Environmental Engineering Reference
In-Depth Information
of oxygen are low, such that there is no competition as an
electron acceptor. For example, this reduction can include
compounds that contain nitrogen, called azo reduction or
aromatic nitro reduction, as well as the reductive
dehalogenation of halogenated compounds (McCutcheon
et al. 2003). Such reductions also can occur for the semi-
oxidized contaminants such as TNT, where nitroreductases
catalyze the reduction of the nitro groups on the central
toluene structure.
The specific enzymatic capability to degrade chlorinated
solvents by the dehalogenase enzyme exists in plants. The
role played by nitroreductase in the roots of leguminous
plants and other plants tissues was previously discussed in
Chap. 11. For example, the kinetics of TCE transformation
in leaf tissue was examined for trees growing above TCE-
contaminated groundwater at the site in Fort Worth, Texas.
All leaf samples collected showed dehalogenase activity.
First-order rate constants of TCE degradation averaged
about 0.049 h
1
for all plant species tested. This gives a
more meaningful TCE half-life of about 14 h (U.S. Environ-
mental Protection Agency 2003).
reactions require enzymes such as GST. Less is known
about the exact pathways of xenobiotic transformation, and
rates of detoxification, though it is known that Phase II
reactions occur at much slower rates in plants than in
mammals.
12.4.2.1 Conjugation
Once a xenobiotic is taken up into a plant, the process of
Phase I and Phase II detoxification can occur (Fig.
12.4
). The
initial step is usually interaction with cytochrome P-450
monooxygenases or POXs, as discussed above. After this
oxidation, conjugation reactions occur where various sugars
or amino acids interact with the activated xenobiotic to form
glycoside compounds; these reactions are mediated by
glycosyltransferases (Schroder and Collins 2002). The result
is an inactivated xenobiotic. These Phase I and Phase II
reactions act to protect the plant by removing the xenobiotic
as quickly as possible by increasing the compound's polar-
ity. Whereas in animals the end product is eliminated by
excretion, in plants these byproducts are stored in vacuoles
or in other organic matter in the plant, which is discussed
below.
The interaction of the intermediate byproducts of Phase I
reactions with a plant- or animal-produced compound is
called a conjugate. Conjugates include plant protein, lignin,
or organic acids. The resulting conjugates often are irrevers-
ibly bound to plant tissue. For example, these organic
compounds cannot be extracted with chemical solvent
extraction techniques. Up to 70% of contaminants that
enter plants are rendered as conjugates (Kvesitadze et al.
2006). On the other hand, other conjugation reactions often
result in the decrease in the toxic effect of the chemical
through increased water solubility and intraplant mobility.
In fact, the process of conjugation often is used in analytical
chemistry to analyze water-insoluble compounds through
derivatization to a more soluble conjugate.
The process of conjugation differs from bioaccumulation
in that the parent compound taken up is changed into a less
harmful form, and the process is regarded as beneficial in
terms of risk reduction. Moreover, in the plant cells exposed
to the contaminant, once the contaminant is conjugated, it no
longer poses a threat to cell metabolism. However, these
compounds are still present in the plant, as no mineralization
occurred.
As was the case for most of the early investigation into
the interaction between plants and xenobiotics, some of the
first evidence of transformation reactions by conjugation
was observed in plants exposed to pesticides. The plant
enzyme GST was identified in the 1960s and 1970s to be
present in both animals and plants. Transferases are enzymes
that catalyze conjugation reactions, which lead to the inter-
action of the byproduct with endogenous plant cellular
material. GSTs can facilitate the reaction between the
12.4.1.7 Hydrolysis
Hydrolysis is the process of splitting a molecule into two
individual molecules. The most important hydrolysis reac-
tion is the splitting of water into H and O during photosyn-
thesis. Hydrolysis also results when functional groups
interact with water, with
OH additions occurring most
commonly. This occurs for organophosphates, carbamates,
as well as esters and ethers, such as the conversion of MTBE
to TBA in low-pH water. Because hydrolysis decreases the
size of the parent compound, the smaller size of each indi-
vidual compound renders it more susceptible to additional
degradation, given the appropriate redox conditions.
The transfer of a glycosyl group to water occurs during
hydrolysis. The glycosides are one of the largest classes of
detoxification compounds in plants. The products of hydro-
lysis typically are further detoxified by Phase II conjugation
reactions.
12.4.2 Phase II Reactions
Phase II reactions occur following oxidation reactions, are
predominated by conjugation reactions, and require energy
in the form of ATP to be expended by the cells (Fig.
12.4
).
The conjugation reactions increase the water solubility of the
oxidized and functionalized compounds following Phase I
reactions, or they produce water insoluble residues that are
irreversibly bound into the plant tissue, such as occurs dur-
ing lignification. Therefore, which pathway a particular con-
taminant will follow has important implications for the
fate of xenobiotics released to groundwater. Conjugation
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