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Antioxidants, nature and chemistry
Submitted by Dr. Tamer
Fouad, M.D.
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Antioxidants are substances
that protect other chemicals of the body from damaging
oxidation reactions
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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
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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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