Natural polyphenols inhibition

In addition to their possible prooxidant activity (see above) polyphenols and flavonoids may influence cancer cells via their antioxidant properties. Recently, Jang et al. [219] studied cancer chemopreventive activity of resveratrol, a natural polyphenolic compound derived from grapes (Chapter 29). These authors showed that resveratrol inhibited the development of preneoplastic lesions in carcinogen-treated mouse mammary glands in culture and inhibited tumorigenesis in a mouse skin cancer model. Flavonoids silymarin and silibinin also exhibited antitumor-promoting effects at the stage I tumor promotion in mouse skin [220] and manifested antiproliferative effects in rat prostate cancer cells [221]. 

Phase 1 clinical trials of the natural polyphenol quercetin (33) against several types of cancer have shown that this compound is well tolerated when administered at a dose of 70 mg/kg by i.v. bolus [63,64]. Further studies revealed that 33 inhibits GST Pl-1 completely after 2 h at a concentration of 25 pM however, addition of GSH partially restores activity. HPLC and LC-MS studies indicate that 33 inhibits GST Pl-1 through the formation of a covalent yet reversible bond with GST Pl-1 cysteine residue at position 47. 

Tannic acid has been reported to inhibit the formation of hydroxyl radical by chelation of Fe2+ [16], Because tannic acid is a plant-derived material, it and other natural polyphenols may be important for subsurface applications of Fenton chemistry. A study of 50 different iron chelators assessed the affect of each chelator on the Fenton process initiated with Fe3+ and hydrogen peroxide [17]. Among the nine classes of chelators tested, results indicated that the chelators ranged from inactive to highly active in terms of hydroxyl radical formation. 

In summary, the natural polyphenols TE3, EGCG, and 5GG exhibit their inhibitory effects on androgen action in prostate cancer cells by two mechanisms, including the inhibition of 5a-reductase activity (figures 12.1 and 12.2) and the suppression of AR protein levels. Therefore, tea polyphenols and 5GG may have the potential to become chemopreventive and chemotherapeutic agents for prostate tumors. 

A new dimeric plant tannin named woodfruticosin A, which is active as an inhibitor of DNA topoisomerase II, has been isolated from a Nepalese medicinal plant. Chelation with the ester groups at 0-1 and 0-6 of penta-O-galloyl- -D-glucose has been suggested as the mode of its complexation with various aromatic substrates (e.g., caffeine, daunomycin, methylene blue). -Pentagalloylglucose has also been reported to specifically inhibit two NADH reductases from E. coli. Two-dimensional H- and C-n.m.r. studies of natural polyphenols, with emphasis on the ester linkages between the phenolic adds and... 

YANG X Q, WANG Y F and XU F (1995) Natural antioxidant tea polyphenols application on oil and food Study on inhibiting the deterioration of salad oil and instant noodles , J UnivAgric Zhejiang, 21 (5), 513-18. 

Tannins are polyphenols that occur only in vascular plants such as leaves, needles, barks heartwood, seeds and flowers. Tannins exist primarily in condensed and hydrolysable forms. Natural tannin extracts have been employed since the turn of the last century in leather industry. Recent development of their industrial uses as adhesives, flocculants, depressants, viscosity modifier agents and more recently as corrosion inhibitors reflects their importance as industrial raw materials. Electrochemical studies have shown that tannins extracted from the barks of mangrove trees are excellent corrosion inhibitors of steel at very low pH. The mechanism of inhibition at this pH was due to the chemisorption of tannin molecules while at higher pH, inhibition was achieved via formation of ferric-tannates [6]. 

Among polyphenols, resveratrol and its derivatives are thought to play a special role. Resveratrol occurs naturally in grapes in both cis-and trans-isomers, and in their respective glucosides (cis- and trans-piceids). All forms inhibit the oxidation of low density lipoprotein (LDL) and additional benefits. Resveratrol occurs in most red wines but is undetectable or occurs in negligible amounts in dry white wines (Pour Nikfardjam, 2002). 

PhIP. The inhibition by tea of these two tests, which are indicators of genotoxicity, suggests that naturally occurring plant extracts can modify metabolic activation of HAAs. Tea polyphenols, epigallocatechin gallate from green tea and theaflavin gal-late from black tea, display similar inhibiting action in both tests (see ref. 318). 

Ascorbic acid browning is also inhibited by the addition of sulfite (Wedzicha and McWeeny, 1974). The same holds for polyphenol oxidase-catalyzed oxidation of natural phenols in fruit. The mechanism of the inhibition is by reaction of oquinone intermediates with sulfite, which leads to nonreactive sulfocatechols (Wedzicha, 1995). 

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