Toxicology 177 (2002) 91 – 104 Prooxidant activity and cellular effects of the phenoxyl radicals of dietary flavonoids and other polyphenolics Giuseppe Galati a , Omid Sabzevari b , John X. Wilson c , Peter J. O’Brien a, * a Department of Pharmacology, Faculty of Pharmacy, Uniersity of Toronto, 19 Russell St., Toronto, Ont., Canada M5S 2S2 b Faculty of Pharmacy, Tehran Uniersity of Medical Sciences, Tehran 14155 /6451, Iran c Department of Physiology, Uniersity of Western Ontario, London, Ont., Canada Abstract Dietary polyphenolics in fruits, vegetables, wines, spices and herbal medicines have beneficial antioxidant, anti-inflammatory and anticancer effects. However, we have observed that dietary polyphenolics with phenol rings were metabolized by peroxidase to form prooxidant phenoxyl radicals which, in some cases were sufficiently reactive to cooxidize GSH or NADH accompanied by extensive oxygen uptake and reactive oxygen species formation. The order of catalytic effectiveness found for oxygen activation when polyphenolics were metabolized by peroxidase in the presence of GSH was phloretin phloridzin 4,2-dihydroxy chalcone p -coumaric acid naringenin apigenin  curcumin resveratrol isoliquiritigenin capsaicin kaempferol. Ascorbate was also cooxidized by the phenoxyl radicals but without oxygen activation. Polyphenolics with catechol rings also cooxidized ascorbate, likely mediated by semiquinone radicals. The order of catalytic effectiveness found for ascorbate cooxidation was fisetin, luteolin, quercetin, eriodictyol, caffeic acid, nordihydroguaiaretic acid catechin taxifolin, catechol. NADH was stoichio- metrically oxidized without oxygen uptake which, suggests that o -quinone metabolites were responsible. GSH was not cooxidized and GSH conjugates were formed, likely mediated by the o -quinone metabolites. Incubation of hepato- cytes with dietary polyphenolics containing phenol rings was found to partially oxidize hepatocyte GSH to GSSG while polyphenolics with a catechol ring were found to deplete GSH through formation of GSH conjugates. Dietary polyphenolics with phenol rings also oxidized human erythrocyte oxyhemoglobin and caused erythrocyte hemolysis more readily than polyphenolics with catechol rings. It is concluded that polyphenolics containing a phenol ring are generally more prooxidant than polyphenolics containing a catechol ring. © 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Flavonoids; Polyphenolics; Phenoxyl radicals; Peroxidase; Prooxidant; Oxidative stress www.elsevier.com/locate/toxicol Abbreiations: Asc, dehydroascorbate; AscH  , semidehydroascorbate radical; AscH 2 , ascorbate; DETAPAC, diethylenetriamine penta-acetic acid; EDTA, ethylenediamine tetra-acetic acid; ESR, electron spin resonance; GS  , thiyl radical; GSH, glutathione (reduced form); GSSG, glutathione (oxidized form); GSSG - , disulfide radical anion; H 2 O 2 , hydrogen peroxide; HRP, horseradish peroxidase; NAD + , -nicotinamide adenine dinucleotide (oxidized form); NAD  , -nicotinamide adenine dinucleotide (cation radical); NADH, -nicotinamide adenine dinucleotide (reduced form); NDGA, nordihydroguaiaretic acid; O 2 - , superoxide radical; PBS, phosphate-buffered saline; PhO  , phenoxyl radical; PhOH, phenolic compound. * Corresponding author. Tel.: +1-416-978-2716; fax: +1-416-978-8511. E-mail address: peter.obrien@utoronto.ca (P.J. O’Brien). 0300-483X/02/$ - see front matter © 2002 Elsevier Science Ireland Ltd. All rights reserved. PII:S0300-483X(02)00198-1