Quercetin, a potent and specific inhibitor of the human P-form phenosulfotransferase

Effects of generally recognized as safe (GRAS) and dietary compounds on phenylephrine metabolism in LS180 human intestinal cells

Phenylephrine (PE) has low and variable oral bioavailability in humans, due in part to presystemic metabolism by sulfation. LS180 cells were used as a model of the human intestinal epithelium to examine phenylephrine metabolism and its inhibition by generally recognized as safe (GRAS) and dietary compounds. Curcumin, zingerone, resveratrol, guaiacol, pterostilbene and isoeugenol significantly inhibited phenylephrine disappearance, while vanillin, propylparaben and eugenol did not. However, when propylparaben was combined with either vanillin or eugenol, the phenylephrine disappearance was significantly inhibited. These data suggest that these compounds or combinations thereof may have potential to improve phenylephrine oral bioavailability.

Sulfonation Disposition of Acacetin: In Vitro and in Vivo

Acacetin, an important component of acacia honey, exerts extensive therapeutic effects on many cancers. However, the sulfonation disposition of acacetin has rarely been reported. Therefore, this study aimed to investigate the sulfonation disposition of acacetin systematically. The results showed that acacetin-7-sulfate was the main metabolite mediated primarily by sulfotransferases (SULT) 1A1. Dog liver S9 presented the highest formation rate of acacetin-7-sulfate. Compared with that in wild-type Friend Virus B (FVB) mice, plasma exposure of acacetin-7-sulfate decreased significantly in multidrug resistance protein 1 knockout (Mrp1^-/-) mice vut increased clearly in breast cancer resistance protein knockout (Bcrp^-/-) mice. In Caco-2 monolayers, the efflux and clearance of acacetin-7-sulfate was reduced distinctly by the BCRP inhibitor Ko143 on the apical side and by the MRP1 inhibitor MK571 on the basolateral side. In conclusion, acacetin sulfonation was mediated mostly by SULT1A1. Acacetin-7-sulfate was found to be transported mainly by BCRP and MRP1. Hence, SULT1A1, BCRP, and MRP1 are responsible for acacetin-7-sulfate exposure in vivo.

Characterization of regio- and stereo-selective sulfation of bufadienolides: exploring the mechanism and providing insight into the structure–sulfation relationship by experimentation and molecular docking analysis

Bufadienolides are a major class of bioactive compounds derived from amphibian skin secretion.

Interactions of cytosolic sulfotransferases with xenobiotics

Cytosolic sulfotransferases are a superfamily of enzymes that catalyze the transfer of the sulfonic group from 3'-phosphoadenosine-5'-phosphosulfate to hydroxy or amine groups in substrate molecules. The human cytosolic sulfotransferases that have been most studied, namely SULT1A1, SULT1A3, SULT1B1, SULT1E1 and SULT2A1, are expressed in different tissues of the body, including liver, intestine, adrenal, brain and skin. These sulfotransferases play important roles in the sulfonation of endogenous molecules such as steroid hormones and neurotransmitters, and in the elimination of xenobiotic molecules such as drugs, environmental chemicals and natural products. There is often overlapping substrate selectivity among the sulfotransferases, although one isoform may exhibit greater enzyme efficiency than other isoforms. Similarly, inhibitors or enhancers of one isoform often affect other isoforms, but typically with different potency. This means that if the activity of one form of sulfotransferase is altered (either inhibited or enhanced) by the presence of a xenobiotic, the sulfonation of endogenous and xenobiotic substrates for other isoforms may well be affected. There are more examples of inhibitors than enhancers of sulfonation. Modulators of sulfotransferase enzymes include natural products ingested as part of the human diet as well as environmental chemicals and drugs. This review will discuss recent work on such interactions.

Mutual interactions between flavonoids and enzymatic and transporter elements responsible for flavonoid disposition via phase II metabolic pathways

Flavonoids, existing mainly as glycosides in nature, have multiple "claimed" beneficial effects in humans. Flavonoids are extensively metabolized in enterocytes and hepatocytes by phase II enzymes such as UGTs and SULTs to form glucuronides and sulfates, respectively. These glucuronides and sulfates are subsequently excreted via ABC transporters (e.g., MRP2 or BCRP). Therefore, it is the interplay between phase II enzymes and efflux transporters that affects the disposition of flavonoids and leads to the low bioavailability of flavonoid aglycones. Flavonoids can also serve as chemical regulators that affect the activity or expression levels of phase II enzymes including UGTs, SULTs and GSTs, and transporters including P-gp, MRP2, BCRP, OATP and OAT. In general, flavonoids may exert the inhibitory or inductive effects on the phase II enzymes and transporters via multiple mechanisms that may involve different nuclear receptors. Since flavonoids may affect the metabolic pathways shared by many important clinical drugs, drug-flavonoid interaction is becoming an increasingly important concern. This review article focused on the disposition of flavonoids and effects of flavonoids on relevant enzymes (e.g. UGTs and SULTs) and transporters (e.g. MRP2 and BCRP) involved in the interplay between phase II enzymes and efflux transporters. The effects of flavonoids on other metabolic enzymes (e.g. GSTs) or transporters (e.g. P-gp, OATP and OAT) are also addressed but that is not the emphasis of this review.

Molecular structure and vibrational spectroscopic studies of Chrysin using HF and Density Functional Theory

In the present study, the molecular confirmation, vibrational and electronic transition analysis of 5,7-dihyroxyflavone (Chrysin) were investigated using experimental techniques (FT-IR, FT-Raman and UV) and quantum chemical calculations by HF and DFT/B3LYP method with 6-31G(d,p) as basis set. The FT-IR and FT-Raman spectra in solid phase were recorded in the region 4000-400cm(-1) and 3500-50cm(-1) respectively. The UV absorption spectra of the title compound dissolved in water, methanol and ethanol were recorded in the range of 200-400nm. The complete vibrational assignments were performed on the basis of total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanic (SQM) method. By using TD-DFT calculation, electronic absorption spectra of the title compound have been predicted and a good agreement with experimental one is established. In addition, the molecular electrostatic potential (MEP), natural bond orbital (NBO) analysis have been investigated using theoretical calculations. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule.

The effect of grapefruit juice on drug disposition

INTRODUCTION

Since their initial discovery in 1989, grapefruit juice (GFJ)-drug interactions have received extensive interest from the scientific, medical, regulatory and lay communities. Although knowledge regarding the effects of GFJ on drug disposition continues to expand, the list of drugs studied in the clinical setting remains relatively limited.

AREAS COVERED

This article reviews the in vitro effects of GFJ and its constituents on the activity of CYP enzymes, organic anion-transporting polypeptides (OATPs), P-glycoprotein, esterases and sulfotransferases. The translational applicability of the in vitro findings to the clinical setting is discussed for each drug metabolizing enzyme and transporter. Reported AUC ratios for available GFJ-drug interaction studies are also provided. Relevant investigations were identified by searching the PubMed electronic database from 1989 to 2010.

EXPERT OPINION

GFJ increases the bioavailability of some orally administered drugs that are metabolized by CYP3A and normally undergo extensive presystemic extraction. In addition, GFJ can decrease the oral absorption of a few drugs that rely on OATPs in the gastrointestinal tract for their uptake. The number of drugs shown to interact with GFJ in vitro is far greater than the number of clinically relevant GFJ-drug interactions. For the majority of patients, complete avoidance of GFJ is unwarranted.

Which sources of flavonoids: complex diets or dietary supplements?

There is increasing interest in the potential health benefits of dietary flavonoids. Fruits and vegetables, tea, and cocoa are rich natural sources of flavonoids. Epidemiological studies have indicated that consumption of these foods is likely to be associated with a reduced risk of cardiovascular disease, but the etiology of this benefit is not yet clearly defined. Furthermore, in some acute interventions, a positive effect of tea and cocoa on vascular function has been reported. An alternative source of flavonoids is dietary supplements, which have become increasingly popular in the recent past. In this context, it needs to be critically evaluated whether vascular health-promoting and other positive properties of flavonoid-rich diets can be replaced by purified flavonoids as dietary supplements. Plant sources of flavonoids contain a complex mixture of secondary plant metabolites and not only flavonoids per se. This complex mixture of secondary plant metabolites cannot be simply exchanged by single purified compounds as dietary supplements. If flavonoids are given as dietary supplements, toxicity issues as well as nutrient drug interactions need to be taken into account. Purified flavonoids given in high doses as dietary supplements may affect trace element, folate, and vitamin C status. Furthermore, they may exhibit antithyroid and goitrogenic activities. In this review article, the available literature on the safety issues surrounding high dose supplemental flavonoid consumption has been summarized.

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.

Sulfation of dietary flavonoids by human sulfotransferases

Dietary flavonoids catechin, epicatechin, eriodictyol, and hesperetin were investigated as substrates and inhibitors of human sulfotransferases (hSULTs). Purified recombinant proteins and human intestine cytosol were used as enzyme sources. hSULT1A1 and hSULT1A3 as well as human intestine cytosol can catalyse the sulfation of the investigated flavonoids. Sulfation of catechin, epicatechin, eriodictyol, and hesperetin by recombinant hSULTs showed substrate inhibition at high flavonoid concentrations. Hesperetin and eriodictyol are potent inhibitors of purified hSULT1A1, hSULT1A3, hSULT1E1, and hSULT2A1. Catechin and epicatechin inhibited hSULT1A1 and hSULT1A3, but not hSULT1E1 and hSULT2A1. The sulfation efficacy and potency of inhibition is related to the C-ring structure of flavonoids. These results suggest that dietary flavonoids may regulate human SULT activity and, therefore, affect the regulation of hormones and neurotransmitters, detoxification of drugs, and the bioactivation of pro- carcinogens and pro-mutagens.

Genistein induction of human sulfotransferases in HepG2 and Caco-2 cells

Sulfotransferases are phase II drug-metabolizing enzymes. While the induction of sulfotransferases by hormones and endogenous molecules is relatively well known, induction by xenobiotics is not well studied. Isoflavones are naturally occurring phyto-oestrogens, mainly existing in soy food products. They have been described as health-promoting, disease-preventing dietary supplements and as agents with cancer-preventive activities. Recently, isoflavones have been reported to interact with nuclear receptors, including those that are known to mediate the induction of drug-metabolizing enzymes. In the present investigation, the isoflavone genistein was shown to be a xenobiotic inducer of human sulfotransferases in transformed human liver cells (HepG2) and colon carcinoma cells (Caco-2). Enzymatic activity assay, Western blot, and real-time reverse transcription-polymerase chain reaction (RT-PCR) results demonstrated that genistein significantly induced protein and mRNA expression of human simple phenol sulfotransferase (hSULT1A1) and human dehydroepiandrosterone sulfotransferase (hSULT2A1) in HepG2 and Caco-2 cells. The induction was time-dependent and dose-dependent. Western blot results agreed well with real-time RT-PCR results, suggesting that induction occurred at the gene transcription level. This isoflavone is the first nutritionally related phyto-oestrogen shown to induce human sulfotransferases in HepG2 and Caco-2 cells.

Inhibition of rat liver sulfotransferases SULT1A1 and SULT2A1 and glucuronosyltransferase by dietary flavonoids

1. Dietary flavonoids including kaempferol, quercetin, genistein and daidzein were tested for their ability to alter the conjugation of oestradiol (E(2)) via rat liver sulfotransferases and glucuronosyltransferase. 2. All four flavonoids inhibited the sulfonation of E(2) via phenol sulfotransferase, SULT1A1 with IC(50)s ranging from 0.29 to 4.61 micro M. Sulfonation of dehydroisoandrosterone (DHEA) via hydroxysteroid sulfotransferase, SULT2A1, was inhibited by higher amounts of the flavonoids (IC(50)s ranging from 34 to 116 micro M). 3. All flavonoids inhibited the formation of E(2)-beta-glucuronides (at carbon atoms 3 and 17) with IC(50)s ranging from 43 to 260 micro M. Glucuronidation of 4-methylumbelliferone (4-MU) was inhibited by high amounts of the flavonoids (IC(50)s ranging from 860 to 1550 micro M). 4. Hydrolysis of sulfonated oestrogens via arylsulfatase-c (ARSC) or 4-methylumbelliferone beta-glucuronidate (MUG) were not inhibited by the flavonoids. 5. It is concluded that SULT1A1 but not SULT2A1 or glucuronosyltransferase is highly sensitive to inhibition by dietary flavonoids. The potency of the inhibition for SULT1A1 (quercetin > kaempferol > genistein > daidzein) suggests a dependency on the number and position of hydroxyl radicals in the flavonoid molecule.

Inhibitory effects of various beverages on human recombinant sulfotransferase isoforms SULT1A1 and SULT1A3

Sulfotransferase (SULT) 1A1 and SULT1A3 play important roles in the presystemic inactivation of beta(2) agonists in the liver and intestine, respectively. The study aimed to investigate the inhibitory effects of grapefruit juice, orange juice, green tea, black tea and oolong tea and their constituents on the activities of SULT1A1 and SULT1A3. The activities of both SULT1A1 and SULT1A3 were significantly inhibited by all the beverages investigated at a concentration of 10%. The beverage constituents were tested in concentration ranges considered to be physiologically relevant. The grapefruit constituent, quercetin, completely inhibited SULT1A1, while quercetin and naringin both partially inhibited SULT1A3. The orange constituents, tangeretin and nobiletin, also completely inhibited SULT1A1. The tea constituents, (-)-epicatechin gallate and (-)-epigallocatechin gallate, both almost completely inhibited SULT1A1 and SULT1A3. Moreover, the theaflavin and thearubigin fractions of black tea both completely inhibited SULT1A1 and strongly inhibited SULT1A3. The inhibitory action of green tea on SULT1A3 was competitive, while that of black tea and oolong tea was mixed competitive/non-competitive. Mechanism-based inhibition was not observed with any beverage. In conclusion, various beverages, especially teas, inhibit the function of SULT1A3, and therefore may have the potential to increase the bioavailability of orally administered substrates of SULT1A3, such as beta(2) agonists.

Health effects of quercetin: from antioxidant to nutraceutical

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

PHARMACOKINETICS AND METABOLISM OF THE FLAVONOID SCUTELLARIN IN HUMANS AFTER A SINGLE ORAL ADMINISTRATION

Scutellarin is widely used in treating various cardiovascular diseases. Few data are available regarding its metabolism and pharmacokinetics in humans. The objectives of this study were to develop methods to identify major metabolites of scutellarin in human urine and plasma and to determine simultaneously the parent drug and its major metabolites in human plasma for pharmacokinetic studies. Four metabolites were detected in urine samples by liquid chromatography coupled with electrospray multi-stage mass spectrometry (MS), but only one of them was found in plasma. Its structure was confirmed as scutellarein 6-O-beta-D-glucuronide by MS, NMR, and UV absorbance spectra. The plasma concentrations of scutellarin and the major metabolite were simultaneously determined using liquid chromatography-tandem MS. After a single p.o. administration of 60 mg of scutellarin to 20 healthy subjects, the plasma concentrations of scutellarin were very low, and its plasma concentration-time curve was also anomalous. Plasma concentration of the major metabolite was comparatively high, and the peak plasma concentration was 87.0 +/- 29.1 ng/ml. The Tmax was late (7.85 +/- 1.62 h), and part of individual pharmacokinetic profiles showed double peaks, which indicated scutellarin could be absorbed into the intestine after hydrolysis to its aglycone by bacterial enzymes. This was followed by reconjugation in the intestinal cell and/or liver with glucuronic acid catalyzed by the phase II enzyme, which showed regioselectivity and species difference. The regioselectivity of glucuronoconjugation for scutellarin may be of importance for pharmacological activity. Plasma concentration of isoscutellarin can be used as a biomarker of scutellarin intake.

Polychlorobiphenylols are selective inhibitors of human phenol sulfotransferase 1A1 with 4-nitrophenol as a substrate

Polychlorobiphenylols (OH-PCBs) were reported as potent inhibitors of estrogen sulfotransferase, thyroid hormone and 3-hydroxybenzo(a)pyrene sulfotransferases. The aim of this study was to examine the effects of selected OH-PCBs on SULT1A1 activity in human liver cytosol, measured with 4microM 4-nitrophenol, a concentration considered to be diagnostic for selectively detecting SULT1A1. All the OH-PCBs studied inhibited the sulfonation of 4-nitrophenol in human liver cytosol. Among the eighteen OH-PCBs studied, 3'-OH-CB3 (4-chlorobiphenyl-3'-ol) was the most potent inhibitor (IC(50): 0.73+/-0.15microM, mean+/-S.D., n=3). The least potent inhibitor studied was 6'-OH-CB35 (3,3',4-trichlorobiphenyl-6'-ol) with IC(50): 49.1+/-10.8microM. The IC(50) values of the other OH-PCBs studied ranged from 0.78 to 3.76microM. Some OH-PCBs with various inhibitory potencies with human liver cytosol were selected for study with recombinant human SULT1A1 and SULT1B1. These OH-PCBs showed more potent inhibition of 4-nitrophenol sulfonation with SULT1A1 than with human liver cytosol. The IC(50) values with human liver cytosol showed a perfect linear correlation with those found with SULT1A1 (r(2)=1), but not with SULT1B1 (r(2)=0.21). The results suggested that in these human samples SULT1A1 was predominantly responsible for the sulfonation of 4-nitrophenol, with very little or no contribution from SULT1B1. The kinetics of inhibition were studied with 4'-OH-CB165, which is similar in structure to OH-PCBs found in human blood. The 4'-OH-CB165 was a mixed noncompetitive-uncompetitive inhibitor (K(i)=1.80+/-0.2microM, K(ies)=0.16+/-0.02microM). Finally, it was demonstrated that the tested OH-PCBs were themselves only slowly sulfonated by human sulfotransferases in the presence of (35)S-PAPS, as measured by the production of (35)S-labeled metabolites. Although this series of 18 OH-PCBs was too small to draw conclusions about structure-potency relationships, this work demonstrated that several OH-PCBs were potent inhibitors of 4-nitrophenol sulfonation but poor substrates in human liver cytosol, and suggested that OH-PCBs may inhibit the sulfation rate of those xenobiotics sulfated by SULT1A1.

Dietary flavonoids: effects on xenobiotic and carcinogen metabolism

Flavonoids are present in fruits, vegetables and beverages derived from plants (tea, red wine), and in many dietary supplements or herbal remedies including Ginkgo Biloba, Soy Isoflavones, and Milk Thistle. Flavonoids have been described as health-promoting, disease-preventing dietary supplements, and have activity as cancer preventive agents. Additionally, they are extremely safe and associated with low toxicity, making them excellent candidates for chemopreventive agents. The cancer protective effects of flavonoids have been attributed to a wide variety of mechanisms, including modulating enzyme activities resulting in the decreased carcinogenicity of xenobiotics. This review focuses on the flavonoid effects on cytochrome P450 (CYP) enzymes involved in the activation of procarcinogens and phase II enzymes, largely responsible for the detoxification of carcinogens. A number of naturally occurring flavonoids have been shown to modulate the CYP450 system, including the induction of specific CYP isozymes, and the activation or inhibition of these enzymes. Some flavonoids alter CYPs through binding to the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, acting as either AhR agonists or antagonists. Inhibition of CYP enzymes, including CYP 1A1, 1A2, 2E1 and 3A4 by competitive or mechanism-based mechanisms also occurs. Flavones (chrysin, baicalein, and galangin), flavanones (naringenin) and isoflavones (genistein, biochanin A) inhibit the activity of aromatase (CYP19), thus decreasing estrogen biosynthesis and producing antiestrogenic effects, important in breast and prostate cancers. Activation of phase II detoxifying enzymes, such as UDP-glucuronyl transferase, glutathione S-transferase, and quinone reductase by flavonoids results in the detoxification of carcinogens and represents one mechanism of their anticarcinogenic effects. A number of flavonoids including fisetin, galangin, quercetin, kaempferol, and genistein represent potent non-competitive inhibitors of sulfotransferase 1A1 (or P-PST); this may represent an important mechanism for the chemoprevention of sulfation-induced carcinogenesis. Importantly, the effects of flavonoids on enzymes are generally dependent on the concentrations of flavonoids present, and the different flavonoids ingested. Due to the low oral bioavailability of many flavonoids, the concentrations achieved in vivo following dietary administration tend to be low, and may not reflect the concentrations tested under in vitro conditions; however, this may not be true following the ingestion of herbal preparations when much higher plasma concentrations may be obtained. Effects will also vary with the tissue distribution of enzymes, and with the species used in testing since differences between species in enzyme activities also can be substantial. Additionally, in humans, marked interindividual variability in drug-metabolizing enzymes occurs as a result of genetic and environmental factors. This variability in xenobiotic metabolizing enzymes and the effect of flavonoid ingestion on enzyme expression and activity can contribute to the varying susceptibility different individuals have to diseases such as cancer. As well, flavonoids may also interact with chemotherapeutic drugs used in cancer treatment through the induction or inhibition of their metabolism.

Sulfonation in pharmacology and toxicology

Sulfonation has a major function in modulating the biological activities of a wide number of endogenous and foreign chemicals, including: drugs, toxic chemicals, hormones, and neurotransmitters. The activation as well as inactivation of many xenobiotics and endogenous compounds occurs via sulfonation. The process is catalyzed by members of the cytosolic sulfotransferase (SULT) superfamily consisting of at least ten functional genes in humans. The reaction in intact cells may be reversed by arylsulafatase present in the endoplasmic reticulum. Under physiological conditions, sulfonation is regulated, in part, by the supply of the co-substrate/donor molecule 3'-phosphadensoine-5-phosphosulfate (PAPS), and transport mechanisms by which sulfonated conjugates enter and leave cells. Variation in the response of individuals to certain drugs and toxic chemicals may be related to genetic polymorphisms documented to occur in each of the above pathways. Sulfonation has a major function in regulating the endocrine status of an individual by modulating the receptor activity of estrogens and androgens, steroid biosynthesis, and the metabolism of catecholamines and iodothyronines Sulfonation is a key reaction in the body's defense against injurious chemicals and may have a major function during early development since SULTs are highly expressed in the human fetus. As with many Phase I and Phase II reactions, sulfonation may also serve as the terminal step in activating certain dietary and environmental agents to very reactive toxic intermediates implicated in carcinogenesis.

Inhibitory Effect of Flavonoids on Sulfo- and Glucurono-Conjugation of Acetaminophen in Rat Cultured Hepatocytes and Liver Subcellular Preparations

A large group of flavonoids was investigated for inhibitory effects on sulfo- and glucurono-conjugation of acetaminophen when added to rat cultured hepatocytes and liver subcellular preparations. The flavonoids inhibited the production of both sulfate and glucuronide conjugates in the cultured cells, with potencies that depended on the specific flavonoid. Among the flavonols, quercetin, kaempferol and galangin were much more effective than myricetin and morin. Flavones including luteolin, apigenin and chrysin were as effective as the corresponding three flavonols above. The inhibition of conjugation by other simple flavones such as 3-, 5-, 7- and 3',4'-OH flavones, and by catechins such as epicatechin and epigallocatechin, was very weak. These data suggest that the presence of both C5 and 7 hydroxyl substitutions on the A-ring in the flavone structure is required for effective inhibitory activity. The effect of flavonoids on sulfo- and glucurono-conjugation was also examined by incubating acetaminophen with isolated liver cytosolic and microsomal preparations, respectively. The active flavonoids in the cells remarkably inhibited the sulfation, but not glucuronidation, in cell-free enzymatic preparations in vitro. The mechanism of inhibition of conjugation by flavonoids in cultured hepatocytes is not likely to depend on the direct inhibition of sulfo- and glucurono-transferase activity by flavonoids.

Glucurono- and sulfo-conjugation of kaempferol in rat liver subcellular preparations and cultured hepatocytes

Glucurono- and sulfo-conjugation of kaempferol in rat liver preparations and cultured hepatocytes were studied using high-performance liquid chromatography (HPLC) with two distinctly different elution solvents. Kaempferol glucuronides and sulfates were produced by treating kaempferol with microsomes plus UDPGA or with cytosol plus PAPS, respectively. HPLC analysis of the conjugates revealed one major and three minor glucuronides with solvent A and one sulfate with solvent B. Kaempferol metabolites produced by cultured hepatocytes also consisted of four glucuronides and one minor sulfate, all of which corresponded to their respective in vitro-produced conjugates in the liver subcellular preparations. The relative proportion of kaempferol sulfate accounted for about 9% of the total conjugates in the cultured hepatocytes. The kinetic data on glucurono- and sulfo-conjugation of kaempferol by the liver subcellular preparations correlated well with the preferential production of kaempferol glucuronides in the cultured hepatocytes. Glucurono- and sulfo-conjugation of 3-, 5- and 7-OH flavones in the liver subcellular preparations were also kinetically characterized. 7-OH flavone was predominantly conjugated to form a glucuronide compared to 3- and 5-OH flavones. These data suggest that glucuronidation at the 7-OH position on the A-ring is a major metabolic pathway of kaempferol in hepatic cells.

Effects of phenolic acids on human phenolsulfotransferases in relation to their antioxidant activity

Sulfate conjugation by phenolsulfotransferase (PST) enzyme is an important process in the detoxification of xenobiotics and endogenous compounds. There are two forms of PST that are specific for the sulfation of small phenols (PST-P) and monoamines (PST-M). Phenoilc acids have been reported to have important biological and pharmacological properties and may have benefits to human health. In the present study, human platelets were used as a model to investigate the influence of 13 phenolic acids on human PST activity and to evaluate the relationship to their antioxidant activity. The results showed that chlorogenic acid, syringic acid, protocatechuic acid, vanillic acid, sinapic acid, and caffeic acid significantly (p < 0.05) inhibited the activities of both forms of PST by 21-30% at a concentration of 6.7 microM. The activity of PST-P was enhanced (p < 0.05) by p-hydroxybenzoic acid, gallic acid, gentisic acid, o-coumaric acid, p-coumaric acid, and m-coumaric acid at a concentration of 6.7 microM, whereas the activity of PST-M was enhanced by gentisic acid, gallic acid, p-hydroxybenzoic acid, and ferulic acid. The phenolic acids exhibited antioxidant activity as determined by the oxygen radical absorbance capacity (ORAC) assay and Trolox equivalent antioxidant capacity (TEAC) assay, especially gallic acid, p-hydroxybenzoic acid, gentisic acid, and coumaric acid, which had strong activity. The overall effect of phenolic acids tested on the activity of PST-P and PST-M was well correlated to their antioxidant activity of ORAC value (r = 0.71, p < 0.01; and r = 0.66, p < 0.01). These observations suggest that antioxidant phenolic acids might alter sulfate conjugation.

Sulfation of bisphenol A abolished its estrogenicity based on proliferation and gene expression in human breast cancer MCF-7 cells

Bisphenol A, an endocrine-disrupting chemical, is widely used in many consumer products. We previously showed the sulfoconjugation of bisphenol A catalyzed by a human thermostable phenol sulfotransferase, ST1A3. The estrogenic potency of bisphenol A sulfate was compared with that of bisphenol A by an E-screen assay using human breast cancer MCF-7 cells. An increase in the expression level of an estrogen-responsive pS2 gene was also examined using MCF-7 cells after exposure to bisphenol A and its sulfate for their estrogenicity. Bisphenol A sulfate did not exhibit estrogenic effects at 0.1 microM (E-screen assay) and 1 mM (pS2 gene expression) compared with bisphenol A, which exhibited the effects at 3 nM (E-screen assay) and 1 microM (pS2 gene expression), respectively. We have therefore evaluated major roles of cytosolic phenol sulfotransferase in the human liver. Bisphenol A sulfation in human liver cytosols was inhibited by more than 90% by p-nitrophenol and quercetin, a typical substrate and specific inhibitor of phenol sulfotransferase, respectively. These results indicated that the estrogenicity of bisphenol A was abolished through its sulfation catalyzed by a human hepatic thermostable phenol sulfotransferase.

Polyphenols: do they play a role in the prevention of human pathologies?

Polyphenols are the most abundant antioxidants in our diets. The main classes of polyphenols are phenolic acids (mainly caffeic acid) and flavonoids (the most abundant in the diet are flavanols (catechins plus proanthocyanidins), anthocyanins and their oxidation products), which account for one- and two-thirds, respectively. Polyphenols are reducing agents, and together with other dietary reducing agents, such as vitamin C, vitamin E and carotenoids, referred to as antioxidants, protect the body's tissues against oxidative stress and associated pathologies such as cancers, coronary heart disease and inflammation. The biological properties, bioavailability, antioxidant activity, specific interactions with cell receptors and enzymes, are related to the chemical structure of polyphenols. It is, therefore, essential to know the nature of the main polyphenols ingested, their dietary origin, the amounts consumed in different diets, their bioavailability and the factors controlling their bioavailability.

Induction of human UDP-glucuronosyltransferase UGT1A1 by flavonoids-structural requirements

Recent studies in our laboratory in the human hepatic and intestinal cell lines Hep G2 and Caco-2 have demonstrated induction of UGT1A1 by the flavonoid chrysin (5,7-dihydroxyflavone) using catalytic activity assays and Western and Northern blotting. In the present study, we examined which features of the flavonoid structures were associated with induction of UGT1A1 and whether common drug-metabolizing enzyme inducers also produce this induction. We also determined whether flavonoid treatment affected sulfate conjugation and CYP1A1 activity. We used intact Hep G2 cells for these studies, with chrysin as the model substrate. Both glucuronidation and sulfation were measured. Hep G2 cells were pretreated for 3 days with 25 microM concentrations of 22 flavonoids (n = 4-12). Only four flavonoids demonstrated induction of glucuronidation similar to that of chrysin (i.e., 3-5-fold in the intact cells). These were acacetin, apigenin, luteolin, and diosmetin, all of which, like chrysin, are 5,7-dihydroxyflavones with varying substituents in the B-ring. 5-Hydroxy-7-methoxyflavone and 5-methyl-7-hydroxyflavone produced a modest 1.5 to 2-fold induction, whereas all other flavonoids examined were without effect. None of the flavonoids caused more than a modest change in sulfation activity (60-140% of control). In contrast, all tested 5,7-dihydroxyflavones and -flavonols induced CYP1A1 activity (ethoxyresorufin deethylation). Of seven common drug-metabolizing enzyme inducers only 3-methylcholanthrene and oltipraz showed modest induction of chrysin glucuronidation but not 2,3,7,8-tetrachlorodibenzo-p-dioxin or phenobarbital. Together, these results strongly suggest that the flavonoid induction of UGT1A1 is through a novel nonaryl hydrocarbon receptor-mediated mechanism.

Urinary excretion of catechin metabolites by human subjects after red wine consumption

Little is known about flavonoid metabolism and excretion in man. In the present study, the urinary excretion of a major flavonoid in wine, catechin, and its metabolites, were measured after nine human subjects each consumed 120 ml red wine (RW) on one day and de-alcoholized red wine (DRW) on a separate day. Both the RW and DRW contained 120 (SEM 3) micromol catechin (35 mg). GC-MS analyses of the trimethylsilylated derivatives of catechin and 3' and 4' methylcatechin were performed before and after hydrolysis of conjugates by beta-glucuronidase and sulfatase. Baseline urine samples collected prior to wine consumption contained 0.013 (SEM 0.005) micromol catechin and metabolites. During the 8 h period following consumption of RW and DRW, 6.6 (SEM 0.9) and 5.3 (SEM 0.6) micromol catechin and metabolites were excreted in 893 (SEM 94) and 740 (SEM 101) ml urine respectively. This corresponded to 3.0-10.3% of the dose after RW and 2.1-8.2% of the dose after DRW. The amount of catechin and metabolites excreted in urine was 20% higher after RW compared with DRW (P=0.06). Catechin in all urine samples was present as metabolites and there were no differences in the proportions of individual metabolites after RW and DRW. As with other flavonoids, the fate of most ingested catechin is not yet known.

Increased cellular apoptosis after chronic aqueous exposure to nonylphenol and quercetin in adult medaka (Oryzias latipes)

Increasing evidence suggests that sublethal effects of natural or xenobiotic chemicals in the environment may be mediated via the stimulation of apoptosis. To investigate whether apoptosis can be induced in fish by weakly estrogenic and androgenic chemicals, adult male Japanese medaka (Oryzias latipes) were exposed to 100 ppb of the estrogenic alkylphenol, 4-nonylphenol, and adult female medaka were exposed to 100 ppb of the aromatase-inhibiting bioflavonoid, quercetin, for 6 weeks. Exposure to nonylphenol and quercetin had no significant effect on the length, weight or condition factors compared to solvent (acetone) controls in male or female medaka. Apoptosis was evaluated in blinded histological sections of whole medaka using terminal dideoxynucleotidyl transferase dUTP nick end labeling (TUNEL) that labels nuclei of cells containing apoptotic (fragmented) DNA. There was a six-fold greater extent of apoptosis in spermatocytes, Sertoli cells and Leydig-homologue cells, but not in spermatids of testes from nonylphenol-exposed male medaka compared to testes of solvent controls. No significant differences in the extent of apoptosis were detected in intestine, liver or kidney from the same male fish. Quercetin-treated female medaka had a significantly increased number of atretic ovarian follicles, but no significant differences in the extent of apoptosis in intestine, liver or kidney. These results suggest that nonylphenol caused testicular degeneration via increased testicular cell apoptosis, while quercetin may be ovotoxic via increased follicular atresia.

Human metabolism of dietary flavonoids: identification of plasma metabolites of quercetin

The position of conjugation of the flavonoid quercetin dramatically affects biological activity in vitro, therefore it is important to determine the exact nature of the plasma metabolites. In the present study, we have used various methods (HPLC with diode array detection, LCMS, chemical and enzymic synthesis of authentic conjugates and specific enzymic hydrolysis) to show that quercetin glucosides are not present in plasma of human subjects 1.5 h after consumption of onions (a rich source of flavonoid glucosides). All four individuals had similar qualitative profiles of metabolites. The major circulating compounds in the plasma after 1.5 h are identified as quercetin-3-glucuronide, 3'-methylquercetin-3-glucuronide and quercetin-3'-sulfate. The existence of substitutions in the B and/or C ring of plasma quercetin metabolites suggests that these conjugates will each have very different biological activities.

Sulfation of flavonoids and other phenolic dietary compounds by the human cytosolic sulfotransferases

The protective effects of diet, especially soya products, tea, and many fruits, against a variety of human cancers, as suggested by epidemiological studies, has focused attention on flavonoids, isoflavonoids, and other phenolic dietary compounds as chemoprotectants. Among the mechanisms suggested for their chemoprotective action, their ability to inhibit the bioactivation of carcinogens by the human cytosolic sulfotransferases (STs) and the direct effects of their sulfoconjugates are being increasingly studied. We report here a systematic study on the sulfation of representative flavonoids, isoflavonoids, anti-oxidants, and other phenolic dietary compounds by all ten known human cytosolic STs. All ten recombinant human cytosolic STs were prepared in a pure form and tested for their sulfating activities with a variety of these compounds. P-form (SULT1A1) phenol ST (PST) showed high sulfating activity with most of these compounds. M-form (SULT1A3) PST showed high activity with the flavonoids but not with the isoflavonoids. SULT1C ST #2 showed high activity with the isoflavonoids and also sulfated most of the other compounds. Possible relevance of these results to the chemoprotective effects of these dietary compounds is discussed.

Carbohydrate sulfotransferases: novel therapeutic targets for inflammation, viral infection and cancer

Effective direct inhibition of adhesion receptors by small molecules has been hampered by extended receptor-ligand interfaces as well as the entropic penalties often associated with inhibition of cell adhesion. Therefore, alternative strategies have targeted enzymes that are centrally involved in the biosynthesis of recognition epitopes, which are crucial for productive adhesion. Two classes of enzymes shown to play a pivotal role in cell-cell and cell-matrix adhesions are the protein-tyrosine and carbohydrate sulfotransferases, which impart crucial sulfate moieties onto glycoproteins. The carbohydrate sulfotransferases will be discussed in terms of target validation and small-molecule inhibitor discovery.

Dietary intake and bioavailability of polyphenols

The main dietary sources of polyphenols are reviewed, and the daily intake is calculated for a given diet containing some common fruits, vegetables and beverages. Phenolic acids account for about one third of the total intake and flavonoids account for the remaining two thirds. The most abundant flavonoids in the diet are flavanols (catechins plus proanthocyanidins), anthocyanins and their oxidation products. The main polyphenol dietary sources are fruit and beverages (fruit juice, wine, tea, coffee, chocolate and beer) and, to a lesser extent vegetables, dry legumes and cereals. The total intake is approximately 1 g/d. Large uncertainties remain due to the lack of comprehensive data on the content of some of the main polyphenol classes in food. Bioavailability studies in humans are discussed. The maximum concentration in plasma rarely exceeds 1 microM after the consumption of 10-100 mg of a single phenolic compound. However, the total plasma phenol concentration is probably higher due to the presence of metabolites formed in the body's tissues or by the colonic microflora. These metabolites are still largely unknown and not accounted for. Both chemical and biochemical factors that affect the absorption and metabolism of polyphenols are reviewed, with particular emphasis on flavonoid glycosides. A better understanding of these factors is essential to explain the large variations in bioavailability observed among polyphenols and among individuals.

Quercetin and resveratrol potently reduce estrogen sulfotransferase activity in normal human mammary epithelial cells

Estrogen sulfotransferase (EST) is the sole sulfotransferase expressed in normal human breast epithelial cells and has an important function in determining free estrogen hormone levels in these cells. In the present study we examined the inhibitory effect of the dietary polyphenols quercetin and resveratrol on EST activity, i.e. 17beta-estradiol (E2) sulfation. Both the compounds potently inhibited recombinant human EST in a competitive fashion with K(i) values of about 1 microM. In fact, both polyphenols could serve as substrates for EST. In order to extend the studies to more physiologically relevant conditions, we examined whether inhibition of EST also occurred in the intact cultured human mammary epithelial (HME) cells. The mean baseline EST activity (E2 sulfate formation) in the HME cells was 4.4 pmol/h per mg protein. The IC(50) for resveratrol was very similar to that for recombinant EST, i.e. about 1 microM. Surprisingly, quercetin was 10 times more potent in the HME cells with an IC(50) of about 0.1 microM, a concentration that should be possible to achieve from the normal dietary content of this flavonoid.

Sulfation of "estrogenic" alkylphenols and 17beta-estradiol by human platelet phenol sulfotransferases

We have investigated the ability of alkylphenols to act as substrates and/or inhibitors of phenol sulfotransferase enzymes in human platelet cytosolic fractions. Our results indicate: (i) straight chain alkylphenols do not interact with the monoamine-sulfating phenol sulfotransferase (SULT1A3); (ii) short chain 4-n-alkylphenols (C < 8) are substrates for the phenol-sulfating enzymes (SULT1A1/2), which exhibit two activity maxima against substrates with alkyl chain lengths of C1-2 and C4-5; (iii) long chain 4-n-substituted alkylphenols (C >/= 8) are poor substrates and act as inhibitors of SULT1A1/2; (iv) human platelets contain two activities, of low and high affinity, capable of sulfating 17beta-estradiol, and 4-n-nonylphenol is a partial mixed inhibitor of the low affinity form of this activity. We conclude that by acting either as substrates or inhibitors of SULT1A1/2, alkylphenols may influence the sulfation, and hence the excretion, of estrogens and other phenol sulfotransferase substrates in humans.

The effects of flavonoids on human phenolsulphotransferases: potential in drug metabolism and chemoprevention

Flavonoids are frequently found in fruits and vegetables, and are currently under investigation as potential chemopreventive agents. The present study reports the inhibitory effects of six flavonoids, quercetin, genistein, daidzein, equol, (+)-catechin and flavone, on sulphation of p-nitrophenol, a model substrate for the P-form of PST (thermostable, TS) and dopamine, a model substrate for the M-form of PST (thermolabile, TL). Kinetic studies of the PST activity and the inhibitory effects of flavonoids on the P-form of PST activity (using Hanes-Wolf and Dixon plots) show low Km and Ki values. Quercetin was found to be the most potent inhibitor and flavone was the least active inhibitor among the flavonoids. Kinetic analysis showed that the inhibition was non-competitive and Ki values were determined for each flavonoid. These observations suggest the potential for clinically important pharmacological and toxicological interactions by flavonoids.

Transport of the flavonoid chrysin and its conjugated metabolites by the human intestinal cell line Caco-2

Chrysin (5,7-dihydroxyflavone), a natural product present in our daily diet, is a potent inhibitor of drug-metabolizing enzymes. However, its oral bioavailability is not known. This study examined the intestinal epithelial transport of chrysin (20 microM), using the human colonic cell line Caco-2 as a model of human intestinal absorption. The apical to basolateral flux of chrysin, with an apparent permeability coefficient (P(app)) during the first hour of 6.9 +/- 1.6 x 10(-6) cm x sec(-1) (mean +/- SEM), was more than 10-fold higher than for the paracellular transport marker mannitol, 0.42 +/- 0.12 x 10(-6) cm x sec(-1). Interestingly, the reverse, basolateral to apical flux of chrysin, P(app) = 14.1 +/- 1.6 x 10(-6) cm x sec(-1), was about 2-fold higher than the apical to basolateral flux (P < 0.01). In transport studies beyond 1 hr, there was a rapid decline in P(app). This correlated with the appearance of two metabolites, M1 (chrysin glucuronide) and M2 (chrysin sulfate), identified by enzymatic hydrolysis procedures and HPLC. Following apical loading of chrysin, as much as 90% of M1 + M2 appeared on the apical side, thus indicating clear efflux of the chrysin metabolites. The addition of the anion transport inhibitor MK-571 (50 microM) on the apical side produced a 71% (P < 0.0001) and 20% (P < 0.05) inhibition of the efflux of M1 and M2, respectively, suggesting the involvement of the multidrug resistance protein MRP2 pump. Indeed, using specific antibodies, MRP2 was in fact detected by western blotting in Caco-2 plasma membranes, whereas MRP1 was not. These observations suggest that chrysin has favorable membrane transport properties but that its intestinal absorption may be seriously limited by surprisingly efficient glucuronidation and sulfation by the enterocytes and almost quantitative efflux by MRP2 of the metabolites formed.

Part of quercetin absorbed in the small intestine is conjugated and further secreted in the intestinal lumen

Rutin and quercetin absorption and metabolism were investigated in rats after in situ perfusion of jejunum plus ileum (15 nmol/min). In contrast to rutin, a high proportion of quercetin (two-thirds) disappeared during perfusion, reflecting extensive transfer into the intestinal wall. Net quercetin absorption was not complete (2.1 nmol/min), inasmuch as 52% were reexcreted in the lumen as conjugated derivatives (7.7 nmol/min). Enterohepatic recycling contribution of flavonoids was excluded by catheterization of the biliary duct before perfusion. After a 30-min perfusion period, 0.71 microM of quercetin equivalents were detected in plasma, reflecting a significant absorption from the small intestine. The differential hydrolysis of effluent samples by glucuronidase and/or sulfatase indicates that the conjugated forms released in the lumen were 1) glucuronidated derivatives of quercetin and of its methoxylated forms (64%) and 2) sulfated form of quercetin (36%). In vitro quercetin glucuronides synthetized using jejunal and ileal microsomal fractions were similar to those recovered in the effluent of perfusion. These data suggest that glucuronidation and sulfatation take place in intestinal cells, whereas no glucurono-sulfoconjugates could be detected in the effluent. The present work shows that a rapid quercetin absorption in the small intestine is very effective together with its active conjugation in intestinal cells.

Fate of the flavonoid quercetin in human cell lines: chemical instability and metabolism

Although cell cultures are increasingly being used as models for studying the biological actions of flavonoids, no information on the fate, such as uptake and metabolism, exists for these natural products in these models. This study examined the elimination of quercetin, one of the most abundant flavonoids, from the cultured human hepatocarcinoma cell line Hep G2 using [14C]-labelled compound with HPLC and LC/MS for structure characterization. These cells showed a 9.6-fold accumulation of quercetin and the formation of an O-methylated metabolite, isorhamnetin. However, a rapid elimination of quercetin, with no unchanged compound present beyond 8 h, was mainly due to oxidative degradation. The initial intermediate reaction appears to involve peroxidation, leading to a dioxetan, as evidenced by a 32-amu increase in the molecular ion by LC/MS. Subsequently, opening of the C-ring leads to the formation of carboxylic acids, the major one identified in this study as protocatechuic acid. A separate reaction results in a polymeric quercetin product which is highly retained on a reversed-phase C18 HPLC column. It is postulated that these degradative and metabolic changes contribute to the multiple biological actions reported for quercetin, using cell culture models. Interestingly, part of the degradative pathway could be inhibited by including nontoxic concentrations of EDTA in the cell culture medium.

The effect of the flavonoids, quercetin, myricetin and epicatechin on the growth and enzyme activities of MCF7 human breast cancer cells

Humans ingest about 1 g of flavonoids daily in their diet, and they are increasingly being associated with cytoprotective antitumour properties. The mechanism(s) responsible for these effects have not yet been elucidated but may involve interaction with xenobiotic metabolising enzymes to alter the metabolic activation of potential carcinogens. We have investigated the effect of the flavonoids, quercetin (Q), myricetin (M) and epicatechin (E) on the growth, morphology and enzyme activities of MCF7 human breast cancer cells. Of the three flavonoids studied only Q caused a decrease in cell protein content and decreased the reduction of MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium). It also inhibited protein, DNA and RNA synthesis to the greatest extent. Q and M increased intracellular reduced glutathione (GSH) content, and Q altered the morphology of the cells after 24 h exposure to 25 microM. E and Q inhibited the O-deethylation of ethoxyresorufin (EROD) catalysed by cytochrome P450 CYPIA. In contrast, M increased the EROD reaction 2-fold. Q increased the activity of DT-diaphorase, NADPH cytochrome c reductase and glutathione reductase, while E increased only NADPH cytochrome c reductase activity. The effects on enzyme activities in vitro suggest that there is not only the potential for flavonoids to alter metabolic activation of carcinogens but also of therapeutically administered drugs in vivo. We are at present investigating the synergy between anti-cancer drugs and flavonoids in terms of anti-tumour efficacy.

Transport of quercetin and its glucosides across human intestinal epithelial Caco-2 cells

There is mounting evidence from human epidemiological, animal in vivo, and in vitro studies to suggest beneficial effects related to the consumption of quercetin and its glucosides. However, there is limited knowledge on the oral bioavailability of these natural products. This study examined the intestinal epithelial membrane transport of quercetin, quercetin 4'-glucoside, and quercetin 3,4'-diglucoside, using the Caco-2 human colonic cell line, a model of human intestinal absorption. The apparent permeability (Papp) of each agent was measured in both apical to basal and basal to apical directions. The apical to basolateral flux of quercetin, Papp 5.8 +/- 1.1 x 10(-6) cm x sec(-1) (mean +/- SEM), was more than 10-fold higher than for the paracellular transport marker mannitol, 0.48 +/- 0.09 x 10(-6) cm x sec(-1) (P < 0.01). Under identical conditions, the Papp for the transcellular marker propranolol was about 5-fold higher than for quercetin (P < 0.001). Interestingly, the reverse, basolateral to apical, flux of quercetin (Papp 11.1 +/- 1.2 x 10(-6) cm x sec(-1)) was almost 2-fold higher than the apical to basolateral flux (P < 0.001). In similar experiments, quercetin 4'-glucoside demonstrated no absorption, Papp < 0.02 x 10(-6) cm x sec(-1) in the apical to basal direction, but did demonstrate basal to apical flux, Papp 1.6 +/- 0.2 x 10(-6) cm x sec(-1). Quercetin 3,4'-diglucoside showed a low apical to basolateral transport (Papp 0.09 +/- 0.03 x 10(-6) cm x sec(-1)); its reverse, basolateral to apical, transport was, however, 4-fold higher (P < 0.05). In these cells, glucose was actively transported with an apical to basolateral Papp of 36.8 +/- 1.1 x 10(-6) cm x sec(-1). These observations suggest facile absorption of quercetin through the human intestinal epithelium, but contrary to a previous proposal, they do not support an active transport process for quercetin glucosides.