NEBRASKA REDOX BIOLOGY CENTER EDUCATIONAL PORTAL

Nebraska Redox Biology Center Educational Portal


Flavonoids

Flavonoids are polyphenols compounds found naturally in plants. Chemically flavonoids are based upon a fifteen-carbon skeleton consisting of two benzene rings linked via a heterocyclic pyrane ring. There are more then 10000 different flavonoids species that have two aromatic benzene rings linked through three carbons that can form an oxygenated heterocycle [ 1, 2 ].

Basic flavonoid structure [ 1, ].


They are divided into several groups such as flavones (flavone, apigenin, and luteolin), flavonols (quercetin, kaempferol, myricetin, and fisetin), flavanones (flavanone, hesperetin, and naringenin). [ 1, 2 ].

Structure of flavonoids [ 1, ].

Flavonoids are the most common and widely distributed group of vascular plant phenolic compounds. They are a major coloring component of flowering plants. Flavonoids cannot be synthesized by animals and are important part of animal diet. Different plant food sources containing different classes of flavonoids. Flavonols are the most abundant group flavonoids in foods. Flavonoids found in the highest amounts in the human diet include the soy isoflavones, flavonols, and the flavones. The absorption of the dietary flavonoids depend on its physicochemical properties such as molecular size, configuration, lipophilicity, solubility, and pKa [ 1, 2, 3, 4 ].

Flavonoids have many biochemical properties, but the best described property of almost every group of flavonoids is their ability to act as antioxidants. The antioxidant activity of flavonoids depends on configuration, substitution, and total number of hydroxyl groups. Such configuration influence their antioxidant activity such as radical scavenging and metal ion chelation. The B ring hydroxyl configuration is the most significant determinant of scavenging of ROS and RNS because it donates hydrogen and an electron to hydroxyl, peroxyl, and peroxynitrite radicals, stabilizing them and giving rise to a relatively stable flavonoids radical [ 1, 5, 6, 7, 8 ]. Flavonoids can can be involved in following redox-related processes: (1) direct scavenging of reactive oxygen species [ 9 ], (2) activation of antioxidant enzymes [ 9 ], (3) chelating of redox active metals [ 10 ], (4) reduction of α-tocopheryl radicals [ 5 ], (5) inhibition of oxidases [ 11 ], (6) mitigation of oxidative stress caused by nitric oxide [ 12 ], (7) regulation of uric acid levels [ 13 ], (8) increase in antioxidant properties of low molecular antioxidants [ 15 ].

Scavenging of reactive oxygen species by flavonoids. The free radical may react with a second radical, acquiring a stable quinone structure [ 8 ].


Specific groups of flavonoids are known to chelate iron and copper, thereby removing a factor for the production of free radicals in Fenton reaction. Quercetin was able to prevent oxidative injury cause released by iron in its free redox active form [ 8, 15 ]. The catechol moiety in the B ring is important for chelating of Cu2+. Quercetin is iron-chelating and iron-stabilizing flavonoid. Morin and quercetin were shown to form complexes with Cd2+ and exhibit strong antioxidant activity [ 8, 16 ].

Binding sites for redox active metals in flavonoids structure [ 8 ].

Similar to other antioxidants flavonoids can act prooxidants and, promote the oxidation of other compounds. Quercetin is a flavonoid with the most pronounced prooxidative activity. Prooxidant activity is thought to be directly proportional to the total number of hydroxyl groups in a flavonoid molecule and seem to be concentration-dependent [ 5, 17, 18 ]. Series of mono- and dihydroxyflavonoids demonstrated no detectable prooxidant activity, while multiple hydroxyl groups, especially in the B-ring, significantly increased production of hydroxyl radicals in Fenton reaction. Baicalein containing a pyrogallol structure in the A-ring which promote hydrogen peroxide production. Flavonoids are capable of Cu2+ reduction to Cu1+ and thus enable formation of initiating radicals by Fenton mechanism [ 5, 19, 20 ].

Prooxidant activity of flavonoids [ 8 ].

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