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Applied and Environmental Microbiology, November 2006, p. 7264-7269, Vol. 72, No. 11
0099-2240/06/$08.00+0     doi:10.1128/AEM.01477-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Mechanism of Augmentation of Organotin Decomposition by Ferripyochelin: Formation of Hydroxyl Radical and Organotin-Pyochelin-Iron Ternary Complex

Guo-Xin Sun¹ and Jian-Jiang Zhong^1,2*

State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People's Republic of China,¹ Key Laboratory of Microbial Metabolism, Ministry of Education, College of Life Science and Biotechnology, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, People's Republic of China²

Received 27 June 2006/ Accepted 10 September 2006

Top Abstract Introduction Materials and Methods Results Discussion References

 
Pyochelin (PCH), a kind of siderophore secreted by Pseudomonas aeruginosa, was recently found to have triphenyltin (TPT)-decomposing capacity. In this work, significant augmentation of TPT decomposition by ferripyochelin (FePCH), the chelating compound of PCH with iron, was demonstrated in Tris-HCl buffer (pH 8.0). The generation of hydroxyl radical (HO·) in the presence of FePCH was observed. Inhibition of HO· generation by adding catalase and HO· scavengers (methanol and dimethyl sulfoxide) decreased TPT decomposition, while an increase in HO· formation in the presence of H₂O₂ enhanced its decomposition. Our findings indicated that HO· generated in the reaction system was responsible for the enhanced TPT decomposition by FePCH versus PCH. The existence of the TPT-pyochelin-iron ternary complex was demonstrated by electron spray ionization-mass spectrometry, tandem mass spectrometry, and ¹H nuclear magnetic resonance. On the basis of the above results, HO· produced in the presence of FePCH was deduced to be in close proximity to TPT and has more opportunity to attack the Sn-C bond, which resulted in the enhanced organotin decomposition. The information obtained may have considerable environmental significance.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Triorganotin compounds, in particular triphenyltin (TPT), have been introduced into the aquatic environment in large quantities due to their extensive use as pesticides in agriculture and as antifouling agents in boat paints (11). As a highly toxic compound, TPT has caused harmful effects on a variety of nontarget organisms, such as mollusks and fish (2, 12), even at aqueous concentrations of a few nanograms per liter (2). Although organotins are banned in most developed countries, they are still being introduced into water and sediment around the world, causing an obvious health risk that has raised public interest in the removal of organotin in and from the environment (14). Microorganisms play a crucial role in organotin removal from the natural environment because of their surprising ability under evolution to degrade xenobiotics (26).

Several strains have been isolated from the sediment or soil, and TPT degradation by these strains was investigated under pure culture conditions (13, 24). Although microbe-mediated dealkylation of organotin has been reported, information about details of the mechanism of microbial TPT degradation/decomposition is still limited (13, 14). Walts and Walsh (25) demonstrated that only a few kinds of organotins were substrates for bacterial organomercurial lyase. Most organotin compounds, such as tributyltin and triethyltin, were not substrates for lyase, and some even caused irreversible inhibition of enzymes. Inoue and coworkers (13, 14) have identified a kind of siderophore, pyoverdine (PVD), which was produced by Pseudomonas chlororaphis and was capable of decomposing triphenyltin and tributyltin. We recently showed that pyochelin (PCH), a siderophore secreted by Pseudomonas aeruginosa, could decompose triphenyltin, and its decomposition mechanism was proposed as chelation of inorganic tin by PCH (22).

In the present work, we found that ferripyochelin (FePCH), the chelate complex of PCH with Fe³⁺, could significantly increase TPT decomposition in comparison with PCH, and further studies were conducted to elucidate the possible mechanism. The formation of HO· in the reaction system through FePCH and the existence of the TPT-pyochelin-iron ternary complex were found to be the key factors responsible for the enhanced TPT decomposition by FePCH.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Chemicals and TPT stock solution.
TPT chloride (95%), diphenyltin (DPT) dichloride (98%), and monophenyltin (MPT) trichloride (96%) were purchased from Aldrich Chemical Co. and used directly without further purification. The TPT stock solution (1 mM) prepared in methanol was kept in the dark at 4°C. Dithiothreitol was obtained from Sino-American Biotechnology Co. (Shanghai, China). Methanol (high-performance liquid chromatography [HPLC] grade) was from Shanghai Laboratory Reagents Co., Ltd. (Shanghai, China). All other chemicals used for mineral salts media and extractions were of analytical grade.

Preparation of PCH and FePCH.
As described earlier (22), P. aeruginosa CGMCC 1.860 was cultured in M9 medium with sodium succinate as a carbon source. PCH was extracted and purified to uniformity from the bacterial growth medium by thin-layer chromatography as described previously (8, 22). FePCH, the iron-binding PCH, was formed by adding FeCl₃ to a methanol solution of PCH and then extracted with ethyl acetate as reported previously (7). The extracts were dried under vacuum and dissolved again in methanol just before use.

Spin trapping.
Desired reaction mixtures (0.5 ml) in Tris-HCl (50 mM, pH 8.0) were assembled in glass tubes and transferred to a quartz electron paramagnetic resonance (EPR) flat cell which was in turn placed into the cavity of the EPR spectrometer (model EMX-8; Bruker, Karlsruhe, Germany). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO; Aldrich Chemical Co.) (100 mM) was used in each tube. The EPR spectra at 25°C were then obtained with a microwave power of 20 mW and at a modulation frequency of 100 kHz.

TPT decomposition and analysis.
TPT (at a final concentration of 80 µM) was added to the Tris-HCl buffer (50 mM, pH 8.0) with PCH (50 µM) or FePCH (50 µM) at 40°C for 12 and 24 h. The reaction was terminated by adding 6 M HCl (200 µl) and methanol (300 µl) at various intervals. TPT decomposition was assayed by HPLC as described previously (22). Catalytic efficiency (as a percentage) is calculated as follows: (1 – C_t/C₀) x 100, where C₀ is the initial concentration of TPT and C_t is the concentration of TPT at reaction time t (h).

Quantitation of HO·.
Benzoic acid was used as a probe for HO·, which has a known reaction rate constant of 4.2 x 10⁹ M^–1 · s^–1 with HO· in aqueous media and produces p-hydroxybenzoic acid (p-HBA) (17). HO· was measured by monitoring the production of p-HBA by high-performance liquid chromatography with a fluorescence detector as follows. Benzoic acid was added into the reaction system containing 50 µM FePCH and 80 µM TPT in the Tris-HCl buffer (pH 8.0). The mixtures were kept in the dark at 40°C, and the samples were prepared by adjusting the pH to 2 to 3 with 2 M HCl. The p-HBA and p-aminohydroxybenzoic acid (p-ABA) were determined by use of Shimadzu (Kyoto, Japan) HPLC equipment (LC-10AT_VP) with fluorescence detection (excitation/emission, 260/335 nm) and a reversed-phase column (250 x 4.6 mm inner diameter; 5 µm; Kromasil C₁₈) (21). The mobile phase consisted of methanol:phosphoric acid (1 mM) (20:30, vol/vol). Chromatography was carried out under isocratic conditions at a flow rate of 0.8 ml/min, and all analyses were carried out at room temperature (about 25°C), and the injected volume was 20 µl each time.

The HO· production rate (P_OH) was obtained by the method of Zhou and Mopper (27). The conversion factor for calculating P_OH from the p-HBA production rate (P_HBA) was 6.07, i.e., P_OH= 6.07 P_HBA, and this value is in agreement with the reported value (27).

Effects of HO· scavengers, catalase, and H₂O₂ on TPT decomposition.
A hydroxyl radical scavenger, methanol or dimethyl sulfoxide (DMSO) (at a final concentration of 200 mM), was added into the Tris-HCl buffer (pH 8.0) to investigate their effects on TPT decomposition (3). Catalase (4,000 U/ml) or H₂O₂ (0.1 mM) was mixed with FePCH for the investigation.

Hydroxyl radicals were chemically generated at 40°C by the iron- plus ascorbate-driven Fenton reaction as described by Regoli and Winston (20). TPT was added at a final concentration of 80 µM into the mixture containing FeCl₃ (0.4 mM), Na₂EDTA (0.4 mM), vitamin C (10 mM), and H₂O₂ (0.1 mM) with a total volume of 0.5 ml. The reaction was terminated by adding 6 M HCl (200 µl) and methanol (300 µl) at various intervals.

MS and ¹H NMR.
The existence of the ternary complex (TPT-pyochelin-iron) was identified by electron spray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS). Electrospray ionization mass spectral analyses were performed on Waters Micromass LCT KC317 mass spectrometer. ¹H nuclear magnetic resonance (NMR) spectra were recorded using an AVANCE-500 NMR spectrometer (Bruker, Germany) at 500 MHz (Analysis & Research Center, ECUST, Shanghai, China). D₂O was used as the solvent.

Analyses of TPT and decomposition products.
TPT and its decomposition products, diphenyltin and monophenyltin, were assayed simultaneously by HPLC as described elsewhere (22).

Inorganic tin (Sn) was prepared as described previously (13). Briefly, the Sn samples were eluted in a void volume by HPLC, dried, and then dissolved in 5 ml 1 M HNO₃. The concentration of Sn was determined by an inductively coupled plasma-atomic emission spectrometer (Perkin-Elmer TJA IRIS1000).

Phenol was identified using HPLC (Shimadzu, Kyoto, Japan) with acetonitrile-water (1:1, vol/vol) as the mobile phase. The flow rate was 0.8 ml/min, and the UV detection wavelength was 220 nm.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Enhanced decomposition of TPT by FePCH versus PCH.
FePCH was prepared as described previously (7). The compound was a wine-red color and nonfluorescent and showed maximum absorption at 237, 255, 325, 425, and 520 nm. All these properties are the same as those of FePCH (7, 8), suggesting that the Fe³⁺-PCH chelate complex, i.e., FePCH, was formed. Our previous work shows that PCH has the ability to decompose TPT (22). As PCH could bind many transition metals, including Fe³⁺, Cu²⁺, Co²⁺, Zn²⁺, and Mn²⁺ (23), the effects of these metal ions on the decomposition of TPT were studied. All metal ions used could improve TPT decomposition by PCH (Fig. 1). Given that the binding affinity of Fe³⁺ with PCH is much higher than other transition metal ions (23), Fe³⁺ was chosen for investigating the detailed decomposition mechanism.

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FIG. 1. Effects of the Fe3+ (), Zn2+ (•), Co2+ (), Mn2+ (), and Cu2+ () metal ions on decomposition of TPT by pyochelin. The methanol solution of PCH (1 mM) was mixed with an equal volume of metal salts (0.5 mM in water) to yield 0.25 mM pyochelin-metal complex in a 2:1 ratio and then diluted to 50 µM with Tris-HCl (50 mM, pH 8.0). TPT was added at 80 µM. The mixture was kept at 40°C for 12 and 24 h, respectively. Other symbols: , samples of buffer plus TPT without PCH and metal ions; , samples of PCH plus TPT without metal ions.

As shown in Table 1, FePCH increased TPT decomposition more than PCH did. In 12 h and 24 h, the decrease of TPT by FePCH was 3.4- and 3.2-fold greater than that by PCH, respectively (Table 1). The results indicated that TPT decomposition was apparently affected by Fe³⁺. The accumulations of decomposition products, i.e., DPT, MPT, and Sn, were detected, and the total values confirmed the mass balance of Sn in organic plus inorganic forms (Table 1).

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TABLE 1. Decomposition of triphenyltin by pyochelin and ferripyochelina

Formation of HO· in the reaction system.
The data of Fig. 1 indicate that the presence of iron enhanced TPT decomposition by PCH. It is reported that some iron chelates are efficient HO· catalysts (9), while HO· may attack TPT and cause its decomposition (18). Thus, confirmation of whether HO· was generated in our cell-free system was done as follows.

Two different methods were used to detect HO·. Spin trapping of HO· in EPR spectra in the presence of FePCH indicated that HO· was produced, while HO· formation was not observed in the absence of FePCH (Fig. 2). When benzoic acid was used to probe HO· in the presence of FePCH (17), the production of p-HBA was detected by HPLC (Fig. 3), which is the reaction product of HO· with benzoic acid (BA) as reported previously (17). The retention time of p-ABA was apparently different from that of p-HBA, and p-ABA was not found in our reaction system (Fig. 3). Here, the formation of p-HBA was used to quantify HO·.

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FIG. 2. Electron paramagnetic resonance spectra were obtained for PCH (50 mM), FePCH (50 mM), Fenton reagents, and the mixture of FePCH (50 mM) with H2O2 (100 mM) or methanol (Me) (100 mM) in the presence of DMPO (100 mM). All reactions were in the Tris-HCl buffer (50 mM, pH 8.0), and this buffer plus DMPO was used as a control.

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FIG. 3. Benzoic acid was used as a probe for detecting HO·. Analyses of BA, p-HBA, and p-ABA were performed by HPLC with a fluorescence detector as described in Materials and Methods. Hydroxyl radicals generated by Fenton reaction also reacted with BA. The reaction product of p-HBA was detected. Mix., mixture.

Role of HO· in TPT decomposition.
Having confirmed that HO· was produced as a consequence of the presence of FePCH, various oxidant scavengers and other inhibitors of HO· production were added into the mixture of TPT and FePCH to provide evidence that HO· formation contributed to TPT decomposition. In the time course experiment, the generation of HO· decreased (Fig. 4), and the TPT decomposition rate was concomitantly reduced. Here, the concentrations of HO· generated by FePCH alone were 20.6 and 30.8 µM at 12 h and 24 h, respectively. Comparison with samples without scavengers shows that scavengers of HO·, methanol and DMSO, reduced TPT decomposition by FePCH (Fig. 4). Similarly, catalase reduced HO· formation as well as the TPT decomposition in the reaction mixture with FePCH (Table 2). In contrast, the addition of H₂O₂ in the reaction mixture increased HO· generation and TPT decomposition (Fig. 2 and Table 2). The catalytic efficiency of FePCH plus H₂O₂ was 59.7% for TPT decomposition, higher than that of FePCH alone (40.3%). From these results, it is concluded that HO· generated in the system with FePCH was responsible for the augmentation of TPT decomposition.

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FIG. 4. HO· scavenger, methanol or DMSO (at a final concentration of 200 mM), was added to the TPT decomposition system. The mixture of TPT and FePCH was used as a control. The concentration of TPT in FePCH (), FePCH plus methanol (), and FePCH plus DMSO (•) was determined by HPLC with a UV detector. Benzoic acid (10 mM) was used as a probe for HO·. The concentrations of HO· in FePCH (), FePCH plus methanol (), and FePCH plus DMSO () were measured by monitoring the production of p-HBA by HPLC with a fluorescence detector.

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TABLE 2. Effects of H2O2 and catalase on TPT (80 µM) decomposition by FePCHa

Existence of the ternary complex of TPT, PCH, and iron.
Given the fact that organotin was reported to form complexes with amino acids or peptides via active ligand with {O, S, N}, {S, N} donor sites (10, 19) and PCH arises from cyclization of cysteine residue containing {S, N} donor atoms as reported previously (8), it is speculated that there was existence of the complex of TPT and FePCH. For the evidence, electrospray ionization mass spectra of TPT, FePCH, and their mixture were obtained. In addition to the typical peaks shown in Table 3, in the mass spectrum of their mixture, there existed an additional peak at m/z 764.1 (FePCH plus TPT plus Cl plus H) besides those of TPT and FePCH, suggesting the formation of TPT and FePCH complex. That additional peak was used as the precursor ion for MS/MS analysis, and the product ion peaks at m/z 351.6 (TPT plus H) and 379.1 (PCH plus Fe) (data not shown), which corresponded to those of TPT and FePCH, respectively, were obtained (Table 3). Therefore, that peak (m/z 764.1) was derived from TPT and FePCH. The ESI-MS and MS/MS data indicated the existence of TPT-FePCH, i.e., the ternary TPT-PCH-iron complex.

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TABLE 3. Typical peaks of electrospray ionization mass spectra

¹H NMR spectra were recorded for aqueous solutions of TPT, TPT plus PCH, and TPT plus PCH plus Fe³⁺ to provide further evidence of the formation of the complex. As shown by the NMR spectra (Fig. 5), comparison with the TPT spectrum (Fig. 5A) shows that although there were no changes in chemical shift, the splitting of TPT peaks was decreased in the presence of PCH (Fig. 5B). In the presence of PCH, TPT peaks did not show changes in chemical shift but splitting of peak was further decreased when Fe³⁺ was also present (Fig. 5C).

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FIG. 5. 1H NMR spectra of (A) TPT, (B) TPT plus PCH, and (C) TPT plus PCH plus Fe3+. D2O was used as the solvent.

Top Abstract Introduction Materials and Methods Results Discussion References

 
The high TPT-decomposing capacity of FePCH, the chelate complex of PCH with Fe³⁺, compared to that of PCH (Fig. 1) suggests that the decomposing ability of TPT was improved in the presence of ferric iron. Pyridine-2,6-bis(thiocarboxylic acid) is a metal-chelating agent obtained from cultures of Pseudomonas stutzeri KC. Its metal complexes with Cu²⁺ also have the capacity to dechlorinate CCl₄ (16). HO· was assayed and identified in the presence of FePCH in the Tris-HCl buffer (pH 8.0), while no HO· was detected when FePCH was replaced by PCH. This indicates that HO· formation was dependent on the presence of iron. Other researchers have found that vanadium can form complexes with PCH and that V-PCH can undergo an oxidative cycle (1). Iron-chelated aminochelin, a kind of siderophore produced by Azotobacter vinelandii, also has the ability to catalyze the formation of HO· (6). HO· formation in the reaction system suggested that FePCH contributed to augmentation of TPT decomposition via the generation of HO·. The addition of HO· scavengers (methanol and DMSO) and antioxidant (catalase) to interrupt the formation of HO·, as expected, decreased the generation of HO·, and TPT decomposition was concomitantly reduced in the reaction system (Fig. 4 and Table 2). In another aspect, the addition of H₂O₂ increased HO· production (Table 2), and in the meantime the capacity of FePCH to decompose TPT was enhanced (Table 2). All results suggest that HO· generated in the reaction mixture with FePCH was involved in TPT decomposition and responsible for its augmentation. Decomposition of TPT was also observed when hydroxyl radical was generated by other HO·-generating systems (Fenton reaction) (Table 2). This was consistent with a previous report (18) and confirmed the role of HO· in TPT decomposition. The work demonstrated that FePCH could promote organotin decomposition by formation of HO·.

The TPT decomposition activity of PVD was markedly decreased rather than increased by Fe³⁺ (14). Furthermore, PVD could not act as a HO· catalyst, and Fe-PVD could not catalyze HO· generation as has been reported in inflammations (5). Obviously, the organotin decomposition mechanisms are quite different for PCH and PVD, although both are siderophores secreted by fluorescent pseudomonads.

Due to its highly reactive and short-lived nature (4), HO· diffused only a negligible distance before encountering an oxidizable substrate (9). HO· must be generated in close proximity to a target to cause its decomposition. It seems that the formation of a complex of TPT with FePCH was a possible explanation for the augmentation of TPT decomposition. The ESI-MS (Table 3) and MS/MS data suggested that the additional peak at m/z 764.1 in a mass spectrum of their mixture was derived from TPT and FePCH, demonstrating the existence of the TPT-FePCH complex. A change of signal splitting in ¹H NMR spectrum can be used as a measure of the degree of complex formation (15). In ¹H NMR spectrum (Fig. 5), in comparison with the spectrum of TPT alone (Fig. 5A), the decrease in the number of split peaks in the presence of PCH (Fig. 5B) indicated that TPT and PCH exhibited noticeable interactions. In other words, PCH was close enough to TPT to induce such a change. This implies that TPT molecules were binding with PCH. The observed decrease in the number of split peaks in the presence of PCH and Fe³⁺ (Fig. 5C) suggested that iron and TPT were in close proximity in this system. These data indicate that a ternary TPT-PCH-Fe complex may exist in solution. It is thought that HO· generated in the presence of FePCH was in close proximity to organotin because of the formation of the ternary complex; therefore, it had more opportunity to attack the Sn-C bond and enhanced TPT decomposition. To our knowledge, this is the first report of the ternary complex of TPT-PCH-Fe.

DPT, MPT, and inorganic tin (Sn) were detected as the decomposition products of TPT by dephenylation (13, 14). Electrospray ionization mass spectrum analyses on the mixture of FePCH with DPT and MPT were conducted (data not shown). The molecular ion peaks of m/z 652 (FePCH plus DPT) and 573 (FePCH plus MPT) in MS spectra indicated that FePCH-DPT and FePCH-MPT complexes were formed. The data suggested that FePCH could bind DPT and MPT. The molecular ion peak of m/z 396.1 (PCH plus Sn plus Na) in MS spectrum indicated that PCH could bind Sn (22), but PCH could not simultaneously bind Sn and Fe and FePCH could not bind Sn. Consistent with a previous report about TPT decomposition by hydroxyl radicals generated from photoinduced Fenton reaction (18), trace amounts of phenol (7.1 µM for 24 h) were assayed as one of decomposition products in our reaction system. PVD degrades TPT to form benzene rather than phenol (13, 14). The difference of their decomposition products once again implies the different mechanisms of TPT decomposition by PCH and PVD.

In summary, hydroxyl radicals were generated in the presence of FePCH, and formation of the complex of TPT with FePCH was confirmed. FePCH can cause organotin decomposition, which may be of considerable environmental significance.

 
Financial support from the National Natural Science Foundation of China (grant 20225619) is gratefully acknowledged.

W. Q. Zhou provided assistance in the early stage of this project.

 
* Corresponding author. Mailing address: Key Laboratory of Microbial Metabolism, Ministry of Education, College of Life Science and Biotechnology, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, People's Republic of China. Phone and fax: 86-21-34204831. E-mail: jjzhong{at}sjtu.edu.cn.

Published ahead of print on 22 September 2006.

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Applied and Environmental Microbiology, November 2006, p. 7264-7269, Vol. 72, No. 11
0099-2240/06/$08.00+0     doi:10.1128/AEM.01477-06
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