Antioxidants, nature and chemistry
Submitted by Dr. Tamer Fouad, M.D.
Retinol, retinoic acid but not retinyl palmitate or retinyl acetate all have antioxidant properties (Prasad, 1989). However, retinoids in general are not classified as antioxidants as they mainly function as antiproliferatives.
GSH is synthesized intracellularly from cysteine, glycine, and glutamate.
In addition to its role as a substrate in GSH redox cycle, GSH is also a scavenger of hydroxyl radicals and singlet oxygen. It is capable of either directly scavenging ROI or enzymatically via glutathione peroxidase, as described previously. In addition, GSH is crucial to the maintenance of enzymes and other cellular components in a reduced state. GSH also has an important role in xenobiotic metabolism and leukotriene synthesis. It is found in millimolar concentration in all human cells (Halliwell, 1994).
The majority of GSH is synthesized in the liver, and approximately 40% is secreted in the bile. The biologic role of GSH in bile is believed to be defence against dietary xenobiotics and lipid peroxidation in the lumen of the gut and protection of the intestinal epithelium from oxygen radical attack (Aw, 1994).
CoQ10 (Coenzyme Q10) is also known as ubiquinone. It is found in almost every living cell (hence the name "ubiquitous") and is essential to energy production by the mitochondria. Far beyond producing energy, CoQ10 can protect the body from destructive free radicals and enhance immune defences.
Acts as an endogenous radical scavenger and antioxidant. It is present in about 0.5 mmol/L in body's fluids and is the end product of purine metabolism. Uric acid is a powerful scavenger of singlet oxygen, peroxyl radical (ROO?) and ?OH radical (Halliwell, 1994).
Depending on the fact that albumin has one sulfhydryl group per molecule, it itself scavenges several free radicals (Halliwell, 1994) and thus can be considered as one of the primary extracellular defense systems.
Albumin is an additional sacrificial antioxidant that can bind copper tightly and iron weakly to its surface. The bound metals would still be on its surface. The bound metals would still be available for participation in Haber-Weiss reaction, but any generated ?OH would immediately react with and be scavenged by albumin. The resultant protein damage is biologically insignificant because of the large amount of available albumin and free radicals would be inactivated before reacting with other more vital protein structures.
Other plasma proteins namely ceruloplasmin and transferrin have also shown antioxidant activity.
Several pharmaceutical agents have been found to exert an antioxidant effect (Reilly et al. 1991):
? Xanthine oxidase inhibitors: e.g. allopurinol, folic acid.
? NADPH inhibitors: e.g. adenosine, calcium channel blockers.
? Superoxide dismutases.
? Inhibitors of iron redox cycling: deferoxamine, apotransferin and ceruloplasmin
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