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Transformation of RDX and other energetic compounds by
xenobiotic reductases XenA and XenB
Fuller, Mark; McClay, Kevin; Hawari, Jalal; Paquet, Louise; Malone,
Thomas; Fox, Brian; Steffan, Robert
Publisher’s version / la version de l'éditeur:
Applied Microbiology Biotechnology, 84, 3, 2009
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Appl Microbiol Biotechnol (2009) 84:535–544
DOI 10.1007/s00253-009-2024-6
APPLIED MICROBIAL AND CELL PHYSIOLOGY
Transformation of RDX and other energetic compounds
by xenobiotic reductases XenA and XenB
Mark E. Fuller & Kevin McClay & Jalal Hawari &
Louise Paquet & Thomas E. Malone & Brian G. Fox &
Robert J. Steffan
Received: 12 November 2008 / Revised: 23 April 2009 / Accepted: 27 April 2009 / Published online: 20 May 2009
# Springer-Verlag 2009
Abstract The transformation of explosives, including
hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), by xenobiotic
reductases XenA and XenB (and the bacterial strains
harboring these enzymes) under both aerobic and anaerobic
conditions was assessed. Under anaerobic conditions,
Pseudomonas fluorescens I-C (XenB) degraded RDX faster
than Pseudomonas putida II-B (XenA), and transformation
occurred when the cells were supplied with sources of both
carbon (succinate) and nitrogen (NH4+), but not when only
carbon was supplied. Transformation was always faster
under anaerobic conditions compared to aerobic conditions,
with both enzymes exhibiting a O2 concentration-dependent
inhibition of RDX transformation. The primary degradation
pathway for RDX was conversion to methylenedinitramine
and then to formaldehyde, but a minor pathway that
M. E. Fuller (*) : K. McClay : R. J. Steffan
Shaw Environmental, Inc,
17 Princess Road,
Lawrenceville, NJ 08648, USA
e-mail: mark.fuller@shawgrp.com
J. Hawari : L. Paquet
Biotechnology Research Institute,
National Research Council Canada,
6100 Royalmount Ave,
Montreal, PQ H4P 2R2, Canada
T. E. Malone
Molecular and Environmental Toxicology Program,
University of Wisconsin,
433 Babcock Drive,
Madison, WI 53706, USA
B. G. Fox
Department of Biochemistry, University of Wisconsin,
433 Babcock Drive,
Madison, WI 53706, USA
produced 4-nitro-2,4-diazabutanal (NDAB) also appeared to
be active during transformation by whole cells of
P. putida II-B and purified XenA. Both XenA and XenB
also degraded the related nitramine explosives octahydro1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 2,4,6,8,10,12hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane. Purified
XenB was found to have a broader substrate range than
XenA, degrading more of the explosive compounds
examined in this study. The results show that these two
xenobiotic reductases (and their respective bacterial
strains) have the capacity to transform RDX as well as a
wide variety of explosive compounds, especially under
low oxygen concentrations.
Keyword Pseudomonas . RDX . Explosive .
Biodegradation . CL-20 . HMX
Introduction
Past and current activities at sites where munitions are
manufactured and tested have resulted in the release of
munition-related compounds. The environmental fate of
these contaminants is an issue of significant concern to
the United States Department of Defense (DoD), regulators, and the public because their mobility and
persistence allow them to contaminate ground water
supplies (Tipton et al. 2003; Yamamoto et al. 2004).
Recently, information describing the extent of soil and
groundwater contamination at military training ranges has
been published (Jenkins et al. 2001; Clausen et al. 2004;
Pennington et al. 2006). Several of these compounds have
been placed on the U.S. Environmental Protection
Agency’s Contaminant Candidate List (http://www.epa.
gov/safewater/ccl/ccl3.html#chemical).
536
Appl Microbiol Biotechnol (2009) 84:535–544
bacterial enzyme systems that degrade RDX have been
identified, including the XplA/XplB system of a number of
geographically dispersed rhodococci (Jackson et al. 2007;
Seth-Smith et al. 2008), the diaphorase of clostridia
(Bhushan et al. 2002), and the type I nitroreductases of
two enterobacteria (Kitts et al. 2000).
The degradation of nitroglycerin and TNT by the
xenobiotic reductases (XenA and XenB) from the aerobes
Pseudomonas putida II-B and Pseudomonas fluorescens
I-C has been explored (Blehert et al. 1999; Pak et al. 2000).
Though XenA and XenB are both members of the Old
Yellow Enzyme family (flavoprotein oxidoreductases) and
catalyze similar reactions, there are significant differences
in the catalytic rates and substrate specificities between the
two. For example, purified XenB catalyzes the transformation of TNT ∼5-fold faster than XenA, whereas the catalytic
rates with nitroglycerin (NG) are approximately equal.
However, XenA preferentially denitrates NG at the terminal
positions (1 and 3 positions), whereas XenB preferentially
denitrates NG at the interior position (2 position). Furthermore, the rate of TNT transformation by XenB was only
slightly enhanced under anaerobic conditions, but the
Extensive research has examined the biological transformation of explosive compounds by pure cultures of bacteria
and mixed consortia in soil and groundwater (see review of
Hawari et al. 2000a). Most research has focused on the
dinitrotoluenes (DNT) and 2,4,6-trinitrotoluene (TNT),
with interest in hexahydro-1,3,5-trinitro-1,3,5-triazine
(RDX) increasing in recent years. RDX biodegradation
has been observed under conditions ranging from fully
aerobic (Binks et al. 1995; Coleman et al. 1998) to strictly
anaerobic (Hawari et al. 2001; Maloney et al. 2002; Adrian
et al. 2003; Pudge et al. 2003; Adrian and Arnett 2004;
Bhatt et al. 2005), and at least three major degradation
pathways have been elucidated (Fig. 1). Anaerobic processes involve either a direct attack on the ring structure or
the successive reduction of the pendant nitro groups
followed by ring cleavage (McCormick et al. 1981; Hawari
et al. 2000a). Many bacterial strains can utilize RDX as a
sole nitrogen source (Boopathy et al. 1998; Zhao et al.
2003; Thompson et al. 2005), but only recently has the use
of RDX as a sole source of carbon, nitrogen, and energy by
a single organism (Thompson et al. 2005) and mixed
cultures (Adrian and Arnett 2006) been reported. Several
O
O
O
N
N
N
N
N
N
N
O
N
O
N
O
N
O
N
Hexahydro-1-nitroso-3,5dinitro-1,3,5-triazine
(MNX)
N
N
N
O
An
ae
ro
bi
c
(I)
N
O
O
O
N
N
NH O
N
O
Hexahydro-1,3-dinitroso5-nitro-1,3,5-triazine
(DNX)
O
Ring cleavage
N
N
CleavageProducts:
Carbon Dioxide
Methanol
Nitrous Oxide
Formaldehyde
O
Hexahydro-1,3,5trinitroso-1,3,5-triazine
(TNX)
O
O
O
O
O
N
NH O
Hydroxymethylnitramine
Methylenedinitramine
(MEDINA)
N
N
O
O
Anaerobic (II)
N
+
OH
O
RDX
N
N
Nitrous
Oxide
Nitramide
+
N
O
O
N
NH2 O
O
N
N
N
NH
HO
H2C O
Formaldehyde
N
O
c
bi
ro
Ae
OH
bis(Hydroxymethyl)nitramine
O
H
O
N
N
N
H2N
N
N
N
HO
N
O
N
O
N
O
N
O
N
H
N
Formaldehyde
Carbon Dioxide
O
Formamide
O
N
OH O
O
+
N
Pentahydro-1,3-dinitro1,3,5-triazine
Tetrahydro-1-nitro1,3,5-triazine
Tetrahydro-1-nitro2,4-dihydroxy-1,3,5triazine
O
NH
NH
O
4-Nitro-2,4diazabutanal
(NDAB)
Fig. 1 Known degradation pathways for RDX. Pathways derived/adapted from Hawari et al. (2000b) and Zhao et al. (2003)
N2
Appl Microbiol Biotechnol (2009) 84:535–544
product distribution resulting from TNT transformation
varied greatly under anaerobic conditions. Transformation
of RDX by these enzymes was not characterized.
In the present study, the effect of decreasing O2 tension
on the catalytic characteristics of XenA and XenB
expressed in their native bacterial hosts and as purified
enzymes were explored. The results reveal that both
enzymes are capable of degrading RDX, octahydro1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and a suite of
related energetic compounds under reduced O2 concentrations, but not necessarily under fully aerobic conditions.
The observation that RDX can be degraded by aerobic
organisms under reduced oxygen tensions could lead to
enhanced bioremediation technologies and a better understanding of natural attenuation process.
Materials and methods
Chemicals All chemicals were reagent grade or purer. The
three nitroso-containing metabolites (hexahydro-1-nitroso3,5-dinitro-1,3,5-triazine; hexahydro-1,3-dinitroso-5-nitro1,3,5-triazine; and hexahydro-1,3,5-trinitroso-1,3,5-triazine)
of RDX were purchased from SRI International (Menlo
Park, CA, USA). RDX (7% HMX as a manufacturing
impurity) was a gift from James Phelan at Sandia National
Laboratories (Albuquerque, NM, USA). HMX was a gift
from Herb Fredrickson at the U.S. Army Engineer Research
and Development Center (Environmental Laboratory,
Vicksburg, MS, USA). [14C]-RDX (specific activity=
60.0 mCi/mmol) was purchased from PerkinElmer Life
Sciences (Boston, MA, USA). The manufacturer stated
the [14C]-RDX radiochemical purity of 99.9% based
on radiochromatography. CL-20 (2,4,6,8,10,12-hexanitro2,4,6,8,10,12-hexaazaisowurtzitane) was obtained from ATK
Launch Systems (Corinne, Utah, USA). Neat standards of
nitroaromatic compounds were purchased from ChemService
(West Chester, PA, USA).
Bacterial strains P. putida II-B and P. fluorescens I-C were
maintained on R2A agar. The two strains were deposited in
the open collection of the Agricultural Research Service
(ARS) Culture Collection, National Center for Agricultural
Utilization Research of the U.S. Department of Agriculture.
The accession number of P. putida II-B is NRRL B-50270,
and the accession number of P. fluorescens I-C is NRRL
B-59269.
Whole cell biotransformation assays P. putida II-B and
P. fluorescens I-C possessing the xenA and xenB genes,
respectively, were screened for transformation of RDX,
HMX, and CL-20. A basal salts medium (BSM) (Hareland
et al. 1975) was used for screening. The carbon source was
537
succinate. Inocula were prepared by growing the strains in
BSM plus succinate overnight, followed by concentration
and washing of the cells twice with nitrogen-free BSM. The
washed cells were used to inoculate vials (40 mL) of BSM
medium (20 mL) amended with sodium succinate (6.2 mM)
and RDX (22 µM) or HMX (∼3 µM). The initial optical
density of the cultures at 550 nm (OD550) was approximately 0.15 (corresponding to 0.3 mg total cell protein per
vial). Vials were incubated at room temperature with
shaking, and samples were removed periodically, passed
through 0.45 µm glass microfiber filters into 2-mL glass
sample vials, and analyzed for RDX, HMX, and breakdown
products by high-performance liquid chromatography
(HPLC; see below). Aerobic vials were equipped with
vents to allow 0.2-μm filtered air to enter and maintain
oxygen concentrations. Experiments performed under anaerobic conditions were prepared, incubated, and sampled
in a Coy anaerobic chamber (Coy Laboratory Products,
Grass Lake, MI, USA).
Transformation of CL-20 (∼2 μM initial concentration)
by the Pseudomonas strains was performed similarly,
except that the screening was performed in polypropylene
tubes instead of glass to prevent abiotic loss of CL-20
(Monteil-Rivera et al. 2004). Anaerobic treatments were
prepared and incubated in an anaerobic glove box. Samples
were removed periodically and centrifuged in polypropylene microfuge tubes to remove biomass, and the supernatant was transferred to polypropylene HPLC vials for
analysis.
The effect that changes in the RDX concentration had on
the rate and extent of transformation by P. putida II-B and
P. fluorescens I-C was examined by adding washed cells to
anaerobic BSM plus succinate amended with RDX at
concentrations of 3, 13, 31, 63, and 126 μM. Samples
were removed periodically and analyzed by HPLC. Direct
toxicity of RDX (at 0, 86, and 153 μM) to these two strains
was examined by monitoring cell density at 550 nm during
aerobic growth in BSM plus succinate (a condition under
which RDX was not degraded).
Production of nitrous oxide (N2O) and nitrite (NO2-)
from RDX were determined by incubating cultures of
P. putida II-B and P. fluorescens I-C with RDX and
periodically removing samples of the headspace and
liquid for analysis (see analytical section below). To
identify the less common RDX breakdown products,
cultures were incubated with ∼90 μM of RDX at room
temperature with shaking, then frozen at −70°C after
approximately 50% of the initial RDX had degraded.
Frozen samples were shipped on dry ice to the Biotechnology Research Institute, National Research Council
Canada for more extensive analysis of RDX breakdown
products according to previously described methods
(Hawari et al. 2000b).
538
Cell-free enzyme assays Several experiments were performed to assess the catalytic properties of the xenobiotic
reductases of P. putida II-B and P. fluorescens I-C, which
were isolated and purified as previously described (Blehert
et al. 1997; Pak et al. 2000). The explosive transformation
assays were performed with the test compounds dissolved
in sodium phosphate buffer (100 mM, pH 7.4). NADPH
was added to a final concentration of 1.5 to 4.5 mM. Vials
were purged with at least 20 volumes of O2-free N2
bubbled through the liquid, then transferred to an
anaerobic chamber where 1 mL of the solutions were
transferred to 2-mL glass screw cap auto-sampler vials (or
polypropylene vials in the case of the explosive CL-20)
and sealed with Teflon lined septa. To examine the effect
of O2 on the rate of RDX and HMX transformation, pure
O2 gas was added via a syringe needle inserted through
the septum of the vial to bring the headspace O 2
concentration up to the desired percentage on a (v/v) basis
with the headspace and vigorously shaken. An assay was
initiated by injecting 1 μL of purified XenB (0.017 mg) or
1 to 5 μL of purified XenA (0.014 to 0.070 mg) through
the septum. For kinetic assays, the vials were automatically and repeatedly analyzed via HPLC (see below).
Negative controls composed of substrate, buffer, and
NADPH were included in all experiments and were used
to detect and adjust for any non-enzymatic substrate
losses.
The aerobic and anaerobic degradation rates of a
range of explosive compounds with XenB were examined. Assays were performed as described, except that
the amount of XenB protein was adjusted as needed to
assure that degradation did not occur faster than could
be measured based on the HPLC analysis times for a
given compound. Assays were performed in duplicate,
and the initial linear rates were calculated as micromoles compound degraded per milligram XenB protein
per minute. Non-kinetic experiments were performed
with the XenA enzyme in which only the 24-h endpoint
result (degradation/no degradation) was measured. Degradation of CL-20 by both XenA and XenB was also
assessed by an endpoint assay.
To determine if RDX was converted to MNX during
transformation by XenB, an experiment utilizing radiolabeled RDX was conducted. Briefly, the enzyme assay
procedure described above was followed, except that the
XenB and XenA enzymes were mixed with 78 μM of
MNX and 54 μM [14C]-RDX. Unlabeled MNX was
included in the assay so that if very small amounts of
MNX were being formed and subsequently degraded by
XenB during the transformation of RDX, the large pool of
unlabelled MNX would slow down the degradation of the
enzymatically formed [14C]-MNX, which could then be
detected using scintillation counting. The reaction vial was
Appl Microbiol Biotechnol (2009) 84:535–544
repeatedly sampled, and the loss of the target substrates
was monitored via HPLC as described below, except that
the HPLC eluant was also collected at 20-s intervals into
scintillation vials pre-filled with 3 mL of Optiphase
HiSafe scintillation cocktail (Perkin-Elmer, Boston MA,
USA). The time of elution of the radioactive peaks was
compared with the elution time of the known explosive
compounds and metabolites (RDX, MNX, DNX, and
TNX) to determine if any of the [14C]-RDX was being
converted to [14C]-MNX or other related compounds.
Under the analytical conditions described below, there is
more than a full minute separating the elution of MNX and
RDX, which would be easily resolved with the described
protocol.
Analytical Other than the analysis for the less common
breakdown products of RDX performed by Biotechnology
Research Institute noted above, the concentrations of the
explosives and their common breakdown products in all
experiments were determined using HPLC according to a
modified EPA Method 8330A (http://www.epa.gov/epa
waste/hazard/testmethods/sw846/pdfs/8330a.pdf) using a
Hewlett-Packard 1100 HPLC equipped with a Allure
C18 column (Restek, Bellefonte, PA, USA) and a UV
detector (230 nm). The mobile phase was 50:50
methanol/water at a flow rate of 0.9 mL/min. The
column temperature was 25°C. The lower detection
limit was approximately 0.1 µM for RDX and 0.25 µM
for the RDX breakdown products. CL-20 was analyzed
on the same system, except that the mobile phase was
adjusted to 55:45 methanol/water, and detection was at
performed at 228 nm. Analytical standards of the parent
explosive compounds (except CL-20) were obtained
from Restek (Bellefonte, PA, USA). The standard for
CL-20 was prepared from the neat material obtained
from ATK Launch Systems (Corinne, Utah, USA). The
standards for MNX, DNX, and TNX were prepared
from the neat material obtained from SRI International
(Menlo Park, CA, USA). Identity of peaks in the
samples was based on retention time matching to peaks
in the standards.
Nitrous oxide was measured using GC-TCD. Nitrite was
determined colorimetrically (Hach Company, Loveland,
CO, USA). Ammonia was measured spectrofluorometrically (Holmes et al. 1999). Hydrogen peroxide production
by purified enzymes in the presence of oxygen was
detected and quantified using either the Quantofix
Peroxide Detection Kit (Macherey-Nagel, Bethlehem,
PA, USA) or the Amplex Red Hydrogen Peroxide/
Peroxidase Assay Kit (Invitrogen, Carlsbad, CA, USA)
with fluorometric analysis using SpectraMax Gemini
fluorescent plate reader (Molecular Devices, Sunnyvale,
CA, USA).
Appl Microbiol Biotechnol (2009) 84:535–544
Results
Transformation of RDX and other explosives by whole
cells During initial experiments, the two Pseudomonas
strains examined here were able to degrade TNT under
aerobic conditions, but no aerobic transformation of RDX,
HMX, or CL-20 was observed (data not shown). Under
anaerobic conditions, transformation of RDX was observed
with P. putida II-B and P. fluorescens I-C (Fig. 2a). HMX
was degraded anaerobically only by P. fluorescens I-C
(Fig. 2b). The apparent first-order rate for RDX disappearance was about 14-fold higher for P. fluorescens I-C as
compared to P. putida II-B (0.0084/h vs. 0.0006/h) at an
initial RDX concentration of 31 μM. The RDX transfor-
Fig. 2 Degradation of a RDX, b HMX (in the presence of RDX), and
c CL-20 by pure cultures of Pseudomonas spp. under anaerobic
conditions. Sterile control (triangles); P. putida II-B (circles); P.
fluorescens I-C (squares). Data points for live cultures represent
average of two replicate vials; a single sterile control vial was used
during RDX and HMX experiments. Error bars represent one standard
deviation of the mean. Note difference in y-axis scales
539
mation rates of P. putida II-B and P. fluorescens I-C
appeared to be concentration dependent. The transformation
rate decreased 3- and 10-fold for P. putida II-B and
P. fluorescens I-C, respectively, as the initial RDX
concentration increased from 3 to 126 μM. However, the
aerobic growth of these two strains was not affected by the
presence of RDX even at 153 μM. When incubated under
conditions in which an initially aerobic medium was
allowed to become O2-depleted during the growth of the
culture, both P. putida II-B and P. fluorescens I-C degraded
RDX, but only P. fluorescens I-C degraded HMX.
Transformation of CL-20 was observed by pure cultures
under anaerobic conditions, with P. fluorescens I-C degrading the compound much faster than P. putida II-B (Fig. 2c).
These findings suggested that O2 either inhibited the
expression or the activity of the catalytic enzymes in these
strains.
RDX transformation by purified XenA and XenB enzymes In
order to assure that results of the whole cell degradation
assays could be attributed to specific enzyme activities,
experiments using purified XenA and XenB were conducted. Initial studies indicated that RDX was not degraded
via a direct reduction of the nitro group (i.e., no nitrosocontaining products were detected by HPLC), so a more
detailed analysis of the transformation products was
performed (Table 1). The product distribution resulting
from RDX degradation differed not only between the XenA
and XenB, but also between the purified enzymes and their
source organisms. With both purified enzymes, the major
products that accumulated indicated that RDX was degraded via the methylenedinitramine (MEDINA) pathway
(Fig. 1, Anaerobic II pathway), yet MEDINA did not
accumulate and was not detected in whole cell incubations.
Formaldehyde was a major product of RDX metabolism by
purified XenA and by XenB, whether assays were
performed with pure enzymes or in whole cells. The carbon
mass balances for the degradation of RDX by the enzymes
and whole cells ranged from 60% to 100% (mole C basis).
With purified XenA, production of trace amounts of 4nitro-2,4-diazabutanal (NDAB) and MNX suggested that
minor reactions occurred with this enzyme that did not
occur with XenB. However, detection of MNX was not
observed using the modified EPA Method 8330 and is
therefore not believed to be produced from RDX by these
enzymes. Indeed, the [14C]-RDX/MNX experiment gave no
evidence that XenA or XenB produced MNX during the
breakdown of RDX. All of the RDX-derived radioactivity
was contained in a broad peak that eluted well before the
unlabelled MNX peak. Furthermore, the rate of degradation
of RDX by XenB is ∼10-fold faster than the degradation of
MNX, which would have been expected to result in a
buildup of [14C]MNX from [14C]RDX. This experiment,
540
Appl Microbiol Biotechnol (2009) 84:535–544
Table 1 Product distribution during degradation of RDX by purified XenA and XenB enzymes (average of duplicate assays) and by whole cells
of P. putida II-B and P. fluorescens I-C (single replicates)
Assay
P. fluorescens I-C
P. putida II-B
XenB
XenA
RDX (µmol)
Products (µmol)
Mass balance (%)
Initial
Residual
MNX
MEDINA
NDAB
HCHO
C
N
21.5
21.5
69.1
69.1
15.0
11.7
29.2
28.9
0.0
0.0
0.0
0.2
1.4
0.4
37.4
23.5
0.0
0.3
0.0
1.5
13.2
2.7
82.2
76.1
92
60
100
98
74
56
78
66
Samples were collected for analysis after about half the initial RDX had been degraded (corresponding to approximately 100 and 400 h for whole
cell assays with P. fluorescens I-C and P. putida II-B, respectively, and 4 and 24 h for XenB and XenA, respectively)
therefore, showed conclusively that MNX was not a typical
product of RDX breakdown by XenB.
Nitrogen mass balances ranged from 56% to 78%. As
shown in Fig. 1 (Anaerobic II pathway), RDX can be
converted to MEDINA and bis(hydroxymethyl)nitramine,
and these compounds decay to form formaldehyde and
nitramide, the latter of which may further break down to
form nitrous oxide and nitrogen gas (Hawari 2004).
Therefore, measurement of these inorganic nitrogenous
products was performed, and percentages were calculated
on the basis of the nitrogen present in the amount of RDX
degraded during a given experiment. Nitrous oxide was not
detected during RDX degradation with the purified
enzymes, but small amounts of nitrous oxide (1 to 2 mol
%) were detected during whole cell assays. Nitrite was
detected during RDX degradation by purified XenB at a
level of ∼17 mol%. Nitrite was detected in whole cell
assays with P. putida II-B and P. fluorescens I-C at levels
∼2 and ∼12 mol%, respectively. Ammonia was detected
during transformation of RDX by whole cells at levels
equal to ∼15 mol% and during degradation of RDX by
XenB (∼23 mol%). However, the possibility that the
assay was actually detecting one or more of the possible
RDX breakdown products (e.g., nitramide) rather than
ammonia could not be ruled out. Inclusion of these
inorganic nitrogenous products increased the nitrogen
mass balances of the products produced during RDX
transformation by P. putida II-B and P. fluorescens I-C to
75% and 102%, respectively (compared to 56% and 74%
based on only the organic products with nitrogen are
considered; Table 1). Similarly, the overall nitrogen mass
balance for RDX degradation by XenB was increased to
118% when both organic and inorganic nitrogenous
products are considered.
Because HMX is a common contaminant of RDX
preparations and because the two compounds are often
found together in the environment, we investigated
whether HMX was degraded sequentially or consecutively
with RDX. With purified XenB, little to no HMX (present
at approximately 10% the RDX concentration) was
degraded in the presence of high concentrations of RDX.
However, when RDX and HMX were present at approximately equal concentrations, HMX and RDX were
degraded simultaneously by XenB. Similarly, in a mixture
of RDX and the common breakdown products MNX,
DNX, and TNX, XenB degraded all the compounds
simultaneously.
Additional experiments were performed to determine the
O2 inhibition characteristics for RDX and HMX transformation. While XenB degraded RDX much faster than
XenA (∼30-fold), both enzyme systems had similar O2
inhibition characteristics (Fig. 3a and b). A similar effect
was noted when HMX served as a substrate for XenB
(Fig. 3c). The percentage of saturation for O2 that resulted
in a 50% reduction in the initial linear degradation rates
(derived from Fig. 3) were 1.5±0.3% and 1.6±0.3% for
RDX degradation by XenA and XenB, respectively, and
2.3±0.4% for HMX degradation by XenB.
Aerobic and anaerobic transformation of various explosives
by XenA and XenB A screening-level study of purified
XenA and XenB was performed to assess their ability to
degrade a range of nitroaromatic and nitramine explosive
compounds under aerobic and anaerobic conditions. With
XenB, ten of the compounds were degraded aerobically and
14 were degraded anaerobically (Table 2). The rates of
degradation of the other compounds relative to the
degradation rate of TNT varied considerably. The rate of
aerobic TNT degradation by the XenB enzyme was
0.155 μmol/mg protein/min, which is within a factor of
four of the specific activity based on previously published
data (Pak et al. 2000). The degradation rate observed for
TNB and tetryl were greater than for TNT under both aerobic
and anaerobic conditions, while all the other nitroaromatic
compounds were degraded slower than TNT. Among the
nitramine compounds, the relative activity of XenB against
Appl Microbiol Biotechnol (2009) 84:535–544
541
and XenB purified enzymes degraded the relatively new
explosive compound CL-20.
Discussion
Fig. 3 Degradation of RDX by purified a XenA and b XenB enzymes
and c HMX by purified XenB enzyme under different initial oxygen
concentrations. The initial concentrations of RDX and HMX were 55,
83, and 9 μM in a, b, and c, respectively. Control contained ambient
oxygen concentration (∼20%). Each line represents data from two
duplicate vials that were alternatingly sampled during the course of the
experiment; therefore, no error bars were calculated. Note difference
in x-axis scales
RDX was the highest and that of HMX was the lowest, in both
the presence and absence of oxygen.
Of the 16 compounds tested by an endpoint assay with
XenA (tetryl was not tested with XenA), six were degraded
by XenA under aerobic conditions, and nine were degraded
anaerobically (data not shown). RDX, TNT, 2,4-DNT,
1,3-DNB, and TNB were degraded by XenA under both
aerobic and anaerobic conditions. The RDX breakdown
products MNX, DNX, and TNX, as well as HMX, were
only degraded under anaerobic conditions by XenA (within
the timeframe of the assays). Additionally, both the XenA
Only a single previous report has described the aerobic
transformation of the nitramine explosive RDX by a
Pseudomonas sp., though the enzymes involved and the
degradation pathway were not discussed (Chang et al.
2004). In our study, RDX and HMX transformation by two
Pseudomonas sp. strains occurred under strictly anaerobic
conditions, as well as under “anoxic” conditions created as
cells consumed dissolved O2 while growing on succinate.
The biodegradation pathway described herein for RDX by
purified xenobiotic reductases and whole cells of P. putida
II-B and P. fluorescens I-C leads to more labile products
(formaldehyde) and less toxic (nitrous oxide) products,
rather than the more toxic nitrosolated compounds like
those produced during other anaerobic processes (Adrian
and Sutherland 1999; Zhang and Hughes 2003) or dead-end
products like NDAB that is produced during aerobic
transformation by some Rhodococcus spp. (Fig. 1, Aerobic
pathway). These products do not persist in the environment
and thus are a more desirable end point for bioremediation
applications.
Unlike previously described Rhodococcus spp. (Coleman
et al. 1998; Nejidat et al. 2008), RDX transformation by pure
cultures in this study was not inhibited, but rather was
facilitated, by the presence of utilizable nitrogen (NH4+).
Transformation rates by whole cells decreased with increasing RDX concentrations, whereas the RDX transformation
rate from purified XenB increased with increasing RDX
concentration. Additionally, the aerobic growth rates of
P. putida II-B and P. fluorescens I-C were not inhibited with
increasing RDX concentration. Taken together, these results
suggest that although RDX itself is not toxic to either the
cells or the degradative enzymes described here, the
breakdown products may exert toxicity by an unknown
mechanism. This finding is in general agreement with
previous results showing toxicity in another pseudomonad
during aerobic transformation of RDX (Chang et al. 2004).
Previous studies with xenobiotic reductases (and related
enzymes) have shown that the presence of O2 can impact
the transformation of explosive compounds in more than
one way. For example, Pak et al. (2000) noted that while
TNT was degraded by XenB both aerobically and anaerobically, the presence of O2 changed the product distribution.
Most notably, certain TNT dimers accumulated, resulting in
the release of nitrite only in the presence of O2 (or other
oxidants such as NADP+) via an abiotic mechanism. In
another study investigating degradation of RDX by three
Enterobacteriaceae isolates, O2 also played a key role in
542
Appl Microbiol Biotechnol (2009) 84:535–544
Table 2 Degradation of nitroaromatic and nitramine explosive compounds by xenobiotic reductase XenB under aerobic and anaerobic conditions
Compound
Nitroaromatics
2,4,6-Trinitrotoluene
2-Nitrotoluene
3-Nitrotoluene
4-Nitrotoluene
2,4-Dinitrotoluene
2,6-Dinitrotoluene
2-Amino-4,6-dinitrotoluene
4-Amino-2,6-dinitrotoluene
Nitrobenzene
1,3-Dinitrobenzene
1,3,5-Trinitrobenzene
N-methyl-N,2,4,6-tetranitroaniline
Nitramines
Hexahydro-1,3,5-trinitro-1,3,5-triazine
Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
Hexahydro-1,3,5-trinitroso-1,3,5-triazine
Hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine
Hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine
Abbreviation
TNT
2-NT
3-NT
4-NT
2,4-DNT
2,6-DNT
2A-4,6-DNT
4A-2,6-DNT
NB
1,3-DNB
TNB
tetryl
RDX
HMX
TNX
DNX
MNX
Initial concentration (µM)
Relative activity (%)
Aerobic
Aerobic
Anaerobic
Anaerobic
36
9
10
8
37
8
9
8
100
0
0
0
385
0
0
0
60
25
96
162
83
49
50
5
57
27
94
165
80
48
50
13
3.7
0.2
0
24
0
17
426
1454
15
4.4
24
36
0.2
24
2243
1520
47
22
49
41
29
47
21
50
41
27
1.5
0
0.4
0
0.1
21
1.6
6.6
3.7
2.1
Rates are presented relative to the rate obtained for TNT under aerobic conditions (0.155 µmol TNT degraded/mg XenB protein/min)
the final outcome, as RDX was degraded only under
oxygen-depleted conditions (Kitts et al. 1994). Similarly,
it was reported that RDX degradation by Klebsiella
pneumoniae strain SCZ-1 was completely quenched by
the presence of O2, though the concentrations of O2
required to stop RDX degradation was not reported (Zhao
et al. 2002).
In the present study, O2 had a large impact on the
activity of XenA and XenB. Several compounds that were
not degraded (or degraded very slowly) aerobically were
degraded under anaerobic conditions, and the rates of
degradation observed during kinetic assays with XenB
were always higher under anaerobic compared to aerobic
conditions. RDX and HMX were among the compounds for
which this was observed and studied in more detail. It is
important to note that O2 did not function as a binary on/off
switch for the transformation of RDX and HMX, but rather
it caused a gradual decrease in RDX transformation as a
function of the initial O2 concentration (Fig. 3). The
mechanism by which oxygen interferes with the degradation of explosives has not yet been conclusively determined. Hydrogen peroxide was produced when XenB was
incubated in the presence of NADPH and oxygen, but it
was not observed in the absence of oxygen (data not
presented). This would seem to indicate that oxygen is
reduced by XenB in the absence of a more preferred
substrate. Therefore, the concentration of oxygen would
compete with explosive compounds for the active site of
XenB and affect the degradation rates.
These results greatly expand the known substrate range
of both XenA and XenB. Although only screening-level
data were obtained and more mechanistic studies are
warranted, some observations and possible explanations
for the observed differences in degradation rates are
presented here. Degradation rates increased as the degree
of nitro-substitution increased for both the toluene (TNT >
DNTs >> NTs) and the benzene (TNB > DNB >> NB)
series under both aerobic and anaerobic conditions. Lack of
activity of XenB against the mononitrotoluenes (2-, 3-, and
4-NT) was previously reported (Pak et al. 2000). It is
interesting to note that the relative activity of XenB against
2,4-DNT was higher compared to 2,6-DNT under both
aerobic and anaerobic conditions. These results also
comport with the work of Pak et al. (2000), as well as
previous literature reports indicating that the 2,4 isomer is
generally more labile than the 2,6 isomer of DNT (Nishino
et al., 1999; Nishino et al. 2000).
Of the nitramine explosives, RDX was degraded the
fastest, followed by the nitroso-amines (in the order
mononitroso > dinitroso > trinitroso) and then HMX. The
decreasing degradation rate with increasing number of
nitroso moieties seems to be counter-intuitive with respect
Appl Microbiol Biotechnol (2009) 84:535–544
to steric controls on the rate since the nitroso groups (–NO)
would be expected to be less bulky than the nitro groups
(–NO2), allowing the enzyme easier access. The observed
rates may reflect the inherent specificity of some part of the
XenB enzyme peripheral to the active site with preferences
for nitro groups over nitroso groups. However, there must
also be enough flexibility in these areas of the enzyme to
allow some recognition of the nitroso moiety or TNX
would not be expected to degrade at all. The difference in
rates between RDX and HMX likely reflects the differences
in the conformation of the heterocyclic rings, which affect
the interaction of the active site of the enzyme with the
molecule.
This work also adds to the information base for the new
energetic compound CL-20. CL-20 was developed as more
powerful and less sensitive replacement for RDX and HMX
(Trott et al. 2003). In general, CL-20 has been found to be
more labile than RDX in soil (Balakrishnan et al. 2004;
Crocker et al. 2005). Our studies have demonstrated anaerobic
transformation of CL-20 by purified xenobiotic reductase
enzymes and whole cells. This new information adds to the
few previously published reports, which demonstrated the
transformation of CL-20 by monooxygenases (Bhushan et al.
2004b), nitroreductases (Bhushan et al. 2004a), and
membrane-associated flavoenzymes (Bhushan et al. 2003).
The addition of RDX, HMX, and CL-20 to the list of
known substrates for the xenobiotic reductases, under
reduced O2 tension, has important implications for bioremediation efforts, assuming that ability to degrade these
explosives is a common characteristic among xenobiotic
reductases. Basic research in environmental microbiology is
often directed toward the isolation and characterization of
bacterial strains that use a target compound as a sole source
nutrient (for carbon, nitrogen, or energy). However, actual
field-scale bioremediation is dominated by biostimulation
approaches (i.e., addition of nutrients to stimulate the
indigenous microbial community) rather than the addition
of specific strains, which derive nutrients from a pollutant.
The results reported here support the practice of general
biostimulation approaches to effect remediation of
explosives-contaminated sites as follows: (1) Transformation occurs under a relatively broad range of O2 concentrations (anoxic to anaerobic); (2) transformation is not
inhibited by the presence of utilizable nitrogen; and (3)
transformation is performed by a class of enzyme that is
widespread among bacterial genera. Several studies in our
laboratory examining the microbial ecology of RDX
biodegradation have detected Pseudomonas spp. 16S rDNA
sequences in RDX-degrading enrichments were derived
from groundwater from an explosives manufacturing site
(unpublished data). Furthermore, given the widespread
distribution of Pseudomonas spp. in the environment, it is
likely that these organisms play a larger role in the
543
transformation of nitramine explosives than previously
thought, which could be further expanded when environmental conditions are manipulated to maximize their
degradative potential.
Acknowledgments The investigators acknowledge and thank the
Strategic Environmental Research and Development Program (Mark
Fuller, P.I., Project ER-1378) and NSF MCB-0316232 (Brian Fox.,
P.I.) for support of this research. Views, opinions, and/or findings
contained herein are those of the authors and should not be construed
as an official department of the army position or decision unless so
designated by other official documentation. Thomas E. Malone was a
trainee of the NIH Institutional Biotechnology Pre-Doctoral Training
Grant T32 GM08349.
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