FLAVONOIDS

1. FLAVONOIDS

2. The flavonoids constitute one of the largest groups of plant pigments – In 1975, the number of identified flavonoids was estimated to be larger than 2000 however, more than 4000 have now been identified. • They occur most commonly in vascular plants where they are normally concentrated in the epidermal cells of the leaves. Here they act as a filter of more than 90% of the damaging ultraviolet B (UV-B) radiation from the sun for the cells of the inner leaf.

3. Other roles they play in the plants include: • They occur as yellow, red or blue pigments (the Latin for yellow is flavus) and this plays a part by • Providing colour to plants and fruits • Attracting insects to the flower • They can act as messenger compounds in plants and are known to be involved in the regulation of the plant growth • They are also believed to be involved in the regulation of plant-microorganism relationships e.g. their astringent / bitter taste can prevent other organisms feeding off them.

4. MAJOR CLASSES • The basic flavonoid skeleton consists of a 3-ring structure:

5. Characteristics(Aldred 2009) • Polyphenols: slightly larger that the quinones and are found in all vascular plants. They can be found in foods such as fruits, vegetables, teas and wines. • All flavonoids have the same basic biosynthetic origin. • Carotenoids are often confused with flavonoids because they both end with –oid. Comparison of structures show the difference between the two.

6. Water soluble • Are not present in algae • Anti-inflammatory  flavonoids influence arachidonic acid metabolism and hence have an anti-inflammatory effect. • Flavonoids are resistant to boiling and fermentation.

7. The capacity of flavonoids to act as antioxidants depends upon their molecular structure. The position of hydroxyl groups and other features in the chemical structure of flavonoids are important for their antioxidant and free radical scavenging activities. Quercetin, the most abundant dietary flavonol, is a potent antioxidant because it has all the right structural features for free radical scavenging activity.

8. Colours associated with flavonoids(Aldred 2009) • Chalcones  yellow or colourless • Flavanones  colourless • Flavones  pure flavones are colourless but the more hydroxyl groups that are present, the more intense the yellow colour. Flavones are co-pigments that protect anthocyanins • Flavonols  colourless pigments that protect anthocyanins • Anthocyanidins  red, blue, purple, violet • Isoflavones  colourless • Catechins  colurless

9. Flavonoids are categorised into 6 major classes: • Read page 75 of Heinrich et al to see how each of the classes are formed and follow the explanation through figure 6.24 on page 76

10. CHALCONES:(Aldred 2009) • Properties: • Antioxidant • Antifungal • Antibacterial • Antitumour • Anti-inflammatory

11. Found in: • Cannabinaceae: Humulus lupulus • Compositae • Leguminoseae: Glycyrrhiza spp. • Piperaceae: Piper methysticum • Intermediate compounds in the biosynthesis of flavonoids • The numbering system is slightly different to the flavonoids in that rings A and B are reversed.

12. Flavanols • This is Catechin, a common flavanol that occurs in many plants. • Also found in chocolate

13. Catechin is found in Green tea, Cocoa powder, Red wine, Hawthorn, Bilberry, Motherwort, and other herbs. • Epicatechin is another common example; it differs from Catechin only in the spatial orientation of its -OH group.

14. Flavonols • Notice this word is spelled with an "o" instead of with an "a" as in "flavanols". This means that the molecule has a double-bonded oxygen atom attached to position 4. They're still "-ols" because they retain the -OH group at position 3 like the flavanols; but they also have a double-bonded oxygen atom, which makes them like another class of flavonoids known as "flavones"

15. Flavonols provide the cream coloured pigments in flowers • Found in  grapes, berries, fruits, cauliflower, cabbages • Properties (Aldred 2009) • Antioxidant – see next slide • Anti-inflammatory

16. One of the most common flavonols is Quercetin. It's  the most abundant flavonol in the diet and is found in hundreds of herbs and foods. Onions are especially rich in Quercetin. It has proven antioxidant effects. (see page 76 for Quercetin’s structure)

17. Flavones • Flavones are like flavonols, without the "-ol." In other words, there is no longer an -OH group at position 3 on the central ring. • Here's the basic flavone skeleton, with the =O at position 4 and the double bond between carbons 2 and 3. • Apigenin is a flavone with -OH groups added to positions 5, 7, and 4'. It's another very common flavonoid, appearing in many medicinal plants and foods such as celery. Another flavone is luteolin, found in sweet red peppers.

18. Found in (Aldred 2009): • Polygonaceaae: Rumex Crispus • Rutaceae: Citrus Spp. • Leguminoseae: Glycyrrhiza spp. • Umbelliferae: Apium graveolens, Petroselinium crispum, • Compositae: Chrysanthemum spp.

19. Properties: • Anti-inflammatory • Anti-allergic • Antithrombotic and vasoprotective • Tumour inhibitor

20. Flavanones • Flavanone:  Take away the double bond between carbons 2 and 3 of the flavone structure, and you have a flavanone. Notice the "o" has changed back to an "a," which indicates that the flavanones have a single bond between carbons 2 and 3. • The basic flavanone skeleton retains the =O, which makes it an "-one." Many flavanones occur as glycosides; for example, hesperitin (aglycone) and hesperidin (glycoside) occur in citrus along with naringenin.

21. Found in (Aldred 2009): • Rutaceae: Citrus species • Properties(Aldred 2009): • Antioxidant • Anti-inflammatory • Cam have an effect on carbohydrate metabolism • Immune system modulator

22. Naringenin, an antioxidant flavanone from citrus species, has   -OH groups attached at positions 5, 7, and 4'. (See page 76 Heinrich et al) Studies have indicated that it has anti-inflammatory, anti-cancer, and liver protective effects.

23. Isoflavones • Isoflavones (also known as isoflavonoids) are very similar to flavones, except the B ring is attached to position 3 of the C ring, rather than to position 2 as in the flavones:

24. Properties of soy isoflavones (Aldred 2009): • Phytoestrogens act like oestrogens in the body working directly on oestrogen receptors. • Fermentation or digestion of soybeans or soy products results in the release of sugar from the compound to leave the free aglycone. • The aglycone of genistin

25. The isoflavone Genistein, is found in Red clover, Alfalfa, Peas, Soy and other legumes. It consists of the basic isoflavone skeleton with -OH groups attached at positions 5, 7, and 4'. Genistein is protective against breast, prostate, and colon cancers and can help with hot flushes and osteoporosis prevention.

26. Anthocyanidins • Anthocyanidin (aglycone) is a kind of molecule that lacks any attached sugars. Notice the -OH group at position 3. Various anthocyanidins have -OH groups at other positions on both the 'A' and 'B' rings.

27. Anthocyanidin is an extended conjugation made up of the aglycone of the glycoside anthocyanins. Next to chlorophyll, anthocyanins are the most important group of plant pigments visible to the human eye. • The anthocyanodins constitute a large family of differently coloured compounds and occur in countless mixtures in practically all parts of most higher plants. They are of great economic importance as fruit pigments and thus are used to colour fruit juices, wine and some beverages.

28. Cyanidine is the most widely distributed anthocyanidin, and is estimated to occur in • 80% of pigmented leaves • 69% of fruit • 50% of flowers

29. Anthocyanidins contain red, yellow and blue pigments which depend on the pH of the sap. The anthocyanidins in Hydrangea, colours it RED in acid soil and BLUE in alkali soil.

30. COMMON DIETARY FLAVONOIDS

33. History(Wohlmuth 1998) • The first flavonoids shown to have pharmacological activity were rutin and hesperidin. They were also called ‘vitamin P’ or bioflavonoids. Today the term bioflavonoid is applied to any flavonoid with biological activity of interest. • The “P” in vitamin P stood for permeability because rutin and hesperidin were found to strengthen capillaries and thus reduce capillary fragility and permeability. • These flavonoids are no longer regarded as vitamins, but they do act synergistically with vitamin C, and they have been used in the treatment of hypertension, radiation injuries, allergies and vascular damage caused by diabetes.

34. Distribution in plants (Wohlmuth 1998) • Most vascular plants contain flavonoids, and we obtain significant amounts in the diet from plant foods. Foodstuffs that are significant contributors to dietary flavonoid intake are: • The predominant dietary flavonoid is quercetin, which forms the aglycone of several flavonoid glycosides including rutin and quercetin.

35. Metabolism and absorption • When examining the absorption and metabolism of flavonoids, it is essential to recognise that most pharmacological studies of activity have been used in in vitro models, often isolating individual enzymes. • However, Mills & Bone (2000, p.31) explains that it is uncertain whether oral doses of flavonoid glycosides or their aglycones, con reach significant concentrations in living animals to reproduce these effects.

36. The strong antioxidant in vitro action is believed to be reduced with metabolism and conjugation during absorption and degradation in the small intestine and colon. Further degradation occurs by the bowel microflora into smaller phenolic acids and it is these which are then absorbed in the major amounts. (Spencer, JPE. 2003, ‘Metabolism of Tea Flavonoids in the Gastrointestinal Tract’, The American Journal of Clinical Nutrition, vol.133, pp.3255S-3261S) • Intestinal disposition of flavonoid metabolism was found to be more important that the hepatic disposition in the first pass metabolism of flavonoids and was found to be a major factor involved in the poor systemic bioavailability of flavonoids.(Chen,J., Lin,H., Hu,M. 2003, ‘Metabolism of flavonoids via enteric recycling: role of intestinal disposition’, The Journal of Pharmacology and Experimental Therapeutics, vol.304, no.3, pp.1228-35)

37. Murota & Terao (2003) found that it was the small intestine cells that provided the most effective absorption site for the quercetin glucosides, as well as being the sites for the metabolic conversion of quercetin and other flavonoids. (Murota,K & Terao,J. 2003, ‘Antioxidant flavonoid quercetin: implications of its intestinal absorption and metabolism’, Archives of biochemistry and biophysics, vol.417, no.1, pp.7-12) • There is now strong evidence for the extensive phase 1 deglycolation and phase 11 metabolism of aglycones across the small intestine, again in the liver and further in the colon where the enzymes of the microflora degrade the flavonoids to simple phenolic acids. These phenolic acids are then reabsorbed and further metabolised by the liver. (Williams,RJ., Spencer,JPE., Rice-EVANS,c. 2004, ‘Flavonoids and Isoflavones (Phytoestrogens): Absorption, Metabolism and Bioactivity’, Free Radical Biology & Medicine, vol.36, no.7, pp.838-849)

38. A further complication to the effective metabolism of flavonoids as antioxidants is the presence of higher concentrations of other small molecule antioxidant nutrients such as ascorbic acid and a-tocopherol (Williams et al 2004). • Therefore, it can be said that the disposition of flavonoids occurs via both the enteric and enterohepatic recycling systems with the intestinal phase 11 enzymes playing a significant role in this process.

39. Certain flavonoids, such as kaempferol and quercetin, are potent stimulators of P-gp as well as CYP450 (3A4). As such, they have the ability to alter the absorption and bioavailability of some drugs and herbs, that are P-gp substrates. (Zhang,S., Yang,X., Morris,ME. 2004, ‘Flavonoids are inhibitors of breast cancer resistance protein (ABCG2) – mediated transport’, Molecular Pharmacology, vol.65, no.5, pp.1208-16) • Flavonoids have an essential role to play on enhancing the bioavailability and activity of hypericum, most likely as a result of the transport of the active constituents through the membranes via interactions with the P-gp multiple drug transporters (Nolder,M & Scholtz,K. 2002, ‘Rutin is essential for the antidepressant activity of Hypericum perforatum extracts in the forced swimming test’, Planta Medica, vol.68, pp.577-80)

40. The flavonoid glabridin from licorice root was found to inactivate or inhibit the activity of several of the CYP450 enzymes – however, this was an in vitro study so the same issues of bioavailability mentioned earlier would need to be considered. (Ute,M., Aviram,M., Rosenblat,M., Hollenberg,PF. 2002, ‘The Licorice Root Derived Isoflavan Glabridin Inhibits the Activities of Human Cytochrome P450S, 3A4, 2B6 and 2C9’, Drug Metabolism and Disposition, vol.30, no.6, pp.709-715) • An understanding of the modulation of the CYP’s, P-gp and MDRP1 explains the beneficial effects of flavonoids in detoxification, chemoprotection and in drug resistance suppression.

41. Pharmacology • Different flavonoids display a variety of pharmacological activities. Some common effects of flavonoids are: • Antioxidant, free radical scavenging activity • Silibin is the most important flavonoid in Silybum marianum. Refer to you text and explain how this works as a flavonoid. • Prevention of LDL oxidation and associated cardiovascular effects • Epidemiological data show a negative correlation between dietary flavonoid intake and coronary heart disease mortality

42. Flavonoids are thought to be at least partly responsible for the ‘French Paradox’, which refers to the low incidence of coronary heart disease in Mediterranean countries despite a high fat intake. • Capillary strengthening activity • Anti-thrombotic (anti-platelet) activity • Relax cardiovascular muscle i.e. hypotensive

43. Antiinflammatory activity • Partly due to their modification of leukotriene and/or prostaglandin synthesis through inhibition of the enzymes 5-lipoxygenase (quercetin) and cyclo-oxygenase (catechin) • Leukotrienes have been implicated in diseases such as asthma, allergic rhinitis, psoriasis, gout, rheumatoid arthritis and inflammatory bowel disease • Quercetin inhibits histamine release from mast cells and basophils • Apigenin is a strongly antiinflammatory flavonoid found in a number of medicinal plants e.g. Chamomilla recutita

44. Antiviral activity • Quercetin has antiviral activity against some enveloped viruses including herpes simplex type 1, respiratory syncytial virus and parainfluenza virus • Flavonoids inhibit reverse transcriptase, suggesting they could play a role in the control of retrovirus infections such as HIV • Neuroprotection read the Vauzour et al. 2008 article and summarise how this occurs.

45. On the left, models of the three human estrogens: estradiol, estrone and estriol. To the right, the isoflavones genistein, daidzein, and glycitein. Notice the general similarity of the structures. These molecules have several features in common which allow them to interact with mammalian oestrogen receptors and to exert hormonal effects.

46. In estrogen-receptor assays, equol exhibits considerably more binding affinity than its precursor; it is roughly equal to genistein in this respect. The differences between the two molecules (blue highlight) seem slight, but equol is more able to fit into and be retained by the oestrogen receptor.