12 be transfered through membrane transport far from the place of its generation. Hydrogen peroxide is not a free radical, but it may have detrimental effects on DNA, proteins, and lipids when its concentration reaches 10 μM (Halliwell and Gutteridge 2007). Nitric oxide radical (NO⋅) is the most common RNS. It is derived by the amino acid L-arginine and is highly reactive. It reacts with Ο2⋅−, generating peroxynitrite anion (ONOO−), which is converted to peroxynitrous acid and finally to harmful OH⋅ and nitrite anion (NO2⋅; Halliwell 2001a). Reactive oxygen and nitrogen species are physiologically produced during metabolic processes and especially during electron transport chain reactions (Di Meo and Venditti 2001). An initial study has reported that 2–5% of O2 used during oxidative phosphorylation in skeletal muscle mitochondria is reduced to O2⋅− at rest (Sjodin et al. 1990). More recent studies have demonstrated that, at rest, reactive species leakage in heart mitochondria ranges from 0.4% to 0.8% (Hansford et al. 1997) or at an even lower percentage (0.15%) in skeletal muscle (St-Pierre et al. 2002). Another internal source of reactive species is peroxisomes, small membrane-enclosed organelles containing enzymes impor- tant for oxidation reactions. Furthermore, the enzymes of the Ρ450 complex generate reactive species during the detoxifi- cation of xenobiotics, such as drugs. There are also external sources of reactive species related to UV radiation, air pollution, smoking, alcohol consumption, and exercise (Halliwell and Gutteridge 2007). Reactive species are often considered to be harmful entities because of their connection to oxidative damage in biomolecules. Nevertheless, this is not always the case. The biology of reactive species follows the phenomenon of hormesis. Accordingly, when reactive species are present in low concentrations, they are necessary for fundamental procedures in the subcellular environment. If their concen- trations exceed a limit, however, they may have detrimental effects on DNA, proteins, and lipids. It appears that human cells generally function in a reduced state, but an amount of reactive species is essential for several biochemical pro- cesses. Signal transduction requires the presence of Ο2⋅− and H2O2 (Ji 2007), whereas NO⋅ is a neurotransmitter and is biologically important for vasoconstriction (Halliwell and Gutteridge 2007). Reactive species are also vital for muscle contraction (Linnane et al. 2002), enzyme activation (Jenkins 1988), and gene expression (Ji et al. 2006), while several gene transcription factors require transient oxidation for their function (Sen 1998). Furthermore, reactive species contribute to the normal function of the immune system via the activation of phagocytes, a common procedure in conditions in which inflammation occurs, such as muscle- damaging (eccentric) exercise (Malm 2001). Additionally, folding of nascent proteins in the endoplasmic reticulum is aided by a more oxidized environment to permit disulfide A.S. Veskoukis et al. bridge formation (Suh et al. 1999). Apoptosis is also accompanied by a degree of intracellular increased oxida- tion, although too much oxidation stops apoptosis by oxidizing and inactivating caspases (Hampton and Orrenius 1998). As previously mentioned, the concentration of reactive species is a key factor in their effect. If the concentration is excessively high, they may become very harmful for macromolecules. Proteins are one of the major targets of reactive species which induce the formation of carbonyl groups (aldehydes and ketones) in those amino acids that are susceptible to oxidation, such as histidine, arginine, lysine, and proline. The carbonyl groups are not metabo- lized in proteasomes and lysosomes, but are accumulated (Levine 2002). Furthermore, the thiol groups (−SH) present in protein molecules are oxidized in thiol radicals (RS⋅). Protein oxidation leads to conformational changes which result in the modification or loss of protein function (Halliwell and Gutteridge 2007). Apart from proteins, lipids are vulnerable in reactive species-induced oxidative damage (Halliwell and Chirico 1993). Polyunsaturated fatty acids (PUFA) are abundant in cellular membranes and are susceptible to oxidation, leading to lipid peroxidation chain reactions (Alessio 1993). Specifically, PUFA oxidation induces the generation of a radical, which reacts with O2 and produces peroxyl radicals (LOO⋅). These radicals further oxidize PUFA and lipid peroxides (LOOH) are produced, which create conjugated dienes as by-products, such as 4-hydroxy-2-nonenal and malonyl dialdehyde (Young and McEneny 2001; Mylonas and Kouretas 1999). Lipid peroxidation increases the permeability of cellular membranes, resulting in cell death. Reactive species also affect DNA by causing chain breaks and damaging its repair mechanism (Jenkins 1988). DNA, and especially guanine, oxidation results in the production of 8- hydroxy-2′-deoxyguanosine. This by-product, if not repaired, induces DNA mutations that may cause aging and carcinogenesis (Radak et al. 1999). In addition, excessive production of reactive species has been implicat- ed in immune system dysfunction (Schneider and Tiidus 2007), muscle damage (Nikolaidis et al. 2007a, b), and fatigue (Betters et al. 2004). Endogenous and dietary antioxidants As explained previously, oxygen, the base element for the favorable aspects of aerobic life, is poisonous in some ways and is linked to the potentially dangerous oxidative damage of macromolecules. Aerobic organisms must therefore adopt ways to survive these harmful effects. This defensive mechanism is the antioxidant system (Halliwell and Gutteridge 2007). There is no simple definition of an