Flavonoids activity

Antioxidant activity of flavonoids has already been shown about 40 years ago [90,91]. (Early data on antioxidant flavonoid activity are cited in Ref. [92].) Flavonoids are polyphenols, and therefore, their antioxidant activity depends on the reactivity of hydroxyl substituents in hydrogen atom abstraction reactions. As in the case of vitamins E and C, the most studied (and most important) reactions are the reactions with peroxyl radicals [14], hydroxyl radicals [15], and superoxide [16]. 

Firmin, J.L. et al., Flavonoid activation of nodulation genes in Rhizobium reversed by other compounds present in plants. Nature, 324, 90, 1986. 

This double bond differentiates between flavones (or flavonols) and flavanones, and different studies show that it enhances flavonoid activity [6, 18, 23, 13]. We can say that flavones are more active than flavanones but we could not generalize because we have also found two flavanones to be more active than the corresponding flavones [12]. But this fact could be explained because these two pairs of flavonoids are methoxylated in position 4 and Mori et al. [6] said, when they studied their flavonoids, that when a B-ring was methoxilated the 2-3 double bond do not increase the activity. 

Harlow GR, Halpert JR. Alanine-scanning mutagenesis of a putative substrate recognition site in human cytochrome P450 3A4 Role of residues 210 and 211 in flavonoid activation and substrate specificity. J Biol Chem 1997 272 5396-402. 

Flavonoids as antioxidants have been reviewed several times 45s including an outline of many claims to their beneficial health effects . Due to their complex structures and different classes (eight thousand different compounds are known ), researchers often resorted to qualitative screening methods to evaluate their antioxidant potentials in mixed aqueous/lipid phases. For example, the so-called Trolox equivalent antioxidant capacity (TEAC), the concentration of Trolox with equivalent antioxidant activity of a 1 mM concentration of the substrate, is frequently used in heterogeneous systems. Unfortunately, this can be an unreliable measure of the activity of the substance, especially if initiation is also carried out in the aqueous phase. Nevertheless, there have been some efforts made to evaluate antioxidant activities of specific flavonoids using more quantitative methods in heterogeneous systems in order to mimic natural environments. A few examples are cited below to illustrate some approaches to determine flavonoid activities in micelles or lipid membranes. 

Flavonoids activate caspases that are specific apoptotic proteases. Cytotoxicity of quercetin may rapidly be reached when it is not sequestered in vacuoles (EC50 = 34 pM, Watjen et al, 2005). 

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