Antioxidant Systems

The presence of free radicals in the cells poses a huge dilemma to the biological systems within for the following reasons:

n Free radicals can randomly inflict damage to essential macromolecules; this damage leads to dysfunction of physiological systems in the organism (47,58-60). n Compelling evidence indicates that free radicals play a positive physiological role in various biological processes such as immune response to pathogens (Chapter 14) and cellular signaling transduction (48,59).

Cells have developed a variety of ways to balance the oxidant level by regulating production of oxidant and removal of oxidant by the antioxidant systems in order to defend themselves from oxidative stress while maintaining normal physiological functions.

One antioxidant mechanism is to scavenge free radicals with various enzymes and proteins, of which one major player is superoxide dismutase (SOD), which is present in all the aerobic organisms (60). SOD is uniquely dedicated to scavenge superoxide radicals by converting them to H2O2 (61-63), which can be further detoxified into water and oxygen by catalase in the cell (47).

Oxidants can also be removed by various other kinds of proteins such as glutathione-S-transferase (64) for detoxification of xenobiotics, endogenous toxins, and hydroperoxides, and ferritin for sequestration of oxidation catalyst iron (62-64).

A second antioxidant system consists of small antioxidant molecules, which include ROS scavengers such as tocopherol, ascorbic acid, glutathione, bilirubin, lipoic acid, and urea (53,65,66). While some of these ROS scavengers are produced in the cells, some of them can only be obtained through food intake. These antioxidant molecules often interact with each other in scavenging oxidants. Such interactions can be exemplified by removal of free radicals in the plasma membrane, in which tocopherols reduce free radicals and then tocopherols can be regenerated by ascorbic acid to take electrons from oxidized tocopherols.

The balance between oxidants and antioxidants is tightly regulated in the cell (Table 5). Increased level of oxidants can lead to increased production of endogenous antioxidants, while increased intake of exogenous antioxidant can result in deceasing production of endogenous antioxidants (47).

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