Evaluation of antioxidant activity of epigallocatechin gallate in biphasicmodel 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 r eactions. EGCG suppressed the initiation rate and prolonged the lag phase d uration of peroxyl radical-induced oxidation in a phospholipid liposome mod el to a greater extent (p < 0.01) compared to both Trolox and alpha -tocoph erol. Effectiveness of these antioxidants to prolong the peroxyl radical-in duced lag phase was inversely related to lipophilic character. EGCG also pr otected against both peroxyl radical and hydroxyl radical-induced supercoil ed DNA nicking. The rate constant describing EGCG reaction against hydroxyl radical was 4.22 +/- 0.07 x 10(10) M-1.sec(-1), which was comparable to th ose of Trolox and alpha -tocopherol, respectively. EGCG exhibited a synergi stic effect with alpha -tocopherol in scavenging 1,1-diphenyl-2-picylhydraz yl (DPPH) radical, thus displaying a direct free radical scavenging capacit y. In vitro Cu2+-induced-human LDL oxidation was accelerated in the presenc e of EGCG and attributed to the conversion of Cu2+ to Cu+. We conclude that the particularly effective antioxidant properties of EGCG noted in both ch emical and biological biphasic systems were related to a unique hydrophilic and lipophilic balance which enabled effective free radical scavenging. Th e same chemical-physical properties of EGCG also enabled prooxidant activit y, only when in contact with unbound transition metal ions in a multiphasic system.