Biomedical Engineering Reference
In-Depth Information
TCE
GSH conjugate
H
Cl
H
Cl
GSH
C=C
C=C
HCl
Cl
Cl
Cl
SG
Figure 7.4. GST-catalyzed dehalogenation of trichloroethene (TCE) in mammals forming a
glutathione conjugate (adapted from Dekant et al.,
1990
). Note: GSH - glutathione.
chloroethanes (Elsner et al.,
2007
; Hirschorn et al.,
2004
). If the first step in the primary
degradation pathway of
cis
-DCE by JS666 involved an epoxidation by a monooxygenase
(C
C cleavage), much smaller fractionation factors would be expected - consistent with
those reported for aerobic VC assimilation (
¼
7.0
‰
) (Chartrand et al.,
2005
; Chu
et al.,
2004
) or cometabolic
cis
-DCE oxidation, which were below detection (Chu et al.,
2004
)or
very small (
8.2 to
9.8
‰
) (Tiehm et al.,
2008
).
Bacteria have not been reported to catalyze an initial C-Cl cleavage in aerobic haloalkene
degradation, which suggests that
cis
-DCE degradation pathways may involve a novel mecha-
nism. In mammalian systems, GST-catalyzed dehalogenation of TCE or PCE involves a C-Cl
cleavage in the first step (Figure
7.4
) (Anders and Dekant,
1998
; Dekant et al.,
1986
,
1990
). In
bacteria, GSTs act as dehalogenases in dichloromethane metabolism (Kohler-Staub and Lei-
singer,
1985
), although an analogous reaction for two-carbon compounds has never been
reported. When degradation proceeds simultaneously in one organism by two distinct path-
ways, the observed carbon isotopic fractionation represents a weighted average of the two
pathways (Elsner et al.,
2008
). When one pathway is dominant, the fractionation effect of the
minor pathway may not be observed. Thus, CSIA results in JS666 indicate that the first step in
the major degradation pathway involves a C-Cl cleavage that is consistent with a first step
involving an enzyme other than a monooxygenase (e.g., GST- or P450-catalyized dehalogena-
tion), but does not rule out the contribution of a monooxygenase to a minor pathway.
Collectively, the results indicate that there may be two
cis
-DCE degradation pathways in
JS666. Work is currently underway to confirm the function of upregulated enzymes and clarify
their role in
cis
-DCE degradation pathways. The presence of two
cis
-DCE degradation path-
ways in JS666 could explain the observation of two degradation phenotypes - one characterized
by growth-coupled
cis
-DCE degradation and the other characterized by cometabolic degrada-
tion, with low rates and specific activity 10-20% of previously reported values (Jennings,
2005
).
The elucidation of degradation pathways in JS666 might allow for the development of
strategies to promote the growth-coupled pathway during bioaugmentation. For example,
once pathway-regulation is better understood, biodegradation could be improved with the
addition of appropriate inducers to stimulate the growth-coupled pathway or by avoiding
substrates and factors that might promote the undesirable, co-metabolic behavior.
7.1 to
7.2.4 Cometabolism of Other Chlorinated Solvents
JS666 is capable of transforming
trans
-1,2-dichloroethene (
trans-
DCE), TCE and VC, but
does not obtain energy from such processes. Previous studies indicated that JS666 does not
grow on 800
mol/bottle) 1,2-dichloroethane (DCA) (Coleman et al.,
2002a
). More
recent studies suggest that JS666 is capable of growth on DCA at lower concentrations
(400
m
M (40
m
M) (S. Nishino and J.C. Spain, Georgia Institute of Technology, unpublished).
The degradation of mixtures of contaminants would be particularly important for bioremedia-
tion applications at contaminated sites.
m
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