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Dietary flavonoids are known to scavenge free radicals but little information is available on their roles in antioxidant protein gene expression. The goal of this paper is to investigate the effect of flavonoid treatment on the antioxidant protein expression in human intestinal Caco-2 cells. The antioxidant proteins of interest were metallothionein (MT), catalase (CAT), and superoxide dismutase (SOD). Treatment of Caco-2 cells with 100 μM genistein, biochanin A, daidzein or kaempferol significantly increased MT mRNA up to 15 fold. On the contrary, CAT mRNA level was not affected by various flavonoids. We also developed gel activity assays to determine the specific activities of CAT and Cu/Zn SOD in flavonoid-treated Caco-2 cells. Compared to the conventional spectrophotometric assays, the gel assays allow a separation of antioxidant activities of the enzymes from that of the flavonoids. CAT and Cu/Zn SOD were …