Xanthine oxidase activity in vitro: effects of food extracts and components.

There is significant interest in the direct antioxidant activities of dietary polyphenols, due to associations between consumption of polyphenol-rich foods, such as fruits and vegetables, and decreased incidence of oxidative-stress related disease. However, indirect antioxidant action, such as the inhibition of ROS-producing enzymes, may be equally relevant to health benefits through a general reduction in oxidative stress in vivo. To this end, the effects of food extracts and individual compounds on the in vitro activity of xanthine oxidase (XO) were assessed, many for the first time. Several compounds were shown to be potent inhibitors in vitro, including hesperetin and theaflavin-3,3'-digallate with IC50 values of 39 and 49 microM, respectively. Of the extracts, cranberry juice, purple grape juice, and black tea were the most potent, with IC50 values of 2.4, 3.5, and 5.8% of extracts, respectively. Some samples were shown to promote XO activity over the concentration ranges tested, including orange juice and pink grapefruit juice. Certain "inhibitors", such as purple grape juice and black tea, promoted XO activity at low concentration. The possible role of dietary inhibitors of XO in reducing oxidative stress in vivo is discussed.