Inhibition of lipid peroxidation by quercetin and quercetin derivatives: antioxidant and prooxidant effects

Flavonoids (polyphenols) are an important class of dietary antioxidants lar gely distributed in plants. In spite of their very fast reaction with lipid peroxyl radicals, flavonoids are relatively modest inhibitors of lipid per oxidation in solution. In addition, the absence of lag phase and the strong dependence of the antioxidant efficiency (the ratio of the rate constant o f chain break to the rate constant of chain propagation) on the antioxidant concentration both point to a nonclassic behaviour. The inhibition of lino leic acid peroxidation by quercetin (3,3',4',5,7-pentahydroxyflavone) and i ts 3-O-glycoside rutin (two ubiquitous dietary flavonoids and potent antiox idants) has been investigated by UV-visible spectroscopy in pH 7.4 micellar solutions of SDS at 37 degrees C. The chain-breaking antioxidant alpha-toc opherol and two synthetic amphiphilic quercetin ethers are also considered for comparison purposes. From the general mathematical treatment developed in this work, flavonoids appear as strong chain-breaking antioxidants whose overall antiperoxidizing activity is, however, strongly restricted by the chain-initiating activity of the flavonoid radicals formed during inhibitio n (prooxidant effect).