Gastrointestinal Metabolism And Toxic Mechanism Of Drinking Water Disinfection Byproduct Halonitromethanes

Halonitromethanes(HNMs),as an emerging class of nitrogenous disinfection by-products(N-DBPs),have been frequently detected in drinking water.A large number of studies have been reported on the detection methods for trace and formation mechanism of HNMs in drinking water,but toxicology researches of HNMs are limited.Based on a small amount of cell tests and Ames tests,HNMs have higher cytotoxicity and genotoxicity compared to some disinfection by-products,such as trihalomethanes and haloacetic acid,which have been regulated in drinking water standards.Therefore,HNMs have received more and more attention and the toxic effects and mechanisms have become the research hotspot.Oral exposure is the main route of exposure to HNMs in drinking water.Digestive enzymes and microbial flora in gastrointestinal tract directly affect the bioavailability and toxic effects of HNMs,but the mechanisms are still unclear.In this paper,trichloromethane(TCNM),bromomethane(BNM)and bromochloromethane(BCNM)are chosen as representative HNMs based on the kind and number of halogen substituents.This paper studies the metabolic processes of HNMs in simulated gastrointestinal tract by constructing human intestinal microbial ecological simulation system(SHIME),and the effects on metabolic processes of HNMs by digestive enzymes and intestinal flora.The toxic effects and its mechanisms of HNMs are studied by metabolomics and other molecular biology techniques in vitro and in vivo.Meanwhile,the effects of the metabolic process of SHIME on the toxicity of HNMs are studied.The results are summarized as follows:(1)HNMs could been rapid dehalogenation and degradation in the SHIME,and most dehalogenation occurred in the stomach and small intestine.The dehalogenation pathways of TCNM should be TCNM?DCNM?CNM?NM.BNM was dehalogenated to NM.BCNM could be dehalogenated to CNM,BNM and NM.The chemicals containing the-SH group,like mucin,cysteine,pancreatin and bile salts could react with HNMs by nucleophilic reactive process(SN2)to perform dehalogenation process,and debromination more readily occurred than dechlorination.Biodehalogenation occurred mainly in the ascending colon due to intestinal flora.Some genera ofAnaeromyxobacter,Desulfovibrio and Pseudomonas as halorespiring bacteria might participate in the HNMs biodehalogenation.HNMs exposure changed the community structure of intestinal flora in SHIME,which could play a fundamental role in human health.(2)In vitro,HNMs significantly increased the intracellular ROS of HepG2 cells,inhibited the transcriptional expression of antioxidant-related genes such as GPX2 and PRDX3 and activity of GSH-Px,and induced oxidative stress.HNMs exposure changed the transcriptional expression of DNA repair genes such as RAD50,CHEK1 and EXO1,and affected the cell DNA repair process.HNMs induced apoptosis and changed the transcriptional expression of Bax,P53 and Bcl-2.Based on the SHIME test,it was found that the toxicity of gastrointestinal dehalogenide was positively correlated with the concentration of HNMs and dehalogenide products,indicating that the metabolic process of gastrointestinal tract significantly reduced the oxidative damage and cytotoxicity induced by HNMs.Based on the above conclusions,it was one of the potential toxic mechanisms of HNMs that cell oxidative stress increasing and DNA damage induce apoptosis with activated P53-dependent apoptosis.(3)In vivo exposure experiment in mice,HNMs caused liver tissue pathological damage,inhibited activity of CAT and GSH-Px,and increased 8-OHdG levels and induced oxidative stress.HNMs changed the mouse serum metabolism,and significantly changed the metabolites,such as serum glutamate,sarcosine,TMAO and?-hydroxybutyrate,which participate in the antioxidant reaction.HNMs disturbed the metabolic pathways,like amino acid metabolism,carbohydrate metabolism,energy metabolism and lipid metabolism.Combined with the results of cell tests,the results showed that oxidative stress was one of the potential toxicity mechanisms of HNMs.In addition,changes of 8-OHdG and serum metabolites contents showed a rank order of effect expressed as BNM>BCNM>TCNM at the same HNMs exposure concentration,which indicated that the toxicity of bromo-HMMs was higher than that of chloro-HMMs.In summary,this study showed that HNMs could react with-SH group through SN2 reaction to perform dehalogenation in the stomach and small intestine,and thus reduce the toxicity of HNMs and change the structure of intestinal microflora.HNMs and dehalogenide products could cause oxidative stress,induce DNA damage and apoptosis and interfere with a variety of mouse metabolic pathways.The conclusion of this study would extend the understanding of the metabolism and toxicity of HNMs in drinking water and provide data support for the risk assessment and regulation of HNMs in drinking water.