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Antioxidants, nature and chemistry

Submitted by Dr. Tamer Fouad, M.D.

Antioxidant enzymes

This includes superoxide dismutase, catalase, and peroxidases.

Superoxide dismutase (SOD)

Source and Nature:

SOD is an endogenously produced intracellular enzyme present in essentially every cell in the body.

Cellular SOD is actually represented by a group of metalloenzymes with various prosthetic groups.  The prevalent enzyme is cupro-zinc (CuZn) SOD, which is a stable dimeric protein (32,000 D).

SOD appears in three forms: (1) Cu-Zn SOD in the cytoplasm with two subunits, and (2) Mn-SOD in the mitochondrion (Mayes, 1993; Warner, 1994). A third extracellular SOD recently has been described contains Copper (CuSOD).

                             2O2?⁻+ 2H⁺ + SOD ? H2O2 + O2

Mechanism of action:

SOD is considered fundamental in the process of eliminating ROI by reducing (adding an electron to) superoxide to form H2O2. Catalase and the selenium-dependent glutathione peroxidase are responsible for reducing H2O2 to H2O.

The respective enzymes that interact with superoxide and H2O2 are tightly regulated through a feedback system. Excessive superoxide inhibits glutathione peroxidase and catalase to modulate the equation from H2O2 to H2O (see Fig.5). Likewise, increased H2O2 slowly inactivates CuZn-SOD. Meanwhile, catalases and glutathione peroxidase, by reducing H2O2, conserve SOD; and SOD, by reducing superoxide, conserves catalases and glutathione peroxidase. Through this feedback system, steady low levels of SOD, glutathione peroxidase, and catalase, as well as superoxide and H2O2 are maintained, which keeps the entire system in a fully functioning state (Fridovich, 1993).

SOD also exhibits antioxidant activity by reducing O2?that would otherwise lead to the reduction of Fe3+ to Fe2+ and thereby promote ?OH formation. When the catalase activity is insufficient to metabolize the H2O2 produced SOD will increase the tissue oxidant activity. Hence, it was found that the antioxidant enzymes function as a tightly balanced system, any disruption of this system would lead to promotion of oxidation.

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