In vitro metabolism of polychlorinated biphenyls and cytochrome P450 monooxygenase activities in dietary-exposed Greenland sledge dogs

Comparative In Vitro and In Vivo Hydroxylation Metabolization of Polychlorinated Biphenyl 101 in Laying Hens: A Pilot Study

Polychlorinated biphenyls (PCBs) can be metabolized into hydroxylated PCBs (OH-PCBs) that exhibit greater toxicity than their parent compounds. In particular, 2,2',4,5,5'-pentachlorobiphenyl (PCB 101) is commonly found in chicken feeds and breeding environments, although information on the biotransformation of this PCB in chickens is lacking. In this study, the hydroxylation metabolization of PCB 101 was assessed based on in vitro trials with Sanhuang chicken liver microsomes and in vivo experiments with Hy-Line Brown hens. The para-substituted metabolite 4'-OH-PCB 101 is the predominant metabolite of PCB 101. 4'-OH-PCB 101 is preferentially retained in the chicken bloodstream and partly distributed into different tissues of laying hens. The blood-brain barrier can effectively prevent the OH-PCB from entering the brain, and the adipose tissue contains a relatively low residue concentration of the OH-PCB. The laying hen can deplete the OH-PCB via laying eggs and excrement. The ratio of 4'-OH-PCB 101/PCB 101 in egg yolk is about 1:2. These results first provide definite evidence for the previous hypothesis of the PCB 101 metabolism by chickens. They could assist in predicting the environmental fate of PCBs, and in the risk assessment of PCBs and OH-PCBs in chicken-based foodstuffs.

Determination of In Vivo Biotransformation Kinetics Using Early-Time Biota Concentrations

Technical challenges have hampered the characterization of biotransformation kinetics-a critical link in understanding and predicting the toxicokinetics and ecotoxicology of organic compounds. A shortcut approach to characterize the in vivo biotransformation rate constant (k_M ) with incomplete pathway or metabolite details was proposed. The value of k_M can be derived as 2tln1fPC(t)) , with f_PC (t) being the molar equivalent fraction of the parent compound (PC) at an early time t in both constant exposure and decay source chemical uptake scenarios. The approximation-based k_M values agreed well with k_M values derived from rigorous fitting or toxicokinetic modeling (n = 42, root mean square error = 0.30) with accuracy exceeding those of typical toxicokinetic or partitioning models. The method is accurate when sampling time is adequately resolved (i.e., t < ln(2)/k_M ) but will likely produce biased k_M values with improper time-averaging. The approximate equation yields consistent theoretical expectations for fast and slow biotransformation reactions and is fully compatible with standard bioaccumulation and toxicity testing protocols. The simplification strategy circumvents statistical complications and numerical issues inherent in regressing or modeling the toxicokinetics of multimetabolite systems and may be adapted to similar problems at other physiological scales or ecotoxicological contexts. The method can help advance interspecies comparison of chemical metabolism and support the development of in vitro-in vivo extrapolations and in silico models needed for building next-generation ecological and health risk-assessment practices. Environ Toxicol Chem 2022;41:148-158. © 2021 SETAC.

In Vitro Metabolism of Photolytic Breakdown Products of Tetradecabromo-1,4-diphenoxybenzene Flame Retardant in Herring Gull and Rat Liver Microsomal Assays

Tetradecabromo-1,4-diphenoxybenzene (TeDB-DiPhOBz) is used as a flame retardant chemical and has been hypothesized to be the precursor of methoxylated polybrominated diphenoxybenzene (MeO-PB-DiPhOBz) contaminants reported in herring gulls from sites across the Laurentian Great Lakes. Here, by irradiating the parent TeDB-DiPhOBz (solution 1) with natural sunlight or UV, we prepared three solutions where solution 2 was dominated by the Br8-11-PB-DiPhOBzs, along with Br5-8-PB-DiPhOBzs (solution 3) and Br4-6-PB-DiPhOBzs (solution 4). The in vitro metabolism of TeDB-DiPhOBz and PB-DiPhOBzs was investigated using harvested wild herring gull (Larus argentatus) and adult male Wister-Han rat liver microsomal assays. After a 90 min incubation period of solution 1 in gull or rat microsomal assays, there was no significant (p > 0.05) depletion of TeDB-DiPhOBz. OH-PB-DiPhOBz metabolites were detectable after gull and rat microsomal assay incubation with solutions 3 or 4, and showed clear species-specific differences. Also detected were two polybrominated hydroxylated metabolites having polybenzofuran structures. Overall, this study suggested that TeDB-DiPhOBz is slowly metabolized in vitro, and also indicated that if wild herring gulls are exposed (e.g., via the diet) to photolytic products of TeDB-DiPhOBz, OH-PB-DiPhOBz and other metabolites could be formed. OH-PH-DiPhOBz are likely precursors to MeO-PB-DiPhOBz contaminants that we reported previously in eggs of wild Great Lakes herring gulls.

Seasonal influences on PCB retention and biotransformation in fish

There is extensive evidence that fish from waters with polychlorinated biphenyls (PCB)-contaminated sediments accumulate PCBs and related chemicals and that people who eat fish from contaminated waters have higher body burdens of PCBs and PCB metabolites than those who do not. PCBs and their metabolites are potentially toxic; thus, it is important to human health to understand the uptake, biotransformation, and elimination of PCBs in fish since these processes determine the extent of accumulation. The intestinal uptake of PCBs present in the diet of fish into fish tissues is a process that is influenced by the lipid composition of the diet. Biotransformation of PCBs in fish, as in mammals, facilitates elimination, although many PCB congeners are recalcitrant to biotransformation in fish and mammals. Sequential biotransformation of PCBs by cytochrome P450 and conjugation pathways is even less efficient in fish than in mammalian species, thus contributing to the retention of PCBs in fish tissues. A very important factor influencing overall PCB disposition in fish is water temperature. Seasonal changes in water temperature produce adaptive physiological and biochemical changes in fish. While uptake of PCBs from the diet is similar in fish acclimated to winter or summer temperatures, there is evidence that elimination of PCBs occurs much more slowly when the fish is acclimated at low temperatures than at warmer temperatures. Research to date suggests that the processes of elimination of PCBs are modulated by several factors in fish including seasonal changes in water temperature. Thus, the body burden of PCBs in fish from a contaminated location is likely to vary with season.

In vitro biotransformation of decabromodiphenyl ether (BDE-209) and Dechlorane Plus flame retardants: a case study of ring-billed gull breeding in a pollution hotspot in the St. Lawrence River, Canada

Decabromodiphenyl ether (deca-BDE) mixture (~97% of BDE-209) is now facing usage restrictions worldwide, which is leading to increased utilization of a series of alternative, replacement flame retardant (FR) products. Among these, Dechlorane Plus (DP) is receiving growing attention as this FR is increasingly being detected in wildlife samples, including birds from North America, Europe and Asia. Recent survey conducted in a known FR hotspot in the St. Lawrence River basin near Montreal (QC, Canada) revealed unexpectedly high detection frequencies and concentrations of BDE-209 and DP isomers (syn- and anti-DP) in the liver of breeding ring-billed gulls (Larus delawarensis) (RBGUs). Despite the global distribution of these current-use FRs, there is to our knowledge no study that has addressed the in vitro biotransformation of BDE-209 and DP isomers in birds. This study aimed at understanding the in vitro metabolism of BDE-209 and syn- and anti-DP using liver microsomes of Montreal-breeding RBGUs. Although BDE-15 (positive assay control) was consistently and positively depleted over the 90-min time frame of the in vitro assay, no depletion was observed for BDE-209 and DP isomers. These results suggest that CYP isoenzyme-mediated reductive dehalogenation of BDE-209 and DP is not likely to be a substantial metabolic pathway in RBGUs. However, investigations on deiodinases (expression, activity) should be considered in future studies as these enzymes have been suggested to be involved in the sequential debromination of BDE-209 in fish and human studies. High levels of BDE-209 determined in liver of RBGUs that strongly correlated with those of known or suggested BDE-209 debromination products (hepta- through nona-BDEs) may thus be indicative of concomitant dietary (e.g., fish consumption) and environmental exposure in the greater Montreal area, combined with poor or lack of metabolic capability toward these FRs.