Flavonoids are a class of secondary plant phenolics with significant antioxidant and chelating properties. In the human diet, they are most concentrated in fruits, vegetables, wines, teas and cocoa. Their cardioprotective effects stem from the ability to inhibit lipid peroxidation, chelate redox-active metals, and attenuate other processes involving reactive oxygen species. Flavonoids occur in foods primarily as glycosides and polymers that are degraded to variable extents in the digestive tract. Although metabolism of these compounds remains elusive, enteric absorption occurs sufficiently to reduce plasma indices of oxidant status. The propensity of a flavonoid to inhibit free-radical mediated events is governed by its chemical structure. Since these compounds are based on the flavan nucleus, the number, positions, and types of substitutions influence radical scavenging and chelating activity. The diversity and multiple mechanisms of flavonoid action, together with the numerous methods of initiation, detection and measurement of oxidative processes in vitro and in vivo offer plausible explanations for existing discrepancies in structure-activity relationships. Despite some inconsistent lines of evidence, several structure-activity relationships are well established in vitro. Multiple hydroxyl groups confer upon the molecule substantial antioxidant, chelating and prooxidant activity. Methoxy groups introduce unfavorable steric effects and increase lipophilicity and membrane partitioning. A double bond and carbonyl function in the heterocycle or polymerization of the nuclear structure increases activity by affording a more stable flavonoid radical through conjugation and electron delocalization. Further investigation of the metabolism of these phytochemicals is justified to extend structure-activity relationships (SAR) to preventive and therapeutic nutritional strategies.

Flavonoid antioxidants: chemistry, metabolism and structure-activity relationships.

There has been increasing interest in the research on flavonoids from plant sources because of their versatile health benefits reported in various epidemiological studies. Since flavonoids are directly associated with human dietary ingredients and health, there is need to evaluate structure and function relationship. The bioavailability, metabolism, and biological activity of flavonoids depend upon the configuration, total number of hydroxyl groups, and substitution of functional groups about their nuclear structure. Fruits and vegetables are the main dietary sources of flavonoids for humans, along with tea and wine. Most recent researches have focused on the health aspects of flavonoids for humans. Many flavonoids are shown to have antioxidative activity, free radical scavenging capacity, coronary heart disease prevention, hepatoprotective, anti-inflammatory, and anticancer activities, while some flavonoids exhibit potential antiviral activities. In plant systems, flavonoids help in combating oxidative stress and act as growth regulators. For pharmaceutical purposes cost-effective bulk production of different types of flavonoids has been made possible with the help of microbial biotechnology. This review highlights the structural features of flavonoids, their beneficial roles in human health, and significance in plants as well as their microbial production.

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Chemistry and Biological Activities of Flavonoids: An Overview

The biochemistry and medical significance of the flavonoids.

Flavonoids: antioxidants or signalling molecules?

Citrus Flavonoids: molecular structure, biological activity and nutritional properties: a review

The effects of lemon flavonoids, as crude flavonoids prepared from lemon juice, were investigated in diabetic rats. The oxidative stress of eriocitrin (eriodictyol 7-O-β-rutinoside) and hesperidin (hesperetin 7-O-β-rutinoside) on streptozotocin-induced diabetic rats was investigated. Diabetic rats were given a diet which contained 0.2% crude flavonoids, 0.2% eriocitrin, and 0.2% hesperidin. After the 28-d feeding period, the concentration of the thiobarbituric acid- reactive substance in the serum, liver, and kidney of diabetic rats administered crude flavonoids, eriocitrin, and hesperidin significantly decreased as compared with that of the diabetic group. The levels of 8-hydroxydeoxyguanosine, which is exchanged from deoxyguanosine owing to oxidative stress, in the urine of diabetic rats administered eriocitrin and hesperidin significantly decreased as compared with that of the diabetic rat group. Crude flavonoids, eriocitrin, and hesperidin suppressed the oxidative stress in the diabetic rats. These results demonstrated that dietary lemon flavonoids of eriocitrin and hesperidin play a role as antioxidant in vivo.

Protective effects of lemon flavonoids on oxidative stress in diabetic rats.

Eriocitrin, a flavonoid glycoside present in lemon fruit, is metabolized in vivo to a series of eriodictyol, methylated eriodictyol, 3,4-dihydroxyhydrocinnamic acid, and their conjugates. Plasma antioxidant activity increased following oral administration of aqueous eriocitrin solutions to rats. Eriocitrin metabolites were found in plasma and renal excreted urine through HPLC and LC-MS analyses. Eriocitrin was not detected in plasma and urine, but eriodictyol, homoeriodictyol, and hesperetin in their conjugated forms were detected in plasma of 4.0 h following administration of eriocitrin. In urine for 24 h, both nonconjugates and conjugates of these metabolites were detected. 3,4-Dihydroxyhydrocinnamic acid, which is metabolized from eriodictyol by intestinal bacteria, was detected in slight amounts with each form in 4.0-h plasma and 24-h urine. Eriocitrin was suggested to be metabolized by intestinal bacteria, and then eriodictyol and 3,4-dihydroxyhydrocinnamic of its metabolite were absorbed. Following administration of eriocitrin, plasma exhibited an elevated resistance effect to lipid peroxidation. Eriocitrin metabolites functioning as antioxidant agents are discussed.

Identification and antioxidant activity of flavonoid metabolites in plasma and urine of eriocitrin-treated rats

Lemon flavonoid, eriocitrin, suppresses exercise-induced oxidative damage in rat liver

An antioxidant was isolated from the peel and juice of lemon fruit (Citrus limon BURM. f.). It was identified as eriocitrin (eriodictyol 7-rutinoside) of the flavanone glycoside by HPLC, 1H-NMR and 13C-NMR analyses. The purified eriocitrin was readily soluble in water, methanol, and ethanol. A water solution of 0.05% eriocitrin was weakly acidic (pH 4.2). Eriocitrin was found to be stable even at high temperature (121°C, 15 min) in acidic solution (pH 3.5). The distribution of eriocitrin in citrus fruits was found to be especially abundant in lemons and limes, however, it was scarcely found in other citrus fruits. In the case of lemon fruit, eriocitrin was primarily distributed in the peel (about 1,500 ppm) composed of the albedo (mesocarp), flavedo (epicarp), and pulp vesicles. It was also significantly present in the juice (about 200 ppm) but was not detected in the seed. Two varieties of lemon fruits, eureka and lisbon, almost had the same eriocitrin content. The antioxidative activity of eriocitrin in the linoleic acid autoxidation system was equal to that of α-tocopherol, and it was enhanced when used together with citric acid. The eriocitrin had a synergistic effect on α-tocopherol.

Isolation of eriocitrin (eriodictyol 7-rutinoside) from lemon fruit (Citrus limon Burm. f.) and its antioxidative activity

Preparation of a Monoclonal Antibody to Nϵ-(Hexanonyl)lysine: Application to the Evaluation of Protective Effects of Flavonoid Supplementation against Exercise-Induced Oxidative Stress in Rat Skeletal Muscle

Kanefumi Yamamoto

Papers

Protective effects of lemon flavonoids on oxidative stress in diabetic rats.

Identification and antioxidant activity of flavonoid metabolites in plasma and urine of eriocitrin-treated rats

Lemon flavonoid, eriocitrin, suppresses exercise-induced oxidative damage in rat liver

Isolation of eriocitrin (eriodictyol 7-rutinoside) from lemon fruit (Citrus limon Burm. f.) and its antioxidative activity

Preparation of a Monoclonal Antibody to Nϵ-(Hexanonyl)lysine: Application to the Evaluation of Protective Effects of Flavonoid Supplementation against Exercise-Induced Oxidative Stress in Rat Skeletal Muscle