Inhibition of the lignin peroxidase of Phanerochaete chrysosporium by hydroxylamino-dinitrotoluene, an early intermediate in the degradation of 2,4,6-trinitrotoluene

Linking Enzymatic Oxidative Degradation of Lignin to Organics Detoxification

The major enzymes involved in lignin degradation are laccase, class II peroxidases (lignin peroxidase, manganese peroxidase, and versatile peroxidase) and dye peroxidase, which use an oxidative or peroxidative mechanism to deconstruct the complex and recalcitrant lignin. Laccase and manganese peroxidase directly oxidize phenolic lignin components, while lignin peroxidase and versatile peroxidase can act on the more recalcitrant non-phenolic lignin compounds. Mediators or co-oxidants not only increase the catalytic ability of these enzymes, but also largely expand their substrate scope to those with higher redox potential or more complicated structures. Neither laccase nor the peroxidases are stringently selective of substrates. The promiscuous nature in substrate preference can be employed in detoxification of a range of organics.

Uptake and effects of 2, 4, 6 - trinitrotoluene (TNT) in juvenile Atlantic salmon (Salmo salar)

Organ specific uptake and depuration, and biological effects in Atlantic salmon (Salmo salar) exposed to 2, 4, 6-trinitrotoluene (TNT) were studied. Two experiments were conducted, the first using radiolabeled TNT (¹⁴C-TNT, 0.16mg/L) to study uptake (48h) and depuration (48h), while the second experiment focused on physiological effects in fish exposed to increasing concentrations of unlabeled TNT (1μg-1mg/L) for 48h. The uptake of ¹⁴C-TNT in the gills and most of the organs increased rapidly during the first 6h of exposure (12h in the brain) followed by a rapid decrease even though the fish were still exposed to TNT in the water. The radioactivity in the gall bladder reached a maximum after 55h, 7h after the transfer to the clean water. A high concentration of ¹⁴C-TNT in the gall bladder indicates that TNT is excreted through the gall bladder. Mortality (2 out of 14) was observed at a concentration of 1mg/L, and the surviving fish had hemorrhages in the dorsal muscle tissue near the spine. Analysis of the physiological parameters in blood from the high exposure group revealed severe effects, with an increase in the levels of glucose, urea and HCO₃, and a decrease in hematocrit and the levels of Cl and hemoglobin. No effects on blood physiology were observed in fish exposed to the lower concentrations of TNT (1-100μg/L). TNT and the metabolites 2-amino-4,6-dinitrotoluene (2-ADNT) and 4-amino-2,6-dinitrotoluene (4-ADNT) were found in the muscle tissue, whereas only 2-ADNT and 4-ADNT were found in the bile. The rapid excretion and estimated bioconcentration factors (range of 2-18 after 48h in gills, blood, liver, kidney, muscle and brain) indicated a low potential for bioaccumulation of TNT.

Biotransformation of trinitrotoluene (TNT) by Pseudomonas spp. isolated from a TNT-contaminated environment

The compound 2,4,6-trinitrotoluene (TNT) is a secondary explosive widely used worldwide for both military and civil purposes. As a result, residual TNT has been detected as an environmental pollutant in both soil and groundwater. The authors have isolated several microbial strains from soil contaminated with TNT by enrichment culture techniques using TNT as a carbon, nitrogen, and energy source. The contaminated soil contained approximately 1860 ppm TNT measured by high-performance liquid chromatography (HPLC). The initial identification of these isolates was determined by 16S rRNA gene comparison. The isolates mainly included species belonging to the genus Pseudomonas. Two strains (Pseudomonas putida strain TP1 and Pseudomonas aeruginosa strain TP6) were selected for further examination. Both strains demonstrated the ability to grow on the medium containing TNT as a carbon, energy, and nitrogen source and also clearly demonstrated the ability to degrade TNT. More than 90% of the TNT in the growth medium was degraded by both strains after 22 d incubation, as determined by HPLC. Additionally, the resting cells of P. putida TP1 and P. aeruginosa TP6 both significantly displayed the ability to transform (metabolize) TNT.

Soils contaminated with explosives: Environmental fate and evaluation of state-of-the-art remediation processes (IUPAC Technical Report)

An explosion occurs when a large amount of energy is suddenly released. This energy may come from an over-pressurized steam boiler, from the products of a chemical reaction involving explosive materials, or from a nuclear reaction that is uncontrolled. In order for an explosion to occur, there must be a local accumulation of energy at the site of the explosion, which is suddenly released. This release of energy can be dissipated as blast waves, propulsion of debris, or by the emission of thermal and ionizing radiation. Modern explosives or energetic materials are nitrogen-containing organic compounds with the potential for self-oxidation to small gaseous molecules (N2, H2O, and CO2). Explosives are classified as primary or secondary based on their susceptibility of initiation. Primary explosives are highly susceptible to initiation and are often used to ignite secondary explosives, such as TNT (2,4,6-trinitrotoluene), RDX (1,3,5-trinitroperhydro-1,3,5-triazine), HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane), and tetryl (N-methyl-N-2,4,6-tetranitro-aniline).

Microbial transformation of 2,4,6-trinitrotoluene in aerobic soil columns

2,4,6-Trinitrotoluene (TNT)-contaminated soil material of a former TNT production plant was percolated aerobically in soil columns. Nineteen days of percolation with a potassium phosphate buffer supplemented with glucose or glucose plus ammonium sulfate caused an over 90% decline in the amount of extractable nitroaromatics in soils containing 70 to 2,100 mg of TNT per kg (dry weight). In the percolation solution, a complete elimination of TNT was achieved. Mutagenicity and soil toxicity were significantly reduced by the percolation process. 4-N-Acetylamino-2-amino-6-nitrotoluene was generated in soil and percolation fluid as a labile TNT metabolite.

Transformation of 2,4,6-Trinitrotoluene by Pseudomonas pseudoalcaligenes JS52

Pseudomonas pseudoalcaligenes JS52 grows on nitrobenzene via partial reduction of the nitro group and enzymatic rearrangement of the resultant hydroxylamine. Cells and cell extracts of nitrobenzene-grown JS52 catalyzed the transient formation of 4-hydroxylamino-2,6-dinitrotoluene (4HADNT), 4-amino-2,6-dinitrotoluene (4ADNT), and four previously unidentified metabolites from 2,4,6-trinitrotoluene (TNT). Two of the novel metabolites were identified by liquid chromatography/mass spectrometry and (sup1)H-nuclear magnetic resonance spectroscopy as 2,4-dihydroxylamino-6-nitrotoluene (DHANT) and 2-hydroxylamino-4-amino-6-nitrotoluene (2HA4ANT). A polar yellow metabolite also accumulated during transformation of TNT by cells and cell extracts. Under anaerobic conditions, extracts of strain JS52 did not catalyze the production of the yellow metabolite or release nitrite from TNT; moreover, DHANT and 2HA4ANT accumulated under anaerobic conditions, which indicated that their further metabolism was oxygen dependent. Small amounts of nitrite were released during transformation of TNT by strain JS52. Sustained transformation of TNT by cells required nitrobenzene, which indicated that TNT transformation does not provide energy. Transformation of TNT catalyzed by enzymes in cell extracts required NADPH. Transformation experiments with (sup14)C-TNT indicated that TNT was not mineralized; however, carbon derived from TNT became associated with cells. Nitrobenzene nitroreductase purified from strain JS52 transformed TNT to DHANT via 4HADNT, which indicated that the nitroreductase could catalyze the first two steps in the transformation of TNT. The unusual ability of the nitrobenzene nitroreductase to catalyze the stoichiometric reduction of aromatic nitro compounds to the corresponding hydroxylamine provides the basis for the novel pathway for metabolism of TNT.

Structure of a laccase-mediated product of coupling of 2,4-diamino-6-nitrotoluene to guaiacol, a model for coupling of 2,4,6-trinitrotoluene metabolites to a humic organic soil matrix

This work presents laccase-mediated model reactions for coupling of reduced 2,4,6-trinitrotoluene (TNT) metabolites to an organic soil matrix. The structure of an isolated coupling product of 2,4-diamino-6-nitrotoluene (2,4-DANT) to guaiacol as humic constituent was determined. Among several structures, the compound was identified conclusively to be the trinuclear coupling product 5-(2-amino-3-methyl-4-nitroanilino)-3,3(prm1)-dimethoxy-4,4(prm1)-diphenoqu inone. The compound has a weight of 409 g mol(sup-1) and may serve as a model reaction for the biogenic formation of bound residues in soil from TNT by coupling aminotoluenes (reduced TNT metabolites) to humic constituents. A linear correlation of the substrate consumption to the enzyme activity was detected. Based on this observation, the described reaction of 2,4-DANT coupling to guaiacol may be used for determination of laccase activity since the reaction was not inhibited by other compounds of culture supernatants. We propose a two-step mechanism for the coupling reaction because 2,4-DANT was not transformed by laccases in the absence of guaiacol and guaiacol oxidation was independent of the presence of 2,4-DANT. The first reaction step is a laccase-mediated dimerization of two guaiacol monomers with subsequent oxidation to a diphenoquinone. The second step is the nucleophilic addition of 2,4-DANT to the ortho position of the carbonyl group of the diphenoquinone structure.

New and classic families of secreted fungal heme peroxidases

Heme-containing peroxidases secreted by fungi are a fascinating group of biocatalysts with various ecological and biotechnological implications. For example, they are involved in the biodegradation of lignocelluloses and lignins and participate in the bioconversion of other diverse recalcitrant compounds as well as in the natural turnover of humic substances and organohalogens. The current review focuses on the most recently discovered and novel types of heme-dependent peroxidases, aromatic peroxygenases (APOs), and dye-decolorizing peroxidases (DyPs), which catalyze remarkable reactions such as peroxide-driven oxygen transfer and cleavage of anthraquinone derivatives, respectively, and represent own separate peroxidase superfamilies. Furthermore, several aspects of the "classic" fungal heme-containing peroxidases, i.e., lignin, manganese, and versatile peroxidases (LiP, MnP, and VP), phenol-oxidizing peroxidases as well as chloroperoxidase (CPO), are discussed against the background of recent scientific developments.

Denitration of 2,4,6-trinitrotoluene in aqueous solutions using small-molecular-weight catalyst(s) secreted by Pseudomonas aeruginosa ESA-5

The denitration of 2,4,6-trinitrotoluene (TNT) can produce mono- or dinitro aromatic compounds susceptible to microbial mineralization. In the present study, denitration of TNT and other nitro aromatic compounds was investigated with a solid-phase extract obtained from the culture supernatant of Pseudomonas aeruginosa ESA-5 grown on a chemically defined aerobic medium. When the C18 solid-phase extract containing extracellular catalysts (EC) was incubated with TNT and NAD(P)H, we observed a significant release of nitrite. The concentration of nitrite released in the reaction medium was strongly dependent on the concentration of NAD(P)H and EC. Denitration also occurred with two TNT-related molecules, 2,4,6-trinitrobenzaldehyde, and 2,4,6-trinitrobenzyl alcohol. The release of nitrite was coupled with the formation of two polar metabolites, and mass spectrometry analyses indicated that each of these compounds had lost two nitro groups from the trinitro aromatic parent molecule. During this process, the production of toxic reduced TNT metabolites was minimal. The incubation of EC with TNT, NAD(P)H, and specific scavengers of reactive oxygen species suggested the involvement of superoxide radicals (O2*-) and hydrogen peroxide in the denitration process. Results obtained in this study reveal for the first time that extracellular small-molecular-weight substance(s) of bacterial origin can serve as green catalyst(s) to initiate TNT denitration. In addition, this study gives clear evidence for the production of a TNT metabolite bearing a single nitro groupfollowing a denitration reaction with catalyst(s) of biotic origin.

Bioremediation of 2,4,6-trinitrotoluene by bacterial nitroreductase expressing transgenic aspen

Trees belonging to the genus Populus are often used for phytoremediation due to their deep root formation, fast growth and high transpiration rates. Here, we study the capacity of transgenic hybrid aspen (Populus tremula x tremuloides var. Etropole) which expresses the bacterial nitroreductase gene, pnrA, to tolerate and take-up greater amounts of the toxic and recalcitrant explosive, 2,4,6-trinitrotoluene (TNT) from contaminated waters and soil. Transgenic aspen tolerate up to 57 mg TNT/L in hydroponic media and more than 1000 mg TNT/ kg soil, whereas the parental aspen could not endure in hydroponic culture with more than 11 mg TNT/L or soil with more than 500 mg TNT/kg. Likewise, the phytotoxicological limit for transgenic plants to a constant concentration of TNT was 20 mg TNT/L while wild-type plants only tolerated 10 mg TNT/L. Transgenic plants also showed improved uptake of TNT over wild-type plants when the original TNT concentration was above 35 mg TNT/L in liquid media or 750 mg TNT/kg in soil. Assays with 13C-labeled TNT show rapid adsorption of TNT to the root surface followed by a slower entrance rate into the plant. Most of the 13C-carbon from the labeled TNT taken up bythe plant (> 95%) remains in the root with little translocation to the stem. Altogether, transgenic aspen expressing pnrA are highly interesting for phytoremediation applications on contaminated soil and underground aquifers.

Participation of oxygen in the bacterial transformation of 2,4,6-trinitrotoluene

The exposure of Bacillus cereus ZS18 cell suspensions to 2,4,6-trinitrotoluene (TNT) in the absence of other oxidizable substrates increases oxygen uptake, exceeding the basal level of respiration of the bacterium 1.5- and 2-fold with 50 and 100 mg/liter of TNT, respectively. The interaction of both living and to less extent dead bacterial cells with TNT results in the accumulation of superoxide anion (O2*-) in the extracellular medium, which was revealed by the EPR spectroscopy. The accumulation of O2*- decreased by 50-70% in the presence of Cu,Zn-superoxide dismutase of animal origin. In the presence of living bacterial cells, the level of TNT decreased progressively, yielding hydroxylaminodinitrotoluenes together with O2*-. In the presence of heat-killed cells, a moderate decrease in TNT was observed, and the appearance of O2*- was not accompanied by the production of any detectable TNT metabolites. Chelating agents inhibited the transformation of TNT and decreased the formation of O2*-. The demonstrated generation of O2*- during the interaction of TNT with K4[Fe(CN)6] together with the observed effects of chelating agents suggest the participation of iron in the one-electron reduction of TNT and the functioning of an extracellular redox cycle with the involvement of molecular oxygen.

Removal characteristics of endocrine-disrupting chemicals by laccase from white-rot fungi

Laccase from 5 white-rot fungal strains (4 Trametes and 1 Pycnoporus strains) were evaluated in the removal spectra with/without mediators against 11 EDCs. Purified laccase from Trametes sp. was also used to reveal the precise degradation spectra and degradation profiles in time course against 20 EDCs with/without mediators. In addition, effectivity of laccase for the purification of complex EDCs contamination was evaluated combining several EDCs. The removal characteristics among tested strains were almost the same and crude/purified laccase could remove various EDCs. Bis(4-hydroxyphenyl)sulfone, diethylhexylphthalate (DEHP), pyrene (PY), anthracene, 3,5-dichlorophenol and pentachlorophenol could not be removed by laccase. DEHP and PY could not be removed even with mediators. Vanillin and vanillic acid revealed to be possible naturally occurring mediators. Laccase-mediator system could expand the degradation spectrum and enhance the EDCs removal ratio and rate. When complex mixtures of EDCs were treated with laccase, the removal ratio was enhanced in comparison to that of single application. Some coexisting EDCs could act as mediators. Thus, the availability of laccase and the effectivity of a mediator on EDCs treatment were indicated.

Production of eight different hydride complexes and nitrite release from 2,4,6-trinitrotoluene by Yarrowia lipolytica

2,4,6-Trinitrotoluene (TNT) transformation by the yeast strain Yarrowia lipolytica AN-L15 was shown to occur via two different pathways. Direct aromatic ring reduction was the predominant mechanism of TNT transformation, while nitro group reduction was observed to be a minor pathway. Although growth of Y. lipolytica AN-L15 was inhibited initially in the presence of TNT, TNT transformation was observed, indicating that the enzymes necessary for TNT reduction were present initially. Aromatic ring reduction resulted in the transient accumulation of eight different TNT-hydride complexes, which were characterized using high-performance liquid chromatography, UV-visible diode array detection, and negative-mode atmospheric pressure chemical ionization mass spectrometry (APCI-MS). APCI-MS analysis revealed three different groups of TNT-hydride complexes with molecular ions at m/z 227, 228, and 230, which correspond to TNT-mono- and dihydride complexes and protonated dihydride isomers, respectively. One of the three protonated dihydride complex isomers detected appears to release nitrite in the presence of strain AN-L15. This release of nitrite is of particular interest since it can provide a pathway towards complete degradation and detoxification of TNT.

Microbial remediation of nitro-aromatic compounds: an overview

Nitro-aromatic compounds are produced by incomplete combustion of fossil fuel or nitration reactions and are used as chemical feedstock for synthesis of explosives, pesticides, herbicides, dyes, pharmaceuticals, etc. The indiscriminate use of nitro-aromatics in the past due to wide applications has resulted in inexorable environmental pollution. Hence, nitro-aromatics are recognized as recalcitrant and given Hazardous Rating-3. Although several conventional pump and treat clean up methods are currently in use for the removal of nitro-aromatics, none has proved to be sustainable. Recently, remediation by biological systems has attracted worldwide attention to decontaminate nitro-aromatics polluted sources. The incredible versatility inherited in microbes has rendered these compounds as a part of the biogeochemical cycle. Several microbes catalyze mineralization and/or non-specific transformation of nitro-aromatics either by aerobic or anaerobic processes. Aerobic degradation of nitro-aromatics applies mainly to mono-, dinitro-derivatives and to some extent to poly-nitro-aromatics through oxygenation by: (i) monooxygenase, (ii) dioxygenase catalyzed reactions, (iii) Meisenheimer complex formation, and (iv) partial reduction of aromatic ring. Under anaerobic conditions, nitro-aromatics are reduced to amino-aromatics to facilitate complete mineralization. The nitro-aromatic explosives from contaminated sediments are effectively degraded at field scale using in situ bioremediation strategies, while ex situ techniques using whole cell/enzyme(s) immobilized on a suitable matrix/support are gaining acceptance for decontamination of nitrophenolic pesticides from soils at high chemical loading rates. Presently, the qualitative and quantitative performance of biological approaches of remediation is undergoing improvement due to: (i) knowledge of catabolic pathways of degradation, (ii) optimization of various parameters for accelerated degradation, and (iii) design of microbe(s) through molecular biology tools, capable of detoxifying nitro-aromatic pollutants. Among them, degradative plasmids have provided a major handle in construction of recombinant strains. Although recombinants designed for high performance seem to provide a ray of hope, their true assessment under field conditions is required to address ecological considerations for sustainable bioremediation.

Explosives: fate, dynamics, and ecological impact in terrestrial and marine environments

An explosive or energetic compound is a chemical material that, under the influence of thermal or chemical shock, decomposes rapidly with the evolution of large amounts of heat and gas. Numerous compounds and compositions may be classified as energetic compounds; however, secondary explosives, such as TNT, RDX, and HMX pose the largest potential concern to the environment because they are produced and used in defense in the greatest quantities. The environmental fate and potential hazard of energetic compounds in the environment is affected by a number of physical, chemical, and biological processes. Energetic compounds may undergo transformation through biotic or abiotic degradation. Numerous organisms have been isolated with the ability to degrade/transform energetic compounds as a sole carbon source, sole nitrogen source, or through cometabolic processes under aerobic or anaerobic conditions. Abiotic processes that lead to the transformation of energetic compounds include photolysis, hydrolysis, and reduction. The products of these reactions may be further transformed by microorganisms or may bind to soil/sediment surfaces through covalent binding or polymerization and oligomerization reactions. Although considerable research has been performed on the fate and dynamics of energetic compounds in the environment, data are still gathering on the impact of TNT, RDX, and HMX on ecological receptors. There is an urgent need to address this issue and to direct future research on expanding our knowledge on the ecological impact of energetic transformation products. In addition, it is important that energetic research considers the concept of bioavailability, including factors influencing soil/sediment aging, desorption of energetic compounds from varying soil and sediment types, methods for modeling/predicting energetic bioavailability, development of biomarkers of energetic exposure or effect, and the impact of bioavailability on ecological risk assessment.

A double mutant of Pseudomonas putida JLR11 deficient in the synthesis of the nitroreductase PnrA and assimilatory nitrite reductase NasB is impaired for growth on 2,4,6-trinitrotoluene (TNT)

Pseudomonas putida JLR11 can grow on 2,4,6-trinitrotoluene (TNT) as the sole nitrogen source. We created nasB (nitrite reductase), pnrA (nitroaromatic reductase) and pnrA nasB mutants and tested their growth with TNT as the sole N source. The nasB and pnrA mutants grew at a reduced rate on TNT, whereas the double nasB pnrA mutant did not. This suggests that P. putida JLR11 carries out multiple enzymatic attacks on TNT-releasing nitrite and/or ammonium. The PnrA nitroreductase plays a key role in the reduction of TNT to 2,6-dinitro-4-hydroxylaminotoluene and the subsequent release of ammonium for growth.

Biotransformation of 2,4,6-trinitrotoluene (TNT) by the fungus Fusarium oxysporum

The fungus Fusarium oxysporum was isolated and identified from the aquatic plant M. aquaticum. The capability of this fungus to transform 2,4,6-trinitrotoluene (TNT) in liquid cultures was investigated TNT was added to shake flask cultures and transformed into 2-amino-4,6-dinitrotoluene (2-A-DNT), 4-amino-2,6-dinitrotoluene (4-A-DNT), and 2,4-diamino-6-nitrotoluene (2,4-DAT) via 2- and 4-hydroxylamino-dinitrotoluene derivatives, which could be detected as intermediate metabolites. Transformation of TNT, 2-A-DNT, and 4-A-DNT was observed by whole cultures and with isolated mycelium. Cell-free protein extracts from the extracellular, soluble, and membrane-bound fractions were prepared from this fungus and tested for TNT-reducing activity. The concentrated extracellular culture medium was unable to transform TNT; however, low levels of TNT transformation were observed by the membrane fraction in the presence of nicotinamide adenine dinucleotide phosphate in an argon atmosphere. A concentrated extract of soluble enzymes also transformed TNT, but to a lesser extent. When TNT toxicity was studied with this fungus, a 50% decrease in the growth of F. oxysporum mycelium was observed when exposed to 20 mg/L TNT.

Transformation and mineralization of 2,4,6-trinitrotoluene by the white rot fungus Irpex lacteus

Unlike other 2,4,6-trinitrotoluene (TNT)-degrading white rot fungi, including Phanerochaete chrysoporium, initial metabolism of TNT by Irpex lacteus was found to occur through two different transformation pathways. Metabolites of the nitro group reduction pathway were confirmed with the standard compounds, and the formation of hydride-Meisenheimer complex of TNT (H(-)-TNT) formed in the denitration pathway was identified with LC/MS and by LC/photodiode array (PDA) detection. The molecular weight of the H(-)-TNT complex was identified as 228 m/z, and the UV-visible absorption spectrum, recorded with a PDA detector, proved the identity of this metabolite (RT, 18.7 min; lambda(max) 254, 474, 557 nm) by comparison with the authentic synthetic H(-)-TNT (RT 18.7 min; lambda(max) 261, 474, 563 nm). Mineralization of [U-(14)C]TNT by I. lacteus was also measured in static and shaken cultures. The mineralization rate of TNT in the static culture was higher than that in the shaken culture, and addition of Tween 80 (final concentration 1%) enhanced the mineralization of TNT in the static culture, resulting in 30.57% of CO(2) evolution from the radiolabeled TNT added. The high TNT mineralization capability of I. lacteus seemed to be the result of simultaneous utilization of the denitration pathway, which is more favorable for the ring cleavage and mineralization of TNT, together with the nitro group reduction pathway.

Models of 2,4,6-trinitrotoluene (TNT) initial conversion by yeasts

The original models of the initial steps of 2,4,6-trinitrotoluene (TNT) conversion by yeasts are presented. Saccharomyces sp. ZS-A1 reduced nitro groups of TNT producing isomeric monohydroxylaminodinitrotoluenes (HADNT) as the key initial metabolites (molar ratio HADNT/TNT was up to 0.81), whereas aminodinitrotoluenes (ADNT) and the hydride-Meisenheimer complex of TNT (H-TNT) were the minor products. Conversely, Candida sp. AN-L13 transformed TNT almost quantitatively into H-TNT, thus realizing the alternative attack, consisting of the TNT aromatic ring reduction. The third type of conversion, revealed in Candida sp. AN-L14, is the combination of both above mechanisms and produces an equimolar mix of HADNT and H-TNT. In the toxicity tests with Paramecium caudatum, the supernatant of Saccharomyces sp. ZS-A1, which converts TNT into HADNT, was most toxic while the supernatant of Candida sp. AN-L13 (TNT --> H-TNT) was least toxic. The microorganisms converting TNT quantitatively to the reactive metabolites can be useful for their immobilization through the detoxifying interaction with the soil components such as humic compounds.

UV-VIS spectroscopy of 2,4,6-trinitrotoluene-hydroxide reaction

Contamination of groundwater, surface water and soil by explosives has occurred at military sites throughout the world as a result of manufacture of explosive compounds, assembly of munitions, and deployment of explosives containing devices. Due to the adverse effects of explosives on humans and other natural receptors, a low cost means of decontaminating these areas of contamination is needed. Base-induced transformation of explosives has shown promise as a rapid, low cost, and minimally resource-intensive technology for detoxifying explosives in soil and water. In order to understand the reaction mechanism, a reaction mixture of 2:1:1 (water:2,4,6-trinitrotoluene (TNT):1 N KOH) was analyzed by UV/VIS spectrometry from 190 to 1,100 nm. Time course measurements were conducted at 25, 20, 15, and 12 degrees C. A factor analysis program was used to analyze the spectral data. Principal component analysis indicated that six principal components explained the spectra to within experimental error, with four factors explaining the majority of the variance. Test spectral vectors for four components were developed, including TNT, two intermediates, and the final product, and were tested against the abstract vectors. Two possible reaction mechanisms were suggested and tested to explain the spectral data.

NAD(P)H:flavin mononucleotide oxidoreductase inactivation during 2,4,6-trinitrotoluene reduction

Bacteria readily transform 2,4,6-trinitrotoluene (TNT), a contaminant frequently found at military bases and munitions production facilities, by reduction of the nitro group substituents. In this work, the kinetics of nitroreduction were investigated by using a model nitroreductase, NAD(P)H:flavin mononucleotide (FMN) oxidoreductase. Under mediation by NAD(P)H:FMN oxidoreductase, TNT rapidly reacted with NADH to form 2-hydroxylamino-4,6-dinitrotoluene and 4-hydroxylamino-2,6-dinitrotoluene, whereas 2-amino-4,6-dinitrotoluene and 4-amino-2,6-dinitrotoluene were not produced. Progressive loss of activity was observed during TNT reduction, indicating inactivation of the enzyme during transformation. It is likely that a nitrosodinitrotoluene intermediate reacted with the NAD(P)H:FMN oxidoreductase, leading to enzyme inactivation. A half-maximum constant with respect to NADH, K(N), of 394 microM was measured, indicating possible NADH limitation under typical cellular conditions. A mathematical model that describes the inactivation process and NADH limitation provided a good fit to TNT reduction profiles. This work represents the first step in developing a comprehensive enzyme level understanding of nitroarene biotransformation.

Biological remediation of explosives and related nitroaromatic compounds

Nitroaromatics form an important group of recalcitrant xenobiotics. Only few aromatic compounds, bearing one nitro group as a substituent of the aromatic ring, are produced as secondary metabolites by microorganisms. The majority of nitroaromatic compounds in the biosphere are industrial chemicals such as explosives, dyes, polyurethane foams, herbicides, insecticides and solvents. These compounds are generally recalcitrant to biological treatment and remain in the biosphere, where they constitute a source of pollution due to both toxic and mutagenic effects on humans, fish, algae and microorganisms. However, relatively few microorganisms have been described as being able to use nitroaromatic compounds as nitrogen and/or carbon and energy source. The best-known nitroaromatic compound is the explosive TNT (2,4,6-trinitrotoluene). This article reviews the bioremediation strategies for TNT-contaminated soil and water. It comes to the following conclusion: The optimal remediation strategy for nitroaromatic compounds depends on many site-specific factors. Composting and the use of reactor systems lend themselves to treating soils contaminated with high levels of explosives (e.g. at former ammunition production facilities, where areas with a high contamination level are common). Compared to composting systems, bioreactors have the major advantage of a short treatment time, but the disadvantage of being more labour intensive and more expensive. Studies indicate that biological treatment systems, which are based on the activity of the fungus Phanerochaete chrysosporium or on Pseudomonas sp. ST53, might be used as effective methods for the remediation of highly contaminated soil and water. Phytoremediation, although not widely used now, has the potential to become an important strategy for the remediation of soil and water contaminated with explosives. It is best suited where contaminant levels are low (e.g. at military sites where pollution is rather diffuse) and where larger contaminated surfaces or volumes have to be treated. In addition, phytoremediation can be used as a polishing method after other remediation treatments, such as composting or bioslurry, have taken place. This in-situ treatment method has the advantage of lower treatment costs, but has the disadvantage of a considerable longer treatment time. In order to improve the cost-efficiency, phytoremediation of nitroaromatics (and other organic xenobiotics) could be combined with bio-energy production. This requires, however, detailed knowledge on the fate of the contaminants in the plants as well as the development of efficient treatment methods for the contaminated biomass that minimise the spreading of the contaminants into the environment during post harvest treatment.

Biological degradation of 2,4,6-trinitrotoluene

Nitroaromatic compounds are xenobiotics that have found multiple applications in the synthesis of foams, pharmaceuticals, pesticides, and explosives. These compounds are toxic and recalcitrant and are degraded relatively slowly in the environment by microorganisms. 2,4,6-Trinitrotoluene (TNT) is the most widely used nitroaromatic compound. Certain strains of Pseudomonas and fungi can use TNT as a nitrogen source through the removal of nitrogen as nitrite from TNT under aerobic conditions and the further reduction of the released nitrite to ammonium, which is incorporated into carbon skeletons. Phanerochaete chrysosporium and other fungi mineralize TNT under ligninolytic conditions by converting it into reduced TNT intermediates, which are excreted to the external milieu, where they are substrates for ligninolytic enzymes. Most if not all aerobic microorganisms reduce TNT to the corresponding amino derivatives via the formation of nitroso and hydroxylamine intermediates. Condensation of the latter compounds yields highly recalcitrant azoxytetranitrotoluenes. Anaerobic microorganisms can also degrade TNT through different pathways. One pathway, found in Desulfovibrio and Clostridium, involves reduction of TNT to triaminotoluene; subsequent steps are still not known. Some Clostridium species may reduce TNT to hydroxylaminodinitrotoluenes, which are then further metabolized. Another pathway has been described in Pseudomonas sp. strain JLR11 and involves nitrite release and further reduction to ammonium, with almost 85% of the N-TNT incorporated as organic N in the cells. It was recently reported that in this strain TNT can serve as a final electron acceptor in respiratory chains and that the reduction of TNT is coupled to ATP synthesis. In this review we also discuss a number of biotechnological applications of bacteria and fungi, including slurry reactors, composting, and land farming, to remove TNT from polluted soils. These treatments have been designed to achieve mineralization or reduction of TNT and immobilization of its amino derivatives on humic material. These approaches are highly efficient in removing TNT, and increasing amounts of research into the potential usefulness of phytoremediation, rhizophytoremediation, and transgenic plants with bacterial genes for TNT removal are being done.

Biotransformation of 2,4,6-trinitrotoluene with Phanerochaete chrysosporium in agitated cultures at pH 4.5

The biotransformation of 2,4,6-trinitrotoluene (TNT) (175 microM) by Phanerochaete chrysosporium with molasses and citric acid at pH 4.5 was studied. In less than 2 weeks, TNT disappeared completely, but mineralization (liberated 14CO2) did not exceed 1%. A time study revealed the presence of several intermediates, marked by the initial formation of two monohydroxylaminodinitrotoluenes (2- and 4-HADNT) followed by their successive transformation to several other products, including monoaminodinitrotoluenes (ADNT). A group of nine acylated intermediates were also detected. They included 2-N-acetylamido-4,6-dinitrotoluene and its p isomer, 2-formylamido-4, 6-dinitrotoluene and its p isomer (as acylated ADNT), 4-N-acetylamino-2-amino-6-nitrotoluene and 4-N-formylamido-2-amino-6-nitrotoluene (as acetylated DANT), 4-N-acetylhydroxy-2,6-dinitrotoluene and 4-N-acetoxy-2, 6-dinitrotoluene (as acetylated HADNT), and finally 4-N-acetylamido-2-hydroxylamino-6-nitrotoluene. Furthermore, a fraction of HADNTs were found to rearrange to their corresponding phenolamines (Bamberger rearrangement), while another group dimerized to azoxytoluenes which in turn transformed to azo compounds and eventually to the corresponding hydrazo derivatives. After 30 days, all of these metabolites, except traces of 4-ADNT and the hydrazo derivatives, disappeared, but mineralization did not exceed 10% even after the incubation period was increased to 120 days. The biotransformation of TNT was accompanied by the appearance of manganese peroxidase (MnP) and lignin-dependent peroxidase (LiP) activities. MnP activity was observed almost immediately after TNT disappearance, which was the period marked by the appearance of the initial metabolites (HADNT and ADNT), whereas the LiP activity was observed after 8 days of incubation, corresponding to the appearance of the acyl derivatives. Both MnP and LiP activities reached their maximum levels (100 and 10 U/liter, respectively) within 10 to 15 days after inoculation.

Enzymatic Combustion of Aromatic and Aliphatic Compounds by Manganese Peroxidase from Nematoloma frowardii

The direct involvement of manganese peroxidase (MnP) in the mineralization of natural and xenobiotic compounds was evaluated. A broad spectrum of aromatic substances were partially mineralized by the MnP system of the white rot fungus Nematoloma frowardii . The cell-free MnP system partially converted several aromatic compounds, including [U- 14 C]pentachlorophenol ([U- 14 C]PCP), [U- 14 C]catechol, [U- 14 C]tyrosine, [U- 14 C]tryptophan, [4,5,9,10- 14 C]pyrene, and [ring U- 14 C]2-amino-4,6-dinitrotoluene ([ 14 C]2-AmDNT), to 14 CO 2 . Mineralization was dependent on the ratio of MnP activity to concentration of reduced glutathione (thiol-mediated oxidation), a finding which was demonstrated by using [ 14 C]2-AmDNT as an example. At [ 14 C]2-AmDNT concentrations ranging from 2 to 120 μM, the amount of released 14 CO 2 was directly proportional to the concentration of [ 14 C]2-AmDNT. The formation of highly polar products was also observed with [ 14 C]2-AmDNT and [U- 14 C]PCP; these products were probably low-molecular-weight carboxylic acids. Among the aliphatic compounds tested, glyoxalate was mineralized to the greatest extent. Eighty-six percent of the 14 COOH-glyoxalate and 9% of the 14 CHO-glyoxalate were converted to 14 CO 2 , indicating that decarboxylation reactions may be the final step in MnP-catalyzed mineralization. The extracellular enzymatic combustion catalyzed by MnP could represent an important pathway for the formation of carbon dioxide from recalcitrant xenobiotic compounds and may also have general significance in the overall biodegradation of resistant natural macromolecules, such as lignins and humic substances.

Initial reductive reactions in aerobic microbial metabolism of 2,4,6-trinitrotoluene

Because of its high electron deficiency, initial microbial transformations of 2,4,6-trinitrotoluene (TNT) are characterized by reductive rather than oxidation reactions. The reduction of the nitro groups seems to be the dominating mechanism, whereas hydrogenation of the aromatic ring, as described for picric acid, appears to be of minor importance. Thus, two bacterial strains enriched with TNT as a sole source of nitrogen under aerobic conditions, a gram-negative strain called TNT-8 and a gram-positive strain called TNT-32, carried out nitro-group reduction. In contrast, both a picric acid-utilizing Rhodococcus erythropolis strain, HL PM-1, and a 4-nitrotoluene-utilizing Mycobacterium sp. strain, HL 4-NT-1, possessed reductive enzyme systems, which catalyze ring hydrogenation, i.e., the addition of a hydride ion to the aromatic ring of TNT. The hydride-Meisenheimer complex thus formed (H-TNT) was further converted to a yellow metabolite, which by electrospray mass and nuclear magnetic resonance spectral analyses was established as the protonated dihydride-Meisenheimer complex of TNT (2H-TNT). Formation of hydride complexes could not be identified with the TNT-enriched strains TNT-8 and TNT-32, or with Pseudomonas sp. clone A (2NT), for which such a mechanism has been proposed. Correspondingly, reductive denitration of TNT did not occur.

Denitration of 2,4,6-trinitrotoluene by Pseudomonas savastanoi

Past disposal of wastewaters containing 2,4,6-trinitrotoluene (TNT) at the former Nebraska Ordnance Plant has resulted in numerous acres of TNT-contaminated soil. Examining the microbial population of these soils revealed several TNT-tolerant Pseudomonas spp. We selected one species, P. savastanoi, to determine its ability to transform TNT. Pure culture experiments were performed in pseudomonas minimal medium containing 0.31 mM TNT (70 mg TNT . L(-1)) under varied nutrient and cell density regimes. Experiments with TNT as a sole C or N source showed that P. savastanoi has the ability to denitrate TNT, as evidenced by production of 2,4-dinitrotoluene (2,4-DNT) and NO2- with time. TNT denitration and formation of 2,4-DNT were enhanced by removing NH4+ and adding NO2- to the growth medium. In all experiments, 2-amino-4,6-dinitrotoluene (2-ADNT) and 4-amino-2,6-dinitrotoluene (4-ADNT) appeared as incidental reduction products. Glucose addition to the medium enhanced 2-ADNT and 4-ADNT production and decreased denitration of TNT. Mid-log phase cells rapidly transformed [ring-14C(U)]TNT but were unable to mineralize significant quantities of TNT, as evidenced by conversion of less than 1% of the label to 14CO2. These results indicate that P. savastanoi is a TNT-tolerant pseudomonad that can promote TNT degradation through reductive denitration and nitro moiety reduction.

Characterization of partial anaerobic metabolic pathway for 2,4,6-trinitrotoluene degradation by a sulfate-reducing bacterial consortium

The anaerobic degradative pathway for metabolism of 2,4,6-trinitrotoluene (TNT) by a consortium of Desulfovibrio spp. isolated from a creek sediment was studied. This consortium has the metabolic capability to degrade TNT to fatty acids. The growth of the consortium and the metabolism of TNT were greatly enhanced in the presence of an additional carbon source like pyruvate. The optimal concentration of pyruvate for the maximum rate of TNT degradation was 15-20 mM. Various intermediates of TNT metabolism were identified. The first step in the pathway was reduction of TNT to 4-amino-2,6-dinitrotoluene and 2-amino-4,6-dinitrotoluene, which were further reduced to 2,4-diamino,6-nitrotoluene. The next intermediate to appear in the culture medium was nitrobenzoic acid, followed by cyclohexanone, 2-methyl pentanoic acid, butyric acid, and acetic acid. A study using radiolabeled TNT showed that no CO2 was produced from TNT during metabolism. The mass balance of the radiolabeled study showed that 49.6% of the TNT was converted to acetic acid, 28% was assimilated into biomass as trichloroacetic acid precipitable materials, and the rest was distributed as various TNT intermediates. Most Desulfovibrio spp. are incomplete oxidizers that are unable to carry out the terminal oxidation of organic substrates. The major end product of TNT metabolism was acetic acid. The bacteria grew on all the TNT intermediates tested as sole source of carbon, except on acetic acid, confirming that the Desulfovibrio spp. have the enzymes necessary for complete degradation of TNT to acetate.

Bioremediation of 2,4,6-trinitrotoluene-contaminated soils by two different aerated compost systems

Two composting systems were compared on a laboratory scale as a bioremediation technology for degradation or immobilization of 2,4,6-trinitrotoluene (TNT) in contaminated soils. The first compost was aerated from the beginning whereas the second compost was only aerated after an anaerobic prephase of 65 days. In the first compost system the TNT concentration declined rapidly by 92% but, at the end, TNT could be partially recovered. During the anaerobic prephase of the second compost system, TNT was almost completely converted to aminodinitrotoluenes, which during the subsequent aeration almost entirely disappeared. In addition, the second compost generated less toxic material than the first one as confirmed by inhibition of bioluminescence of Vibrio fischeri. These data show that microbiological TNT-degradation systems can be successfully designed which are prerequisite for an efficient bioremediation of contaminated soils.

Bioconversion of 2,4-diamino-6-nitrotoluene to a novel metabolite under anoxic and aerobic conditions

Under nitrate-reducing, nongrowth conditions, a Pseudomonas fluorescens species reduced 2,4,6-trinitrotoluene to aminodinitrotoluenes, which were then further reduced to diaminonitrotoluenes. 2,4-Diamino-6-nitrotoluene (2,4-DANT) was further transformed to a novel metabolite, 4-N-acetylamino-2-amino-6-nitrotoluene (4-N-AcANT), while 2,6-diamino-4-nitrotoluene (2,6-DANT) was persistent. Efforts to further degrade 2,4-DANT and 2,6-DANT under aerobic, nitrogen-limited conditions were unsuccessful; 2,6-DANT remained persistent, and 2,4-DANT was again transformed to the 4-N-AcANT compound.

Degradation of nitroaromatic compounds by microorganisms

Nitroaromatic compounds are abundantly present in nature, but are in most cases highly toxic to living organisms. Several microorganisms, however, are capable of mineralizing or converting these compounds. Until now four pathways for the complete degradation of nitroaromatics have been described, which start with either the oxygenolytic or reductive removal of the nitro group from the aromatic ring or with this removal by means of replacement reactions. Besides these conversions many organisms are able to reduce nitroaromatics. The degradation of nitroaromatic compounds does not only occur in pure cultures but also in situ, for example in soil, water and sewage. However, several problems are associated with the application of microorganisms in the bioremediation of contaminated sites, as nitroaromatics or their conversion products may chemically interact with soil particles and cells. Besides the possibilities of applying microorganisms in the cleaning of sites contaminated with nitroaromatics, the use of microorganisms or enzymes in the biocatalytic production of industrially valuable products from nitroaromatics is also discussed.