Organophosphate flame retardants (OPFRs) are one class of flame retardants that have a variety of applications in a wide range of products after the phased out Polybrominated diphenyl ethers (PBDEs). Several toxicological studies have shown that exposure to OPFRs can potentially cause adverse reproductive effects, endocrine disruptive effects and systemic effects in fish, birds, rodents and humans.
Understanding the accumulation and metabolism of OPFRs in the body can give insight into the toxicology and the ecological risk of OPFRs, especially for the Alkyl-OPFRs. Therefore, in this study, the rare minnow (Gobiocypris rarus) was selected as a model fish to investigate the metabolic process of Alkyl-OPFRs and the tissue distribution and elimination of their primary metabolites. A pilot field investigation of tissue distribution profile of OPFRs and di-alkyl phosphate (DAPs) were also studied in dominant freshwater fish collected from bodies of water in Beijing, China. The major results are as follows.
(1) The metabolic process of Alkyl-OPFRs in liver of rare minnow were investigated using a liquid chromatography coupled to a quadrupoletime-of-flight mass spectrometer (UPLC-QTOF-MS), after cleanup procedure. The major metabolites of the Alkyl-OPFRs were DAPs, the products of hydroxylation and of glucuronide conjugation. The phase I metabolism pathways of Alkyl-OPFRs in fish involving O-dealkylation and hydroxylation, and the phase II metabolism pathways is the glucuronide conjugation.
(2) The rapid metabolism of Alkyl-OPFRs in fish liver microsomes were both best fitted to the Michaelis–Menten model (at administrated concentrations ranging from 0.5 to 200 μM), with a CLint (intrinsic clearance) of 3.9 and 3.1 μl/min/mg protein, respectively for TBOEP and TNBP. But no significant (P > 0.05) biotransformation was observed for these compounds in intestinal microsomes. The results suggests that the fish liver make a primary contribution to the metabolism of Alkyl-OPFRs. In fish liver microsomes assay, bis(2-butoxyethyl) hydroxyethyl phosphate (BBOEHEP) and bis(2-utoxyethyl) 3-hydroxyl-2-butoxyethyl phosphate (3-OH-TBOEP) were the most abundant metabolites of TBOEP, and dibutyl-3-hydroxybutyl phosphate (3-OH-TNBP) was the predominant metabolite of TNBP. CYP3A4 was the significant CYP450 isoforms catalyzing the formation of the hydroxylated and DAPs metabolites.
(3) After exposure, TBOEP and TNBP could be mainly accumulated in liver and kidney of rare minnow. A physiologically based toxicokinetic (PBTK) model was estabolished to discribe the metabolic process of TBOEP and TNBP. The estimated partition coefficients between kidney and blood were higher than those between other tissues and blood. The metabolites were also accumulated more in liver, intestine and kidney. BBOEP and DNBP were the most aboundant metabolites in various tissues for TBOEP and TNBP, respectively. BBOEHEP, the major in vitro metabolite of TBOEP , was lower accumulated than BBOEP and 3-OH-TBOEP in tissues of fish.This may because BBOEHEP was the metabolic intermediates of TBEOP in fish.Metabolite parent concentration factor (MPCF) were calculated for the metabolites.All the metabolites reached lower accmulations than their parents, characterized by the low MPCF.
(4) The accumulation and distribution of 8 OPFRs and their 4 DAPs metabolites were first investigated in whole-body samples and various tissues of three freshwater fish species (topmouth gudgeon, crucian carp and loach) with different feeding habits from locations around Beijing, China. Accumulation of OPFRs was relatively higher in freshwater fish across all sampling locations (264.7-1973 ng/g lw), in which TNBP,TCEP, TCIPP and TEHP were the main contributors to the total OPFR levels. The DAPs concentrations were observed to be relatively large in fish, which were 0.10-1.12 times of OPFRs concentrations. With respect to their distribution in different tissues, both the parent OPFRs and DAPs were found at relatively higher levels in the liver than in other tissues (muscle, intestine, kidney and ovary). This study emphasizes the importance of monitoring DAPs in biota, and future study need improve the understanding of the toxicology and ecological risk of this OPFRs metabolites.