**10.2 Second line of defense**

In second line of defense include retinol, α-tocopherol, uric acid, bilirubin, albumin and thiols. This anti-oxidant scavenges the active radicals and breaks the chain propagation reaction and suppresses chain initiation.

## **10.3 Third line of defense**

Third line of defense includes proteolytic enzymes, proteinases and peptidases present in the cytosols and in the mitochondria of mammalian cells, etc. they act by repair deoxyribonucleic acid.

#### **10.4 Fourth line of defense**

Fourth line of defense include adaptation, in this signals for the production and reaction of free radicals induce the formation of antioxidant and they also transport the suitable antioxidant to the right place.

Various types of natural antioxidant fount in nature and they are differ in their specifications, mechanism of action and their composition, etc. [55–59]

#### **11. Enzymes**

These enzymes act as antioxidant and act by converting reactive oxygen species and reactive nitrogen species into stable compound and is important in the repairing of damaged DNA, Proteins and oxidized peroxides.

#### **11.1 Superoxide dismutase (SOD)**

 Superoxide dismutase catalyzes the breakdown of superoxide anion into hydrogen peroxide and oxygen. These enzymes are present in aerobic cells as well as in extra cellular fluids. In plants, superoxide dismutase are present in chloroplast, peroxisomes and apoplast in cytosol, while in human body superoxide dismutase-1 present in cytoplasm and superoxide dismutase-2 in mitochondria and superoxide dismutase-3 in extracellular [59–64].

#### **11.2 Catalase**

 Catalase is an enzyme and present in almost all living organism which are exposed to oxygen and its function is to catalyze the decomposition of hydrogen peroxide into water and oxygen. As we know that hydrogen peroxide is harmful substance and produced by various metabolic processes and to prevent damage from hydrogen peroxide, catalase converts the hydrogen peroxide into water and oxygen [65].

#### **11.3 Glutathione**

Glutathione is a cysteine containing peptide found naturally in aerobic organism and it is not required in daily diet because it is synthesized in cells from amino acids [66]. Thiol group in its cysteine moiety of glutathione is responsible for antioxidant activity. Glutathione is one of the most important cellular antioxidant enzymes which present in high concentration and play an important role in maintain redox state of cell [67].

#### **11.4 High molecular weight compounds**

Metal catalyzed free radicals production is restricted by few high molecular weight compounds like albumin, transferrin, ceruplasmin, etc. [68]

#### **11.5 Low molecular weight compound**

 Low molecular weight compounds like Tocopherol, quinines, bilirubin are lipid soluble antioxidant while ascorbic acid, uric acid are water soluble antioxidants [69].
