Enantioselective degradation of the organophosphorus insecticide isocarbophos in Cupriavidus nantongensis X1T: Characteristics, enantioselective regulation, degradation pathways, and toxicity assessment

https://doi.org/10.1016/j.jhazmat.2021.126024Get rights and content

Highlights

  • Strain X1T degraded R isocarbophos isomer (R-ICP) 42-fold faster than S-ICP.

  • R-ICP is less toxic to insects, but more toxic to humans than S-ICP.

  • Divalent metal cations could increase the degradation ability of strain X1T.

  • Enantioselective uptake determined the degradation selectivity.

  • A novel hydrolysis pathway of ICP was proposed.

Abstract

The chiral pesticide enantiomers often show selective efficacy and non-target toxicity. In this study, the enantioselective degradation characteristics of the chiral organophosphorus insecticide isocarbophos (ICP) by Cupriavidus nantongensis X1T were investigated systematically. Strain X1T preferentially degraded the ICP R isomer (R-ICP) over the S isomer (S-ICP). The degradation rate constant of R-ICP was 42-fold greater than S-ICP, while the former is less bioactive against pest insects but more toxic to humans than the latter. The concentration ratio of S-ICP to R-ICP determines whether S-ICP can be degraded by strain X1T. S-ICP started to degrade only when the ratio (CS-ICP/CR-ICP) was greater than 62. Divalent metal cations could improve the degradation ability of strain X1T. The detected metabolites that were identified suggested a novel hydrolysis pathway, while the hydrolytic metabolites were less toxic to fish and green algae than those from P-O bond breakage. The crude enzyme degraded both R-ICP and S-ICP in a similar rate, indicating that enantioselective degradation was due to the transportation of strain X1T. The strain X1T also enantioselectively degraded the chiral organophosphorus insecticides isofenphos-methyl and profenofos. The enantioselective degradation characteristics of strain X1T make it suitable for remediation of chiral organophosphorus insecticide contaminated soil and water.

Introduction

Organophosphorus insecticides (OPs) are among the most commonly used pesticides worldwide to control agricultural pest insects (Y.N. Zhang et al., 2020). In 2018, about 300,000 tons of pesticides were applied in China and more than 90% could be finally entered to the soil and water, causing environmental pollution and endangering human health (Silver et al., 2017, Gao et al., 2020). OPs are highly toxic and can inhibit acetylcholinesterase (AChE) in the human nervous system (Jiang et al., 2019). OPs can also induce blood lymphocytes cells to cause DNA damage and lead to organ dysfunction (Sandhu et al., 2013, Karami-Mohajeri et al., 2017).

Isocarbophos (O-2-isopropoxycarbonylphenyl O-methyl phosphoramidothioate, ICP) is one of the most used chiral OPs in rice fields to control chewing and sucking insects (Yao et al., 2015). ICP has a chiral center of P atom and contains a pair of enantiomers, R-isocarbophos (R-ICP) and S-isocarbophos (S-ICP) (Fig. 1) (Zhang et al., 2012). The insecticidal activities and toxicity of ICP enantiomers for target or non-target organisms have been fully studied. The S-ICP was 100-fold more toxic than R-ICP to Nilaparvata lugens (rice pest, Hemiptera: Delphacidae) and Chilo suppressalis (rice pest, Lepidoptera: Crambidae), while S-ICP was more persistent than R-ICP in rice cultivation (Di et al., 2019). On the contrary, the R-ICP was 2-fold more toxic than S-ICP to HepG2 cells (human liver tissue cells) and R-ICP could induce the production of reactive oxygen species (Liu et al., 2010). However, most chiral-OPs are used in racemic-form (Sanganyado et al., 2017). Applications of the R-ICP that has low insecticidal activity but high mammalian toxicity can cause environmental pollution and increase the risk to human health.

Enantioselective degradation of chiral pesticides, especially in soil, is often attributed to microorganisms and the degradation processes can be affected by soil type, pH, and microbial populations (Li et al., 2012, Zhang et al., 2018, Kaziem et al., 2020). In paddy soil under natural conditions, the degradation rate of RS/SR-paichongding was higher than that of RR/SS-paichongding. After adding the degradation strain Sphingobacterium sp. P1–3, the degradation rate of RR/SS-paichongding was significantly increased (Chen et al., 2017). In the paddy soil collected in Shaoxing (Zhejiang, China), the half-life of R-ICP was shorter than that of S-ICP, which was caused by soil microorganisms (Di et al., 2019). A-(−)-Fosthiazate (an organophosphorus insecticides) was preferentially enriched in Phaseolus vulgaris Linn soil, while A-(+)-fosthiazate was preferentially accumulated in Vicia faba Linn soil (Di et al., 2021). However, very few enantioselective degradation strains have been isolated from soils. The detailed mechanism of enantioselective microbial degradation of pesticides in the environment is still unclear.

Some ICP-degrading strains were isolated from ICP-polluted soil and identified via cultivation in the past few years. Arthrobacter sp. scl-2, Bacillus megaterium CM-Z19 and Bacillus pumilus SALL-7 could use ICP as the only carbon source and energy material for growth, and could degrade 50–100 mg/L ICP in 18 h−14 d (Chen et al., 2017, Zhu et al., 2019). However, these strains could not selectively degrade ICP enantiomers.

The chiral-selective degradation enzymes are considered to be the main reason for microbial enantioselective degradation (Huang et al., 2020). For example, the degradation enzymes RdpA and SdpB from Sphingobium herbicidovorans MH and Delftia acidovorans MC1 could preferentially degrade R-dichlorprop and S-dichlorprop, respectively (Nielsen et al., 2017). Some degrading strains could control the enantiomeric degradation rate through selective absorption, and the degradation enzyme itself did not have enantioselectivity (Lv et al., 2017, Fang et al., 2020, Zhang et al., 2020a). However, no selective absorption transporters or chiral-selective degradation enzymes for OPs have been reported.

The toxicity, risk assessment and environmental behavior of ICP enantiomers have been investigated in numerous studies (Liu et al., 2015). However, the mechanism of enantioselective degradation of ICP was rarely reported. We previously isolated and identified an OPs degrading strain, Cupriavidus nantongensis X1T (Sun et al., 2016). The degradation rate of strain X1T for some chiral OPs was initially fast and then became slow. Strain X1T showed apparent enantioselectivity for these chiral enantiomers (Fang et al., 2020). To further research the characteristics and mechanism of enantioselectivity, strain X1T was used to (1) characterize the degradation kinetics of Rac-ICP, R-ICP and S-ICP; (2) measure the effects of pH and metal ions on the enantioselectivity; (3) determine and identify the degradation metabolites; (4) evaluate the toxicity of ICP and its metabolites; (5) determine the enantioselective degradation characteristics of a variety of chiral OPs and (6) explore the mechanism of preferential degradation of R-ICP by strain X1T.

Section snippets

Chemicals and media

The racemic isocarbophos (Rac-ICP, CAS 24353–61–5) standard and its metabolites, isopropyl salicylate and salicylic acid were purchased from Sigma-Aldrich (Shanghai, China). Isocarbophos chiral enantiomers, R-ICP and S-ICP, were kindly gifted by Professor Minghua Wang (Nanjing Agricultural University, China). Rac-isofenphos-methyl and Rac-profenofos standard were purchased from Dr. Ehrenstorfer (Augsburg, Germany). KCl, CaCl2, CrCl3, FeCl3.6H2O, Al2(SO4)3.18H2O, MnCl2.4H2O, MgCl2.6H2O, ZnCl2,

R-ICP was degraded favorably over S-ICP by strain X1T

The degradation kinetics of 50 mg/L Rac-ICP by strain X1T were shown in Fig. 2A. Strain X1T completely degraded R-ICP and 30% of S-ICP in 12 h. The degradation curves were all well fitted with the first-order kinetics equation (R>0.95). Meanwhile, strain X1T showed high enantiomer selectivity. R-ICP in the racemic mixture was preferentially degraded over S-ICP. The degradation rate constant of R-ICP was 42-fold higher than that of S-ICP. Interestingly, when R-ICP was almost degraded, the S-ICP

Conclusion

This was the first report describing enantioselective degradation of chiral ICP. R-ICP was preferentially degraded over S-ICP by strain X1T, while the former has lower bioactivity to pest insects and higher toxicity to the non-target organisms. Metabolites and toxicity analyses suggested a novel hydrolysis pathway by strain X1T under an alkaline condition, which was safer for non-target organisms. The enantioselectivity of strain X1T to R-ICP and S-ICP occurred in the absorption process. This

CRediT authorship contribution statement

Liancheng: Fang: Visualization, Formal analysis, Writing - original draft preparation, Writing - review & editing. Luyuan Xu: Resources, Data curation. Qiongying Shi: Resources, Data curation. Taozhong Shi: Software, Conceptualization, Validation. Xin Ma: Validation. Xiangwei Wu: Methodology. QingX. Li: Supervision, Writing - review & editing. Rimao Hua: Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported in part by grants from the National Natural Science Foundation of China (31972314), Collaborative Innovation Projects of Anhui Province (GXXT-2019–034), High-level Talent Introduction Project of Anhui Agricultural University (rc522101) and the USDA (Hatch project HAW5032-R).

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