Structure-antioxidant activity relationships of flavonoids and phenolic acids

Polyphenols constitute one of the most numerous and ubiquitous groups of plant metabolites and are an integral part of both human and animal diets. Ranging from simple phenolic molecules to highly polymerized compounds with molecular weights of greater than 30,000 Da, the occurrence of this complex group of substances in plant foods is extremely variable. Polyphenols traditionally have been considered antinutrients by animal nutritionists, because of the adverse effect of tannins, one type of polyphenol, on protein digestibility. However, recent interest in food phenolics has increased greatly, owing to their antioxidant capacity (free radical scavenging and metal chelating activities) and their possible beneficial implications in human health, such as in the treatment and prevention of cancer, cardiovascular disease, and other pathologies. Much of the literature refers to a single group of plant phenolics, the flavonoids. This review offers an overview of the nutritional effects of the main groups of polyphenolic compounds, including their metabolism, effects on nutrient bioavailability, and antioxidant activity, as well as a brief description of the chemistry of polyphenols and their occurrence in plant foods.

Polyphenols: Chemistry, Dietary Sources, Metabolism, and Nutritional Significance

The biochemistry and medical significance of the flavonoids.

Flavonoids are a group of polyphenolic compounds, diverse in chemical structure and characteristics, found ubiquitously in plants. Therefore, flavonoids are part of the human diet. Over 4,000 different flavonoids have been identified within the major flavonoid classes which include flavonols, flavones, flavanones, catechins, anthocyanidins, isoflavones, dihydroflavonols, and chalcones. Flavonoids are absorbed from the gastrointestinal tracts of humans and animals and are excreted either unchanged or as flavonoid metabolites in the urine and feces. Flavonoids are potent antioxidants, free radical scavengers, and metal chelators and inhibit lipid peroxidation. The structural requirements for the antioxidant and free radical scavenging functions of flavonoids include a hydroxyl group in carbon position three, a double bond between carbon positions two and three, a carbonyl group in carbon position four, and polyhydroxylation of the A and B aromatic rings. Epidemiological studies show an inverse correlation between dietary flavonoid intake and mortality from coronary heart disease (CHD) which is explained in part by the inhibition of low density lipoprotein oxidation and reduced platelet aggregability. Dietary intake of flavonoids range between 23 mg/day estimated in The Netherlands and 170 mg/day estimated in the USA. Major dietary sources of flavonoids determined from studies and analyses conducted in The Netherlands include tea, onions, apples, and red wine. More research is needed for further elucidation of the mechanisms of flavonoid absorption, metabolism, biochemical action, and association with CHD.

Flavonoids—Chemistry, metabolism, cardioprotective effects, and dietary sources

The relative antioxidant activities, against radicals generated in the aqueous phase, of a range of plant-derived polyphenolic flavonoids, constituents of fruit, vegetables, tea and wine, have been assessed. The results show that compounds such as quercetin and cyanidin, with 3′,4′ dihydroxy substituents in the B ring and conjugation between the A and B rings, have antioxidant potentials four times that of Trolox, the vitamin E analogue. Removing the ortho-dihydroxy substitution, as in kaempferol, or the potential for electron deloculisation by reducing the 2.3 double bond in the C ring, as in catechin and epicatechin, decreases the antioxidant activity by more than 50%. but these structures are still more effective than α-tocopherol or ascorbate. The relative significance of the positions and extents of hydroxylation of the A and B rings to the total antioxidant activity of these plant polyphenols is demonstrated.

The relative antioxidant activities of plant-derived polyphenolic flavonoids

Flavonoids as superoxide scavengers and antioxidants.

Phenolic compounds and an analog as superoxide anion scavengers and anti oxidants

Five phenylpropanoid glycosides from Pedicularis, isoacteoside (1), acteoside (2), echinacoside (3), pedicularioside A (4), cistanoside D (5), and a chemically synthesised compound, permethylacteoside (6), as phenolic compounds can protect against oxidative hemolysis. The inhibitory activity is related to the number of phenolic hydroxy groups. Compounds 1, 2, 3, and 4, possessing four phenolic hydroxy groups, have stronger inhibitory activities than 5 possessing only two phenolic hydroxy groups, and compound 6 with no phenolic hydroxy group inhibited oxidative hemolysis weakly.

Protection of phenylpropanoid glycosides from Pedicularis against oxidative hemolysis in vitro.

Fast repair of thymine-hydroxyl radical adduct by phenylpropanoid glycosides

In vitro effects of prostaglandin E2 or indomethacin on the proliferation of lymphokine-activated killer cells and their cytotoxicity against bladder tumor cells in patients with bladder cancer

Zheng Rongliang

Papers

Flavonoids as superoxide scavengers and antioxidants.

Phenolic compounds and an analog as superoxide anion scavengers and anti oxidants

Protection of phenylpropanoid glycosides from Pedicularis against oxidative hemolysis in vitro.

Fast repair of thymine-hydroxyl radical adduct by phenylpropanoid glycosides

In vitro effects of prostaglandin E2 or indomethacin on the proliferation of lymphokine-activated killer cells and their cytotoxicity against bladder tumor cells in patients with bladder cancer