Environmental Engineering Reference
In-Depth Information
formic acids. PAHs such as naphthalene also can undergo
hydroxylation, and the intermediates then undergo conjuga-
tion as part of Phase II reactions. For chlorinated solvents,
Newman et al. (1997) reported the presence of di- and
trichloroacetic acids in plants exposed to the uptake of
TCE. As such, these bound compounds are the hydroxylated
byproducts of plant-cell TCE oxidation.
antioxidant (Purvis 1997). Plants use the same glutathione
and POX as antioxidants that humans use to combat oxida-
tive stresses.
Another way that antioxidants work is by donating
electrons to the oxidants, almost as surrogates, rather that
having the cell DNA being oxidized. Such compounds
include the vitamins C and E, as the carotenoids, beta-
carotenoids, luteins, lycopenes, and terpenes found in
many food crops. Limonene, found in oranges, stimulates
the processes in Phase I and II, as does broccoli. Other
antioxidant compounds include the flavonoids (cocoa,
green teas), and catechins. These compounds are abundant
in many fruits, such as pomegranates, grapes, blackberries,
cranberries, and blueberries.
Because part of the plant's detoxification system in
Phase I results in the generation of activated oxygen species,
such as hydrogen peroxide, trees have various self-produced
antioxidants to limit the damage. Such antioxidants include
thiol-containing compounds such as the cysteine and gluta-
thione previously discussed, and lipid-soluble compounds
such as tocopherol (Vitamin E) and carotene.
The presence of antioxidant compounds in plant leaves
and fruits can be explained by their interaction with the
energy of the sun during photosynthesis. Singlet oxygen
can be formed from oxygen in leaves by the input of too
much solar radiation. Because this can kill cells by chloro-
plast oxidation, plants use these antioxidants to decrease the
concentration of these oxygen singlets (Halliwell 2006).
Others include those previously discussed in this section,
such as the flavonoids and various vitamins. Flavonoids
and bioflavonoids are plant pigments, and were called vita-
min P by the discoverer of vitamin C, Albert Szent-Gyorgyi,
in the 1930s.
Extracts of leaves and beans, such as tea and coffee, are
used as beverages around the world. Leaves contain high
concentrations of compounds such as flavonoids that act to
protect the leaves from the damaging rays of the sun (since
they can't put on sunscreen to absorb UV radiation like we
can). These compounds are in highest concentration when
the leaves are at their most vulnerable, when they first
emerge from the winter buds. This is why green teas, the
young leaves of the tea plants, are used for teas. These
compounds protect the plant from the damaging rays of the
sun, which can lead to the formation of free radicals that can
cause cellular and genetic damage.
12.4.1.3 Decarboxylation
Decarboxylation is an oxidative reaction where a carboxyl
functional group
COOH is replaced with hydrogen and
CO
2
is released. Perhaps the most famous decarboxylation
reaction is the conversion of pyruvate into acetyl-CoA at the
beginning of the Kreb's cycle of aerobic respiration.
12.4.1.4 Dealkylation
Dealkylation is an oxidative reaction where compounds that
contain
S can have their alkyl functional
group removed by addition of hydrogen atoms. These
reactions are catalyzed by P-450 enzymes.
N,
O, or
12.4.1.5 Antioxidants
Plants synthesize many compounds that are not used by the
plant for energy or growth. This is a consequence of the plant
evolutionary production of defensive processes for protec-
tion against both oxygen as a strong oxidant and the free
radicals produced by ambient reactions, such as Fe(II) and
H
2
O
2
. Oxidative stresses to plants are the result of the
production of reactive oxygen species (ROS). These stresses
are
suppressed
by
the
production
of
antioxidants
compounds.
Free radicals, such as the hydroxyl radical (OH), are
harmful to living tissue because they contain at least one
unpaired electron. Interaction of plant, or mammalian,
tissues with free radicals is responsible for cellular damage,
degeneration, and eventually cell death. The flow of
electrons during oxidation is from the element being
oxidized to the element accepting the electron. This reaction
can occur spontaneously and abiotically, as with rusting
metal, the chemical oxidation of Fe(II) to Fe(III) by atmo-
spheric oxidation. The rate of this reaction can be enhanced
by the presence of water and electrolytes.
Antioxidant compounds inhibit the oxidation process. As
stated above, plants inhabit an essentially harsh environ-
ment, replete with ozone, UV radiation, oxidative chemicals,
and free radicals, all which can destroy cells through damage
to DNA. As well, a consequence of aerobic respiration in
plants and humans, the Krebs cycle typically results in the
reduction of free oxygen to water. However, if some oxygen
is not reduced to water, this excess oxygen and can form
superoxide anions (O
2
). This can produce hydrogen perox-
ide and induce tissue damage, ultimately attacking DNA. To
remove some of these threats, glutathione can be used as an
12.4.1.6 Reduction
As discussed above, the primary utilization of P-450
enzymes is as oxidative enzymes. These enzymes also can
be used, however, to perform reductive reactions. Take, for
example, the dehalogenases. In this capacity, rather than
facilitating the addition of oxygen to a xenobiotic, they add
hydrogen. These reactions happen best when concentrations
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