 micromol/L, respectively. The inhibition was shown to be concentration, NADPH, and time dependent and involved an attack on the reduced COOH-terminal -Cys-Sec-Gly active site of TrxR. Oxygen-derived superoxide anions enhanced the inhibitory effect whereas anaerobic conditions attenuated inhibition. Spectral analysis suggested that the flavonols might perform their inhibitory effects via semiquinone radicals. Additionally, the flavonols had the potential to inhibit the growth of A549 cells with the same potency as inhibition of TrxR. TrxR activity in the cell lysates was reduced on treatment with myricetin >50 micromol/L, which coincided with the oxidization of Trx. The cell cycle was arrested in S phase by quercetin and an accumulation of cells in sub-G1 was observed in response to myricetin. Thus, the anticancer activity of quercetin and myricetin may be due to inhibition of TrxR, consequently inducing cell death.

Red wine alcohol promotes quercetin absorption and directs its metabolism towards isorhamnetin and tamarixetin in rat intestine in vitro

Moderate consumption of red wine has been associated with beneficial effects on human health, and this has been attributed to the flavonoid content. Factors that influence the bioavailability of this group of polyphenolic compounds are therefore important. Using the rat cannulated everted jejunal sac technique, we have investigated the effect of alcohol on the intestinal absorption of quercetin and its 3-O-glucoside from red wine. Tissue preparations were incubated in whole or dealcoholised red wine, diluted 1 : 1 with Krebs buffer for 20 min at 37 degrees C, after which the mucosa was removed and processed for HPLC analysis. Tissues exposed to red wine had significantly higher amounts of both quercetin (x 3; P < 0.001) and quercetin-3-O-glucoside (x 1.5; P < 0.01) associated with them, compared with sacs incubated in the dealcoholised equivalent. In addition, both tamarixetin (T) and isorhamnetin (I), in the mucosal tissue from sacs exposed to the whole wine, were significantly elevated approximately two fold (P < 0.05; P < 0.01, respectively). Similar results were obtained when sacs were incubated in Krebs buffer containing a mixture of pure quercetin and quercetin-3-O-glucoside with or without alcohol, and, although effects on the apparent absorption of Q and Q-3-G were not so marked, concentrations of the metabolites quercetin-3-O-glucuronide and I were significantly increased by the presence of alcohol (P < 0.01 and P < 0.001, respectively). It is therefore plausible that the moderate alcohol content of red wine contributes to its beneficial health effects in humans by both increasing the absorption of quercetin and quercetin-3-O-glucoside and by channelling their metabolism towards O-methylation to yield compounds (T and I), which have potential protective effects against cancer and cardiovascular diseases.

Consequences of quercetin methylation for its covalent glutathione and DNA adduct formation

This study investigates the pro-oxidant activity of 3'- and 4'-O-methylquercetin, two relevant phase II metabolites of quercetin without a functional catechol moiety, which is generally thought to be important for the pro-oxidant activity of quercetin. Oxidation of 3'- and 4'-O-methylquercetin with horseradish peroxidase in the presence of glutathione yielded two major metabolites for each compound, identified as the 6- and 8-glutathionyl conjugates of 3'- and 4'-O-methylquercetin. Thus, catechol-O-methylation of quercetin does not eliminate its pro-oxidant chemistry. Furthermore, the formation of these A-ring glutathione conjugates of 3'- and 4'-O-methylquercetin indicates that quercetin o-quinone may not be an intermediate in the formation of covalent quercetin adducts with glutathione, protein and/or DNA. In additional studies, it was demonstrated that covalent DNA adduct formation by a mixture of [4-(14)C]-3'- and 4'-O-methylquercetin in HepG2 cells amounted to only 42% of the level of covalent adducts formed by a similar amount of [4-(14)C]-quercetin. Altogether, these results reveal the effect of methylation of the catechol moiety of quercetin on its pro-oxidant behavior. Methylation of quercetin does not eliminate but considerably attenuates the cellular implications of the pro-oxidant activity of quercetin, which might add to the mechanisms underlying the apparent lack of in vivo carcinogenicity of this genotoxic compound. The paper also presents a new mechanism for the pro-oxidant chemistry of quercetin, eliminating the requirement for formation of an o-quinone, and explaining why methylation of the catechol moiety does not fully abolish formation of reactive DNA binding metabolites.

Formation of Transient Covalent Protein and DNA Adducts by Quercetin in Cells with and without Oxidative Enzyme Activity

This study investigates the role of cellular tyrosinase and/or peroxidase-like oxidative enzyme activity in the covalent binding of quercetin to glutathione, protein, and DNA, as well as the stability of quercetin DNA adducts in time. This was done by studying the formation of glutathionyl quercetin adducts in various in vitro models, and the covalent binding of radiolabeled quercetin to protein and DNA in cells with elevated peroxidase or tyrosinase levels and in cells devoid of nucleotide excision repair (NER). Cells with elevated tyrosinase or peroxidase levels contained approximately 2 times higher levels of covalent quercetin adducts than cells without detectable levels of these oxidative enzymes. However, this difference was smaller than expected based on the differences in tyrosinase and/or peroxidase levels, indicating that these types of oxidative enzyme activities do not play a major role in the cellular pro-oxidant activity of quercetin. Furthermore, quercetin DNA adducts were of transient nature, independent of the presence of NER, suggesting chemical instability of the adducts. Whether this transient nature reflects real reversibility or formation of genotoxic, depurinated sites remains to be investigated at the molecular level. Together, these data indicate that formation of covalent quercetin adducts can be expected in all cells, independent of their oxidative enzyme levels, whereas the transient nature of the DNA adducts formed may limit or cause their ultimate biological impact. If the transient nature represents chemical reversibility of the adduct formation, it would provide a possible explanation for the apparent lack of in vivo carcinogenicity of this in vitro mutagen. Therefore, in vitro mutagenicity studies should focus more on the transient nature of DNA adducts responsible for the mutagenicity in vitro, since this transient nature of DNA adducts may play an essential role in whether the genotoxicity observed in vitro will have any impact in vivo.

Human NAD(P)H:quinone oxidoreductase inhibition by flavonoids in living cells

Procedures for assessing enzyme inhibition in living cells are an important tool in the study of the relevance of enzyme-catalyzed reactions and interactions in the human body. This paper presents the effects of flavonoids on NAD(P)H:quinone oxidoreductase 1 (NQO1) activity, by a newly developed method to measure NQO1 inhibition in intact cells. The principle of this method is based on the resorufin reductase activity of NQO1. The change in fluorescence in time was used to determine NQO1 activity in intact Chinese hamster ovary (CHO) cells genetically engineered to overexpress human NQO1. Applying this method to determine the inhibitory effects of reported in vitro NQO1 inhibitors (dicoumarol, 7,8-dihydroxyflavone, chrysin) showed that for all inhibitors tested, the IC50 in intact cells was at least 3 orders of magnitude higher than the IC50 in cell lysates. This result demonstrates that in vitro studies with purified NQO1 or with extracts from disrupted tissues are of limited value for obtaining insight into the situation in living cells. Possible factors underlying this discrepancy are being discussed. For the first time, we determined NQO1 inhibition by flavonoids in cells without disruption of the cells or addition of cofactors, enabling the assessment of enzymatic activity and the interaction of modulators of enzymatic activity in an intracellular situation.

Flavonoids and alkenylbenzenes: mechanisms of mutagenic action and carcinogenic risk

The present review focuses on the mechanisms of mutagenic action and the carcinogenic risk of two categories of botanical ingredients, namely the flavonoids with quercetin as an important bioactive representative, and the alkenylbenzenes, namely safrole, methyleugenol and estragole. For quercetin a metabolic pathway for activation to DNA-reactive species may include enzymatic and/or chemical oxidation of quercetin to quercetin ortho-quinone, followed by isomerisation of the ortho-quinone to quinone methides. These quinone methides are suggested to be the active alkylating DNA-reactive intermediates. Recent results have demonstrated the formation of quercetin DNA adducts in exposed cells in vitro. The question that remains to be answered is why these genotoxic characteristics of quercetin are not reflected by carcinogenicity. This might in part be related to the transient nature of quercetin quinone methide adducts, and suggests that stability and/or repair of DNA adducts may need increased attention in in vitro genotoxicity studies. Thus, in vitro mutagenicity studies should put more emphasis on the transient nature of the DNA adducts responsible for the mutagenicity in vitro, since this transient nature of the formed DNA adducts may play an essential role in whether the genotoxicity observed in vitro will have any impact in vivo. For alkenylbenzenes the ultimate electrophilic and carcinogenic metabolites are the carbocations formed upon degradation of their 1'-sulfooxy derivatives, so bioactivation of the alkenylbenzenes to their ultimate carcinogens requires the involvement of cytochromes P450 and sulfotransferases. Identification of the cytochrome P450 isoenzymes involved in bioactivation of the alkenylbenzenes identifies the groups within the population possibly at increased risk, due to life style factors or genetic polymorphisms leading to rapid metaboliser phenotypes. Furthermore, toxicokinetics for conversion of the alkenylbenzenes to their carcinogenic metabolites and kinetics for repair of the DNA adducts formed provide other important aspects that have to be taken into account in the high to low dose risk extrapolation in the risk assessment on alkenylbenzenes. Altogether the present review stresses that species differences and mechanistic data have to be taken into account and that new mechanism- and toxicokinetic-based methods and models are required for cancer risk extrapolation from high dose experimental animal data to low dose carcinogenic risks for man.

Cellular uptake and metabolism of flavonoids and their metabolites: implications for their bioactivity

Flavonoids have been proposed to act as beneficial agents in a multitude of disease states, including cancer, cardiovascular disease, and neurodegenerative disorders. The biological effect of these polyphenols and their in vivo circulating metabolites will ultimately depend on the extent to which they associate with cells, either by interactions at the membrane or more importantly their uptake. This review summarises the current knowledge on the cellular uptake of flavonoids and their metabolites with particular relevance to further intracellular metabolism and the generation of potential new bioactive forms. Uptake and metabolism of the circulating forms of flavanols, flavonols, and flavanones into cells of the skin, the brain, and cancer cells is reviewed and potential biological relevance to intracellular formed metabolites is discussed.