There is a paradox in the metabolism of organisms. Although most organisms on earth need oxygen to survive, oxygen is a highly reactive molecule that can destroy organisms by producing reactive oxygen species. Therefore, a complex network system composed of antioxidant metabolites and enzymes has been established in the organism. Through the synergistic cooperation between antioxidant metabolic intermediates and products and enzymes, important cellular components such as DNA, protein and Lipids are protected from oxidative damage. The antioxidant system basically achieves antioxidant effects in two ways, one is to prevent the production of active oxygen substances, and the other is to eliminate these active substances before they cause damage to important components of the cell to achieve anti-oxidation. Effective. However, these reactive oxygen species also have important cellular functions, such as acting as redox signaling molecules in biochemical reactions. Therefore, the role of the antioxidant system in the organism is not to completely remove all oxidizing substances, but to keep these substances at an appropriate level.
The reactive oxygen species produced in cells include hydrogen peroxide (H2O2), hypochlorous acid (HClO), free radicals such as hydroxyl radicals (·OH) and superoxide anions (O2). Hydroxyl radicals are particularly unstable and can react with most biomolecules quickly and without specificity. Such species are mainly produced by metal-catalyzed hydrogen peroxide reduction (such as Fenton reaction). These oxidants destroy cells by initiating chain reactions such as lipid peroxidation, or oxidizing DNA and proteins. If the damaged DNA is not repaired, it can cause mutations and induce cancer. Damage to the protein can inhibit the activity of the enzyme, and the protein will be denatured or degraded.
The human body needs to consume oxygen to generate reactive oxygen species in the process of energy production. In this process, several steps in the electron transport chain can produce by-product superoxide anions. It is particularly important that the coenzyme Q in complex III becomes a highly active free radical intermediate (Q·) during the reduction process. This unstable intermediate will cause electronic "leakage" (loss of electrons), and the "leaked" electrons will jump out of the normal electron transport chain and directly reduce oxygen molecules to produce superoxide anions. Peroxide can also be produced by the oxidation of reduced flavoproteins such as complex I. However, although these enzymes produce oxidants, it is not clear whether the electron transport chain is more important than other biochemical processes that can also produce peroxides. In the process of photosynthesis of plants, algae and cyanobacteria, especially under high irradiation intensity, reactive oxygen species are also produced, but carotenoids are used as photoprotective agents to absorb excessive strong light to protect cells. The algae and cyanobacteria contain A large amount of iodine and selenium can also offset the oxidative damage caused by high radiation intensity. Carotenoids, iodine and selenium act as antioxidants to avoid the production of reactive oxygen species by reacting with the over-reduced photosynthetic reaction center.
