Pharmacokinetics of quercetin absorption from apples and onions in healthy humans

A high-throughput method for the extraction and analysis of quercetin in human plasma using 96-well SPE and LC-(ESI)MS/MS (7 min/run) is described. Quercetin exists as a range of glycosides in foods. The dominant types of quercetin glycosides vary depending on genetics (i.e., species and cultivar). Dietary sources include onions and apples (i.e., the peel). Herein the quercetin glycoside composition was determined in a composite standard of dried apple peel and in onion powder. The predominant forms of quercetin in apple peel include quercetin O-arabinoside, 3-O-galactoside, 3-O-glucoside, and 3-O-rhamnoside. In the onion powder, quercetin occurred as the quercetin 3,4'-O-glucoside and 4'-O-glucoside. Pharmacokinetics relating to absorption (C(max), t(max), and AUC(0-24?h)) and elimination (k(el) and t(1/2)) were compared after the consumption of apple peel powder (AP), onion powder (OP), or a mixture of the apple peel and onion powder enriched applesauce (MP) by healthy volunteers (eight females and eight males). The enriched applesauce delivered ～100 mg of quercetin aglycone equivalents. Consumption of the OP resulted in C(max) = 273.2 ± 93.7 ng/mL, t(max) = 2.0 ± 1.7 h, and t(1/2) = 14.8 ± 4.8 h, whereas the AP resulted in C(max) = 63.8 ± 22.4 ng/mL, t(max) = 2.9 ± 2.0 h, and t(1/2) = 65.4 ± 80.0 h. The MP resulted in an intermediate response with C(max) = 136.5 ± 45.8 ng/mL, t(max) = 2.4 ± 1.5 h, and t(1/2) = 18.7 ± 6.8 h. Consumption of the OP led to faster absorption, higher concentration, and greater bioavailability as compared to the AP. No significant gender-related differences were observed in the absorption of quercetin, whereas significant gender-related differences in the elimination half-time (t(1/2)) were observed.     

Flavonoids in vegetable foods commonly consumed in Brazil and estimated ingestion by the Brazilian population

The objective of this work was to quantify the flavonoids present in foods most commonly consumed by the Brazilian population. The predominant flavonoids found in largest abundance in all of the analyzed vegetables were glycosides of quercetin. In lettuce, a small amount of luteolin was also detected. In sweet pepper, quercetin and luteolin were both present. White onion [48-56 mg/100 g of fresh weight (FW), expressed as aglycon], red onion (40-100 mg/100 g of FW), red lettuce (67-67.2 mg/100 g of FW), arugula (41-118 mg/100 g of FW), and chicory (18-38 mg/100 g of FW) were highest in total flavonoids. In fruits, the highest concentrations of flavonoids were found in the peel (125-170 mg/100 g of FW) and pulp (35-44 mg/100 g of FW) of oranges and in some apple varieties (14-36 mg/100 g of FW). Variability in flavonoid content due to time of harvesting was high for leafy vegetables and red onions. The estimated ingestion by Brazilian population ranged from 60 to 106 mg/day.     

Apples prevent mammary tumors in rats

Regular consumption of fruits and vegetables has been consistently shown to be associated with reduced risk of developing chronic diseases such as cancer and cardiovascular disease. Apples are commonly consumed and are the major contributors of phytochemicals in human diets. It was previously reported that apple extracts exhibit strong antioxidant and antiproliferative activities and that the major part of total antioxidant activity is from the combination of phytochemicals. Phytochemicals, including phenolics and flavonoids, are suggested to be the bioactive compounds contributing to the health benefits of apples. Here it is shown that whole apple extracts prevent mammary cancer in a rat model in a dose-dependent manner at doses comparable to human consumption of one, three, and six apples a day. This study demonstrated that whole apple extracts effectively inhibited mammary cancer growth in the rat model; thus, consumption of apples may be an effective strategy for cancer protection.     

Protective effects of quercetin and vitamin C against oxidative stress-induced neurodegeneration

Clinical trials of several neurodegenerative diseases have increasingly targeted the evaluation of various antioxidants' effectiveness. The human diet contains several thousand phytochemicals, many of which have significant bioactivities. Vitamin C, a naturally occurring antioxidant, is known to reduce the risk of neurodegenerative disorders such as Alzheimer's disease. Quercetin, one of the major flavonoids in some fruits and vegetables, has much stronger antioxidative and anticarcinogenic activities than vitamin C. Therefore, we investigated the protective effects of quercetin on hydroxy peroxide-induced neurodegeneration. To determine the protective effects, PC12 cells were preincubated with quercetin and vitamin C before H(2)O(2) treatment for 2 h. Results showed that cell viability was clearly improved with quercetin, and quercetin showed a higher protective effect than vitamin C. Because oxidative stress is known to increase neuronal cell membrane breakdown, we further investigated lactate dehydrogenase and trypan blue exclusion assays. We observed that quercetin decreased oxidative stress-induced neuronal cell membrane damage more than vitamin C. These results suggest that quercetin, in addition to many other biological benefits, contributes significantly to the protective effects of neuronal cells from oxidative stress-induced neurotoxicity, such as Alzheimer disease.     

Varietal differences in phenolic content and antioxidant and antiproliferative activities of onions

Epidemiological studies have indicated that the consumption of fruits and vegetables is associated with a reduced risk for the development of chronic diseases, such as cardiovascular disease and cancer. Phytochemicals, including phenolics and flavonoids, are suggested to be the major bioactive compounds contributing to the health benefits of fruits and vegetables. Onions are a major source of dietary flavonoids; however, there may exist varietal differences in composition, concentration, and beneficial activities. To characterize these differences, shallots and 10 onion (Allium cepa L.) varieties commonly available in the United States (Western Yellow, Northern Red, New York Bold, Western White, Peruvian Sweet, Empire Sweet, Mexico, Texas 1015, Imperial Valley Sweet, and Vidalia) were evaluated for total phenolic and flavonoid contents and antioxidant and antiproliferative activities. Shallots contained the highest total phenolic content (114.7 +/- 10.0 mg/100 g of sample) among the varieties tested, with a 6-fold difference observed when compared to the variety with the lowest phenolic content (Vidalia, p < 0.05). Western Yellow onion variety exhibited the highest total flavonoid content (69.2 +/- 3.7 mg/100 g of onion) of the varieties tested, with an 11-fold difference when compared to the variety with the lowest flavonoid content (Western White, p < 0.05). Shallots exhibited the highest total antioxidant activity (45.5 +/- 2.1 micromol of vitamin C equiv/g of onion), followed by Western Yellow, New York Bold, Northern Red, Mexico, Empire Sweet, Western White, Peruvian Sweet, Texas 1015, Imperial Valley Sweet, and Vidalia. For all varieties, both total phenolic and flavonoid contents were strongly correlated with total antioxidant activity (R (2) = 0.9668, p < 0.05; and R (2) = 0.7033, p < 0.05, respectively). The proliferation of HepG(2) and Caco-2 cells was significantly inhibited in a dose-dependent fashion after exposure to the Western Yellow, shallots, New York Bold, and Northern Red extracts, with Western Yellow, shallots, and New York Bold exhibiting the highest antiproliferative activity against HepG(2) cells and New York Bold and Western Yellow exhibiting the highest antiproliferative activity against Caco-2 cells. However, the varieties of Western White, Peruvian Sweet, Empire Sweet, Mexico, Texas 1015, Imperial Valley Sweet, and Vidalia demonstrated weak antiproliferative activity against both HepG(2) and Caco-2 cells. These results may influence consumers toward purchasing onion varieties exhibiting greater potential health benefits and may significantly affect future breeding efforts to enhance onion nutritional qualities.     

Assessment of carotenoid bioavailability of whole foods using a Caco-2 cell culture model coupled with an in vitro digestion

Epidemiological studies have shown that consumption of carotenoid-rich fruits and vegetables is associated with a reduced risk of developing chronic diseases. beta-Carotene, alpha-carotene, and beta-cryptoxanthin are precursors of vitamin A, a nutrient essential for human health. However, little is known about the bioavailability of carotenoids from whole foods. This study characterized the intestinal uptake performance of carotenoids using monolayers of differentiated Caco-2 human intestinal cells and mimicked human digestion to assess carotenoid absorption from carrots and corn. Results showed that Caco-2 cellular uptake of beta-carotene and zeaxanthin was higher than that of lutein. Uptake performances of pure carotenoids and carotenoids from whole foods by Caco-2 cells were both curvilinear, reaching saturated levels after 4 h of incubation. The time kinetics and dose response of carotenoid uptake presented a similar pattern in Caco-2 cells after plating for 2 and 14 days. Furthermore, the applicability of this new model was verified with whole grain corn, showing that cooked corn grain significantly enhanced carotenoid bioavailability. These results support the feasibility of the in vitro digestion cell model for assessing carotenoid absorption from whole foods as a suitable and cost-effective physiological alternative to current methodologies.     

Cellular antioxidant activity (CAA) assay for assessing antioxidants, foods, and dietary supplements

A cellular antioxidant activity (CAA) assay for quantifying the antioxidant activity of phytochemicals, food extracts, and dietary supplements has been developed. Dichlorofluorescin is a probe that is trapped within cells and is easily oxidized to fluorescent dichlorofluorescein (DCF). The method measures the ability of compounds to prevent the formation of DCF by 2,2'-azobis(2-amidinopropane) dihydrochloride (ABAP)-generated peroxyl radicals in human hepatocarcinoma HepG2 cells. The decrease in cellular fluorescence when compared to the control cells indicates the antioxidant capacity of the compounds. The antioxidant activities of selected phytochemicals and fruit extracts were evaluated using the CAA assay, and the results were expressed in micromoles of quercetin equivalents per 100 micromol of phytochemical or micromoles of quercetin equivalents per 100 g of fresh fruit. Quercetin had the highest CAA value, followed by kaempferol, epigallocatechin gallate (EGCG), myricetin, and luteolin among the pure compounds tested. Among the selected fruits tested, blueberry had the highest CAA value, followed by cranberry > apple = red grape > green grape. The CAA assay is a more biologically relevant method than the popular chemistry antioxidant activity assays because it accounts for some aspects of uptake, metabolism, and location of antioxidant compounds within cells.     

Effect of reddening-ripening on the antioxidant activity of polyphenol extracts from cv. 'Annurca' apple fruits

Apple is among the most consumed fruits worldwide, and several studies suggest that apple polyphenols could play a role in the prevention of degenerative diseases. 'Annurca' apple fruit undergoes, after harvest, a typical reddening treatment to turn the apples' skin red, and it is noted for its high firmness. This paper reports the effect of reddening-ripening treatment on polyphenol concentration and antioxidant activity of both peel and flesh extracts. The in vitro antioxidant properties have been compared with the protective effect against the cytotoxic effects of reactive oxygen species using Caco-2 cells as model system. Pretreatment of cells with different polyphenolic apple extracts provides a remarkable protection against oxidative damage. This effect seems to be associated with the antioxidant activity of 'Annurca' apple polyphenolic compounds. The flesh has antioxidant properties comparable to those possessed by the peel. Neither the reddening nor the fruit conservation causes changes in the antioxidant properties possessed by this apple variety. The data indicate that polyphenolic compounds in 'Annurca' apples are relatively stable in the peel and also in the flesh; therefore, the health benefits of polyphenols should be maintained during long-term storage. Finally, a diet rich in apple antioxidants could exert a beneficial effect in the prevention of intestinal pathologies related to the production of reactive oxygen species.     

Apple peels as a value-added food ingredient

There is some evidence that chronic diseases, such as cancer and cardiovascular disease, may occur as a result of oxidative stress. Apple peels have high concentrations of phenolic compounds and may assist in the prevention of chronic diseases. Millions of pounds of waste apple peels are generated in the production of applesauce and canned apples in New York State each year. We proposed that a valuable food ingredient could be made using the peels of these apples if they could be dried and ground to a powder without large losses of phytochemicals. Rome Beauty apple peels were treated with citric acid dips, ascorbic acid dips, and blanches before being oven-dried at 60 degrees C. Only blanching treatments greatly preserved the phenolic compounds, and peels blanched for 10 s had the highest total phenolic content. Rome Beauty apple peels were then blanched for 10 s and dried under various conditions (oven-dried at 40, 60, or 80 degrees C, air-dried, or freeze-dried). The air-dried and freeze-dried apple peels had the highest total phenolic, flavonoid, and anthocyanin contents. On a fresh weight basis, the total phenolic and flavonoid contents of these samples were similar to those of the fresh apple peels. Freeze-dried peels had a lower water activity than air-dried peels on a fresh weight basis. The optimal processing conditions for the ingredient were blanching for 10s and freeze-drying. The process was scaled up, and the apple peel powder ingredient was characterized. The total phenolic content was 3342 +/- 12 mg gallic acid equivalents/100 g dried peels, the flavonoid content was 2299 +/- 52 mg catechin equivalents/100 g dried peels, and the anthocyanin content was 169.7 +/- 1.6 mg cyanidin 3-glucoside equivalents/100 g dried peels. These phytochemical contents were a significantly higher than those of the fresh apple peels if calculated on a fresh weight basis (p < 0.05). The apple peel powder had a total antioxidant activity of 1251 +/- 56 micromol vitamin C equivalents/g, similar to fresh Rome Beauty peels on a fresh weight basis (p > 0.05). One gram of powder had an antioxidant activity equivalent to 220 mg of vitamin C. The freeze-dried apple peels also had a strong antiproliferative effect on HepG(2) liver cancer cells with a median effective dose (EC(50)) of 1.88 +/- 0.01 mg/mL. This was lower than the EC(50) exhibited by the fresh apple peels (p < 0.05). Apple peel powder may be used in a various food products to add phytochemicals and promote good health.     

Antioxidant and antiproliferative activities of common vegetables

Epidemiological studies have shown that consumption of fruits and vegetables is associated with reduced risk of chronic diseases. Increased consumption of fruits and vegetables containing high levels of phytochemicals has been recommended to prevent chronic diseases related to oxidative stress in the human body. In this study, 10 common vegetables were selected on the basis of consumption per capita data in the United States. A more complete profile of phenolic distributions, including both free and bound phenolics in these vegetables, is reported here using new and modified methods. Broccoli possessed the highest total phenolic content, followed by spinach, yellow onion, red pepper, carrot, cabbage, potato, lettuce, celery, and cucumber. Red pepper had the highest total antioxidant activity, followed by broccoli, carrot, spinach, cabbage, yellow onion, celery, potato, lettuce, and cucumber. The phenolics antioxidant index (PAI) was proposed to evaluate the quality/quantity of phenolic contents in these vegetables and was calculated from the corrected total antioxidant activities by eliminating vitamin C contributions. Antiproliferative activities were also studied in vitro using HepG(2) human liver cancer cells. Spinach showed the highest inhibitory effect, followed by cabbage, red pepper, onion, and broccoli. On the basis of these results, the bioactivity index (BI) for dietary cancer prevention is proposed to provide a simple reference for consumers to choose vegetables in accordance with their beneficial activities. The BI could be a new alternative biomarker for future epidemiological studies in dietary cancer prevention and health promotion.     

Comparative effects of food-derived polyphenols on the viability and apoptosis of a human hepatoma cell line (HepG2)

Consumption of fruits and vegetables, which are rich in polyphenols, has been associated with a reduced risk of chronic diseases such as cancer. Dietary polyphenols have antioxidant and antiproliferative properties that might explain their beneficial effect on cancer prevention. The aim of this study was to investigate the effects of different pure polyphenols [quercetin, chlorogenic acid, and (-)-epicatechin] and natural fruit extracts (strawberry and plum) on viability or apoptosis of human hepatoma HepG2 cells. The treatment of cells for 18 h with quercetin and fruit extracts reduced cell viability in a dose-dependent manner; however, chlorogenic acid and (-)-epicatechin had no prominent effects on the cell death rate. Similarly, quercetin and strawberry and plum extracts, rather than chlorogenic acid and (-)-epicatechin, induced apoptosis in HepG2 cells. Moreover, quercetin and fruit extracts arrested the G1 phase in the cell cycle progression prior to apoptosis. Quercetin and strawberry and plum extracts may induce apoptosis and contribute to a reduced cell viability in HepG2 cells.     

Antiproliferative effects of dietary phenolic substances and hydrogen peroxide

There has been controversy as to whether the antiproliferative activity of dietary phenolic substances on cancer cells is due to the bioactivities of phenolics or the generation of hydrogen peroxide (H2O2) in media as an artifact. This study was to investigate whether the formation of H2O2 by different phenolics induces acute toxicity and carcinogenicity in normal rat liver epithelial cells. Gallic acid, one of the major antioxidants present in fruits and vegetables, dose-dependently generated considerably more H2O2 in DMEM media without cells than did quercetin. Gallic acid exerted stronger antiproliferative activity than quercetin on both Caco-2 human colon cancer cells (Caco-2 cells) and WB-F344 normal rat liver epithelial cells (WB cells) cultured in DMEM media, and the effect was partially reduced by catalase. Furthermore, gallic acid (but not quercetin) also inhibited gap-junction intercellular communication (GJIC; a carcinogenic phenomenon), which was in part protected by the addition of catalase. Exogenous H2O2 addition also inhibited the proliferation of both Caco-2 cells and WB cells and inhibited GJIC in a dose-dependent manner, but these effects were almost abolished by the treatment with catalase. From these results it is concluded that the antiproliferative effects of some antioxidants on cancer cells are partially due to their prooxidant actions.     

Apple peels, from seven cultivars, have lipase-inhibitory activity and contain numerous ursenoic acids as identified by LC-ESI-QTOF-HRMS

Apple peel contains numerous phytochemicals, many of which show bioactivity. This study investigated the identity of triterpenoid compounds contained in ethanolic extracts of peel from seven apple cultivars. Using HPLC-ESI-QTOF-HRMS, accurate mass information was obtained for 43 compounds, and chemical identity was inferred from the calculated elemental composition, fragment masses, ms/ms, and a limited set of authentic standards. Compounds were identified as triterpene acids and tentatively identified as ursenoic (or oleanoic) acid derivatives containing hydroxyl, oxo, and coumaroyloxy groups. These apple skin extracts exhibited lipase-inhibitory activity, which may be linked to the ursenoic acid content. Furthermore, both triterpene content and lipase-inhibitory activity varied by cultivar.     

Transepithelial transport of microbial metabolites of quercetin in intestinal Caco-2 cell monolayers

m-Hydroxyphenylacetic acid (mHPA), 3,4-dihydroxyphenylacetic acid (DHPA), and 4-hydroxy-3-methoxyphenylacetic acid (HMPA) are major microbial metabolites of quercetin. After administration of quercetin to human subjects, these metabolites are readily detected in blood and urine. mHPA, DHPA, and HMPA are thought to exert protective biological activity within the body due to their antioxidant properties. However, very little work has been published concerning their absorption. I have examined the absorption characteristics of the quercetin metabolites in Caco-2 cells by a coulometric detection method using HPLC-ECD. All of them exhibited nonsaturable transport in Caco-2 cells up to 30 mM, whereas HMPA and mHPA also showed proton-coupled polarized absorption. The proton-coupled directional transport of HMPA and mHPA was inhibited by the substrate of the monocarboxylic acid transporter (MCT). A considerable amount of apically loaded HMPA and mHPA was taken up and transported through to the basolateral side, while almost all of the apically loaded DHPA was retained on the apical side. Furthermore, the transepithelial flux of DHPA was inversely correlated with the paracellular permeability of Caco-2 cells, although those of HMPA and mHPA were almost constant. These results indicate that transport of DHPA was mainly via paracellular diffusion, although HMPA and mHPA were absorbed to some extent by the MCT.     

Influence of dietary quercetin on glutathione redox status in mice

Flavonoids such as quercetin have been shown to serve as a protective defense against oxidative damage in vivo. However, the bioavailability of quercetin depends on the food source and type of glycosidic moiety linked to the molecule. In this study, mice were fed 1 mg/day quercetin in the form of quercetin aglycone, rutin, apple, or onion, and reduced glutathione (GSH), oxidized glutathione (GSSG), and protein-GSH mixed disulfides were determined to investigate the influence of dietary quercetin on the GSH redox status in metabolically active tissues, mitochondria, and plasma of mice. All quercetin treatment groups produced increases in the GSH:GSSG ratio and decreases in mixed disulfide levels in hepatic tissue. Cardiac tissue did not change in response to dietary quercetin; however, cardiac mitochondria demonstrated a reduction in the GSH:GSSG ratio and an increase in protein mixed disulfide levels. No significant changes were observed in the plasma GSH:GSSG ratio, but mixed disulfide levels were decreased for all of the diets. The changes in plasma redox status did not parallel the changes in the tissues. Onion fed mice demonstrated the greatest increases in GSH:GSSG ratios and the greatest decreases in protein mixed disulfide levels of all diets compared. For all treatment groups, increases in the GSH:GSSG ratios corresponded with decreases in protein mixed disulfide levels. The results of this study indicate that quercetin influences GSH:GSSG ratios and protein thiolation in a tissue-specific manner and that these effects are dependent on food source and bioavailability.     

Apple and pear peel and pulp and their influence on plasma lipids and antioxidant potentials in rats fed cholesterol-containing diets

The aim of this study was to assess the bioactive compounds of apple and pear peel and pulp in vitro and their influence on plasma lipids and antioxidant potentials in vivo. The antioxidant potentials measured by 1,1-diphenyl-2-picrylhydrazyl (DPPH), beta-carotene bleaching (beta-carotene), and nitric oxide inhibition radical scavenging (NO) tests in apple peel and pulp were significantly higher than in pear peel and pulp, respectively. The ethanol extract of apple peels showed the strongest inhibition of lipid peroxidation as a function of its concentration and was comparable to the antioxidant activity of butylated hydroxyanisole. The pear pulp extract had the weakest antioxidant ability, whereas other extracts such as apple pulp and pear peel were nearly equal. The antioxidant activities comprised contributions from polyphenols, phenolic acids, and flavonoids and correlated well with polyphenols and flavonoids. The correlation coefficients between polyphenols and antioxidant activities by DPPH, beta-carotene, and NO were as follows: 0.9207, 0.9350, and 0.9453. Contrarily, the correlation coefficient between the content of dietary fiber and the antioxidant activities test was low. The content of all studied indices in apple and pear peel was significantly higher than in peeled fruits (p < 0.05). Diets supplemented with fruit peels exercised a significantly higher positive influence on plasma lipid levels and on plasma antioxidant capacity of rats than diets with fruit pulps.     

Polyphenolic profiles in eight apple cultivars using high-performance liquid chromatography (HPLC)

Polyphenolic compounds of apple may play an important role in physiologic functions related to human health. Different polyphenolics may have varied biological activities including antioxidant activity. The objective of this study was to investigate the profiles of polyphenolic compounds in different apple varieties and different parts of an apple. The total and individual polyphenolics differed significantly among the eight apple cultivars grown in Ontario, and the peels had higher concentrations than the flesh. Among the tested cultivars, Red Delicious and Northern Spy had the highest concentrations and Empire the lowest. Five major polyphenolic groups with a total of 16 identified individual compounds were found, among which the dihydroxycinnamic acid esters, phloretin glycosides, and flavan-3-ols were found in both flesh and peel, whereas quercetin glycosides were almost exclusively found in the peel. Cyanidin 3-galactoside was unique to and found only in red apple peels. In both apple peel and flesh, the predominant group of polyphenolics was the procyanidins, followed by quercetin glycosides in the peel and hydroxycinnamic acid esters in the flesh. 3-Hydroxyphloretin 2'-xyloglucoside was newly identified in apple. The results obtained in this study will further the understanding of the polyphenolic composition of apples and their roles in health-promoting physiological functions.     

Antioxidant activity of apple peels

Consumption of fruits and vegetables has been shown to be effective in the prevention of chronic diseases. These benefits are often attributed to the high antioxidant content of some plant foods. Apples are commonly eaten and are large contributors of phenolic compounds in European and North American diets. The peels of apples, in particular, are high in phenolics. During applesauce and canned apple manufacture, the antioxidant-rich peels of apples are discarded. To determine if a useful source of antioxidants is being wasted, the phytochemical content, antioxidant activity, and antiproliferative activity of the peels of four varieties of apples (Rome Beauty, Idared, Cortland, and Golden Delicious) commonly used in applesauce production in New York state were investigated. The values of the peels were compared to those of the flesh and flesh + peel components of the apples. Within each variety, the total phenolic and flavonoid contents were highest in the peels, followed by the flesh + peel and the flesh. Idared and Rome Beauty apple peels had the highest total phenolic contents (588.9 +/- 83.2 and 500.2 +/- 13.7 mg of gallic acid equivalents/100 g of peels, respectively). Rome Beauty and Idared peels were also highest in flavonoids (306.1 +/- 6.7 and 303.2 +/- 41.5 mg of catechin equivalents/100 g of peels, respectively). Of the four varieties, Idared apple peels had the most anthocyanins, with 26.8 +/- 6.5 mg of cyanidin 3-glucoside equivalents/100 g of peels. The peels all had significantly higher total antioxidant activities than the flesh + peel and flesh of the apple varieties examined. Idared peels had the greatest antioxidant activity (312.2 +/- 9.8 micromol of vitamin C equivalents/g of peels). Apple peels were also shown to more effectively inhibit the growth of HepG(2) human liver cancer cells than the other apple components. Rome Beauty apple peels showed the most bioactivity, inhibiting cell proliferation by 50% at the low concentration of 12.4 +/- 0.4 mg of peels/mL. The high content of phenolic compounds, antioxidant activity, and antiproliferative activity of apple peels indicate that they may impart health benefits when consumed and should be regarded as a valuable source of antioxidants.     

Phytochemicals of apple peels: isolation, structure elucidation, and their antiproliferative and antioxidant activities

Bioactivity-guided fractionation of Red Delicious apple peels was used to determine the chemical identity of bioactive constituents, which showed potent antiproliferative and antioxidant activities. Twenty-nine compounds, including triterpenoids, flavonoids, organic acids and plant sterols, were isolated using gradient solvent fractionation, Diaion HP-20, silica gel, and ODS columns, and preparative HPLC. Their chemical structures were identified using HR-MS and 1D and 2D NMR. Antiproliferative activities of isolated pure compounds against HepG2 human liver cancer cells and MCF-7 human breast cancer cells were evaluated. On the basis of the yields of isolated flavonoids (compounds 18- 23), the major flavonoids in apple peels are quercetin-3-O-beta-D-glucopyranoside (compound 20, 82.6%), then quercetin-3-O-beta-D-galactopyranoside (compound 19, 17.1%), followed by trace amounts of quercetin (compound 18, 0.2%), (-)-catechin (compound 22), (-)-epicatechin (compound 23), and quercetin-3-O-alpha-L-arabinofuranoside (compound 21). Among the compounds isolated, quercetin (18) and quercetin-3-O-beta-D-glucopyranoside (20) showed potent antiproliferative activities against HepG2 and MCF-7 cells, with EC 50 values of 40.9 +/- 1.1 and 49.2 +/- 4.9 microM to HepG2 cells and 137.5 +/- 2.6 and 23.9 +/- 3.9 microM to MCF-7 cells, respectively. Six flavonoids (18-23) and three phenolic compounds (10, 11, and 14) showed potent antioxidant activities. Caffeic acid (10), quercetin (18), and quercetin-3-O-beta-D-arabinofuranoside (21) showed higher antioxidant activity, with EC 50 values of <10 microM. Most tested flavonoids and phenolic compounds had high antioxidant activity when compared to ascorbic acid and might be responsible for the antioxidant activities of apples. These results showed apple peel phytochemicals have potent antioxidant and antiproliferative activities.