Antioxidants

A new picture is emerging based on extensive research that cells harbor a highly complex network of redox-sensitive processes, which are constantly adjusting the metabolism and physiology of cells to the prevailing conditions of the environment. Antioxidants, whether synthesized endogenously or exogenously administered, are reducing agents that neutralize oxidants before they can cause damage to biomolecules. These substances act through powerful antioxidant defense systems within the cell to minimize the damaging effects caused by reactive species (RS). The first line of this defense system is the deployment of small antioxidant substances such as vitamin C (ascorbic acid), vitamin E (tocopherols), glutathione (GSH), coenzyme Q10 (CoQ10), and carotenoids. Thereafter, a battery of antioxidant enzymatic defenses (superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), thioredoxin (Trx), and catalase (CAT)) prevents the free radical (FR) cascade and terminates the lipid peroxidation cycle. The enzymatic defenses are regarded as crucial in that only they can prevent FR toxicity when the antioxidant molecule supply is exhausted. Unfortunately, the imbalance between pro-oxidant and antioxidant systems in cells in favor of the former usually results in oxidative stress. In this chapter, we review the antioxidant defenses of cells. Extensive coverage is given to enzymatic antioxidant that acts in a concerted and sequential manner to terminate lipid peroxidation chain reaction and to prevent oxidative damage to proteins. We provided the reaction schemes and kinetics of these enzymes, the preference and range of substrates catalyzed, the structure of the native protein and subunits, and the diversity of isoenzymes. The chapter also gives an excellent account of antioxidant compounds involved in the protection of the human body. It serves as an overview of antioxidant molecules that are components of natural products.