Enzyme superoxide radicals (O2-°) as by-products of different

Enzyme reaction of SODRos are produced in both unstressed
and stressed cells. Plants have well-developed defense systems against Ros,
involving both limiting the formation of Ros as well as instituting its
removal. Under unstressed conditions, the formation and removal of O2
are in balance. However, the defense system, when presented with increased Ros
formation under stress conditions, can be overwhelmed. Plants respond to a rise
in Ros that the defense system is unable to remove with increased enzymatic or
non-enzymatic antioxidant processes (Alscher and Hess, 1993), but the
mechanisms underlying these processes is not well understood.Superoxide anion radical (O2-°)
is very reactive molecule which acts as progenitor of destructive chain
reactions that finally result in the damaging of the molecules close to the
site of O2·¯ production (Berlett and Stadtman, 1997). Organisms continuously produce
superoxide radicals (O2-°) as by-products of different
metabolic pathways. Free radicals start a series of chain reactions and are
sometimes transformed into other kinds of molecules. (Fernandez, Carlos et
al., 2009)One of the biochemical changes
occurring when plants are subjected to biotic or abiotic stresses is the production
of Ros (Allen, 1995). The mitochondria and the chloroplast are important
intracellular generators of Ros. In chloroplasts, Ros can be generated by direct
transfer of excitation energy from chlorophyll to produce single oxygen, or by
univalent oxygen reduction of photosystem I, in the Mehler reaction (Asada,
1999). Superoxide dismutases (SOD, EC
1.15.1.1) constitute a family of important enzymes that scavenge oxygen free
radicals. As the first defence in cells to counteract the toxicity of active
oxygen, these enzymes exist in various aerobic organisms (Sun et al.,
2013; Miller al., 2012). SODs are classified into four groups
according to their binding metal ions: Fe, Ni, Mn, and Cu-Zn. MnSOD and
Cu-ZnSOD are found in eukaryotes and in most prokaryotes. FeSOD, however, is
found mostly in prokaryotes and in a few plants, and NiSOD is only found in Streptomyces
and cyanobacteria (Bannister et al., 1987; Bafana, et al.,
2011; Dong, et al., 2013).Superoxide dismutase is considered to
be an important enzyme in protection of aerobes against oxidant damage, and
increased tolerance to oxidant stress is associated with induction of this
enzyme. Application of SODDifferent researchers have also
demonstrated a beneficial effect of SODs supplementation on cell resistance.
The inhibition of damage on DNA plasmids incubated with SOD suggests a decrease
in apoptosis following incubation with SODs (Takehara et al., 1994). This observation suggests that the
administration of SODs could inhibit the induction of tumours. Moreover, the
prevention of tumour progress (induced by inflammation) has been shown in mice
following oral administration of melon SODs (Okada et al., 2006).The positive effects of SODs
administration on nervous tissue damage and higher brain functions. Wengenack et al. 1997 have shown the improved survival
of rat CA1 neurons, following global cerebral ischemia, after an intravenous
injection of bovine Cu/Zn-SOD (Wengenack et al., 1997). An
increase in neurogenesis has also been demonstrated in a mouse model of
restraint stress after oral melon SODs administration (Nakajima et al.,
2009).SOD, the first enzyme in the
detoxifying process, converts superoxide anion radicals  (O2-°) to hydrogen
peroxide (H2O2), and APX reduces H2O2
to water using ascorbic acid as a specific electron donor (Asada, 1992; Foyer
et al., 1994; Asada, 1999). Chloroplasts,
the major component of photosynthetic tissue, is highly sensitive to damage by reactive
oxygen species, which are frequently generated by the reaction of chloroplast O2
and electrons that escape from the photosynthetic electron transfer system (Foyer
et al., 1994). Location of SOD in plant cellsCopper, zinc superoxide dismutase
(Cu, Zn SOD; EC 1.15.1.1) catalyzes a two-step dismutation of the toxic
superoxide radical (O2-) to molecular oxygen and hydrogen
peroxide through alternate reduction and oxidation of copper ion at
diffusion-limited catalytic rate (Liochev and Fridovich, 2000).Superoxide dismutase (E.C1.15.1.1) an
ubiquitous antioxidant enzyme catalyzes the dismutation of the super oxide
radical into hydrogen peroxide and molecular oxygen and protects
oxygen-metabolizing cells against harmful effects of superoxide free-radicals (Chaudhary, Chand and Kaur, 2013).

            2O2- + 2H? SOD
O2 + H2O2Cu, Zn SOD forms the first line of
defense in all aerobic organisms against reactive oxygen radicals and is vital
to the survival of cells (Halliwell and Gutteridge, 1990). Therefore,
SOD is an important enzyme for aerobic organisms including plants and humans
and also in those applications which require scavenging of O2-
(Bafana et al., 2011).SOD (EC 1.15.1.1), the first enzyme
in the detoxifying process, converts O2- radicals to H2O2
In plants, Cu/Zn SOD isoforms are found primarily in chloroplasts and in the
cytosol, and Mn SODs are located primarily in mitochondria (Rabinowitch and
Fridovich, 1983). In addition, peroxisomal localization of Mn SODs has been
reported in pea (Sandalio et al,.
1987). Tobacco (Nicotiana tabacum) plants also contain
chloroplast-localized Fe SOD (Van Camp et
al., 1990). In chloroplasts, H2O2 is reduced by
APX (EC 1.11.1.11) using ascorbate as an electron donor (Bowler et
al., 1992).Cu-Zn SODs are found throughout the
plant cell. There are two different groups of Cu-Zn SODs. The first group
consists of cytoplasmic and periplasmic forms, which are homodimeric. The
second group comprises the chloroplastic and extracellular Cu-Zn SODs, which
are homotetrameric (Bordo et al.,
1994). Efforts to fortify antioxidant
defenses in chloroplasts or in the cytosol include overexpression of CuZnSOD or
APX, which provides enhanced tolerance to oxidative stress in plants (Gupta et al., 1993a, b; Perl et al., 1993; Mittler et al., 1999; Murgia et al. 2004). In certain cases,
however, overexpression of a single antioxidative enzyme did not provide
protection against oxidative or abiotic stresses (Tepperman and Dunsmuir,
1990; Pitcher et al., 1991;
Torsethaugen et al., 1997),
suggesting that overexpression of one enzyme may not alter the function of the
entire anti-oxidant pathway. The plant SOD isoenzymes also differ
in their subcellular location. Typically, FeSOD MnSOD is mitochondrial, is
plastidic, and CuZnSOD can be plastidic or cytosolic (Bowler et al., 1992). There are also
reports of peroxisomal and extracellular SODs (Streller and Wingsle, 1994;
Bueno et al., 1995). Numerous attempts have been made to
enhance stress tolerance in plants by modifying the production of SOD enzymes.
Ectopic production of cytosolic Cu/Zn SOD improved stress tolerance in tobacco (Faize
et al., 2011), potato (Perl et al.,
1993), sugar beet (Tertivanidis et al.,
2004) and plum (Diaz-Vivancos et al.,
2013). Antioxidants such as ?avonoids, carotenoids,
vitamins, phenols and dietary glutathiones are capable of acting as free
radical scavengers, peroxide decomposers, singlet and triplet oxygen quenchers,
enzyme inhibitors and synergists (Singh et al., 2000). The
antioxidant activity of phenolic compounds are attributed due to their redox
properties, which can play an important role in adsorbing and quenching singlet,
neutralizing free radicals and triplet oxygen or decomposing peroxides and have
health functional properties that may protect humans from various diseases (Larson
al., 1988; Heinonen et al., 1998).Plants have developed various
mechanisms such as modulating the expression of stress tolerance genes and synthesizing
compatible solutes (Ahmad et al., 2010) to cope with the
oxidative stresses caused by unfavorable environments. Antioxidant defense
systems are well known for scavenging reactive oxygen species (Ros) produced in
different stressful conditions, such as activation of the antioxidant enzymes
superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and peroxidase
(POD) (Allen et al., 1997; Kwon et al., 2002).Plants have both enzymatic and
nonenzymatic antioxidant systems to prevent or alleviate the damage from Ros.
Several enzymes can efficiently detoxify Ros. Superoxide radicals are
disproportionately detoxified by superoxide dismutase (SOD), and hydrogen
peroxide is destroyed by catalase (CAT) and different kinds of peroxidases such
as guaiacol peroxidase (GPX), etc. A major hydrogen peroxide-detoxifying system
in plants is the ascorbate-glutathione cycle that includes ascorbate peroxidase
(APX) and glutathione reductase (GR) (Asada et al., 1994). Abiotic stresses are known to act as
a catalyst in producing free radical reactions resulting in oxidative stress in
various plants where reactive oxygen species (Ros) such as superoxide radical
(O2-), hydroxyl radical (OH), hydrogen peroxide (H2O2)
and alkoxyl radical (RO.) The first reaction in the
“ascorbate-glutathione” cycle is transformation of two molecules of
superoxide anion radical (O2-°), with two protons (H+),
into one molecule of hydrogen peroxide (H2O2) and one
molecule of oxygen (O2). The enzyme that catalyses this reaction is
superoxide dismutase (SOD):

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O2-°
+ O2-° + 2H+ SOD H2O2 + O2
(Goran, Z. et al., 2008)

Free radicals (such as the superoxide
anion) are chemical species containing one or more unpaired electrons (O2
+ e = O2-°), and therefore are extremely reactive. Free
radicals act on several cell components producing damage and modifying cell
functions. The targets for these dangerous molecules are basically
polyunsaturated fatty acids, some proteins and genetic material.