Introductory Chapter: Role of Fenton and Haber-Weiss Reaction in Epilepsy

1. Introduction

Epilepsy is one of the most widespread brain diseases worldwide. It has high morbidity and mortality rates and affects around 70 million people globally. Epileptic seizures have several cognitive impairments and psychosocial consequences in patients. The occurrence of epilepsy is a self-facilitated pathological process triggered by the initial brain damage, ultimately leading to the loss of excitatory and inhibitory neurons in specific areas of the brain. Decades of research failed to fully illuminate its etiology, whereas preventive or disease-modifying therapies are still missing. New insights into the mechanisms of epilepsy are required to create the effective treatments.

Oxidative stress is a contributing factor to the onset and evolution of epilepsy. The association between free radical production and oxidative stress is regarded as a possible mechanism involved in epileptogenesis.

In the following pages of this chapter, we will be looking at the role of Fenton and Haber-Weiss reaction in triggering epilepsy leading to the loss of excitatory and inhibitory neurons in specific regions of the brain.

Brain is the most vulnerable organ to oxidative stress due to its high oxygen intake and reduced antioxidative protection [1]. Excessive oxidative stress is one of the main causes of epileptic seizures. The association between free radical production and oxidative stress is regarded as a possible mechanism involved in epileptogenic seizures [2, 3].

The Fenton and Haber-Weiss reaction has a significant role in the generation of free radicals such as superoxide and highly toxic hydroxyl ions causing epileptic episodes [4, 5].

In this chapter, we discussed the role of the Fenton reaction and Haber-Weiss reaction in ferroptosis and epilepsy that provide a new direction for understanding the underlying mechanisms of epilepsy leading to new therapeutic targets.

2. Chemistry of Fenton and Haber-Weiss reaction

H.J. Fenton first described the oxidation of tartaric acid by hydrogen peroxide in the presence of ferrous irons, and then, it is known as the Fenton reaction [6, 7]. Fenton reaction can be carried out by two pathways: radical and non-radical systems for Fenton reaction [8, 9].

2.1 Radical system for Fenton reaction

Hydroxyl radical (OH^.) is mainly produced by the reaction between ferrous iron and hydrogen peroxide. The Fenton reaction requires a set of conditions such as pH, temperature, concentration of hydrogen peroxide, and iron [10].

The Fenton reaction needs acidic conditions (pH = 3–4) for its catalytic activities, which gradually decreases due to the precipitation of iron as Fe (OH)₃ and the degradation of H₂O₂ into O₂ and H₂O [8, 11]. At the higher temperature, the rate of the reaction is increased, along with the increased decomposition rate of hydrogen peroxide [10, 12]. Increasing the concentration of Fe²⁺ leads to higher reaction rates until it reaches a certain concentration above which all rate increases appear to be marginal [13]. An adequate concentration of H₂O₂ is also needed for the reaction. Moreover, the Haber-Weiss reaction was first proposed by F. Haber and J J Weiss in 1932. Fe³⁺ is reduced to Fe²⁺ through the reaction with superoxide (O₂^.−) and H₂O₂. Finally, OH^., OH⁻, and oxygen are produced:

$$\begin{array}{c}{\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+}+\mathrm{OH}+\mathrm{O}{\mathrm{H}}^{-}\left[\text{Fenton reaction}\right]\\ \mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}+{\mathrm{H}}_2\mathrm{O}\\ {\mathrm{O}}_2^{-}+{\mathrm{H}}_2{\mathrm{O}}_2\stackrel{{\mathrm{Fe}}^{3+}/{\mathrm{Fe}}^{2+}}{\to }\mathrm{OH}+\mathrm{O}{\mathrm{H}}^{-}+{\mathrm{O}}_2\left[\text{Haber}‐\text{Weiss reaction}\right]\end{array}$$E1

Due to the presence of multivalency of iron, iron can react with H₂O₂ by one- or two-electron transfer. Several studies indicate that as a classical Fenton reaction, Fe²⁺ reacts with H₂O₂ by the outer sphere electron transfer with no direct bonding interactions between the electron donor and the acceptor [8]. Other studies also indicate that metal-centered Fenton reaction occurs by the direct bonding between iron and H₂O₂ by inner sphere electron transfer mechanisms. This interaction could produce a metal-peroxo complex, Fe (II)HOO, which may react further to generate either HO· radicals (one-electron oxidant) or Fe (IV)O (two-electron oxidant) (Figure 1).

Figure 1.

The reaction mechanism of the classical and metal-centered Fenton reaction [8].

2.2 Non-radical system for Fenton reaction

Non-radical Fenton reaction begins with the reversible reaction between Fe²⁺ and H₂O₂. [Fe²⁺. H₂O₂], [FeO²⁺], and [FeOFe]⁵⁺ are the most important intermediate products of this reaction. The reaction is carried out either through oxidation or reduction reaction of iron ions and addition or subtraction of H₂O₂ reaction [8, 9]. The overall reaction is represented in Figure 2.

Figure 2.

Reaction mechanism of non-radical Fenton reaction [8, 9].

In addition, the Fenton reaction is catalyzed by several transition metals such as iron, zinc, copper, cobalt, manganese [14] The overall reaction is represented as follows:

$${\mathrm{Mn}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2={\mathrm{Mn}}^{+}+{\mathrm{OH}}^{-}+\mathrm{OH}$$E2

The capacity of metal ions to induce epilepsy is well known. The concentration of Fe²⁺, Zn²⁺, Cu²⁺, and CO²⁺ are higher in the epileptic human brain compared with the healthy brain. The abnormal levels of trace metals may be epileptogenic, and they enhance excitatory synaptic mechanisms and reduce inhibitory processes. These metal ions also produce higher concentrations of hydroxyl radicals by the Fenton and Haber-Weiss reaction in epilepsy [5, 15, 16].

3. Fenton and Haber-Weiss reaction in epilepsy

Reactive oxygen species (ROS) play a major role in epilepsy [2, 16]. ROS is generated by many cellular processes such as mitochondrial metabolism, cellular respiration, metabolism of organic matter through a redox reaction, and tissue homeostasis. High-reactive hydroxyl radical is produced by the Fenton and Haber-Weiss reaction in the presence of suitable transition metal. Iron is abundant in the epileptic brain and is involved in the formation of hydroxyl radicals [5, 15].

In 2012, Dixon et al. first discovered iron-dependent cell death by the accumulation of iron-dependent free radicals, and this process is known as “ferroptosis” [17]. Therefore, the imbalance of ROS production is an important factor in ferroptosis. Burdened iron is a common cause of hemorrhagic post-stroke epilepsy and post-traumatic epilepsy [18, 19]. Enormous evidence suggests that a chronic epileptic animal model is created by the injection of hemoglobin or iron into the cortex of an animal [20, 21]. Higher levels of intracellular superoxide and hydroxyl radicals are found in the cerebral cortex after ferric chloride injection [22]. Other studies showed that concentrations of transferrin are markedly higher in patients with epilepsy [23]. It boosts iron intake into the cell and accelerates ferroptosis. In addition, sudden unexpected death in epilepsy is caused by cardiomyocyte ferroptosis in the heart through excess production of ROS [24]. Therefore, activation of the ferroptosis pathway is implicated in the pathogenesis of epilepsy and epileptic neuronal death. Excess iron ions generate hydroxyl radicals, which have high reactivity with proteins, lipids, and nucleic acids, leading to lipid peroxidation and promoting ferroptosis in epilepsy (Figure 3).

Figure 3.

The association between the Fenton and Haber-Weiss reaction and epilepsy [5, 15].

3.1 Fenton and Heber-Weiss reactions and Ferroptosis

Iron is a crucial element in cellular metabolism, energy generation, and growth in organisms. It participates in various oxidation-reduction reactions. Iron tends to be stored and transported in the Fe³⁺ form. In the blood, Fe³⁺ binds to transferrin (Tf) to form a complex which can be delivered into the cells by binding to transferrin receptor-1 (TFR1) in the cell membrane and then transported to the endosome [25]. Then, Fe³⁺ is converted to Fe²⁺ by an oxidation-reduction process with the help of six-transmembrane epithelial antigen of prostate 3 (STEAP3) and divalent metal transporter 1 (DMT1) and then released into the labile iron pool of mitochondria, lysosome, cytosol, and the nucleus [26]. Iron can also be exported by ferroprotein, an iron efflux pump in the cellular membrane, which can oxidize Fe²⁺ to Fe³⁺. Excess Fe²⁺ reacts with H₂O₂ and produces OH⁻ anion and OH^. radical by the Fenton reaction in ferroptosis. Moreover, the Haber-Weiss cycle showed that Fe³⁺ is reduced to Fe²⁺ through the reaction with superoxide (O₂^.), and Fe²⁺ reacts with H₂O₂ and forms OH^., OH⁻, and Fe^3+. Thus, Fe²⁺ is conducive to the production of ROS and promotes ferroptosis [15]. Autophagy can modulate the sensitivity to ferroptosis via the selective autophagy of ferritin; this process is called “ferritinophagy.” Nuclear receptor coactivator 4 (NCOA4) binds to ferritin and then delivers it to autophagosomes for lysosomal degradation. Fe²⁺ is released in the cell by degradation, which promotes ferroptosis (Figure 4).

Figure 4.

Schematic representation of iron metabolism and ferroptosis associated with Fenton and Haber-Weiss reaction [15].

3.2 Fenton and Haber-Weiss reaction and lipid peroxidation

Numerous lipid species are distributed in intra- or extra-cellular areas and play important roles in the energy supply and structural components of the intracellular membrane system. Cell membranes are sensitive to radical damage due to the presence of polyunsaturated fatty acids (PUFAs). Free radical oxidizes PUFAs, leading to the formation of hydroperoxides lipid and alkyl radical. This lipoperoxidation alters membrane structure, damages its fluidity integrity, and finally causes ferroptosis [27]. Due to the presence of double bonds, PUFAs are one of the most reactive substrates toward free radicals mainly hydroxyl radicals. Hydroxyl-dependent and hydroxyl-independent pathways are the main routes for the lipid peroxidation process [28]. The Fenton reaction and Haber-Weiss reaction are involved in a hydroxyl-dependent pathway, whereas Fe²⁺ accelerates hydroxyl-independent lipid peroxidation. As a result, ferroptosis also gets accelerated [5, 15, 28].

The lipid peroxidation process is initiated by the attack of hydroxyl radicals at bis-allelic positions in the fatty acid side chains, leading to generating of an alkyl radical. The radical is stabilized by the resonance with the double bond. Then, a chain reaction occurs with the extension of the damage and formation of further radical spices, and this process is known as the propagation phase. A newly formed radical reacts with oxygen and forms a peroxyl radical (LOO^.), which can react with other adjacent PUFAs to form a hydroperoxide and an alkyl radical, and it causes a chain reaction and damages more fatty acids [28]. In lipid peroxidation, the fatty acid undergoes a further reaction with oxygen and produces hydroperoxynonenal and then hydroxynonenal (Figure 5).

Figure 5.

Lipid peroxidation mechanism by hydroxyl radical [28].

In a nutshell, in the presence of excess iron ions, lipid peroxidation forms more lipid-free radicals and serves as a trigger for ferroptosis.

3.3 Fenton and Haber-Weiss reaction and DNA damage

Mitochondrial DNA (mtDNA) is mainly susceptible to ROS due to its proximity, despite being packaged with proteins as protective covering. Its mutation can lead to a variety of diseases such as epilepsy.

In DNA, ROS reacts with nitrogenous bases and deoxyribose. This can lead to mutations, carcinogenesis, apoptosis, and necrosis. Hydroxyl radical causes direct damage to DNA, mainly by standard excision, and causes oxidative damage to the pyrimidine and purine bases. This process starts with the radical-induced abstraction of a proton from any position of the deoxyribose and can result in many products (Figure 6). In thymine, the abstraction of methyl hydrogen from the 5-position by the hydroxyl radical generates a resonance-stabilized carbon radical, which provides the hydroxymethylene derivative, after treatment with oxygen and followed by reduction.

Figure 6.

Mechanism of oxidative damage to DNA-deoxyribose by Fenton and Haber-Weiss reaction [28].

3.4 Protein oxidation and Fenton Haber-Weiss reaction

Proteins are encoded by nuclear and mitochondrial DNA, which have numerous functions in the cells. Their function and regulation depend on their structures. Oxidative stress damages their structural integrity, causes loss of catalytic activity, and dysregulates the metabolic pathways [28, 29]. The protein oxidation is initiated by the abstraction of hydrogen from the protein by the hydroxyl radical and generates the protein radicals. It is stabilized by the resonance with the carboxyl group of protein. Then this protein radical reacts with oxygen and forms the protein peroxyl radical.

4. Conclusion

In summary, we described the regulatory mechanism of ROS production (mainly hydroxyl radicals) by Fenton and Haber-Weiss reaction. Created hydroxyl radicals facilitate ferroptosis, lipid peroxidation, DNA damage, and protein oxidation leading to epileptic episodes. Increasing evidence demonstrated that epilepsy is closely related to ferroptosis and iron metabolism. Ferroptosis is also accelerated by hydroxyl radical, which is mainly formed by Fenton and Haber-Weiss reaction. Therefore, antioxidant therapy, free radical scavenger therapy, and metal chelator therapy may be novel approaches to slow the progression of epilepsy. However, further investigation is needed for understanding new treatment strategies based on Fenton and Haber-Weiss’s reaction to neurological diseases such as epilepsy.