Redox and Other Biological Activities of Tea Catechins That May Affect Health: Mechanisms and Unresolved Issues

The beneficial health effects of green tea have been attributed to tea catechins. However, the molecular mechanisms of action, especially those in vivo, remain unclear. This article reviews the redox and other activities of tea catechins, using (-)-epigallocatechin-3-gallate (EGCG), as an example. EGCG is a well-known antioxidant. However, EGCG can be oxidized to generate reactive oxygen species and EGCG quinone. We propose that EGCG quinone can react with Keap-1 to activate Nrf2-regulated cytoprotective enzymes. Tissue levels of catechins are important for their biological activities; a section is devoted to reviewing the biological fates of tea catechins after ingestion. Possible EGCG oxidation in vivo and whether the oligomeric forms are biologically active in animals are discussed. We also review the effects of EGCG on the activities of enzymes, receptors, and other signaling molecules through binding and raise a question about whether the autoxidation of EGCG in vitro may lead to artifacts or misinterpretation in some studies. Finally, we discuss the challenges in the extrapolation of in vitro results to situations in vivo and the translation of laboratory studies to humans.

Automated summary

This article reviews the redox and other activities of tea catechins, focusing on (-)-epigallocatechin-3-gallate (EGCG), and discusses their molecular mechanisms of action in vivo. The article also explores the fate of tea catechins in the body after ingestion, potential oxidation of EGCG in vivo, and the biologically active forms in animals. Additionally, the effects of EGCG on enzymes, receptors, and signaling molecules are reviewed, along with the possible artifacts or misinterpretation in studies due to EGCG autoxidation in vitro. Challenges in extrapolating in vitro results to in vivo situations and translating laboratory studies to humans are also discussed.