**2. Redox reactions on ectotherms versus endotherms**

## **2.1 Definition of oxidative stress**

Oxygen Free Radicals are highly reactive species which are known to be the major factor in oxidative cell injury via the oxidation and subsequent functional impairment of lipids, carbohydrates, proteins and DNA. In the 1950s, free radicals were first identified in biological systems and were proposed to be involved in pathological processes [3]. The major source of intracellular free radicals is mitochondria due to the presence of an electron transport chain [4, 5], which consumes 85–90% of the oxygen utilized by cell [4, 6]. While passing through the mitochondrial electron transport chain, up to 2% of the total oxygen consumed undergoes one-electron reduction to generate superoxide anion radicals (O2•<sup>−</sup>) and hydrogen peroxide (H2O2). This hydrogen peroxide may lead to hydroxyl radical (OH• ), the most reactive free radical produced in biological systems, with the participation of transition metals in the Haber-Weiss reaction [7]. In addition, different stressors, particularly those induced by environmental physical and chemical factors were reported to increase levels of free radicals. As research on free radicals focused on oxygen radicals, with some other forms of non-radical active oxygen, they are collectively referred to as reactive oxygen species (ROS).

As the formation of reactive oxygen species (ROS) is a part of natural cellular oxidative metabolism, the question if living organisms possess regulated enzymatic systems to defend against ROS, suddenly arises. This was first confirmed by McCord and Fridovich [8] who discovered the enzyme superoxide dismutase (SOD) and demonstrated that living organisms have developed protective mechanisms against ROS. Over time, this was supported by continuing discoveries of several mechanisms by which ROS can be neutralized: antioxidant enzymes and low molecular mass antioxidants. During normal oxidative metabolism ROS are produced continually, but they are scavenged by superoxide dismutase (SOD), glutathione peroxidase and catalase [9]. Other small molecular antioxidants: glutathione, ascorbic acid and a-tocopherol are also involved in the detoxification of free radicals. Those reported evidences lead to Helmut Sies [10] to first propose the following definition of oxidative stress as 'imbalance between oxidants and antioxidants in favour of the oxidants, potentially leading to damage'. Oxidative stress was considered to be harmful, while antioxidants provided defence and prevention of tissue damage. However, ROS were recently found to play signalling roles not only in ROS-related processes, but in many basic functions such as fertilization, growth, and differentiation [11–14].

**95**

**Figure 1.**

*Homeostasis in redox balance.*

*Redox Balance Affects Fish Welfare*

**2.2 Redox balance**

biotopes [17].

fish species [21].

*DOI: http://dx.doi.org/10.5772/intechopen.89842*

We have described how reactive oxygen species (ROS) can be both harmful and beneficial. Consequently, under physiological conditions, the cellular redox equilibrium is tightly regulated on the one hand by pro-oxidants and on the other by enzymatic and non-enzymatic antioxidants (**Figure 1**). Due to the central role of ROS in many pathologies, restoring the redox balance forms an innovative target in the development of new strategies for treating several conditions. For example, Coenzyme-Q and its redox status -that was mostly found in the reduced form- have been proposed as an adaptation to different thermal environments in Antarctic fishes [15], and to reflects species-specific ecological habits and physiological constraint associated with oxygen demand represent an adaptation to environmental oxygen availability in coral reef fishes [16]. Recently, the determination of the redox balance of liver Coenzyme-Q from fish has been observed to have a potential, based on physiological principles, to be used as a practical biomarker for polycyclic aromatic hydrocarbon (PAH) contamination in aquatic

The main marker of oxidative damage in fish tissues have been considered as lipid peroxidation (LPO, usually measured as ThioBarbituric Reactive Substances, TBARS) [18–20]. However, the application of mammals' techniques to measure the direct oxidation of the amino acid side chains, so-called the Advanced Oxidation Protein Products, (AOPPs) or/and the tissue accumulation of 4-hydroxinonenal (4-HNE) cross-linked proteins are novel approach in the study of oxidative insult in

To cope with the oxidative damage resulting from metabolism, animals use non-enzymatic defences, such as thiol groups and glutathione, and enzymes with antioxidant activity [1, 20, 22]. Thus, in fish as in mammals, antioxidant liver capacity has been classically measured via total glutathione, (tGSH) and its oxidized and reduced forms (GSH and GSSG, respectively), and via the main antioxidant enzyme activities: superoxide dismutase (SOD), catalase (CAT), glutathione

peroxidase (GPX) and glutathione reductase (GR).
