Agriculture Reference
In-Depth Information
7.4.4 rOS and oxidative damage
to biomolecules
To avoid oxidative stress, production and removal of
ROS should be strictly controlled. The cell is said to be in
a state of 'oxidative stress' when production of ROS
exceeds the capacity of the defence mechanisms to
remove them. Numerous biotic and abiotic stresses such
as salinity, drought, high light levels, heavy metal tox-
icity, pesticides and pathogens disturb the equilibrium/
balance between production and scavenging of ROS.
Increased levels of ROS can cause damage to biomole-
cules such as lipids, proteins and DNA. This imbalance
disturbs various membrane properties like fluidity and
ion transport, and causes loss of enzyme activity, protein
cross-linking, inhibition of protein synthesis, DNA
damage and ultimately death of the cell.
PUFA
+→
2
O UFAOO
− •
(7.10)
The peroxy radical formed is highly reactive and able to
propagate the chain reaction:
PUFA
OO
+ → +
PUFA
OOH
PUFA
OOH
PUFA
(7.11)
Conjugated dienes are formed when free radicals attack
the hydrogens of methylene groups, separating double
bonds and rearranging them with the help of metals like
Fe 2+ . Lipid hydroperoxide undergoes reductive cleavage as
per the following reaction (Recknagal & Glende, 1984):
Fe
2
+
complexPUFA
+
OOH
(7.12)
Fe
3
+
complexPUFAO
+
− •
Decomposition of lipid hydroperoxide results in the
formation of several reactive species like lipid alkoxyl
radicals, aldehydes (malondialdehyde, acrolein and cro-
tonaldehyde), alkanes, lipid epoxides and alcohols
(Davies,2000).PUFA-O•caninitiateadditionalchain
reactions (Buettner, 1993):
PUFA
7.4.4.1 ROS and lipids
Increased ROS levels will enhance lipid peroxidation
rates in both cellular and organellar membranes, ulti-
mately affecting normal cellular functioning. Lipid
peroxidation increases the oxidative stress by producing
various lipid-derived radicals, which react with each
other and damage proteins and DNA (Sharma & Dubey,
2005; Han et al., 2009; Tanou et al., 2009; Mishra et al.,
2011). Increased levels of lipid peroxidation have been
used as an indicator of cellular damage due to ROS pro-
duction under various biotic and abiotic stresses. It has
been reported that under various environmental
stresses, lipid peroxidation (degradation) is increased.
Malondialdehyde (MDA) is one of the final end-prod-
ucts of peroxidation in phospholipids and is actively
responsible for cell damage (Halliwell & Gutteridge,
1989). Double bonds between carbon atoms and ester
linkages between glycerol and fatty acids are common
sites of ROS attack. Fatty acids present in membrane
phospholipids are very sensitive to ROS attack; a single
hydroxyl ion can cause peroxidation of many polyun-
saturated fatty acids because of a cyclic chain reaction.
The overall process of lipid peroxidation involves three
distinct stages: initiation, progression and termination.
The initial phase of lipid peroxidation involves activation
of O 2 ,whichisarate-limitingstep.TheROS•O 2 - and•OH
have been found to react with methylene groups of poly-
unsaturated fatty acids (PUFA) forming conjugated dienes,
lipid peroxy radicals and hydroperoxides (Smirnoff, 1995):
PUFAHX
O UFAH PUFA
+
OH
+
PUFA
(7.13)
Peroxidation of PUFAs by ROS can lead to chain break-
age and thereby increased membrane fluidity and
permeability.
7.4.4.2 ROS and proteins
ROS attack on proteins may cause a number of modifi-
cations, both direct and indirect. Modulation in protein
activity through nitrosylation, carbonylation, disulphide
bond formation and glutathionylation are some of the
direct alterations of proteins due to ROS attack. Indirect
modifications of proteins involve conjugation with
breakdown products of fatty acid peroxidation,
aggregation of cross-linked reaction products, peptide
chain fragmentation, modification of electric charge and
increased susceptibility to proteolysis (Yamauchi et  al.,
2008). Increased concentrations of carbonylated pro-
teins have been observed in tissues under oxidative
stress, and these are often used as markers of protein
oxidation (Parween et  al., 2011). Amino acids exhibit
differential responses to ROS attack, with thiol groups
and sulphur-containing amino acids being susceptible
sites in any peptide chain. ROS can abstract an H atom
from cysteine residues to form a thiol radical that will
cross-link to a second thiol radical to form a disulphide
bridge. Like pesticides, several metals like Cd, Pb and Hg
have also been shown to cause the depletion of
(7.9)
+→ +
PUFA
XH
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