**Table 1.**

The most important free radicals that occur are:


#### **3.1 Superoxide radicals (O2 − )**

The superoxide anion radical (O2 − ) is produced by the single electron reduction of oxygen which acts as an intermediate in a number of biochemical reactions in body [13] and is a weak oxidant that cannot cause serious cell damage by itself.

However, it may lead to the initiation of a series of reactions that can lead to oxidative stress [6, 14, 15]. One of the main points of superoxide production is Coenzyme Q, and this anion is formed at other points in the electron transport chain as well as in the mitochondrial electron transport chain. Another ROS is produced by the O2 − radical, which does not leak far from where it originates [12, 16].

$$\begin{aligned} \mathbf{O}\_2 + \acute{\mathbf{e}} &\rightarrow \mathbf{O}\_2^- \text{ (superoxide radical)}\\ \mathbf{H}\_2\mathbf{O}\_2 + \mathbf{O}\_2 &\rightarrow \mathbf{HO}^- + \mathbf{OH}^- + \mathbf{O}\_2 \end{aligned} \tag{1}$$

The OH− radicals produced are highly reactive and can cause significant damage by reacting with structures such as DNA [6, 17, 18].

The half-lives of superoxide radicals that produce H2O2 and oxygen by the dismutation reaction are quite short. This reaction occurs spontaneously and is catalyzed by the Superoxide Dismutase (SOD) enzyme [6].

$$\rm O\_2^- + O\_2^- + 2H + SOD \to H\_2O\_2 + O\_2 \tag{2}$$

In natural conditions, O2 − can be produced in muscle tissues in a variety of ways. One of the sources of O2 in muscle tissues are various components of the electron transport chain in mitochondria, such as NADPH-linked dehydrogenase and ubiquinone, which can leak electrons into O2. Autoxidation of heme proteins [19, 20] and metabolic enzymes such as xanthine oxidase [21] are other sources of O2. With the ingestion of bacteria, the activation of several leukocytes in the vasculature of the muscle tissue causes the production of O2 − , one of the major bactericides [22].

#### **3.2 Hydrogen peroxide (H2O2)**

Aerobic cells naturally contain low concentrations of hydrogen peroxide (H2O2) as a metabolite. In an O2 forming system, it is expected to give H2O2 catalyzed by nonenzymatic or superoxide dismutase (SOD) [23]. Although it is not free radical, hydrogen peroxide reacts with a transition metal (e.g. Fe+2) to form a free radical [16].

H2O2 production has been detected in mitochondria, microsomes, peroxisomes and phagocytic cells. Also, many enzymes, such as xanthine oxidase, aldehyde oxidase, urate oxidase, glucose oxidase, glycolate oxidase, and D-amino acid oxidase,

#### *Lipid Peroxidation DOI: http://dx.doi.org/10.5772/intechopen.95802*

can directly produce H2O2 [23]. It has been reported that H2O2 is produced at a hemoglobin rate of approximately 3.9 × 10-9 M/hg and the concentration of H2O2 in red blood cells in a steady state is 2 × 10-10 M [24]. It has been reported that H2O2 formed during oxidation of oxymyoglobin plays an important role in lipid peroxidation [25]. Furthermore, it was reported that turkey muscle tissues stored at 37°C for 30 minutes produced approximately 14.0 nmol H2O2 per gram fresh weight, and its formation increased with storage at 4°C [26].

H2O2, which lacks unpaired electrons, is not a radical and, unlike charged O2, shows limited reactivity and permeability to the membrane [27]. Nevertheless, H2O2 can have devastating effects by generating more reactive species such as OH by catalysis of Fe (II) [28]. In addition, H2O2, depending on its concentration, can denature heme proteins to release iron and heme group or to convert heme protein to ferryl or perferryl radical [20].

#### **3.3 Hydroxyl radicals (HO<sup>−</sup> )**

The hydroxyl radical (OH) is the most reactive oxygen radical [29]. It is the most powerful free radical hydroxyl radical found in biological systems. In tissues exposed to radiation, a large part of the energy is absorbed by the water inside the cell and the radiation creates a covalent bond between oxygen and hydrogen, forming hydrogen (H<sup>−</sup> ) and hydroxyl radical (OH<sup>−</sup> ).

$$\mathrm{H}-\mathrm{O}-\mathrm{H}\rightarrow\mathrm{H}^{-}+\mathrm{OH}^{-} \text{(Hydroxyl radical)}\tag{3}$$

OH<sup>−</sup> radicals, which can provide radical formation and participate in a series of reactions, cause strand breaks in DNA by joining the structure of bases in DNA and RNA, which they do by causing a lot of damage to the bases and sugars of DNA. If the damage is very severe, it may not be repaired by cellular protective systems and as a result, mutations and cell death occur [14, 17, 30].

The steady-state concentration of the OH<sup>−</sup> radical in vivo is zero because it reacts with every molecule in the living cell, such as DNA, protein, phospholipid, amino acid, and sugar, at or near the place of formation. The high reactivity of the OH− radical is thought to result from the extraordinary combination of three properties. These properties include high electrophilicity, high thermochemical reactivity, and the ability to form near target molecules [31]. OH<sup>−</sup> formation was achieved in living erythrocytes under the effect of adriamycin using the spin trap electron paramagnetic resonance (EPR) technique [32]. Most of the OH− produced in vivo or in situ was obtained from the decomposition of H2O2 [33] by Fe (II) catalysis. Additionally, OH<sup>−</sup> can be produced by various sources: sunlight (Joseph JM, Aravindakumar), ultraviolet radiation [34], ionizing irradiation [35], reaction of hypochlorous acid with O2- [36] and sonolysis of water (ultrasound) [37].

The reaction of the OH<sup>−</sup> radical can be inhibited by OH<sup>−</sup> scavengers such as methanol, ethanol, 1-butanol, mannitol, formate, thiourea, dimethylthiourea, glucose, tris-buffer, or sorbitol [23]. Although OH<sup>−</sup> scavengers prevent OH<sup>−</sup> from reacting with other molecules, including lipid molecules, they are not always effective. There are several reasons to consider:


macromolecules can react with metal-binding molecules. It has been reported that as a result of the formation of the Fe (II) ion and 2-deoxyreebose complex, the Fe (II) ion that binds to DNA interacts with H2O2 to form OH− , which instantly damages DNA [39]. It was determined that the Fe (III) ion binds to the membrane and then forms free radicals in the binding site. It has been suggested that iron is accepted as the main binding site of the sulfone group with the carboxyl groups of sialic acids to the membrane, the sulfate group of glycolipids and the phosphate head group of glycoproteins and phospholipids [40]. On the other hand, it has also been reported that OH− scavengers effectively inhibit OH<sup>−</sup> formation in the presence of EDTA. Indeed, EDTA allows Fe (II) ions to be removed from these binding sites [41]. Thus, the toxicity of O2 and H2O2 may be due to the presence and distribution of metal ion catalysts to form OH− in cells.
