Free radical

Generally, free radicals are atoms, molecules or ions with unpaired electrons on an otherwise open shell configuration. As unpaired electrons are usually highly reactive, free radicals are likely to participate in chemical reactions. It has been found that free radicals are involved in many diseases, and that free radical-induced injury is one of the most important molecular mechanisms of tissue injury.

Oxidative stress

In the process of oxidation metabolism, molecular oxygen generates radicals. Normally, there is a dynamic balance between the oxygen radicals and anti-oxidants. However, when the generation of oxygen radicals exceeds the elimination capacity of antioxidant system, oxidative stress occurs in the body.

Harmful effects of oxidative stress

• 1）. Damage of DNA. Oxygen radicals can damage the basic group, deoxyribose and phosphodiester bond, resulting in DNA mutation and necrosis. It is estimated that the frequency of oxidant injury of DNA in a cell reaches 10 000 times per genome in a single day.
  
• 2）. Damage of protein. Protein is usually one of the substances which are most likely to be attacked by free radicals, especially tryptophan residue, phenylalanine residue, and methionine residue. The reaction can lead to the modification of amino acid residue, the cross-linking and interruption of peptide chain, and protein denaturation.
  
• 3）. Damage of lipid. A large amount of lipids are contained in cell membranes. Lipid is another substance likely to be attacked by free radicals, and such an attack leads to lipid peroxidation and subsequently cell damage.
  

Antioxidation mechanism of GSH

Mainly, there are two kinds of antioxidants, enzymes and non-enzyme antioxidants. GSH is one of the non-enzyme antioxidants. GSH, as a reversible hydrogen donor, protects the nucleic acid, protein and lipid from free radical injury through the redox state conversion of sulfhydryl.