What Are Reactive Oxygen Species?
The mitochondrial electron transport chain — which generates ATP (cellular energy) — inevitably leaks electrons to molecular oxygen, producing superoxide radical (O₂•⁻) as a by-product. Cells have extensive antioxidant systems to manage this: superoxide dismutase (SOD) converts O₂•⁻ to hydrogen peroxide (H₂O₂); catalase then converts H₂O₂ to water. The dangerous step is the Fenton reaction: Fe²⁺ reduces H₂O₂ to the extremely reactive hydroxyl radical (•OH), which attacks DNA, lipids, and proteins non-specifically at near-diffusion-limited rates.
How EGCG Scavenges Radicals
EGCG and other polyphenols with catechol or galloyl moieties (adjacent hydroxyl groups on aromatic rings) can donate hydrogen atoms to radical species, effectively quenching them. The resulting EGCG radical (semiquinone) is relatively stable and less reactive than the original radical it quenched. For superoxide, hydroxyl radical, and peroxyl radicals, this mechanism is well-supported as an in vitro mechanism.
The metal chelation mechanism may be more physiologically relevant. By chelating Fe²⁺ and Cu²⁺ — the metals that catalyse hydroxyl radical formation — catechins prevent the most harmful radical chain reactions before they start. This mechanism does not require the catechins to be present at the precise location of the reaction in the same way that direct radical scavenging would.
🧠 Expert Tip: The Nrf2 Pathway: Indirect but Potent
EGCG activates the Nrf2 transcription factor, which binds to antioxidant response elements (ARE) in DNA, upregulating a battery of protective enzymes: SOD, catalase, glutathione peroxidase (GPx), heme oxygenase-1 (HO-1), and glutathione synthesis. This indirect mechanism may deliver more sustained antioxidant protection than direct scavenging, because it amplifies the cell's own protective capacity rather than just adding an external donor.
The Pro-Oxidant Paradox
At high concentrations — typically above 100 µmol/L in cell culture, or with high-dose supplement use — EGCG undergoes auto-oxidation, producing hydrogen peroxide and EGCG quinone radicals. This pro-oxidant activity has caused concern about the safety of very-high-dose EGCG supplements. At normal dietary concentrations from brewed tea (typically 1–50 µmol/L in plasma), pro-oxidant activity is not clinically relevant. The concern is specifically for concentrated EGCG supplements (400–800mg+ doses per day) taken without food over long periods.
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