Implications of the Mechanisms of Action of Tea Polyphenols as Antioxidants in vitro for Chemoprevention in Humans

Considerable evidence points to the health benefits of the phytochemical constituents of fruit, vegetables, beverages, and grains in protection against cardiovascular disease and certain cancers. This may be accounted for through contributions from their constituent antioxidants (vitamin E, vitamin C, β-carotene, other carotenoids and the non-nutrient flavonoids, ubiquitous phytochemical components of the diet), as well as through other anticarcinogenic and cardioprotective effects independently of their antioxidant effects. Indeed, the epidemiological evidence for a risk-reducing role of vitamin C for cancer is not as strong as that for fruit and vegetables (1). Dietary components may contribute to antioxidant function in several ways that relate to their structural chemistry (2): i) by directly scavenging free radicals through hydrogen/electron donation, depending on their reducing properties, or reduction potentials; ii) by intercepting the propagatory chain reactions of lipid peroxidation and scavenging peroxyl radicals, also dependent on their reducing properties. Their effectiveness as inhibitors of lipid peroxidation will also relate to their accessibility to the site of action, defined by their partition coefficients; iii) by scavenging reactive nitrogen species and competitively inhibiting tyrosine nitration and DNA deamination; and iv) as preventative antioxidants by chelating transition metal ions and inhibiting the formation of iron-induced free radicals, and the iron-mediated propagation of free radical reactions. Many of the polyphenols, especially those as constituents of tea (Table I), exhibit all these antioxidant properties. This review will focus on the polyphenolic constituents of tea and their biological effects in inhibiting damage induced by reactive oxygen and nitrogen species in vitro, processes that have implications for chemoprevention in humans. The reducing properties of polyphenols are defined by the number and the structural arrangement of their phenolic hydroxyl groups. The structures of the tea catechins are shown in Figure 1.