Antioxidant and Pro-Oxidant Activities of Green Tea Polyphenols in Cancer Prevention

Green tea (Camellia sinensis) is a widely consumed beverage that has shown preventive effects against chronic diseases including cancer. Green tea polyphenols are believed to be responsible for this cancer preventive effect and modulation of redox activity has been suggested as a potential mechanism of action. Data from various laboratories have suggested that green tea polyphenols can act as direct antioxidants by scavenging reactive oxygen species or chelating transition metals; as potent pro-oxidants which generate reactive oxygen species via a transition metal catalyzed reaction; or indirectly to modulate redox status by upregulating of phase II and antioxidant enzymes. Considerable data has been generated using in vitro systems, but these results must be interpreted with caution because of the relatively high oxygen concentrations available in vitro compared to in vivo. In the present review, we discuss the available data regarding the pro-oxidant and antioxidant effects of tea polyphenols and provide suggestions for future research.