Bioaccumulation And Biotransformation Of Polybrominated Diphenyl Ethers (BDE15, BDE47)by Crucian Carp

Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardants that are frequently applied to a number of different consumer products such as plastics, electronics, polyurethane foam, textiles, and so on. High concentrations of PBDEs have been found in various abiotic environmental sectors, including water, sediments, air, indoor dust, and in different organisms. As a kind of global environment contaminants, PBDEs have aroused increasing concerns on account of their widespread use, ubiquitous environmental distribution, persistence, great bioaccumulation potential, and possible toxicity. As the PBDE derivatives, hydroxylated (OH-) PBDEs are likely to be more potent in causing toxic effects than the parent PBDEs. These derivatives have been shown to possess antiestrogenic, antiandrogenic, or to disrupt thyroid hormone homeostasis. Significant biotransformation of some PBDEs via reductive debromination has been observed. The results from exposed mammals demonstrated that OH-PBDEs were formed as metabolites from parent PBDEs in vivo. However, little is known about the fate of lower brominated BDEs in fish, specifically about their oxidation metabolism.In this thesis, the aim was to simulate the exposure of PBDEs to assess the in vivo biotransformation behavior of lower brominated BDEs in freshwater fish. We exposed the crucian carp (Carassius auratus) to aqueous and dietary PBDEs (4,4’-dibromodiphenyl ether, BDE15and2,2’,4,4’-dibromodiphenyl ether, BDE47) and then evaluated the tissue distribution, bioaccumulation, excretion, depuration and biotransformation of PBDEs under controlled laboratory conditions. Identification of the biotransformation products was also performed by using a gas chromatography-mass spectrometry (GC-MS/MS). Main contents and results are as follows:(1) A method was established for determination of PBDEs and their metabolites in fish tissues. Accelerated solvent extraction (ASE) assisted extraction was used to extract PBDEs, and the cleanup procedure includes gel permeation chromatography (GPC) and acidic silica. Gas chromatography (GC) or gas chromatography-mass spectrometry (GC-MS) were used to determine and identify the amount of PBDEs and their metabolic products.(2) Following aqueous exposure, bioaccumulation parameters of BDE15were calculated and the results showed a rapid uptake by carp. BDE15was most concentrated in the gill and liver, with bioconcentration factor values (BCFs) of1.69×105and1.28×105, respectively, which were higher than that of muscle. In particular, five biotransformation products of BDE15in carp were identified using GC-MS/MS. Among the5metabolites, two were debrominated metabolites, and the other three were mono-OH-BDE15, diOH-BDE15and bromophenol, respectively. These results unequivocally indicated that oxidation metabolism did occur via the formation of hydroxylated (OH-) metabolites in freshwater fish crucian carp exposed in vivo.(3) Following dietary exposure, bioaccumulation parameters were also calculated. Unlike the aqueous exposure, BDE15and BDE47were most concentrated in the liver, with their biomagnification factors (BMFs) of1.083and1.145, respectively, which were also higher than that of muscle. Similarly, several metabolites of BDE47were also detected, which further indicated that debromination was not the only metabolic pathway in the fish. Four other metabolic pathways:hydroxylation, debromination/hydroxylation, methoxylation and cleavage of the diphenyl ether bond were also occurred, formed OH-tetraBDEs, OH-triBDEs and dibromophenol. These findings firmly elucidated the in vivo metabolic pathways of PBDEs in fish.(4) To evaluate the potential toxicity of PBDEs on fish, activities of hepatic ethoxyresorufin-O-deethylase (EROD), ethoxycoumarin-O-deethylase (ECOD), glutathione-S-transferase (GST), and levels of serum total thyroine (T4), triiodothyronine (T3) of three PBDEs (BDE15, BDE47and decabromodiphenyl ether, BDE209) were studied in carp exposed to spiked contaminated food. EROD and ECOD activities were both induced, while GST activity was markedly inhibited. Reductions of T3and T4were noted in the later period of the exposure. These results indicated that the decrease of thyroid hormone concentrations may be attributed to the formation of oxidized metabolites through oxygenase catalized reaction involving CYP450.(5) Using results from a small scale survey in the residents of Nanjing Chemical Industrial Park, the upper-bound value of daily intakes (EDIs) of PBDEs for residents were estimated as1.63ng·kg-1·bw-1d·-1, which was lower than the lowest observed adverse effect level (LOAEL)1mg·kg-1·bw-1·d-1recommended by Darnerud. The results imply that the health risk of PBDEs to residents in Nanjing through dietary exposure is very low.In summary, PBDEs can rapidly uptake by Carassius auratus. Hydroxylated PBDE metabolites have been detected in both routes of aqueous and dietary exposure and these metabolites are likely generated by phase I enzyme mediated metabolism of the parent compounds. Further studies are needed to investigate the metabolic mechanisms, and the key tissues and enzyme systems responsible for catalyzing these metabolic pathways in fish.