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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 stoichiometrically 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 hepatocytes
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.