Secondary Metabolites of Fruits and Vegetables with Antioxidant Potential

*Ravneet Kaur, Shubhra Shekhar and Kamlesh Prasad*

### **Abstract**

An antioxidant is of great interest among researchers, scientists, nutritionists, and the public because of its ability to prevent oxidative damage, as indicated by various studies. This chapter mainly focuses on the free radicals and their types; antioxidants and their mode of action against free radicals; fruits, vegetables, and their byproducts as a source of antioxidants; and various analytical methods employed for assessing antioxidant activity. Antioxidants discussed in this chapter are ascorbic acid, Vitamin E, carotenoids and polyphenols, and their mechanism of action. Different antioxidant activity assay techniques have been reported. Fruits and vegetables are abundant sources of these secondary metabolites. The waste generated during processing has many bioactive materials, which possibly be used in value-added by-products.

**Keywords:** antioxidant, free radical, oxidative stress, secondary metabolite, ascorbic acid, carotenoids, polyphenol, degenerative diseases

### **1. Introduction**

The word antioxidant is commonly heard nowadays, especially whenever there comes a topic of health concern. People consume antioxidants as a symbol of a healthy lifestyle to fight against various health problems, better skin, and anti-aging benefits. What makes antioxidants so important? The trait responsible for such importance of antioxidants is their ability to stop free radical reactions that can have potentially deleterious effects [1]. This gives rise to various questions, such as What are the free radicals? What are the sources of free radicals? What are their harmful effects? What are antioxidants? What are the common sources of antioxidants? How do they work against free radicals? Answers to these questions are discussed in the present chapter.

#### **2. Free radicals**

Free radicals are those atoms or molecules with an unpaired electron in their outer orbit [2]. Any electron present alone in an orbital is referred to as an unpaired electron, and it is accountable for the reactive and unstable state of the free radical.

The vital class of free radicals generated in a living system is usually derived from oxygen and reactive oxygen species (ROS) [3]. Hydroperoxyl (HO2 o ), alkoxy (RO<sup>o</sup> ), peroxyl (RO2 o ), hydroxyl (OH<sup>o</sup> ), and superoxide radical (HO2 o ) are common among oxygen free radicals. Nitrosative stress is the condition that occurs due to the overproduction of reactive nitrogen species (RNS) [3, 4]. Nitric oxide (NO<sup>o</sup> ) and nitrogen dioxide (NO2 o ), the nitrogen-free radicals can also be converted into other nonreactive species under the antioxidant-dependent reactions. Thus, ROS and RNS include radicals and nonradical species, such as hydrogen peroxide, singlet oxygen, ozone, organic peroxide, peroxynitrite, nitrosyl cation, nitroxyl cation, dinitrogen trioxide, and nitrous acid [5]. When reactive oxygen species (ROS) react with thiols, they give rise to reactive sulfur species (RSS) [6].

The most reactive hydroxyl free radical is formed by exposure to ionizing radiations. These radiations lead to the formation of Ho and OHo by causing the fission of OH bonds in water.

$$\text{H}\_2\text{O} \rightarrow \text{H}^\text{o} + \text{OH}^\text{o} \tag{1}$$

Harmful effects are initiated when the hydroxyl radical reacts with macronutrients such as carbohydrates, protein, and lipids along with DNA, the genetic material [7].

Molecular oxygen receives one electron and is converted to superoxide anion, a reduced form [8]. Superoxide anion is formed in the mitochondria during the initial step of the electron transport system [9]. Oxygen is reduced to water during the electron chain reaction. The electrons escape a chain reaction and react directly with oxygen in its formation [8].

$$\begin{array}{ccccc} \mathbf{O}\_{2} & \xrightarrow{\mathbf{e}^{-}} & \mathbf{H}\mathbf{O}\_{2}\mathbf{^{o}} & \xrightarrow{\mathbf{e}^{-}} & \mathbf{H}\_{2}\mathbf{O}\_{2} & \xrightarrow{\mathbf{e}^{-}} & \mathbf{O}\mathbf{H}^{\mathbf{o}} & \xrightarrow{\mathbf{e}^{-}}\\ & & & \text{radical} & & \text{Fydrogen} & \text{H}^{+} & \text{Hydroayly} & \text{H}^{+} & \text{Water} \\ & & & & \text{radical} & & & \text{radial} & & \end{array}$$

Many other reactive oxygen species are also formed in the living system by the formed superoxide anions. These include hydrogen peroxide, hydroxyl radicals, or singlet oxygen [10].

Hydrogen peroxide (H2O2) is a nonradical that is formed by the superoxide radical when it undergoes nonenzymatic or enzyme-catalyzed (superoxide dismutase, SOD) dismutation reaction. It is very diffusible within and between the cells [11].

$$\begin{array}{cccc} \text{2HO}\_2^{\text{o}} & + & \text{2H}^+ & \xrightarrow{\text{C}} & \text{H}\_2\text{O}\_2 & + & \text{O}\_2\\ \text{Superoxide} & & & \text{Hydrogen} & & \text{Oxygen} \\ \text{radial} & & & & \text{peroxide} \end{array} \tag{3}$$

In the presence of metal ions and superoxide anion, hydrogen peroxide generates hydroxyl radical.

$$\begin{array}{ccccc} \text{HO}\_2^{\text{o}} & + & \text{H}\_2\text{O}\_2 & \xrightarrow{\text{---}} & \text{OH}^{\text{o}} & + & \text{OH}^- & + & \text{O}\_2\\ \text{Superoxide} & & \text{Hydrogen} & & & \text{Hydroxyl} & & \text{Oxygen} &\\ \text{ radical} & & & & & \text{radical} & & & \end{array}$$

Nitric oxide is formed during the metabolization of arginine to citrulline by the enzyme nitric oxide synthase (NOSs) via five electron oxidative reactions [12]. Nitric oxide readily diffuses through cytoplasm and plasma membranes due to its solubility in both liquid and liquid media [13].
