Dr. Donald R. Buhler

Antioxidant Activities of Flavonoids

Department of Environmental and Molecular Toxicology
Oregon State University

Dr. Cristobal Miranda

Flavonoids are polyphenolic compounds that are ubiquitous in nature and are categorized, according to chemical structure, into flavonols, flavones, flavanones, isoflavones, catechins, anthocyanidins and chalcones. Over 4,000 flavonoids have been identified, many of which occur in fruits, vegetables and beverages (tea, coffee, beer, wine and fruit drinks). The flavonoids have aroused considerable interest recently because of their potential beneficial effects on human health-they have been reported to have antiviral, anti-allergic, antiplatelet, anti-inflammatory, antitumor and antioxidant activities. 

Antioxidants are compounds that protect cells against the damaging effects of reactive oxygen species, such as singlet oxygen, superoxide, peroxyl radicals, hydroxyl radicals and peroxynitrite. An imbalance between antioxidants and reactive oxygen species results in oxidative stress, leading to cellular damage. Oxidative stress has been linked to cancer, aging, atherosclerosis, ischemic injury, inflammation and neurodegenerative diseases (Parkinson's and Alzheimer's). Flavonoids may help provide protection against these diseases by contributing, along with antioxidant vitamins and enzymes, to the total antioxidant defense system of the human body. Epidemiological studies have shown that flavonoid intake is inversely related to mortality from coronary heart disease and to the incidence of heart attacks. 

The recognized dietary antioxidants are vitamin C, vitamin E, selenium, and carotenoids. However, recent studies have demonstrated that flavonoids found in fruits and vegetables may also act as antioxidants. Like alpha-tocopherol (vitamin E), flavonoids contain chemical structural elements that may be responsible for their antioxidant activities. A recent study by Dr. van Acker and his colleagues in the Netherlands suggests that flavonoids can replace vitamin E as chain-breaking anti- oxidants in liver microsomal membranes. The contribution of flavonoids to the antioxidant defense system may be substantial considering that the total daily intake of flavonoids can range from 50 to 800 mg. This intake is high compared to the average daily intake of other dietary antioxidants like vitamin C (70 mg), vitamin E (7-10 mg) or carotenoids (2-3 mg). Flavonoid intake depends upon the consumption of fruits, vegetables, and certain beverages, such as red wine, tea, and beer. The high consumption of tea and wine may be most influential on total flavonoid intake in certain groups of people. 

The oxidation of low-density lipoprotein (LDL) has been recognized to play an important role in atherosclerosis. Immune system cells called macrophages recognize and engulf oxidized LDL, a process that leads to the formation of atherosclerotic plaques in the arterial wall. LDL oxidation can be induced by macrophages and can also be catalyzed by metal ions like copper. Several studies have shown that certain flavonoids can protect LDL from being oxidized by these two mechanisms. 

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. 

Recently, chalcone and flavanone flavonoids with prenyl or geranyl side chains have been identified in hops and beer by Dr. Fred Stevens and Dr. Max Deinzer at Oregon State University. Hops are used in beer for flavor. Xanthohumol (a chalcone) and isoxanthohumol and 6-prenylnaringenin (flavanones) are the major prenyl-flavonoids found in beer. Although the antioxidant activities of these compounds have not been studied, these flavonoids may be responsible for the antioxidant activity of lager beer, which is higher than that of green tea, red wine, or grape juice as reported earlier by Dr. Joe A. Vinson from the University of Scranton in Pennsylvania. Xanthohumol is found only in beer but in small concentrations. 

To assess the antioxidant activity of the prenylated flavonoids, we-in collaboration with LPI researchers-evaluated the capacity of these flavonoids to inhibit the oxidation of LDL by copper. The antioxidant properties of the prenylflavonoids were compared to those of quercetin (a flavonol), genistein (the major isoflavone in soy), chalconaringenin (a non-prenylated chalcone), naringenin (a non-prenylated flavanone), and vitamin E. The possible interaction of xanthohumol, the major prenylchalcone in beer, with vitamin E to inhibit LDL oxidation induced by copper was also examined. 

Our results showed that the prenylchalcones and prenylflavones are effective in preventing LDL oxidation  initiated by copper and that the prenylchalcones generally have greater antioxidant activity than the prenylflavanones. Xanthohumol, the major prenylchalcone in hops and beer, is a more powerful antioxidant than vitamin E or genistein. However, xanthohumol was less potent than quercetin. The potency of xanthohumol as an antioxidant is markedly increased when combined with an equivalent amount of vitamin E. 

As reported in the Journal of Agricultural and Food Chemistry, we also found that the prenyl group plays an important role in the antioxidant activity of certain flavonoids. A flavonoid chalcone (chalconaringenin) and a flavanone (naringenin) with no prenyl groups act as pro-oxidants, i.e. they promote rather than limit the oxidation of LDL by copper. However, adding a prenyl group to these flavonoid molecules counteracted their pro-oxidant activities. 

Our work reveals that there are unique flavonoids in hops and beer that may be potentially useful in the preventionof human disease attributed to free radical damage. The observation that prenyl groups are important in conferring antioxidant activity to certain flavonoids may lead to the discovery or synthesis of novel prenylated flavonoids as preventive or therapeutic agents against human diseases associated with free radicals. Our encouraging results with xanthohumol suggest that this prenylchalcone should be further studied for its antioxidant action and protective effects against free radical damage in animals and humans. Preliminary studies have shown that xanthohumol is absorbed from the digestive tract in rats, and more studies are needed to evaluate the bioavailability of these interesting flavonoids in people. 

Further studies are also needed to establish the safety of xanthohumol or other flavonoids for use as dietary supplements since high doses of these compounds may produce adverse effects in humans, according to recent findings by Dr. Martyn Smith, professor of toxicology, University of California at Berkeley