VIEWS: 35 PAGES: 16 POSTED ON: 3/9/2010 Public Domain
Choosing antioxidants for food and medical applications Dr Karen
Choosing antioxidants for food and medical applications Dr Karen Schaich, from Rutgers University in New Jersey, looks at choosing antioxidants for food and medical applications Interest in natural antioxidants for both health and for improved food stabilisation has intensified dramatically over the past ten years. Food as medicine is a current hot trend that is capturing everyone’s imagination with images of a new “magic bullet” or “fountain of youth”. Antioxidants that that have traditionally been used to inhibit oxidation in foods also quench dreaded free radicals and stop oxidation chains in vivo, so they have become viewed by many as nature’s answer to environmental and physiological stress, aging, atherosclerosis, and cancer. For the food industry, moving to natural antioxidants is a potential goldmine that offers a “green” label for food stabilisers plus intriguing new opportunities for formulating for health and specific medical benefits. In this context, our mothers’ admonitions to “Eat your fruits and vegetable!” and the old adage the “You are what you eat!” take on dramatic new meaning. The nutraceutical trend towards doubling the impact of natural antioxidants that stabilise food AND maximise health impact presents distinct challenges in evaluating antioxidant activity of purified individual compounds, mixed extracts, and endogenous food matrices and optimising applications. Determining antioxidant capacity has thus become a very active research topic, and an alphabet soup of assays has evolved to screen natural materials and identify likely candidates that will extend the shelf life and quality of both foods and human beings. The question is, what do these assays really tell us and which assay(s) will most accurately reflect antioxidant effectiveness in both foods and animals? The answer is still anything but clear. The most popular screening assays have been developed to be fast, easy, and use commonly available instrumentation, but they don’t all measure the same chemistry. ORAC (Oxygen Radical Absorbance Capacity)1-3, TRAP (Total Radical-Trapping Antioxidant Parameter)4-6, CL (chemiluminescence)7-9, TOSC (Total Oxidant Scavenging Capacity)10,11 and TAC (Total Antioxidant Capacity)12-14 assays measure abilities of compounds to quench radicals by transferring hydrogen atoms to reform the original compounds. In FRAP (Ferric Reducing Antioxidant Power)15-17 and CUPRAC (copper reduction)18,19 assays, compounds transfer electrons to reduce radicals to ions. These assays paradoxically also reveal pro-oxidant potential since reduced metals are active propagators of radical chains. To complicate matters further, TEAC (Trolox Equivalent Antioxidant Capacity)20- 23 and DPPH (diphenylpicrylhydrazyl)24-27 assays, based on reactions of stable free radicals, act by both mechanisms depending on the compound and the reaction conditions. Not surprisingly, tests of all kinds of plants with these assays have documented strong activity in brightly coloured red, purple, yellow, orange, and deep green materials that have high polyphenol concentrations3,28-37 (as mothers instinctively know). However, interpreting results superficially without careful consideration of reaction details in individual systems, and extrapolating results to more complex systems indiscriminately without considering critical differences, present several dilemmas that can limit the usefulness and accuracy of these assays beyond screening. Dilemma 1: Inconsistent antioxidant activity in different assays Antioxidant activity and mechanisms are system-dependent and vary with radical targets, individual and total antioxidant concentrations, solvent, antioxidant phase localisation26, presence of competing compounds including metals, sometimes pH, and presence of oxygen38. An antioxidant may act by one mechanism in system A and another mechanism at a different rate in System B; it may be catalytic at high concentration but protective at low levels. Mismatch between antioxidant mechanisms and assay reactions is one reason why total phenolic content often does not correlate with measured antioxidant capacity of natural extracts. However, such differences can be exploited to advantage. Integrating results from multiple assays with different endpoints can elucidate subtle but important differences in reactivity between compounds, as well as changes in reaction rates and mechanisms with solvent, environment, and antioxidant concentration. It can also reveal conditions under which antioxidants should not be used!39 Dilemma 2: Different activities in intact materials vs. mixed extracts vs. purified individual compounds There is a tendency to expect that if a compound is found to be the “active” component of a natural material by a given assay, it must be more effective if isolated and concentrated in pure form. However, individual antioxidants often behave differently in intact materials (e.g. ingested food), extracts containing multiple antioxidants with different solubilities and reactivities, and isolated form, so which is correct? When multiple components are synergistic, enhancing solubility and providing complementary reaction mechanisms, an antioxidant may be more effective in whole foods and extracts alone. In contrast, when multiple components are competitive, fighting for the same assay substrates and binding sites, activity of individual compounds increases with isolation. These differences need to be recognised and considered when interpreting results and developing applications. Dilemma 3: Results from different labs not comparable in format or values Even when established methods are supposedly followed, variations in details of operating procedures, methods of calculation, and reporting format from lab to lab contribute to inconsistent and contradictory reports of actual and relative antioxidant “capacities” of natural materials and make it often impossible to compare results between labs. The problem has become especially critical since manufacturers are now using ORAC values in advertising and product claims. ORAC units as area under the curve vary with each recorder and integrator, so unstandardised values are meaningless! International efforts to standardise assay methods in two International Congresses on Antioxidant Methods (2004 and 2005) are a step in the right direction40,41, but consistent and reproducible results will also require much more deliberate consideration of the chemistries involved in each reaction and system than are usually given.39,42 Dilemma 4: Assays often poorly predict antioxidant effectiveness in real systems in vitro It is tempting to extrapolate results of antioxidant assays to guide effective stabilisation of foods and cosmetics in vitro or to design nutraceuticals or pharmaceuticals for in vivo therapies. However, screening assays that monitor quenching of a single target radical under limited reaction conditions are poor models for antioxidants or antioxidant mixtures that must control multiple oxidative reactions simultaneously active in the complex systems of foods and biological tissues.39 Phase partitioning of radicals and antioxidants between lipid and water in real systems introduces further complications.42 In foods, the dominant radicals arise from oxidising lipids, but aqueous radicals may also arise from metals, photoinitiators, and perhaps also proteins. A very hydrophobic antioxidant will localise in the lipid phase and inhibit radical chains that are already active, but will not stop initiations. Hydrophilic antioxidants are more efficient in blocking hydroxyl radicals, superoxide anion, and other radicals in the aqueous phase but have little influence on reactions in the lipid phase once they are initiated. Some antioxidants partition between water and lipid and change their reaction depending on the solvent. Curcumins, for example, scavenge radicals rapidly in lipids but when water is present metal complexation dominates.43 Currently, only one version of the ORAC assay differentiates hydrophilic and lipophilic radical scavenging44, and no assay investigates solvent effects. Thus, predicting effectiveness in complex systems or designing applications strictly from assay results are often not successful. Chemistry is only a small part of antioxidant bioactivity. Moving up another level of complexity, chemical antioxidant assays conducted in the test tube are poor models of how antioxidants act in cells and tissues where radical generation is compartmentalised, antioxidants must be able to reach the radical source to be effective, and absorption processes thus become the controlling issue. Perhaps more importantly, phenolic antioxidants have many effects beyond free radical scavenging, so when the bioactivity being screened involves other mechanisms than, or in addition to, free radical scavenging, correlation with chemical assays is poor. Because of these disconnects, cell cultures should be viewed as their own separate level of antioxidant assay with their own quirks and advantages. Cells are particularly useful for monitoring how much of the antioxidant is taken up and by what pathway, determining reaction mechanisms and dose-response relationships – how much antioxidant is needed to induce an action and changes in response with dose level, and for observing the range of cellular responses to various challenges. Nonetheless, cell behaviour is closely linked to cell growth cycle, number of passes in cell culture, and source of cells especially for the popular Caco-2 intestinal cells where flavonoids alter proliferation and differentiation45, so problems with within lab and between-lab reproducibility can be significant. A final precaution -- neither chemical nor cell assays extrapolate to in vivo applications where what happens in the stomach and intestine determines antioxidant access to other tissues. Dilemma 5: Assays have questionable relevance and extrapolatability to bioactivity in vivo In vitro chemical assays of free radical scavenging are poor surrogates for biological activity in vivo because they provide no information about absorption, metabolism, tissue distribution, and excretion; they do not account for indirect action at a distance; and they assume that radical scavenging is the only antioxidant action while in fact it may be among the least important. Furthermore, when adapted to test antioxidant capacity of body fluids or tissues, they are plagued by interferences from cellular reducing agents and proteins and their interpretation is hampered by not knowing the sample composition. Cell cultures are only one step better: they do provide absorption and metabolism information45-49 but the doses applied directly are usually several orders of magnitude higher than could be expected to reach cells after absorption in vivo. Whether activity under such conditions accurately reflects what happens in vivo is thus open to question. Despite this shortcoming, antioxidant action in cell culture is cited in the literature almost universally as if were in vivo. An explosion of new research on antioxidant bioavailability and metabolism shows that while antioxidant vitamins are fully available, uptake of small phenols is lower and variable, and absorption of larger polyphenol molecules is very low to negligible in most cases, with most flavonoids remaining in the intestine.30,50,51 Phenolic compounds that are absorbed appear to be rapidly metabolised, appearing in the urine as methylated, glucuronidated, or sulfated conjugates within hours52-55. The greatest problem is caused by sloppy sensationalism in reporting and interpreting results. Contemporary technology allows very sensitive detection of ever tinier amounts – now picograms or less of material can be accounted for -- and mere “detection” is often presented without absorption calculations as if the full dose were absorbed. Surprisingly, molar concentrations (M) and mole amounts (mol) appear in to be used interchangeably all too frequently, so actual doses and concentrations are unclear. Absorption reported as concentrations in tissues, e.g. ng/ml plasma or g tissue are difficult to convert to total amounts absorbed, which would give a more precise and honest accounting. Methods for detecting trace levels of compounds may also need re-evaluation for accuracy since after-the-fact estimates of total absorption using average rat and human plasma volumes and tissue weights give yields substantially higher than doses in some studies. Clearly, standardisation is needed for in vivo methodology as well as chemical assays. This skepticism aside, critically needed scientifically rigorous studies of antioxidant absorption, distribution, metabolism, and excretion that are beginning to appear show a pattern of very low or selective absorption followed by rapid conjugation and elimination of what little gets through, particularly for flavonoids and other polyphenols.54,55 Improved analytical instrumentation and recent observations that sugars attached to flavonoids increase their absorption may change this picture somewhat. Nevertheless, these observations raise serious questions about the rationale currently underlying antioxidant testing: physiological responses55,56 ranging from inhibition of inflammation and edema, urinary tract infections, cancer, and aging are either exquisitely sensitive to a few molecules or they cannot be explained by direct action of the antioxidants and simple radical quenching alone. We thus need to look beyond traditional thinking to evaluate other mechanisms. To be sure, it is not easy to track the effects of antioxidants in living animals. Test tube and cell studies have revealed that, in addition to reduction of oxidative stress, (poly)phenols complex metals,57, 58 bind to proteins59, 60 and digestive secretions61, and both activate and inactivate enzymes that mediate a wide range of cell responses.62,63 These activities need to be verified in animals. In vivo, polyphenols block estrogen56,64,65 and other66 receptors, and binding to proteins in the intestinal epithelium may alter other receptors and unleash a signal transduction cascade67,68 that leads to systemic response, e.g. massive induction of endogenous antioxidants such as uric acid69 and tocopherol.70,71 Microbial flora digestion of polyphenols is also important,48,72,73 and absorption and bioactivity of these products need to be evaluated. Rational standardised protocols must be developed to determine the full role these various actions play in overall “antioxidant action” of individual and mixed antioxidant compounds and to establish the concentration limits controlling each. Eradicating the dilemmas Antioxidant research had its childhood in finding antioxidants in nearly all natural materials and its puberty in discovering that antioxidants have important bioactivities. Now it is time for antioxidant research to grow up as a field, to move beyond easy screening and shift focus to the more difficult work of systematically elucidating details of how antioxidants work so that when they are used in food formulations, their effectiveness in both foods and in vivo can be maximised. We can eliminate the five dilemmas of antioxidant research listed above first by thinking more about what information is really needed rather continuously running extracts through screening assays just to generate numbers for publications. Next, abandon the “quick and dirty” approach. The complexities of food and physiological applications of antioxidants, separately and combined, require rigorous consideration and analysis of all aspects of the (bio)chemistry, operative reaction mechanisms, and reaction/radical/target specificity in various test systems, as well as careful and accurate quantitation of all reactants and products involved. To do this, some old tests must be abandoned and some will remain useful if more depth and control is incorporated; but, in addition, new approaches must be adapted or developed to provide greater detail of action at the molecular level and account for the multiple complex actions of antioxidants in both foods and living systems. Accomplishing this will reveal the full power of antioxidants, in all its forms, and point the way to more effective utilisation of natural antioxidants in foods, nutrition, and medicine. References 1. Cao G, Sofic E, Prior RL  Antioxidant capacity of tea and common vegetables. Journal of Agricultural and Food Chemistry 44, 3426-3431. 2. Huang D, Ou B, Hampsch-Woodill M, Flanagan JA, Prior RL [2002a] High- throughput assay of oxygen radical absorbance capacity (ORAC) using a multichannel liquid handling system coupled with a microplate fluorescence reader in 96-well format. Journal of Agricultural and Food Chemistry 50, 4437-4444. 3. Ou B, Huang D, Hampsch-Woodill M, Flanagan JA, Deemer EK [2002b] Analysis of antioxidant activities of common vegetables employing oxygen radical absorbance capacity (ORAC) and ferric reducing power (FRAP) assays: a comparative study. Journal of Agricultural and Food Chemistry 50, 3122-3128. 4. Lissi E, Salim-Hanna M, Pascual C, del Castillo MD  Evaluation of total antioxidant potential (TRAP) and total antioxidant reactivity from luminol-enhanced chemiluminescence measurements. Free Radical Biology & Medicine 18, 153-158. 5. Campos AM, Escobar J, Lissi EA  The total reactive antioxidant potential (TRAP) and total antioxidant reativity (TAR) of Ilex paraguayensis extracts and red wine. Journal of Brazilian Chemical Society 7, 43-49. 6. Sanchez-Moreno C  Methods used to evaluate the free radical scavenging activity in foods and biological systems. Food Science & Technology International 8, 121-137. 7. Robinson EE, Maxwell SRJ, Thorpe GHG  An investigation of the antioxidant activity of black tea using enhanced chemiluminescence. Free Radical Research 26, 291- 302. 8. Kondo Y, Ohnishi M, Kawaguchi M  Detection of lipid peroxidation catalyzed by chelated iron and measurement of antioxidant activity in wine by a chemiluminescence analyzer. Journal of Agricultural and Food Chemistry 47, 1781-1785. 9. Waring WS, Mishra V, Maxwell SRJ  Comparison of spectrophotometric and enhanced chemiluminescent assays of serum antioxidant capacity. Clinica Chimica Acta 338, 67-71. 10. Winston GW, Regoli F, Dugas AJ Jr., Fong JH, Blanchard KA  A rapid gas chromatographic assay for determining oxyradical scavenging capacity of antioxidants and biological fluids. Free Radical Biology & Medicine 24, 480-493. 11. Regoli F & Winston GW  Quantification of total oxidant scavenging capacity of antioxidants for peroxynitrite, peroxyl radicals, and hydroxyl radicals. Toxicology and Applied Pharmacology 156, 96-105. 12. Ghiselli A, Serafini M, Natella F, Scaccini C  Total antioxidant capacity as a tool to assess redox status: critical review and experimental data. Free Radical Biology & Medicine 29, 1106-1114. 13. Kampa M, Nistikaki A, Tsaousis V, Maliataki N, Notas G, Castanas E  A new automated method for the determination of the total antioxidant capacity (TAC) of human plasma, based on the crocin bleaching assay. BMC Clinical Pathology 2, http://www.biomedcentral.com/1472-6890/1472/1473. 14. Erel O  A novel automated direct measurement method for total antioxidant capacity using a new generation, more stable ABTS radical cation. Clinical Biochemistry. 37, 277-285. 15. Benzie IF & Strain JJ  The ferric reducing ability of plasma (FRAP) as a measure of "antioxidant power": the FRAP assay. Analytical Biochemistry 239, 70-76. 16. Langley-Evans SC  Antioxidant potential of green and black tea determined using the ferric reducing power (FRAP) assay. International Journal of Food Science & Nutrition. 51, 181-188. 17. Pulido R, Bravo L, Saura-Calixto F  Antioxidant activity of dietary polyphenols as determined by a modified ferric reducing/antioxidant power assay. Journal of Agricultural and Food Chemistry 48, 3396-3402. 18. Apak R, Güçlü KG, Özyürek M, Karademir SE  Novel total antioxidant capacity index for dietary polyphenols and vitamins C and E, using their cupric iron reducing capability in the presence of neocuproine: CUPRAC method. Journal of Agricultural and Food Chemistry 52, 7970 - 7981. 19. Zaporozhets OA, Krushynska OA, Lipkovska NA, Barvinchenko VN  A new test method for the evaluation of total antioxidant activity of herbal products. Journal of Agricultural and Food Chemistry 52, 21-25. 20. van den Berg R, Haenen GRMM, van den Berg H, Bast A  Applicability of an improved Trolox equivalent antioxidant capacity (TEAC) assay for evaluation of antioxidant capacity measurements of mixtures. Food Chemistry 66, 511-517. 21. van den Berg R, Haenen GRMM, van den Berg H, van der Vigh W, Bast A  The predictive value of the antioxidant capacity of structurally related flavonoids using the trolox equivalent antioxidant capacity (TEAC) assay. Food Chemistry 70, 391-395. 22. Arts MJTJ, Dallinga JS, Voss HP, Naenen GRMN, Bast A  A critical appraisal of the use of the antioxidant capacity (TEAC) assay in defining optimum antioxidant structures. Food Chemistry 80, 409-414. 23. Arts MJTJ, Dallinga JS, Voss H-P, Haenen GRMM, Bast A [2004a] A new approach to assess the total antioxidant capacity using the TEAC assay. Food Chemistry 88, 567- 570. 24. Papariello GJ & Janish MAM  Diphenylpicrylhydrazyl as an organic analytical reagent in the spectrophotometric analysis of phenols. Analytical Chemistry 38, 211-214. 25. Bondet V, Brand-Williams W, Berset C  Kinetics and mechanisms of antioxidant activity using the DPPH free radical method. Lebensmittel Wissenschaft und Technologie 30, 609-615. 26. Barclay LRC, Edwards CE, Vinqvist MR  Media effects on antioxidant activities of phenols and catechols. Journal of the American Chemical Society 121, 6226- 6231. 27. Litwinienko G & Ingold KU  Abnormal effects on hydrogen atom abstractions. 1. The reactions of phenols with 2,2-diphenyl-1-picrylhydrazyl (DPPH.) in alcohols. Journal of Organic Chemistry 68, 3433-3438. 28. Miller NJ, Sampson J, Candeias LP, Bramley PM, Rice-Evans CA  Antioxidant activities of carotenes and xanthophylls. Federation of European Biochemical Societies Letters 384, 240-242. 29. Cherubini A, Beal MF, Frei B  Black tea increases the resistance of human plasma to lipid peroxidation in vitro but not ex vivo. Free Radical Biology & Medicine 27, 381-387. 30. Scalbert A & WIlliamson G  Dietary intake and bioavailability of polyphenols. Journal of Nutrition 130, 2073S-2085S. 31. Chen Z-H, Zhou B, Yang L, Wu L-M, Liu Z-L  Antioxidant activity of green tea polyphenols against lipid peroxidation initiated by lipid-soluble radicals in micelles. Journal of the Chemical Society, Perkin Transactions 2, 1835-1839. 32. Ehlenfeldt MK & Prior RL  Oxygen radical absorbance capacity (ORAC) and phenolic and anthocyanin concentrations in fruit and leaf tissues of highbush blueberry. Journal of Agricultural and Food Chemistry 49, 2222-2227. 33. Roberts WG & Gordon MH  Determination of the total antioxidant activity of fruits and vegetables by a liposome assay. Journal of Agricultural and Food Chemistry 51, 1486-1493. 34. Zheng W & Wang SY  Oxygen radical absorbing capacity of phenolics in blueberries, cranberries, chokeberries, and lingonberries. Journal of Agricultural and Food Chemistry 51, 502-509. 35. Chen I-C, Chang H-C, Yang H-W, Chen G-L  Evaluation of total antioxidant activity of several popular vegetables and Chinese herbs: A fast approach with ABTS/H2O2/HRP system in microplates. J. Food Drug Anal. 12, 29-33. 36. Wu X, Beecher GR, Holden JM, Haytowitz DB, Gebhardt SE, Prior RL  Lipophilic and hydrophilic antioxidant capacities of common foods in the United States. Journal of Agricultural and Food Chemistry 52, 4026-4037. 37. Toor RK & Savage GP  Antioxidant activity in different fractions of tomatoes. Food Research International 38, 487-494. 38. Rice-Evans CA, Miller NJ, Paganga G  Structure-antioxidant activity relationships of flavonoids and phenolic acids. Free Radical Biology & Medicine 20, 933-956. 39. Schaich KM  Developing a rational basis for selection of antioxidant screening and testing methods, In: Proceedings of Bakto Interantional Symposium on Natural Preservatives in Food Systems, Princeton, NJ, March 2005, in press. 40. Finley JW  Introduction: White Papers from the "First International Congress on Antioxidant Methods". Journal of Agricultural and Food Chemistry 53, 4288-4289. 41. Prior RL, Wu X, Schaich KM  Standardized Methods for the Determination of Antioxidant Capacity and Phenolics in Foods and Dietary Supplements. Journal of Agricultural and Food Chemistry 53, 4290-4302. 42. Frankel EN & Meyer AS  The problems of using one-dimensional methods to evaluate multifunctional food and biological antioxidants. J. Sci. Food Agric. 80, 1925- 1941. 43. Schaich KM, Fisher C, King RT  Formation and reactivity of free radicals in curcuminoids: an EPR study, In: ACS Symposium Series 547: Food Phytochemicals for Cancer Prevention II: Teas, Spices, and Herbs, ed. Ho CT, Huang MT, Rosen RT, American Chemical SOciety, Washington, DC, pp. 204-221. 44. Prior RL, Hoang A, Gu L, Wu X, Bacchiocca M, Howard L, Hampsch-Woodill M, Huang D, Ou B, Jacob R  Assays for hydrophilic and lipophilic antioxidant capacity (oxygen radical absorbance capacity (ORACFL)) of plasma and other biological and food samples. Journal of Agricultural and Food Chemistry 51, 3273-3279. 45. Laurent C, Besançon P, Auger C, Rouanet J-M, Caporiccio B  Grape seed extract affects proliferation and differentiation of human intestinal Caco-2 cells. Journal of Agricultural and Food Chemistry 52, 3301-3308. 46. Lin Y-L, Juan I-M, CHemn Y-L, KLiang Y-C, Lin J-K  Composition of polyphenols in fresh tea leaves and association of their oxygen-radical-absorbing capacity with antiproliferative actions in fibroblast cells. Journal of Agricultural and Food Chemistry 44, 1387-1394. 47. Walle UK, Galijatovic A, Walle T  Transport of the flavonoids chrysin and its conjugated metabolitesby the human intestinal cell line Caco-2. Biochemical Pharmacology 58, 431-438. 48. Aura A-M, O'Leary KA, Williamson G, Ojala M, Bailey M, Puupponen-Pimiä R, Nuutila AM, Oksman-Caldentey K-M, Poutanen K  Quercetin derivatives are deconjugated and converted to hydroxyphenylacetic acids but not methylated by human fecals flora in vitro. Journal of Agricultural and Food Chemistry 50, 1725-1730. 49. Murota K & Terao J  Antioxidative flavonoid quercetin: implication of its intestinal absorption and metabolism. Archives of Biochemistry and Biophysics 417, 12- 17. 50. Scalbert A, Morand C, Manach C, Rémésy C  Absorption and metabolism of polyphenols in the gut and impact on health. Biomedical Pharmacotherapy 56, 276-282. 51. Asensi M, Medina I, Ortega A, Carretero J, Baño MC, Obrador E, Estrela JM  Inhibition of cancer growth by resveratrol is related to its low bioavailability. Free Radical Biology & Medicine 33, 387-398. 52. Manach C, Texier O, Morano C, Crespy V, Régérat F, C. D, Rémésy C  Comparison of the bioavailability of quercetin and catechin in rats. Free Radical Biology & Medicine 27, 1259-1266. 53. Ader P, Wessmann A, Wolffram S  Bioavailability and metabolism of the flavonol quercetin in the pig. Free Radical Biology & Medicine 27, 1056-1067. 54. Kuhnle G, Spencer JPE, Schroeter H, Shenoy B, Debnam E, Srai SKS, Rice-Evans CA, Hahn U  Epicatechin and catechin are o-methylated and glucuronidated in the small intestine. Biochemical and Biophysical Research Communications 277, 507-512. 55. Walle T  Absorption and metabolism of flavonoids. Free Radical Biology & Medicine 36, 829-837. 56. Birt DF, Hendrich S, Weiqun W  Dietary agents in cancer prevention: flavonoids and isoflavonoids. Pharmacology & Therapeutics 90, 157-177. 57. Fernandez MT, Mira ML, Florenzio MH, Jennings KR  Iron and copper chelation by flavonoids - an electrospray mass spectrometry study. Journal of Inorganic Biochemistry 92, 105-111. 58. Paiva-Martins F & Gordon MH  Interactions of ferric ions with olive oil phenolic compounds. Journal of Agricultural and Food Chemistry 53, 2704-2709. 59. Arts MJTJ, Haenen GRMM, Wilms LC, Beetstra SAJN, Heijnen CGM, Voss H-P, Bast A  Interaction between flavonoids and proteins: effect on the total antioxidant capacity. Journal of Agricultural and Food Chemistry 50, 1184-1187. 60. Chen Y & Hagerman AE [2004a] Quantitative examination of oxidized polyphenol- protein complexes. Journal of Agricultural and Food Chemistry 52, 6061-6067. 61. Laurent C, Besançon P, Caporiccio B  Flavonoids from a grape seed extract interact with digestive secretions and intestinal cells as assessed in an in vitro digestion/Caco-2 cell culture model. Food Chemistry, in press, available on line at www.sciencedirect.com. 62. Middleton E, Jr., Kandaswami C, Theoharides T  The effects of plant flavonoids on mammalian cells: implications for inflammation, heart disease, and cancer. Pharmacological Reviews 52, 673-751. 63. Chaudhary A & Willett KL  Inhibition of human cytochrome CYP1 enzymes by flavonoids of St. John's wort. Toxicology 217, 194-205. 64. Miyamoto M, Matsushita Y, Kiyokawa A  Prenylflavonoids: a new class of non-steroidal phytoestrogen (Part 2). Estrogenic effects of 8-isopentenylnaringenin on bone metabolism. Planta Medica 64, 516-519. 65. Breinholt V, Hossaini A, Svendsen GW, Brouwer C, Neilsen SE  Estrogenic activity of flavonoids in mice. The importance of estrogen receptor distribution, metabolism, and bioavailability. Food and Chemical Toxicology 38, 555-564. 66. Sachinidis A, Skach RA, Seul C, Ko Y, Hescheler J, Ahn H-Y, Fingerle J  Inhibition of the PDGF b-receptor tyrosine phosphorylation and it downstream intracellular signal transduction pathway in rat and human vascular smooth muscle cells by different catechins. FASEB Journal 16, 893-895. 67. Williams RJ, Spencer JPE, Rice-Evans CA  Flavonoids: antioxidants or signalling molecules? Free Radical Biology & Medicine 36, 838-849. 68. Kong AN, Yu R, Chen C, Mandlekar S, Primiano T  Signal transduction events elicited by natural products: role of MAPK and caspase pathways in homeostatic response and induction of apoptosis. Archives of Pharmacy Research 23, 1-16. 69. Lotito SB & Frei B  The increase in human plasma antioxidant capacity after apple consumption is due to the metabolic effect of fructose on urate, not apple-derived antioxidant flavonoids. Free Radical Biology & Medicine 37, 251-258. 70. Frank J, Kamal-Eldin A, Lundh T, Määttä K, Törrönen R, Vessby B  Effects of dietary anthocyanins on tocopherols and lipids in rats. Journal of Agricultural and Food Chemistry 50, 7226-7230. 71. Simonetti P, Ciappellano S, Gardana C, Bramati L, Pietta P  Procyanidins from Vitis vinifera seeds: In vivo effects on oxidative stress. Journal of Agricultural and Food Chemistry 50, 6217-6221. 72. Bokkenheuser V, Shackleton CHL, Winter J  Hydrolysis of dietary flavonoid glycosides by strains of intestinal Bacteroides from humans. Biochemical Journal, 248- 953. 73. Winter J, Moore LH, Dowell VR, Bokkenheuser VD  C-ring cleavage of flavonoids by human intestinal bacteria. Applied Environmental Microbiology 55, 1203- 1208.
Pages to are hidden for
"Choosing antioxidants for food and medical applications Dr Karen"Please download to view full document