Structure-antioxidant activity relationships of flavonoids and phenolic acids

Antioxidant properties of phenolic compounds

The main dietary sources of polyphenols are reviewed, and the daily intake is calculated for a given diet containing some common fruits, vegetables and beverages. Phenolic acids account for about one third of the total intake and flavonoids account for the remaining two thirds. The most abundant flavonoids in the diet are flavanols (catechins plus proanthocyanidins), anthocyanins and their oxidation products. The main polyphenol dietary sources are fruit and beverages (fruit juice, wine, tea, coffee, chocolate and beer) and, to a lesser extent vegetables, dry legumes and cereals. The total intake is approximately 1 g/d. Large uncertainties remain due to the lack of comprehensive data on the content of some of the main polyphenol classes in food. Bioavailability studies in humans are discussed. The maximum concentration in plasma rarely exceeds 1 microM after the consumption of 10-100 mg of a single phenolic compound. However, the total plasma phenol concentration is probably higher due to the presence of metabolites formed in the body's tissues or by the colonic microflora. These metabolites are still largely unknown and not accounted for. Both chemical and biochemical factors that affect the absorption and metabolism of polyphenols are reviewed, with particular emphasis on flavonoid glycosides. A better understanding of these factors is essential to explain the large variations in bioavailability observed among polyphenols and among individuals.

Dietary Intake and Bioavailability of Polyphenols

Epidemiological and medical anthropological investigations suggest that flavanol-rich foods exert cardiovascular health benefits. Endothelial dysfunction, a prognostically relevant key event in atherosclerosis, is characterized by a decreased bioactivity of nitric oxide (NO) and impaired flow-mediated vasodilation (FMD). We show in healthy male adults that the ingestion of flavanol-rich cocoa was associated with acute elevations in levels of circulating NO species, an enhanced FMD response of conduit arteries, and an augmented microcirculation. In addition, the concentrations and the chemical profiles of circulating flavanol metabolites were determined, and multivariate regression analyses identified (–)-epicatechin and its metabolite, epicatechin-7-O-glucuronide, as independent predictors of the vascular effects after flavanol-rich cocoa ingestion. A mixture of flavanols/metabolites, resembling the profile and concentration of circulating flavanol compounds in plasma after cocoa ingestion, induced a relaxation in preconstricted rabbit aortic rings ex vivo, thus mimicking acetylcholine-induced relaxations. Ex vivo flavanol-induced relaxation, as well as the in vivo increases in FMD, were abolished by inhibition of NO synthase. Oral administration of chemically pure (–)-epicatechin to humans closely emulated acute vascular effects of flavanol-rich cocoa. Finally, the concept that a chronic intake of high-flavanol diets is associated with prolonged, augmented NO synthesis is supported by data that indicate a correlation between the chronic consumption of a cocoa flavanol-rich diet and the augmented urinary excretion of NO metabolites. Collectively, our data demonstrate that the human ingestion of the flavanol (–)-epicatechin is, at least in part, causally linked to the reported vascular effects observed after the consumption of flavanol-rich cocoa.

/pdf/epicatechin-mediates-beneficial-effects-of-flavanol-rich-5b5eggk05z.pdf

(–)-Epicatechin mediates beneficial effects of flavanol-rich cocoa on vascular function in humans

Role of antioxidants in prophylaxis and therapy: A pharmaceutical perspective.

Following administration of a single dose of [U14C]cyanidanol-3 to human volunteers, a mean of 55% of the dose of 14C was excreted in urine; 90% of urine 14C was excreted within 24 h of drug administration. The major urinary metabolites were the glucuronides of (+)-catechin and 3'-O-methyl-(+)-catechin, and the sulphate of the latter. These three conjugates collectively accounted for three quarters of urine 14C. Urinary excretion of unchanged (+)-cyanidanol-3 was 0.1-1.4% dose. (+)-Cyanidanol-3 and metabolites containing the intact flavanol ring system accounted for 90% of urine 14C. Ring scission was, under these conditions, a minor metabolic pathway resulting in the excretion of small amounts of 3-hydroxybenzoic acid, 3-hydroxyhippuric acid and 3-hydroxyphenylpropionic acid. Unchanged (+)-cyanidanol-3 was detected in plasma between 30 min and 12 h after administration. Metabolites (as total 14C) persisted in plasma for at least 120 h after administration.

The metabolism and excretion of (+)-[14C]cyanidanol-3 in man following oral administration.

The major urinary metabolites of (+)-catechin (cyanidanol-3) in the rat were (+)-catechin glucuronide, 3′-O-methyl-(+)-catechin glucuronide and 3′-O-methyl-(+)-catechin sulphate. The latter conjugate was the major metabolite in marmoset urine.

3′-O-Methyl-(+)-catechin glucuronide and 3′-O-methyl-(+)-catechin sulphate: new urinary metabolites of (+)-catechin in the rat and the marmoset

1. Three young male volunteers excreted 51·5, 33·6 and 55·9% of a single 2 g oral dose of radiolabelled 3-O-methyl-(+)-catechin in urine within 120 h of administration. Excretion of the unchanged compound, however, accounted for only 0·7, 0·1 and 0·2% of the dose.2. The major urinary metabolites were glucuronides of 3,3′-O-dimethyl-(+)-catechin (15·8% dose) and a glucuronide and a sulphate of 3-O-methyl-(+)-catechin (11·4% and 10·6% dose, respectively). No evidence for demethylation was obtained.3. 3-O-Methyl-(+)-catechin was detected and measured in plasma by h.p.l.c. 0·5-12 h after administration. Peak levels (11·18 μg/ml) were attained within two hours of administration and the half-life of removal from plasma was approx. 140 min.4. Metabolism of 3-O-methyl-(+)-catechin in man was found to vary significantly from other species, as methylated metabolites (3′-O-methyl ethers) in man represented only about 53% of the urinary metabolites, whereas in the mouse, rat and marmoset, 3′-O-methylation was almost ...

The quantitative disposition of 3-O-methyl-(+)-[U-14C]catechin in man following oral administration

1. The major biliary metabolites of flavanones in the rat have been identified by chromatographic and spectral methods.2. Evidence is presented that flavanones and flavanone glycosides, following oral or parenteral administration, undergo glucuronylation and are selectively excreted via the bile.3. Flavanone glycosides but not unconjugated aglycones may be excreted to a significant extent in bile.4. The percentage of parenterally administered flavanones excreted in bile varies inversely with the amount administered.

The bilary excretion of flavanones in the rat.

1. Oral administration of 3-palmitoyl-(+)-[U-14C]catechin to rats resulted in the excretion of 63% of the dose in urine, 24% in faeces and 7% as 14CO2 in the 17 days following dosing. Persistent levels of 14C were noted in tissues, and 28 days after dosing 3.7% of the dose was present in the animal body. 2. Biliary excretion accounted for 28.2% of the dose in 48h after dosing. The major biliary metabolite was 3'-O-methyl-(+)-catechin glucuronide. These experiments also demonstrated the dependence of 3-palmitoyl-(+)-catechin on the presence of bile in the intestine for absorption. 3. Studies in vitro showed that intestinal micro-organisms were unable to metabolize 3-palmitoyl-(+)-catechin. The compound did, however, undergo deesterification in plasma and also in liver-perfusion studies. In the latter experiments, conjugates of the liberated (+)-catechin were also formed. 4. Urinary metabolites were predominantly (80%) conjugates of (+)-catechin and 3'-O-methyl-(+)-catechin. Ring scission products and 14CO2 were also excreted but not from rats with ligated bile ducts. These metabolites are therefore considered to arise from biliary metabolites not containing the ester linkage.

The metabolism and excretion of 3-palmitoyl-(+)-catechin in the rat

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