Evaluation of antioxidant activity of epigallocatechin gallate in biphasic model systems in vitro.

The antioxidant activity of epigallocatechin gallate (EGCG) was studied in different in vitro model systems, which enabled evaluation of both chemical and physical factors involved in assessing the role of EGCG in oxidative reactions. EGCG suppressed the initiation rate and prolonged the lag phase duration of peroxyl radical-induced oxidation in a phospholipid liposome model to a greater extent (p < 0.01) compared to both Trolox and alpha-tocopherol. Effectiveness of these antioxidants to prolong the peroxyl radical-induced lag phase was inversely related to lipophilic character. EGCG also protected against both peroxyl radical and hydroxyl radical-induced supercoiled DNA nicking. The rate constant describing EGCG reaction against hydroxyl radical was 4.22+/-0.07 x 10(10) M(-1) x sec(-1), which was comparable to those of Trolox and alpha-tocopherol, respectively. EGCG exhibited a synergistic effect with alpha-tocopherol in scavenging 1,1-diphenyl-2-picylhydrazyl (DPPH) radical, thus displaying a direct free radical scavenging capacity. In vitro Cu2+-induced-human LDL oxidation was accelerated in the presence of EGCG and attributed to the conversion of Cu2+ to Cu+. We conclude that the particularly effective antioxidant properties of EGCG noted in both chemical and biological biphasic systems were related to a unique hydrophilic and lipophilic balance which enabled effective free radical scavenging. The same chemical-physical properties of EGCG also enabled prooxidant activity, only when in contact with unbound transition metal ions in a multiphasic system.