**3. Antioxidant activity of anthocyanins**

Free radical is an uncoupled electron group or atoms that can independently exist, including superoxide anion radicals, hydroxyl radicals, lipoxyl radicals, nitric *Anthocyanins: Novel Antioxidants in Diseases Prevention and Human Health DOI: http://dx.doi.org/10.5772/intechopen.89746*

**Figure 2.**

*The structure of anthocyanin.*

oxide radicals, and other radicals. Free radicals are the normal metabolic products in an organism and also undertake the important function such as being responsible for transferring energy in the redox process of organism substance metabolism. However, the production of free radicals may be increased due to light, heat, radiation, and other factors. The excessive free radicals in the body are unstable and can capture the electron and show a strong oxidative ability and destroy the cell membrane, proteins, DNA, RNA, and other molecules, finally, which may result in aging and various diseases.

Oxygen free radicals, produced by oxidation respiratory chain, have a strong oxidation property [9]. The first way to remove oxygen free radicals is dependent on the endogenous free radical scavenging system in the body, which includes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and other antioxidants. The related reactions are as follows [10]:


In addition, there are other antioxidants such as reduced GSH, vitamin C, and vitamin E, which can also scavenge free radicals. These antioxidant enzymes and antioxidants can effectively eliminate free radicals to form lines of defense to prevent free radicals attacked to the body and maintain the balance of free radicals and finally to ensure the health of the body.

$$\begin{array}{rcl} \mathsf{2O}. \ \mathsf{ + 2H^{+}} \ \stackrel{\mathsf{5OD}}{\longrightarrow} & \mathsf{H}\_{2}\mathsf{O}\_{2} \ \mathsf{+ O}\_{2} \end{array}$$

**Figure 3.** *Catalytic reaction of SOD.*

$$\begin{array}{rcl} \mathsf{2H}\_2\mathrm{O}\_2 & \xrightarrow{\mathsf{CAT}} & \mathrm{O}\_2 + \mathsf{2H}\_2\mathrm{O} \end{array}$$

**Figure 4.** *Catalytic reaction of CAT.*

**Figure 5.**

*Catalytic reaction of GSK-Px.*

When there is an abnormal increase of free radicals in the body for some reasons, the body is unable to maintain its balance. The exogenous free radicals scavenger is needed to supplement and to help maintain the balance of free radicals to the body. The additional exogenous reduced antioxidants are oxidized by the increased free radicals and prevent the abnormal increased free radicals to attack the biomembrane and other bio-molecules. A series of studies have shown that anthocyanins exhibit its antioxidant activity from three aspects: the first is to directly scavenge free radicals, another is to regulate the activity of antioxidant enzymes, and the third is to chelate with metal ions. These three properties of anthocyanins are derived from the strong reducibility of the polyhydroxyl structure of anthocyanins. Many studies indicated that the ability of *Morus alba* L. anthocyanin, *Glycine max* (L) *Merr* anthocyanin, and *Bletilla striata (Thunb) Reichb.f.* anthocyanin for removing oxygen free radicals and hydroxyl free radicals was positively correlated with the content of anthocyanins, and their effects were higher than Vc [11–13]. Proanthocyanidins isolated from *Hippophae rhamnoides* L. and *Nelumbo nucifera* Gaertn can reduce malondialdehyde (MDA) level in serum and skin tissue of healthy rats, raise SOD and GSH-Px activities in serum and skin tissue, and significantly alleviate the liver lipid peroxidation injury by CCl4 [14]. In addition, metal ions, especially Fe2+, can catalyze the transformation of oxygen radicals into hydroxyl radicals. When a substance can chelate with metal ions, the transformation from oxygen radicals to hydroxyl radicals can be blocked. Moreover, some anthocyanins have the ability to chelate with metal ions and play an antioxidant role. Procyanidins possess the "catechol" structure [10] and can strongly chelate with metal ions. The result of chelation is procyanidins can decrease free radical production from Fenton and Haber-Weiss reaction which is dependent on the necessary iron ions. Procyanidins block the free radical chain reaction and exert a strong antioxidant property. Oxygen free radical reacts with unsaturated fatty acids of the cell membrane to produce lipid peroxidation and the product to be MDA. MDA has a high toxicity and strong destructiveness to the cell membrane, which can change the cellular membrane fluidity and permeability and abnormal inside and outside ion distribution of cells and then destroy the function of various tissues and organs, which eventually leads to the cell irreversible damage and serious diseases. The polyhydroxyl structure of anthocyanins makes them have strong reducibility, which can play an important role in human health care and disease prevention by scavenging free radicals, regulating antioxidant enzyme activity and chelating metal ions in the body.
