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

A detailed overview of quercetin: implications for cell death and liver fibrosis mechanisms

Background

Quercetin, a widespread polyphenolic flavonoid, is known for its extensive health benefits and is commonly found in the plant kingdom. The natural occurrence and extraction methods of quercetin are crucial due to its bioactive potential.

Purpose

This review aims to comprehensively cover the natural sources of quercetin, its extraction methods, bioavailability, pharmacokinetics, and its role in various cell death pathways and liver fibrosis.

Methods

A comprehensive literature search was performed across several electronic databases, including PubMed, Embase, CNKI, Wanfang database, and ClinicalTrials.gov, up to 10 February 2024. The search terms employed were "quercetin", "natural sources of quercetin", "quercetin extraction methods", "bioavailability of quercetin", "pharmacokinetics of quercetin", "cell death pathways", "apoptosis", "autophagy", "pyroptosis", "necroptosis", "ferroptosis", "cuproptosis", "liver fibrosis", and "hepatic stellate cells". These keywords were interconnected using AND/OR as necessary. The search focused on studies that detailed the bioavailability and pharmacokinetics of quercetin, its role in different cell death pathways, and its effects on liver fibrosis.

Results

This review details quercetin's involvement in various cell death pathways, including apoptosis, autophagy, pyroptosis, necroptosis, ferroptosis, and cuproptosis, with particular attention to its regulatory influence on apoptosis and autophagy. It dissects the mechanisms through which quercetin affects these pathways across different cell types and dosages. Moreover, the paper delves into quercetin's effects on liver fibrosis, its interactions with hepatic stellate cells, and its modulation of pertinent signaling cascades. Additionally, it articulates from a physical organic chemistry standpoint the uniqueness of quercetin's structure and its potential for specific actions in the liver.

Conclusion

The paper provides a detailed analysis of quercetin, suggesting its significant role in modulating cell death mechanisms and mitigating liver fibrosis, underscoring its therapeutic potential.

A critical examination of human data for the biological activity of quercetin and its phase-2 conjugates

This critical review examines evidence for beneficial effects of quercetin phase-2 conjugates from clinical intervention studies, volunteer feeding trials, and in vitro work. Plasma concentrations of quercetin-3-O-glucuronide (Q3G) and 3'-methylquercetin-3-O-glucuronide (3'MQ3G) after supplementation may produce beneficial effects in macrophages and endothelial cells, respectively, especially if endogenous deglucuronidation occurs, and lower blood uric acid concentration via quercetin-3'-O-sulfate (Q3'S). Unsupplemented diets produce much lower concentrations (<50 nmol/l) rarely investigated in vitro. At 10 nmol/l, Q3'S and Q3G stimulate or suppress, respectively, angiogenesis in endothelial cells. Statistically significant effects have been reported at 100 nmol/l in breast cancer cells (Q3G), primary neuron cultures (Q3G), lymphocytes (Q3G and3'MQ3G) and HUVECs (QG/QS mixture), but it is unclear whether these translate to a health benefit in vivo. More sensitive and more precise methods to measure clinically significant endpoints are required before a conclusion can be drawn regarding effects at normal dietary concentrations. Future requirements include better understanding of inter-individual and temporal variation in plasma quercetin phase-2 conjugates, their mechanisms of action including deglucuronidation and desulfation both in vitro and in vivo, tissue accumulation and washout, as well as potential for synergy or antagonism with other quercetin metabolites and metabolites of other dietary phytochemicals.

Cryopreservation of bioflavonoid-rich plant sources and bioflavonoid-microcapsules: emerging technologies for preserving bioactivity and enhancing nutraceutical applications

Bioflavonoids are natural polyphenolic secondary metabolites that are medicinal. These compounds possess antitumor, cardioprotective, anti-inflammatory, antimicrobial, antiviral, and anti-psoriasis properties to mention a few. Plant species that contain bioflavonoids should be preserved as such. Also, the bioactivity of the bioflavonoids as neutraceutical compounds is compromised following extraction due to their sensitivity to environmental factors like light, pH, and temperature. In other words, the bioflavonoids' shelf-life is affected. Scientists noticed that bioflavonoids have low solubility properties, poor absorption, and low bioavailability following consumption. Researchers came up with methods to encapsulate bioflavonoids in order to circumvent the challenges above and also to mask the unpleasant order these chemicals may have. Besides, scientists cryopreserve plant species that contain bioflavonoids. In this review, we discuss cryopreservation and bioflavonoid microencapsulation focusing mainly on vitrification, slow freezing, and freeze-drying microencapsulation techniques. In addition, we highlight bioflavonoid extraction techniques, medicinal properties, challenges, and future perspectives of cryopreservation and microencapsulation of bioflavonoids. Regardless of the uniqueness of cryopreservation and microencapsulation as methods to preserve bioflavonoid sources and bioflavonoids' bioactivity, there are challenges reported. Freeze-drying technology is costly. Cryoprotectants damage the integrity of plant cells, to say the least. Researchers are working very hard to overcome these challenges. Encapsulating bioflavonoids via coaxial electrospray and then cryopreserving the micro/nanocapsules produced can be very interesting.

Anticancer potential of Nymphaea nouchali Brum flowers against Ehrlich ascites carcinoma cell lines

Background

Nymphaea nouchali Brum is exotic and medicinal plant in India.

Aim of the Study

The main of this study is to evaluate the anticancer properties of Nymphaea nouchali Brum flowers against Ehrlich ascites carcinoma (EAC)-induced Swiss albino mice.

Materials and Methods

The anticancer properties of Nymphaea nouchali Brum dry and fresh methanol extracts was investigated using EAC in Swiss albino mice. After inoculation of EAC cells into mice, treatment with NNDM flower extract (200 and 400 mg/kg) and standard drug 5-Fluorouracil (20 mg/kg) was continued for 9 days. The evaluation of the effect of drug response was made by the study of tumor growth response including increase in lifespan, the study of hematological parameters, biochemical estimations, and antioxidant assay of liver tissue compared to EAC control. The viability of cancer cell lines (such as HeLa, MCF-7, and MDA-MB 231 cells) was evaluated by 3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide (MTT) assay.

Results

Therefore, from the results of the present study, it can be concluded that NNDM exhibited significant antitumor activity against EAC in Swiss albino mice. The effect of NNDM on viability of cancer cell lines (such as HeLa, MCF-7, and MDA-MB 231 cells) was evaluated by MTT assay, apoptosis in HeLa cell lines was evaluated by DNA laddering assay, HeLa cells treated with NNDM exhibited a characteristic "ladder" pattern after separation of the fragments by agarose gel electrophoresis and subsequent visualization, by ethidium bromide staining. NNDM exhibited a significant effect on cell viability.

Conclusions

Based on results, it was concluded that NNDM exhibited cytotoxic effect on cancer cells and, from DNA laddering assay, it can be concluded that NNDM-induced apoptosis in EAC cells.

Recent insights into oxidative metabolism of quercetin: catabolic profiles, degradation pathways, catalyzing metalloenzymes and molecular mechanisms

Quercetin is the most abundant polyphenolic flavonoid (flavonol subclass) in vegetal foods and medicinal plants. This dietary chemopreventive agent has drawn significant interest for its multiple beneficial health effects ("polypharmacology") largely associated with the well-documented antioxidant properties. However, controversies exist in the literature due to its dual anti-/pro-oxidant character, poor stability/bioavailability but multifaceted bioactivities, leaving much confusion as to its exact roles in vivo. Increasing evidence indicates that a prior oxidation of quercetin to generate an array of chemical diverse products with redox-active/electrophilic moieties is emerging as a new linkage to its versatile actions. The present review aims to provide a comprehensive overview of the oxidative conversion of quercetin by systematically analyzing the current quercetin-related knowledge, with a particular focus on the complete spectrum of metabolite products, the enzymes involved in the catabolism and the underlying molecular mechanisms. Herein we review and compare the oxidation pathways, protein structures and catalytic patterns of the related metalloenzymes (phenol oxidases, heme enzymes and specially quercetinases), aiming for a deeper mechanistic understanding of the unusual biotransformation behaviors of quercetin and its seemingly controversial biological functions.

Metabolic Forest: Predicting the Diverse Structures of Drug Metabolites

Adverse drug metabolism often severely impacts patient morbidity and mortality. Unfortunately, drug metabolism experimental assays are costly, inefficient, and slow. Instead, computational modeling could rapidly flag potentially toxic molecules across thousands of candidates in the early stages of drug development. Most metabolism models focus on predicting sites of metabolism (SOMs): the specific substrate atoms targeted by metabolic enzymes. However, SOMs are merely a proxy for metabolic structures: knowledge of an SOM does not explicitly provide the actual metabolite structure. Without an explicit metabolite structure, computational systems cannot evaluate the new molecule's properties. For example, the metabolite's reactivity cannot be automatically predicted, a crucial limitation because reactive drug metabolites are a key driver of adverse drug reactions (ADRs). Additionally, further metabolic events cannot be forecast, even though the metabolic path of the majority of substrates includes two or more sequential steps. To overcome the myopia of the SOM paradigm, this study constructs a well-defined system-termed the metabolic forest-for generating exact metabolite structures. We validate the metabolic forest with the substrate and product structures from a large, chemically diverse, literature-derived dataset of 20 736 records. The metabolic forest finds a pathway linking each substrate and product for 79.42% of these records. By performing a breadth-first search of depth two or three, we improve performance to 88.43 and 88.77%, respectively. The metabolic forest includes a specialized algorithm for producing accurate quinone structures, the most common type of reactive metabolite. To our knowledge, this quinone structure algorithm is the first of its kind, as the diverse mechanisms of quinone formation are difficult to systematically reproduce. We validate the metabolic forest on a previously published dataset of 576 quinone reactions, predicting their structures with a depth three performance of 91.84%. The metabolic forest accurately enumerates metabolite structures, enabling promising new directions such as joint metabolism and reactivity modeling.

Formation and biological targets of botanical o-quinones

The formation of o-quinones from direct 2-electron oxidation of catechols and/or two successive one electron oxidations could explain the cytotoxic/genotoxic and/or chemopreventive effects of several phenolic botanical extracts. For example, poison ivy contains urushiol, an oily mixture, which is oxidized to various o-quinones likely resulting in skin toxicity through oxidative stress and alkylation mechanisms resulting in immune responses. Green tea contains catechins which are directly oxidized to o-quinones by various oxidative enzymes. Alternatively, phenolic botanicals could be o-hydroxylated by P450 to form catechols in vivo which are oxidized to o-quinones. Examples include, resveratrol which is oxidized to piceatannol and further oxidized to the o-quinone. Finally, botanical o-quinones can be formed by O-dealkylation of O-alkoxy groups or methylenedioxy rings resulting in catechols which are further oxidized to o-quinones. Examples include safrole, eugenol, podophyllotoxin and etoposide, as well as methysticin. Once formed these o-quinones have a variety of biological targets in vivo resulting in various biological effects ranging from chemoprevention - > no effect - > toxicity. This U-shaped biological effect curve has been described for a number of reactive intermediates including o-quinones. The current review summarizes the latest data on the formation and biological targets of botanical o-quinones.

Safety Aspects of the Use of Quercetin as a Dietary Supplement

The flavonoid quercetin is frequently found in low amounts as a secondary plant metabolite in fruits and vegetables. Isolated quercetin is also marketed as a dietary supplement, mostly as the free quercetin aglycone, and frequently in daily doses of up to 1000 mg d^-1 exceeding usual dietary intake levels. The present review is dedicated to safety aspects of isolated quercetin used as single compound in dietary supplements. Among the numerous published human intervention studies, adverse effects following supplemental quercetin intake have been rarely reported and any such effects were mild in nature. Published adequate scientific data for safety assessment in regard to the long-term use (>12 weeks) of high supplemental quercetin doses (≥1000 mg) are currently not available. Based on animal studies involving oral quercetin application some possible critical safety aspects could be identified such as the potential of quercetin to enhance nephrotoxic effects in the predamaged kidney or to promote tumor development especially in estrogen-dependent cancer. Furthermore, animal and human studies with single time or short-term supplemental quercetin application revealed interactions between quercetin and certain drugs leading to altered drug bioavailability. Based on these results, some potential risk groups are discussed in the present review.

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.

Molecular docking studies of quercetin and its analogues against human inducible nitric oxide synthase

Nitric oxide synthases (NOS) catalyze to produce nitric oxide (NO) from L-arginine. The isoform of NOS i.e. inducible nitric oxide synthases (iNOS) expression is observed in various human malignant tumors such as breast, lung, prostate and bladder, colorectal cancer, and malignant melanoma. Also an increased level of iNOS expression and activity has been found in the tumor cells of gynecological malignancies, stroma of breast cancer and tumor cells of head and neck cancer. Because of its importance in causing tumors and cancer, iNOS enzyme has become a new target in finding novel inhibitors as anti cancer agents. The present work focuses on the molecular docking analysis of quercetin and its analogues against iNOS enzyme. Earlier there are reports of quercetin inhibiting iNOS enzyme in certain experiments as anti cancer agent. But the clinical use of quercetin is limited by its low oral bioavailability and therefore needed its molecular modification to improve its pharmacological properties. In the present study ten analogues of quercetin were found to be docked at the active site cavity with favorable ligand-protein molecular interaction and interestingly from the ADME-Toxicity analysis these analogues have enhanced pharmacological properties than quercetin.

Mutagenicity of flavonoids assayed by bacterial reverse mutation (Ames) test

The mutagenicity of ten flavonoids was assayed by the Ames test, in Salmonella typhimurium strains TA98, TA100 and TA102, with the aim of establishing hydroxylation pattern-mutagenicity relationship profiles. The compounds assessed were: quercetin, kaempferol, luteolin, fisetin, chrysin, galangin, flavone, 3-hydroxyflavone, 5-hydroxyflavone and 7-hydroxyflavone. In the Ames assay, quercetin acted directly and its mutagenicity increased with metabolic activation. In the presence of S9 mix, kaempferol and galangin were mutagenic in the TA98 strain and kaempferol showed signs of mutagenicity in the other strains. The absence of hydroxyl groups, as in flavone, only signs of mutagenicity were shown in strain TA102, after metabolization and, among monohydroxylated flavones (3-hydroxyflavone, 5-hydroxyflavone and 7-hydroxyflavone), the presence of hydroxyl groups only resulted in minor changes. Luteolin and fisetin also showed signs of mutagenicity in strain TA102. Finally, chrysin, which has only two hydroxy groups, at the 5-OH and 7-OH positions, also did not induce mutagenic activity in any of the bacterial strains used, under either activation condition. All the flavonoids were tested at concentrations varying from 2.6 to 30.7 nmol/plate for galangin and 12.1 to 225.0 nmol/plate for other flavonoids. In light of the above, it is necessary to clarify the conditions and the mechanisms that mediate the biological effects of flavonoids before treating them as therapeutical agents, since some compounds can be biotransformed into more genotoxic products; as is the case for galangin, kaempferol and quercetin.

125I-labeled quercetin as a novel DNA-targeted radiotracer

Quercetin is a major flavonoid that is found in most plants; it can intercalate with DNA. The purpose of this study was to investigate radiolabeling of qurecetin with (125)I, DNA binding and cellular process. In this work, quercetin was labeled with Auger emitting nuclide (125)I using chloramine-T. DNA binding of (125)I-quercetin ((125)I-Q) was investigated using cell-free in vitro assay with naked human genomic DNA in agarose plugs. Cellular uptake and nuclei accumulation were evaluated in human prostate cancer cell lines (DU 145). The internalization of (125)I-Q was evaluated with fluorescence microscopy. Cellular damage was monitored by using apoptosis assay. Quercetin was successfully labeled with (125)I, and it was taken up rapidly with cells and accumulated in the cellular nuclei. (125)I-Q-DNA binding was tight with long retention time, and it potentially induced DNA damage. These findings provide for using of (125)I-labeled quercetin as a carrier of Auger electron emitting radionuclide to the cell nuclei for targeted radiotherapy.

(-)-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.

Antioxidant properties of quercetin

Quercetin, a plant-derived aglycone form of flavonoid glycosides, has been used as a nutritional supplement and may be beneficial against a variety of diseases, including cancer. We examined the antioxidant properties of quercetin. The reduction potential of quercetin was measured at various pH values using voltammetric methods, and its total antioxidant capacity (TAC) was measured using the phosphomolybdenum method. The effect of quercetin on production of reactive oxygen species (ROS) and nitric oxide (NO) in LPS-stimulated human THP-1 acute monocytic leukemia cells was determined by flow cytometry using CM-H2DCFDA dye. The results were compared with curcumin, a natural product exhibiting a similar range of reported health benefits.

RESULTS

1) Quercetin has a higher reduction potential compared with curcumin at three different pH settings and is comparable to Trolox at pH 7-9.5; 2) its TAC is 3.5 fold higher than curcumin; 3) it reduced LPS-induced ROS to near normal levels; 4) it reduced LPS-induced NO production. These data provide a physico-chemical basis for comparing antioxidants, with potential benefits individually or in combination.

Autoxidative and cyclooxygenase-2 catalyzed transformation of the dietary chemopreventive agent curcumin

The efficacy of the diphenol curcumin as a cancer chemopreventive agent is limited by its chemical and metabolic instability. Non-enzymatic degradation has been described to yield vanillin, ferulic acid, and feruloylmethane through cleavage of the heptadienone chain connecting the phenolic rings. Here we provide evidence for an alternative mechanism, resulting in autoxidative cyclization of the heptadienone moiety as a major pathway of degradation. Autoxidative transformation of curcumin was pH-dependent with the highest rate at pH 8 (2.2 μM/min) and associated with stoichiometric uptake of O(2). Oxidation was also catalyzed by recombinant cyclooxygenase-2 (COX-2) (50 nm; 7.5 μM/min), and the rate was increased ≈10-fold by the addition of 300 μM H(2)O(2). The COX-2 catalyzed transformation was inhibited by acetaminophen but not indomethacin, suggesting catalysis occurred by the peroxidase activity. We propose a mechanism of enzymatic or autoxidative hydrogen abstraction from a phenolic hydroxyl to give a quinone methide and a delocalized radical in the heptadienone chain that undergoes 5-exo cyclization and oxygenation. Hydration of the quinone methide (measured by the incorporation of O-18 from H(2)(18)O) and rearrangement under loss of water gives the final dioxygenated bicyclopentadione product. When curcumin was added to RAW264.7 cells, the bicyclopentadione was increased 1.8-fold in cells activated by LPS; vanillin and other putative cleavage products were negligible. Oxidation to a reactive quinone methide is the mechanistic basis of many phenolic anti-cancer drugs. It is possible, therefore, that oxidative transformation of curcumin, a prominent but previously unrecognized reaction, contributes to its cancer chemopreventive 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.

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.

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.

Cytological and biochemical effects of St. John's Wort supplement (a complex mixture of St. John's Wort, Rosemary and Spirulina) on somatic and germ cells of Swiss Albino mice

Commercially available St. John’s wort supplement (SJWS) composed of an herbal mixture of St. John’s Wort (SJW), Rosemary (RM) and Spirulina (SP) is used as a dietary supplement for the treatment of psychiatric disorders. Although the minor ingredients, (RM and SP) are proven antioxidants, their quantity is quite insignificant as compared to the SJW, which is the major ingredient. Most of the toxic effects of SJWS are attributed to the main constituents of SJW which differ due to the influence of light (hypericin) and variations in temperature above freezing point (hyperforin). However, there are no reports on toxicity of SJWS maintained at room temperature in pharmacies and supermarkets. In view of the folkloric importance, immense (prescribed or unprescribed) use and a paucity of literature on SJWS, it was found worthwhile to (1) determine the genotoxic effects of SJWS in somatic and germ cells of mice and (2) investigate the role of biochemical changes, as a possible mechanism. The protocol included the oral treatment of mice with different doses (380, 760 and 1520 mg/kg/day) of SJWS for 7 days. The following experiments were conducted: (i) cytological studies on micronucleus test, (ii) cytogenetic analysis for meiotic chromosomes, (iii) cytological analysis of spermatozoa abnormalities, (iv) quantification of proteins and nucleic acids in hepatic and testicular cells and (v) estimation of malondialdehyde (MDA) and nonprotein sulfhydryl (NP-SH) in hepatic and testicular cells. The treatment increased the frequency of micronuclei in polychromatic erythrocytes (PCE) in the femora. It caused aberrations in chromosomes of testes and induced spermatozoa abnormalities. These changes might be attributed to the epigenetic mechanisms as revealed by an increase in concentrations of MDA and depletion of nucleic acids and NP-SH levels in both hepatic and testicular cells observed in the present study. Since, the samples of SJWS used were not drawn from extremities of light and temperature; the observed effect might not be related to the main constituents of SJW. However, these changes might be ascribed to the combined effect of terpenes, tannins, quercetin and flavonoids present in SJW.

Quality assurance and safety of herbal dietary supplements

Since the U.S. Congress passed the Dietary Supplement Health and Education Act (DSHEA) in 1994, use of herbal products has been growing rapidly worldwide. To ensure consumer health protection, the quality and safety of herbal plants, particularly those used for dietary supplement preparations, must be determined. To date, toxicological data on the identification of genotoxic and tumorigenic ingredients in many raw herbs and their mechanisms of action are lacking. Thus, identification of carcinogenic components in herbal plants is timely and important. In this review, the issues of quality control and safety evaluation of raw herbs and herbal dietary supplements are discussed. Two examples of tumorigenicity and mechanism of tumor induction are discussed: aristolochic acid and riddelliine, both of which have been detected in Chinese herbal plants. It is proposed that an organized effort with international participation on cancer risk assessment should be actively pursued so that the safety of commercial herbal plants and herbal dietary supplements can be ensured.

Antioxidant effects of flavonoid from Croatian Cystus incanus L. rich bee pollen

Oxidant/antioxidant status, estrogenic/anti-estrogenic activity and gene expression profile were studied in mice fed with Cystus incanus L. (Cistaceae) reach bee pollen from location in Central Croatia's Dalmatia coast and offshore islands. Seven phenolic compounds (out of 13 tested) in bee pollen sample were detected by high performance liquid chromatography (HPLC) analysis. Phenolics detected in C. incanus L. bee pollen belong to flavonol (pinocembrin), flavanols (quercetin, kaempferol, galangin, and isorhamnetin), flavones (chrysin) and phenylpropanoids (caffeic acid). Bee pollen as a food supplement (100mg/kgbw mixed with commercial food pellets) compared to control (commercial food pellets) modulated antioxidant enzymes (AOE) in the mice liver, brain and lysate of erythrocytes and reduced hepatic lipid peroxidation (LPO). Bee pollen induced 25% of anti-estrogenic properties while no estrogenic activity was found. Differential gene expression profile analyses after bee pollen enriched diet identify underexpressed gene Hspa9a, Tnfsf6 (liver) and down-regulated gene expression of Casp 1 and Cc121c (brain) which are important in the apoptosis pathway and chemotaxis. These results indicate that used bee pollen possess a noticable source of compounds with health protective potential and antioxidant activity.

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.

Inhibitory effects of diesel exhaust components and flavonoids on 20alpha-hydroxysteroid dehydrogenase activity in mouse tissues

The inhibitory effects of diesel exhaust components and flavonoids on 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) activity were examined in cytosolic fractions from the liver, kidney and lung of male mice. 9,10-Phenanthrenequinone (9,10-PQ) and 1,2-naphthoquinone (1,2-NQ), which are contained in diesel exhaust particles (DEPs), potently inhibited 20alpha-HSD activity in liver cytosol. 9,10-PQ also inhibited the enzyme activity in lung cytosol. However, 20alpha-HSD activity in kidney cytosol was little inhibited by 9,10-PQ or 1,2-NQ. Flavonoids such as quercetin, fisetin and kaempferol exhibited high inhibitory potencies for 20alpha-HSD activity in liver cytosol, whereas these flavonoids were poor inhibitors for the enzyme activity in kidney cytosol. It is likely that several diesel exhaust components and flavonoids augment the signaling of progesterone in the liver cells, by potently inhibiting 20alpha-HSD activity in mouse liver cytosol. The possibility that there are distinct enzymes catalyzing 20alpha-HSD activity in the non-reproductive tissues of male mice is also discussed.

Lifespan of etoposide-treated human neutrophils is affected by antioxidant ability of quercetin

Neutropenia is the primary dose-limiting effect of etoposide toxicity resulting in a decreased efficiency of cancer treatment. Hence, the protection of neutrophils has important clinical implications. We investigated whether quercetin, due to its antioxidant properties, is able to modulate the damaging activity of etoposide. DNA damage, evaluated by the comet assay, and apoptosis, determined by FACScan flow cytometry using Annexin/PI, increased with etoposide doses. The intracellular level of reactive oxygen species (ROS) was enhanced in resting neutrophils incubated with etoposide at concentrations up to 25 microM; above this concentration etoposide revealed antioxidant properties. Only in latex-activated neutrophils, i.e. with latex-stimulated respiratory burst was the ROS production inhibited, as assessed by the luminol amplified chemiluminescence. The characteristic electron spin resonance (ESR) signal of etoposide phenoxyl radical, which occurs in the presence of myeloperoxidase, H2O2 and etoposide, was quenched by quercetin in a dose-dependent manner (0.1-0.5 microM). Quercetin also inhibited DNA damage induced by etoposide and enhanced the inhibitory action of etoposide on the ROS formation in neutrophils. However, quercetin (1 microM) lowered early and late apoptosis/necrosis only when apoptosis was induced by 25 microM etoposide; at higher etoposide concentration apoptosis was enhanced. Summing up, antioxidant adjuvant therapy using quercetin can be beneficial in prolonging neutrophils' lifespan in peripheral blood only when etoposide plasma concentration is low.

Ginkgo biloba leave extract: biological, medicinal, and toxicological effects

Ginkgo biloba leave extract is among the most widely sold herbal dietary supplements in the United States. Its purported biological effects include: scavenging free radical; lowering oxidative stress; reducing neural damages, reducing platelets aggregation; anti-inflammation; anti-tumor activities; and anti-aging. Clinically, it has been prescribed to treat CNS disorders such as Alzheimer's disease and cognitive deficits. It exerts allergy and changes in bleeding time. While its mutagenicity or carcinogenic activity has not been reported, its components, quercetin, kaempferol and rutin have been shown to be genotoxic. There are no standards or guidelines regulating the constituent components of Ginkgo biloba leave extract nor are exposure limits imposed. Safety evaluation of Ginkgo biloba leave extract is being conducted by the U.S. National Toxicology Program.

A critical review of the data related to the safety of quercetin and lack of evidence of in vivo toxicity, including lack of genotoxic/carcinogenic properties

Quercetin is a naturally-occurring flavonol (a member of the flavonoid family of compounds) that has a long history of consumption as part of the normal human diet. Because a number of biological properties of quercetin may be beneficial to human health, interest in the addition of this flavonol to various traditional food products has been increasing. Prior to the use of quercetin in food applications that would increase intake beyond that from naturally-occurring levels of the flavonol in the typical Western diet, its safety needs to be established or confirmed. This review provides a critical examination of the scientific literature associated with the safety of quercetin. Results of numerous genotoxicity and mutagenicity, short- and long-term animal, and human studies are reviewed in the context of quercetin exposure in vivo. To reconcile results of in vitro studies, which consistently demonstrated quercetin-related mutagenicity to the absence of carcinogenicity in vivo, the mechanisms that lead to the apparent in vitro mutagenicity, and those that ensure absence of quercetin toxicity in vivo are discussed. The weight of the available evidence supports the safety of quercetin for addition to food.

Water and methanolic extracts of Salvia officinalis protect HepG2 cells from t-BHP induced oxidative damage

Common sage (Salvia officinalis L., Lamiaceae) is an aromatic and medicinal plant well known for its antioxidant properties. Some in vivo studies have shown the biological antioxidant effects of sage. However, the intracellular antioxidant mechanisms of action are still poorly understood. In this study, we evaluated the cytoprotective effects of two sage extracts (a water and a methanolic extract) against tert-butyl hydroperoxide (t-BHP)-induced toxicity in HepG2 cells. The most abundant phenolic compounds present in the extracts were rosmarinic acid and luteolin-7-glucoside. Both extracts, when co-incubated with the toxicant, protected significantly HepG2 cells against cell death. The methanolic extract, with a higher content of phenolic compounds than the water extract, conferred better protection in this in vitro model of oxidative stress with liver cells. Both extracts, tested in a concentration that protects 80% against cell death (IC(80)), significantly prevented t-BHP-induced lipid peroxidation and GSH depletion, but not DNA damage assessed by the comet assay. The ability of sage extracts to reduce t-BHP-induced GSH depletion by 62% was probably the most relevant contributor to the observed cytoprotection. A good correlation between the above cellular effects of sage and the effects of their main phenolic compounds was found. When incubated alone for 5h, sage extracts induced an increase in basal GSH levels of HepG2 cells, which indicates an improvement of the antioxidant potential of the cells. Compounds present in sage extracts other than phenolics may also contribute to this latter effect. Based in these results, it would be of interest to investigate whether sage has protective effects in suitable in vivo models of liver diseases, where it is known that oxidative stress is involved.

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.