Inhibitory effect of flavonoids

In addition to xanthine oxidase, flavonoids are able to inhibit the activity of a wide range of enzymes. These inhibitory effects of flavonoids may depend both on their free radical scavenging and chelating properties. Thus, it has been shown that flavonoids inhibit... 

The effects of flavonoids on in vitro and in vivo lipid peroxidation have been thoroughly studied [123]. Torel et al. [124] found that the inhibitory effects of flavonoids on autoxidation of linoleic acid increased in the order fustin < catechin < quercetin < rutin = luteolin < kaempferol < morin. Robak and Gryglewski [109] determined /50 values for the inhibition of ascorbate-stimulated lipid peroxidation of boiled rat liver microsomes. All the flavonoids studied were very effective inhibitors of lipid peroxidation in model system, with I50 values changing from 1.4 pmol l-1 for myricetin to 71.9 pmol I 1 for rutin. However, as seen below, these /50 values differed significantly from those determined in other in vitro systems. Terao et al. [125] described the protective effect of epicatechin, epicatechin gallate, and quercetin on lipid peroxidation of phospholipid bilayers. 

The above findings are supported in the other studies of the inhibitory effects of flavonoids on iron-stimulated lipid peroxidation. Quercetin was found to be an inhibitor of iron-stimulated hepatic microsomal lipid peroxidation (/50 = 200 pmol I ) [134]. Flavonoids eriodictyol, luteolin, quercetin, and taxifolin inhibited ascorbate and ferrous ion-stimulated MDA formation and oxidative stress (measured by fluorescence of 2,7,-dichlorodihydro-fluorescein) in cultured retinal cells [135]. It should be mentioned that in recent work Heijnen et al. [136] revised the structure activity relationship for the protective effects of flavonoids against lipid peroxidation. 

It must be noted that the inhibitory effects of flavonoids and other antioxidants in nonhomogenous biological systems can depend not only on their reactivities in reactions with free radicals (the chain-breaking activities) but also on the interaction with biomembranes. Thus, Saija et al. [137] compared the antioxidant effects and the interaction with biomembranes of four flavonoids quercetin, hesperetin, naringen, and rutin in iron-induced... 

It is known that the toxic effects of some solid particles and fibers (latex, zeolite, asbestos fibers, etc.) depend on the stimulation of free radical production. Therefore, the inhibitory effects of flavonoids on particle-mediated damaging processes might be expected. Thus, rutin... 

Kitagawa S, Nabekura T, Takahashi T, Nakamura Y, Sakamoto H, Tano H, Hirai M, Tsukahara G. 2005. Structure-activity relationships of the inhibitory effects of flavonoids on P-glycoprotein-mediated transport in KB-C2 cells. Biol Pharm Bull 28 2274-2278. 

Flavonoids are well known as inhibitors of hyaluronidase. Li 1950 Rodney et al, reported the effect of several flavonoids on hyaluronidase using in vitro and in vivo methods [79], Knppusamy et al. investigated the structure-activity relationship of die flavonoids as inhibitors of testicular hyaluronidase [80] and hyaluronidase from snake, scorpion, and bee venom [81]. They showed that flavones. flavanols. and chalcones possess the general ability to inhibit hyaluronidase Kinetic investigations showed that these compounds acted as competitive inhibitors [80]. Several hyaluronidase inhibitors were shown to be able to delay the action of snake venom when injected into mice [8ll One caimnt rule out the possible interaction of the flavonoids with other components of the venom, as the inhibitory effects of flavonoids are not specific to hyaluronidase... 

Nishibe, S. Ono, K. Nakane, H. Kawamura, T. Noro, Y. Tanaka, T. Studies of the Plantaginis herba. 9. Inhibitory Effects of Flavonoids from Plantago Species on HIV Reverese Transcriptase Activity. Nat. Med. 1997, 51, 547-549. 

In addition to free radical scavenging, the biological activity of flavonoids involves their action as protein kinase inhibitors and transcriptional regulators. However, the inhibitory effects of flavonoids on DNA modulating enzymes emphasize that the use of these substances as dietary supplements may also increase the risk of disease development (Moskaug, 2001). 

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