Different antioxidant effects of polyphenols on lipid peroxidation and hydroxyl radicals in the NADPH-, Fe-ascorbate- and Fe-microsomal systems

Abstract

Antioxidant and pro-oxidant effects of 14 naturally occurring polyphenols (PP) on rat liver microsomal lipid peroxidation (LP) and hydroxyl radical ( OH) production were studied in NADPH-dependent, 50 μM Fe 2+–500 μM ascorbate (Fe–AA) or 50 μM Fe 2+ system, respectively. LP determined by the thiobarbituric acid method was inhibited in the NADPH system by flavonols and trans-resveratrol that were more effective than other flavonoids and derivatives of benzoic and cinnamic acid and were mostly more efficient than in the Fe–AA system. Inhibition of LP in the Fe system was higher by one order of magnitude than in the Fe–AA system. OH production in the NADPH system, measured by formaldehyde production, was decreased by myricetin, fisetin and quercetin, but increased by kaempferol, morin and trans-resveratrol, indicating that OH played a minor role in LP, which all of these PP inhibited. None of these PP at up to 40 μM concentration quenched OH in the Fe–AA system. All tested PP, except trans-resveratrol and gentisic acid, spectrally interacted with Fe 2+ or Fe 3+, indicating formation of complexes or oxidation of PP. In contrast to the NADPH system we found no correlation between Fe 2+ chelation and inhibition of Fe–AA- or Fe-dependent LP indicating that iron chelation did not play a significant role in the two latter systems. It is concluded that the inhibition of LP by PP was apparently due to their hydrogen donating properties rather than chelation of iron.