Natural Antioxidants from Plants and Their Mechanisms of Action
Chi-Tang Ho
Department of Food Science,
Cook College, Rutgers University, NJ
It is generally accepted that reactive oxygen species (ROS) may be very damaging in vivo. They can attack lipids, protein and DNA to induce oxidation. This oxidative damage is considered to be the major cause of aging and several degenerative diseases such as heart diseases and cancer. Dietary antioxidants are suggested to be important to overcome the actions of ROS in vivo. The most important dietary antioxidants are certainly plant polyphenols. Many studies have suggested that plant polyphenols exhibit strong antioxidant activity, which is due to their ability to reduce free radical formation and to scavenge free radicals.
Natural phenolic antioxidants isolated from tea, rosemary, sage, tumeric, ginger as well as other fruits, vegetables and herbs have proven to be powerful to preserve the freshness of processed foods. Some of them are potent cancer preventive agents.
Recently, tea catechins have attracted considerable interest because of their associated beneficial health properties, including strong antioxidant activity and remarkable cancer preventive effects in several animal models. A number of studies attributed the cancer preventive effects to their antioxidant actions. It is supposed that tea catechins can react with reactive oxygen species, which may play important roles in carcinogenesis, thus terminating those chain reactions. Previous structure-activity studies indicated that flavonoids with an o-dihydroxy or trihydroxy B ring are the most effective antioxidants. Moreover, a gallate ester moiety at the 3-position of catechins (ECG and EGCG) has the highest antioxidant activity. It is believed that characteristic reaction products may provide novel markers for antioxidant reactions of tea catechins in living systems. Therefore, detailed studies of the specific mechanisms of the antioxidation of catechins in different oxidation systems are of great scientific interest. Our studies show that using different oxidants can result in the formation of different oxidation products from catechins and that the main site of antioxidant action of catechins seems to not only depend on the oxidant used, but also on the structures of catechins. The identification of oxidant-specific products may provide analytical approaches to evaluating the antioxidant actions of catechins in biological systems. Thus, it may provide a unique tool in the study of possible anticancer activity of catechins in the living system.
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