Environmental Engineering Reference
In-Depth Information
9.8
Enzymatic and Nonenzymatic Antioxidants
Nickel is known to be a redox-inactive metal. Therefore, it cannot directly generate
ROS (Boominathan and Doran
2002
). However, many nonenzymatic antioxidants
such as GSH and AsA (Freeman et al.
2004
) and enzymatic antioxidants such as
superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glu-
tathione
S
-transferase (GST) have been shown to be stimulated by Ni stress (Gabbrielli
et al.
1999
; Rao and Sresty
2000
; Baccouch et al.
2001
; Gajewska and Skłodowska
2008
). For example, Gajewska and Skłodowska (
2007
) reported a 250% increase in
the concentration of free radicals, such as O
2
−
and H
2
O
2,
in the leaves of Ni-stressed
wheat plants after a 3-day exposure to externally applied Ni. They observed a decrease
in antioxidative enzymes, such as SOD and CAT, from Ni stress, but observed a sig-
niicant increase in APX and POD activities at the same exposure. However, lipid
peroxidation in leaf tissue membranes remained unchanged after application of Ni.
This indicates that APX and POD may have eficiently removed ROS, thereby pre-
venting lipid peroxidation of biomembranes. Similar results were presented by
Gajewska et al. (
2006
); these authors found decreased SOD and CAT activities, but
increased POD and GST under conditions of increased Ni stress. Randhawa et al.
(
2001
) experienced contrasting results while working with the green alga
Scenedesmus
acutus.
These authors reported an increase in GSH, CAT, and SOD activities under
Ni stress. Wang et al. (
2010
) also reported a signiicant increase in the activity of
CAT, GPX, PAL, and SOD in cotyledons, stems, and roots of Ni-stressed
Luffa cylin-
drica
seedlings. It is now well known that SOD catalyzes disproportionation of O
2
−
to
H
2
O
2
and O
2
, and hence is considered to be a irst line of defense against ROS.
Harmful concentrations of H
2
O
2
are scavenged by catalase (CAT). CAT converts
H
2
O
2
to H
2
O and O
2
, whereas the alternative scavenger, ascorbate peroxidase (APX)
catalyzes a reduction of H
2
O
2
using ascorbate as an electron donor. Other peroxidases
like POD are also capable of eliminating H
2
O
2
by oxidizing phenolic compounds at
the expense of H
2
O
2
. These enzymes constitute a second line of defense against the
ROS production (Gaspar et al.
1991
). The results of the foregoing papers make it
clear that the antioxidative enzymatic system is stimulated under Ni stress and plays
a key role in helping crop plants tolerate excessive Ni levels.
9.9
Soluble Proteins
Heavy metals reduce total soluble protein levels in many plant species (Kastori et al.
1992
). In the presence of Ni stress, this reduction generally results from reduced
synthesis or accelerated hydrolysis of proteins. Kevresan et al. (
1998
) reported sugar
beet as experiencing a signiicant decrease in protein content when exposed to Cd
and Ni. However, occasionally, soluble protein levels may increase under metal
stress. Ewais (
1997
) reported increased protein content in roots and shoots of
Cyperus difformis
after exposure to Cd, Ni, and Pb.
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