Let us throw light on the mechanistic action of antioxidants
In order to prevent oxidative damages, the antioxidant compounds react in one-electron reactions with free radicals in vivo/in vitro. In this section, we will discuss the mechanistic action of the natural antioxidant compounds and assays for the evaluation of their antioxidant activities as it is very important to understand the reaction mechanism of antioxidants with the free radicals. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated in usual biochemical reactions with increased exposure to the environment. These are responsible for the oxidative stress in different pathophysiological conditions which in turn are liable for alterations of cellular constituents of our body and result in various disease states. There are certain compounds acting as antioxidants to enhance cellular defense mechanism to reduce the oxidative stress.
Depending on their activities, we can categorize antioxidants as enzymatic and non-enzymatic antioxidants. The process of enzymatic antioxidants involve breaking down and removing free radicals and these are also involved for the conversions of oxidative products into hydrogen peroxide which is then converted to water in presence of cofactors such as copper, zinc, manganese, and iron via a multi-step process. On the other hand, non-enzymatic antioxidants (classic examples involve vitamin C, vitamin E, plant polyphenol, carotenoids, and glutathione) involve in the process by interrupting free radical chain reactions.
ROS are generated on activation of NADPH oxidase with fast uptake of oxygen and initially superoxide anion radical (O2˙−) is generated. This is then converted to Hydrogen peroxide by SOD (superoxide dismutase).
On the other hand, RNS such as nitric oxide (NO˙), are generated by the enzyme nitric oxide synthase from arginine. It is very much evident that an inducible nitric oxide synthase (iNOS) is competent of constantly producing huge quantity of NO˙, which act as a O2˙− quencher as NO˙ and O2˙− can react together to produce peroxynitrite (ONOO−), a very strong oxidant. Neither NO˙ nor O2˙− can be considered to be a strong oxidant but peroxynitrite is a strong and versatile oxidant that can attack a broad array of biological targets. Peroxynitrite thus formed, reacts with the aromatic amino acid residues present in the enzyme resulting in the nitration of the aromatic amino acids and results in the enzyme inactivation.
Several research reports clearly reveal that oxygen metabolism generates ˙OH, O2˙−, and the non-radical H2O2 and among these ˙OH is extremely reactive and it reacts with biological molecules such as DNAs, proteins, and lipids, which results in the huge to moderate chemical alterations of these molecules.
Activated oxygen species and metal ions such as Fe2+ and Cu2+ come together to damage the proteins oxidatively. Lysine, proline, arginine and histidine are extremely sensitive towards such oxidative damage. Recent studies have revealed that a wide array of residue modifications can take place obviously including the formation of peroxides and carbonyls and usually oxidative damage to tissue indicates in the increased amount of oxidized protein. Body protects itself from ROS by using enzymatic antioxidant mechanisms by reducing the levels of lipid hydroperoxide and H2O2 and they are important in the prevention of lipid peroxidation and maintaining the structure and function of cell membranes. Classic examples involve superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSHPx) and peroxiredoxin I–IV (I–IV).
SOD’s are situated in the cytosol and mitochondria and they catalytically convert the O2˙− into oxygen and H2O2 in company of the metal ion cofactors such as copper (Cu), zinc (Zn) or manganese (Mn). Now, the enzyme CAT is frequently present in the peroxisome, converts H2O2 to water and oxygen. On the other hand, GSHPx are usually found both in the cytoplasm and extracellularly in almost every human tissue and they are capable convert H2O2 into water. There is no scarcity of research articles and reviews that reveal the enzymatic antioxidants and their mechanisms.
Nonenzymatic antioxidants involve vitamin C, vitamin E, plant polyphenol, carotenoids and glutathione.
