**1.1 Free radicals and oxidative stress**

Free radicals are short-lived, reactive molecules with unpaired electrons in their outer orbits. The most important of the free radicals are superoxide radical (O2 • −), hydroxyl radical (OH), singlet oxygen (1 O2) and non-radical hydrogen peroxide (H2O2) and peroxynitrite (ONOO-) and they are known as "reactive oxygen species (ROS)". ROS can respond easily to biological molecules such as fats, nucleic acids, proteins and carbohydrates in the body. Therefore, they are held responsible for many diseases such as aging, cancer, cardiovascular diseases, immune system diseases, cataracts, diabetes, kidney and liver diseases [1, 2]. The main reason for the harmful effects of reactive species is that they are radicals, which may lead to the formation of radicals or have a higher oxidation potential [3]. Since the reaction between reactive oxygen species and

#### **Figure 1.**

*Indicates the effect of oxidative stress and how aging interacts with age-related illnesses. Summarizes the dietary intake of anti-oxidants on oxidative stress in aging [4, 5].*

bio-molecules is in the form of a radical chain reaction, oxidative damage is also in the form of chains. This chain reaction produces new reactive species that, in turn, damage other biomolecules (**Figure 1**) [1, 2].

Free radicals; It can be caused by activating phagocytes, antineoplastic agents, radiation, habit-forming substances, environmental agents and stress, autooxidation of small molecules, enzymes and proteins, mitochondrial electron transport systems, peroxisomes, plasma membrane and oxidative stressors [6]. Free radical formation happens continuously in cells due to enzymatic and non-enzymatic reactions. Enzymatic reactions that serve as sources of free radicals include reactions involved in the respiratory chain, phagocytosis, prostaglandin synthesis, and the cytochrome P-450 system [5, 7, 8]. Free radicals can also occur in non-enzymatic reactions of oxygen.

Some self-generated sources of free radicals [9].


*Antioxidants from Plant Sources and Free Radicals DOI: http://dx.doi.org/10.5772/intechopen.100350*

Sources of external-produced free radicals include:


Research shows that free radicals have a significant effect on aging, free radical damage can be controlled with adequate antioxidant defense, and optimal antioxidant nutrient intake can contribute to improved quality of life. Recent studies show that antioxidants can positively influence life expectancy. Numerous experiments have shown that DNA and RNA are susceptible to oxidative lesions. It has been reported that DNA is considered as the main target, especially in aging and cancer [10]. It has been found that oxidative nucleotides such as glycol, ditch and 8-hydroxy-2-deoxyguanosine increase during oxidative damage to DNA under UV radiation or It has been suggested that 8-hydroxy-2-deoxyguanosine can be used as a biomarker for oxidative stress [11].

#### **1.2 Oxidative changes in DNA**

DNA oxidation, along with DNA hydrolysis and DNA methylation, is a major contributor to genome instability and degradation. Spontaneous mutagenesis under aerobic conditions is greater than under anaerobic conditions, and deletion of the OxyR regulon, which prevents DNA damage in bacteria, significantly increases spontaneous mutations [11]. Among the DNA bases, guanine is the most susceptible to oxidative damage. The major mutagenic lesion is 8-Oxo-7,8-dihydroguanine (also called 8-oxoguanine or 8-hydroxyguanine), which forms base pairs with adenine instead of cytosine and therefore produces transversion mutations after replication [12]. Numerous DNA damage reactions and their degradation products have been studied [13]. The accumulation of 8-oxoguanine causes mitochondrial dysfunction and is oncogenic [14] and the enzyme human mutt homolog (MTH1), which detoxifies oxidized nucleotides, is a potential target in cancer therapy [15, 16] (**Figure 2**).

Reactive oxygen and nitrogen species such as superoxide anion, hydrogen peroxide, hydroxyl radical and nitric oxide and their biological metabolites also play an important role in carcinogenesis. The reaction of free radicals with DNA induces ROS DNA damage as it involves chain breakage, base modification and DNA protein cross-links. Numerous researchers have suggested the involvement of free radicals in carcinogenesis, mutation and transformation; their presence in the biosystem can lead to mutation, transformation and ultimately cancer [12]. The induction of mutagenesis, the best known of the biological effects of radiation, occurs mainly by DNA damage by H2O. Radical and other species are produced by radiolysis, and also direct radiation action on DNA, reaction effects on DNA. The radicals are mainly added to the double bond of the pyrimidine bases and hydrogen is abstracted from the sugar portion of the DNA

#### **Figure 2.**

*Redox processes have fundamental implications in biology. Oxidative stress, either as reversible (oxidative distress) pervades all principal levels, from genome integrity and maintenance to the redox metabolism [12].*

resulting in a chain reaction. These effects cause cell mutagenesis and carcinogenesis, and lipid peroxides are responsible for the activation of carcinogens [11, 17–19].

Free radicals and antioxidant balance in healthy people, this balance in unhealthy people in terms of free radicals. However, when antioxidant mechanisms are activated or this balance is shifted towards antioxidants, it becomes easier to cope with the complications of diseases caused by imbalance. They stand out as compounds that are effective in preventing diseases. Today, it has been a subject of interest, especially in the determination of the antioxidant capacity of dietary foods. In this review, some important antioxidant foods and their properties, which are consumed extensively in our country and especially in our region, will be explained [20, 21].

### **2. Antioxidants**

There are many defense mechanisms to prevent the formation of reactive oxygen species (ROS) and the damage they cause. These mechanisms are known as "antioxidant defence systems" or "antioxidants" for short.

Antioxidants act in four ways. (1) It is the scavenging effect by affecting the free oxygen radicals, keeping them or converting them to weaker new molecules. Antioxidants, tracheobronchial mucus and small molecules exert this type of effect. (2) Reducing their activity or inactivating them by interacting with free oxygen radicals and transferring hydrogen to them is the suppressive effect. Vitamins, flavonoids have such an effect. (3) The chain-breaking effect is the effect of binding free oxygen radicals and breaking their chains and preventing their functions. Hemoglobin, ceruloplasmin and minerals show chain-breaking effects. (4) Repairing the damage caused by free radicals is a restorative effect [22].

Antioxidants can be of endogenous or exogenous origin. Antioxidants react with free radicals (bonding with them) and prevent them from damaging cells. These features increase the chance of living a healthier life with minimal effects on aging, as they reduce the risk of cells becoming abnormal and eventually forming tumors, as well as reducing cell destruction [23].

The overload of free radicals poses a danger to the body. However, they are also necessary for the body to see its functions and to be protected from diseases. Free radicals are controlled in a very delicate balance in the body. Some mechanisms prevent, destroy or reduce oxidative damage in cells. Substances that inactivate oxidants by direct action are called antioxidants. All antioxidants exert their effects in four main ways:

1.Collector effect

2. suppressive effect

3.Chain braking effect

4.Restorative effect

Antioxidants can be divided into two main groups: natural (endogenous) and unnatural (exogenous) antioxidants [24].
