pH-dependent redox properties of epigallocatechin gallate (EGCG) and its effect on respiration, photosynthesis and cell death in pea

The redox properties of the green tea component epigallocatechin gallate (EGCG) in vitro were studied and its effect on plant (pea) cells was tested. EGCG exhibited both pro- and antioxidant properties. In solutions, EGCG was oxidized by oxygen at physiological (slightly alkaline) pH values. Lowering the pH slowed down this process. Oxidation of EGCG was accompanied by the formation of O2-• and H2O2. In addition, EGCG functioned as an electron donor for peroxidase and, in combination with it, utilized H2O2. Exposure of EGCG to pea cells (leaf cuts or epidermis) suppressed respiration, reduced the transmembrane electric potential difference (Δψ) in mitochondria, and inhibited electron transfer in the photosynthetic electron transfer chain. Among the sites of the photosynthetic redox chain, Photosystem II was the least sensitive to the action of EGCG. EGCG reduced the rate of formation of reactive oxygen species in the epidermis, which was induced by NADH treatment and was registered using 2′,7′-dichlorofluorescine diacetate. In vivo, EGCG at concentrations from 10 µM to 1 mM suppressed KCN-induced death of guard cells in the epidermis, which was determined from the destruction of cell nuclei. EGCG at a concentration of 10 mM disrupted the barrier function of the guard cell plasma membrane, increasing its permeability to propidium iodide.