Primary role of cytochrome P450 2B6 in the oxidative metabolism of 2,2',4,4',6-pentabromodiphenyl ether (BDE-100) to hydroxylated BDEs

Mechanistic insight into biotransformation of novel triazine-based flame retardant 1,3,5-tris(2,3-dibromopropyl)-1,3,5-triazinane-2,4,6-trione by human cytochrome P450s

The escalating focus on the environmental occurrence and toxicology of emerging pollutants underscores the imperative need for a profound exploration of their metabolic transformations mediated by human CYP450 enzymes. Such investigations have the potential to unravel the intricate metabolite profiles, substantially altering the toxicological outcomes. In this study, we integrated the computational simulations with in vitro metabolism experiments to investigate the metabolic activity and mechanism of an emerging pollutant, 1,3,5-tris(2,3-dibromopropyl)-1,3,5-triazinane-2,4,6-trione (TDBP-TAZTO), catalyzed by human CYP450s. The results highlight the important contributions of CYP2E1, 3A4 and 2C9 to the biotransformation of TDBP-TAZTO, leading to the identification of four distinct metabolites. The effective binding conformations governing biotransformation reactions of TDBP-TAZTO within active CYP450s are unveiled. Structural instability of primary hydroxyTDBP-TAZTO products suggests three potential outcomes: (1) generation of an alcohol metabolite through successive debromination and reduction reactions, (2) formation of a dihydroxylated metabolite through secondary hydroxylation by CYP450, and (3) production of an N-dealkylated metabolite via decomposition and isomerization reactions in the aqueous environment. The formation of a desaturated debrominated metabolite may arise from H-abstraction and barrier-free Br release during the primary oxidation, potentially competing with the generation of hydroxyTDBP-TAZTO. These findings provide detailed mechanistic insight into TDBP-TAZTO biotransformation by CYP450s, which can enrich our understanding of the metabolic fate and associated health risk of this chemical.

Update of the risk assessment of polybrominated diphenyl ethers (PBDEs) in food

The European Commission asked EFSA to update its 2011 risk assessment on polybrominated diphenyl ethers (PBDEs) in food, focusing on 10 congeners: BDE-28, -47, -49, -99, -100, -138, -153, -154, -183 and ‑209. The CONTAM Panel concluded that the neurodevelopmental effects on behaviour and reproductive/developmental effects are the critical effects in rodent studies. For four congeners (BDE-47, -99, -153, -209) the Panel derived Reference Points, i.e. benchmark doses and corresponding lower 95% confidence limits (BMDLs), for endpoint-specific benchmark responses. Since repeated exposure to PBDEs results in accumulation of these chemicals in the body, the Panel estimated the body burden at the BMDL in rodents, and the chronic intake that would lead to the same body burden in humans. For the remaining six congeners no studies were available to identify Reference Points. The Panel concluded that there is scientific basis for inclusion of all 10 congeners in a common assessment group and performed a combined risk assessment. The Panel concluded that the combined margin of exposure (MOET) approach was the most appropriate risk metric and applied a tiered approach to the risk characterisation. Over 84,000 analytical results for the 10 congeners in food were used to estimate the exposure across dietary surveys and age groups of the European population. The most important contributors to the chronic dietary Lower Bound exposure to PBDEs were meat and meat products and fish and seafood. Taking into account the uncertainties affecting the assessment, the Panel concluded that it is likely that current dietary exposure to PBDEs in the European population raises a health concern.

New insight into molecular mechanism of P450-Catalyzed metabolism of emerging contaminants and its consequence for human health: A case study of preservative methylparaben

Hydroxylated metabolites in the living body are considered as a potential biomarker of exposure to emerging contaminations (ECs) and breast cancer, but their formation mechanism has not received enough attention. Besides, the adverse impacts of metabolites during the metabolic transformation of ECs largely remain unknown. In this study, we employed a density functional calculation combing with in-vitro incubation of human liver microsomes to explore the bio-transformation of preservative methylparaben (MPB) in human bodies. Our results showed that hydroxylated metabolites of MPB (OH-MPB) were observed experimentally, while a formation mechanism was revealed at the molecular level. That is, hydroxylated metabolite was exclusively formed via the hydrogen abstraction from the phenolic hydroxyl group of MPB followed by the OH-rebound pathway, rather than the direct hydroxylation on the benzene ring. The increasing of hydroxyl groups on ECs could improve the metabolisms. This was confirmed in the metabolism of ECs without hydroxyl group and with multiple-hydroxyl groups, respectively. Furthermore, toxicity assessments show that compared to parent MPB, the hydroxylated metabolites have increased negative impacts on the gastrointestinal system and liver. A semiquinone product exhibits potential damage in the cardiovascular system and epoxides are toxic to the blood and gastrointestinal system. The findings deepen our insight into the biotransformation of parabens in human health, especially by providing health warnings about the potential impacts caused by semiquinone and epoxides.

Polybrominated diphenyl ethers in type 2 diabetes mellitus cases and controls: Repeated measurements prior to and after diagnosis

BACKGROUND

Previous studies have reported associations between certain persistent organic pollutants (POPs) and type 2 diabetes mellitus (T2DM). Polybrominated diphenyl ethers (PBDEs) are a class of POPs that are found in increasing concentrations in humans. Although obesity is a known risk factor for T2DM and PBDEs are fat-soluble, very few studies have investigated associations between PBDEs and T2DM. No longitudinal studies have assessed associations between repeated measurements of PBDE and T2DM in the same individuals and compared time trends of PBDEs in T2DM cases and controls.

OBJECTIVES

To investigate associations between pre- and post-diagnostic measurements of PBDEs and T2DM and to compare time trends of PBDEs in T2DM cases and controls.

METHODS

Questionnaire data and serum samples from participants in the Tromsø Study were used to conduct a longitudinal nested case-control study among 116 T2DM cases and 139 controls. All included study participants had three pre-diagnostic blood samples (collected before T2DM diagnosis in cases), and up to two post-diagnostic samples after T2DM diagnosis. We used logistic regression models to investigate pre- and post-diagnostic associations between PBDEs and T2DM, and linear mixed-effect models to assess time trends of PBDEs in T2DM cases and controls.

RESULTS

We observed no substantial pre- or post-diagnostic associations between any of the PBDEs and T2DM, except for BDE-154 at one of the post-diagnostic time-points (OR = 1.65, 95% CI: 1.00, 2.71). The overall time trends of PBDE concentrations were similar for cases and controls.

DISCUSSION

The study did not support PBDEs increasing the odds of T2DM, prior to or after T2DM diagnosis. T2DM status did not influence the time trends of PBDE concentrations.

Multidisciplinary Insights into the Structure-Function Relationship of the CYP2B6 Active Site

Cytochrome P450 2B6 (CYP2B6) is a highly polymorphic human enzyme involved in the metabolism of many clinically relevant drugs, environmental toxins, and endogenous molecules with disparate structures. Over the last 20-plus years, in silico and in vitro studies of CYP2B6 using various ligands have provided foundational information regarding the substrate specificity and structure-function relationship of this enzyme. Approaches such as homology modeling, X-ray crystallography, molecular docking, and kinetic activity assays coupled with CYP2B6 mutagenesis have done much to characterize this originally neglected monooxygenase. However, a complete understanding of the structural details that make new chemical entities substrates of CYP2B6 is still lacking. Surprisingly little in vitro data has been obtained about the structure-function relationship of amino acids identified to be in the CYP2B6 active site. Since much attention has already been devoted to elucidating the function of CYP2B6 allelic variants, here we review the salient findings of in silico and in vitro studies of the CYP2B6 structure-function relationship with a deliberate focus on the active site. In addition to summarizing these complementary approaches to studying structure-function relationships, we note gaps/challenges in existing data such as the need for more CYP2B6 crystal structures, molecular docking results with various ligands, and data coupling CYP2B6 active site mutagenesis with kinetic parameter measurement under standard expression conditions. Harnessing in silico and in vitro techniques in tandem to understand the CYP2B6 structure-function relationship will likely offer further insights into CYP2B6-mediated metabolism. SIGNIFICANCE STATEMENT: The apparent importance of cytochrome P450 2B6 (CYP2B6) in the metabolism of various xenobiotics and endogenous molecules has grown since its discovery with many in silico and in vitro studies offering a partial description of its structure-function relationship. Determining the structure-function relationship of CYP2B6 is difficult but may be aided by thorough biochemical investigations of the CYP2B6 active site that provide a more complete pharmacological understanding of this important enzyme.

Treatment of PBDEs from Soil-Washing Effluent by Granular-Activated Carbon: Adsorption Behavior, Influencing Factors and Density Functional Theory Calculation

Soil-washing is a potential technology for the disposal of soil contaminated by e-waste; however, the produced soil-washing effluent will contain polybrominated diphenyl ethers (PBDEs) and a large number of surfactants, which are harmful to the environment, so the treatment of PBDEs and the recycling of surfactants are the key to the application of soil-washing technology. In this study, coconut shell granular-activated carbon (GAC) was applied to remove PBDEs from Triton X-100 (TX-100) surfactant which simulates soil-washing effluent. The adsorption results show that, GAC can simultaneously achieve effective removal of 4,4′-dibromodiphenyl ether (BDE-15) and efficient recovery of TX-100. Under optimal conditions, the maximum adsorption capacity of BDE-15 could reach 623.19 μmol/g, and the recovery rate of TX-100 was always higher than 83%. The adsorption process of 4,4′-dibromodiphenyl ether (BDE-15) by GAC could best be described using the pseudo-second-order kinetic model and Freundlich isothermal adsorption model. The coexistence ions had almost no effect on the removal of BDE-15 and the recovery rate of TX-100, and the solution pH had little effect on the recovery rate of TX-100; BDE-15 had the best removal effect under the condition of weak acid to weak base, indicating that GAC has good environmental adaptability. After adsorption, GAC could be regenerated with methanol and the adsorption effect of BDE-15 could still reach more than 81%. Density functional theory (DFT) calculation and characterization results showed that, Van der Waals interaction and π–π interaction are dominant between BDE-15 and GAC, and hydrogen bond interactions also exist. The existence of oxygen-containing functional groups is conducive to the adsorption of BDE-15, and the carboxyl group (-COOH) has the strongest promoting effect. The study proved the feasibility of GAC to effectively remove PBDEs and recover surfactants from the soil-washing effluent, and revealed the interaction mechanism between PBDEs and GAC, which can provide reference for the application of soil-washing technology.

Insights into the metabolic mechanism of PBDEs catalyzed by cytochrome P450 enzyme 3A4: A QM/MM study

Elucidating the metabolic mechanism and the derivatives of polybrominated diphenyl ethers (PBDEs) is significant to risk assessment. This study delineated the metabolic mechanism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) catalyzed by P450 enzymes using a combination of molecular dynamic (MD), quantum mechanics/molecular mechanics (QM/MM) and density functional theory (DFT). The calculation results reveal that the electrophilic addition is the main pathway for the biotransformation of BDE-47 catalyzed by P450 enzymes. 6-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH-BDE-47) is a more kinetically preferable product than 5-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (5-OH-BDE-47). Electrophilic addition reaction can lead to the formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). The ecotoxicity assessment indicates that the final products of BDE-47 are still toxic to aquatic organisms, but the solubility increase of the hydroxylated products can accelerate their excretion from the body. We expect that the established metabolic mechanism and the derivatives will be used to predict the biotransformation of other PBDE congeners catalyzed by P450 enzymes in human livers.

An inadvertent issue of human retina exposure to endocrine disrupting chemicals: A safety assessment

Endocrine disrupting chemicals (EDCs) are a group of chemical compounds that present a considerable public health problem due to their pervasiveness and associations with chronic diseases. EDCs can interrupt the endocrine system and interfere with hormone homeostasis, leading to abnormalities in human physiology. Much attention has been focused on the adverse effects EDCs have on the reproductive system, neurogenesis, neuroendocrine system, and thyroid dysfunction. The eye is usually directly exposed to the surrounding environment; however, the influences of EDCs on the eye have received comparatively little attention. Ocular diseases, such as ocular surface diseases and retinal diseases, have been implicated in hormone deficiency or excess. Epidemiologic studies have shown that EDC exposure not only causes ocular surface disorders, such as dry eye, but also associates with visual deficits and retinopathy. EDCs can pass through the human blood-retinal barrier and enter the neural retina, and can then accumulate in the retina. The retina is an embryologic extension of the central nervous system, and is extremely sensitive and vulnerable to EDCs that could be passed across the placenta during critical periods of retinal development. Subtle alterations in the retinal development process usually result in profound immediate, long-term, and delayed effects late in life. This review, based on extensive literature survey, briefly summarizes the current knowledge about the impact of representative manufactured EDCs on retinal toxicity, including retinal structure alterations and dysfunction. We also highlight the potential mechanism of action of EDCs on the retina, and the predictive retinal models of EDC exposure.

Occupational and dietary differences in hydroxylated and methoxylated PBDEs and metals in plasma from Puget Sound, Washington, USA region volunteers

Electronic waste (E-waste) recycling is a rapidly growing occupation in the USA with the potential for elevated exposure to flame retardants and metals associated with electronic devices. We previously measured polybrominated diphenyl ethers (PBDEs) in plasma from E-waste workers and found them similar to non-E-waste workers. This study focused on structurally related PBDE derivatives, the hydroxylated (OH-PBDEs) and methoxylated (MeO-PBDEs) forms along with metals known to occur in E-waste. Humans can metabolize PBDEs and some MeO-PBDEs into OH-PBDEs, which is a concern due to greater health risks associated with OH-PBDEs. We measured 32 different OH-PBDEs and MeO-PBDEs in plasma samples provided by 113 volunteers living in the greater Puget Sound region of Washington State, USA. We measured 14 metals in a subset of 10 E-waste and 10 non-E-waste volunteers. Volunteers were selected based on occupational and dietary habits: work outdoors and consume above average amounts of seafood (outdoor), electronic waste recycling (E-waste) or non-specific indoor occupations (indoor). A two-week food consumption diary was obtained from each volunteer prior to blood sampling. OH-PBDEs were detected in all volunteers varying between 0.27 and 102 ng/g/g-lipid. The MeO-PBDEs were detected in most, but not all volunteers varying between n.d. - 60.4 ng/g/g-lipid. E-waste recyclers had OH-PBDE and MeO-PBDE plasma levels that were similar to the indoor group. The outdoor group had significantly higher levels of MeO-PBDEs, but not OH-PBDEs. Comparison of plasma concentrations of BDE-47 with its known hydroxylated metabolites suggested OH-PBDE levels were likely determined by biotransformation and at least two subpopulations identified differing in their apparent rates of OH-PBDE formation. The metals analysis indicated no significant differences between E-waste workers and non-E-waste workers. Our results indicate E-waste workers do not have elevated plasma levels of these contaminants relative to non-E-waste workers.

Retention of polybrominated diphenyl ethers and hydroxylated metabolites in paired human serum and milk in relation to CYP2B6 genotype

Polybrominated diphenyl ethers (PBDEs) and their hydroxylated metabolites (OH-BDEs) are endocrine disrupting compounds prevalent in human serum and breast milk. Retention of PBDEs and OH-BDEs in humans may be affected by differences in PBDE metabolism due to variants in cytochrome P450 2B6 (CYP2B6). The objectives of this study are to assess the partitioning profiles of PBDEs and OH-BDEs in forty-eight paired human serum and milk samples, and to evaluate the relationship between variants in CYP2B6 genotype and PBDE and OH-BDE accumulation in humans. Results show that the geometric mean (GM) concentrations of PBDEs are similar in serum (GM = 43.4 ng/g lipid) and milk samples (GM = 52.9 ng/g lipid), while OH-BDEs are retained primarily in serum (GM = 2.31 ng/g lipid), compared to milk (GM = 0.045 ng/g lipid). Participants with CYP2B6*6 genotype had a greater relative retention of PBDEs in serum and milk, and significant relationships (p <  0.05) were also observed for PBDE-47, 5-OH-BDE-47 and 6-OH-BDE-47 concentrations relative to CYP2B6*5 and CYP2B6*6 genotypes. These results are the first to show that CYP2B6 genotype is significantly related to the relative retention of PBDEs in humans, which may have direct implications for variability in the susceptibility of individuals to the potential adverse effects of these contaminants.

Degradation of 2,2',4,4'-tetrabromodiphenyl ether by Pycnoporus sanguineus in the presence of copper ions

The degradation of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by Pycnoporus sanguineus was investigated in order to explore the impact of the heavy metal Cu²⁺ on BDE-47 decomposition and the subsequent formation of metabolites, as well as to further elucidate the degradation mechanism of BDE-47. An increase in degradation rate from 18.63% to 49.76% in the first four days and its stabilization at (51.26 ± 0.08)% in the following days of BDE-47 incubation were observed. The presence of Cu²⁺ at 1 and 2 mg/L was found to promote the degradation rate to 56.41% and 60.79%, respectively, whereas higher level of Cu²⁺ (≥5 mg/L) inhibited the removal of BDE-47. The similar concentration effects of Cu²⁺ was also found on contents of fungal protein and amounts of metabolites. Both intracellular and extracellular enzymes played certain roles in BDE-47 transportation with the best degradation rate at 27.90% and 27.67% on the fourth and third day, individually. During the degradation of BDE-47, four types of hydroxylated polybrominated diphenyl ethers (OH-PBDEs), i.e., 6'-OH-BDE-47, 5'-OH-BDE-47, 4'-OH-BDE-17, 2'-OH-BDE-28, and two bromophenols, i.e., 2,4-DBP and 4-BP were detected and considered as degradation products. These metabolites were further removed by P. sanguineus at rates of 22.42%, 23.01%, 27.04%, 27.96%, 64.21%, and 40.62%, respectively.

Assessing Surface Sediment Contamination by PBDE in a Recharge Point of Guarani Aquifer in Ribeirão Preto, Brazil

Polybrominated diphenyl ethers (PBDEs) are used as flame retardants in several products, although they can act as neurotoxic, hepatotoxic and endocrine disruptors in organisms. In Brazil, their levels in aquatic sediments are poorly known; thus, concerns about the degree of exposure of the Brazilian population to PBDEs have grown. This study aimed to quantify the presence of PBDEs in sediment samples from an important groundwater water supply in Ribeirao Preto, Brazil, and to contribute to studies related to the presence of PBDEs in Brazilian environments. Gas chromatography coupled with Electron Capture Detection (GC-ECD) was used for quantification after submitting the samples to ultrasound-assisted extraction and clean-up steps. Results showed the presence of six PBDE, BDE-47 being the most prevalent in the samples, indicating a major contamination of the penta-PBDE commercial mixture. The concentration of ΣPBDEs (including BDE-28, -47, -66, -85, -99, -100, -138, -153, -154 and -209) varied between nd (not detected) to 5.4 ± 0.2 ng g−1. Although preliminary, our data show the anthropic contamination of a direct recharge area of the Aquifer Guarani by persistent and banned substances.

Sulfonation and glucuronidation of hydroxylated bromodiphenyl ethers in human liver

Hydroxylated bromodiphenyl ethers (OH-BDEs) can arise from monooxygenation of anthropogenic BDEs or through natural biosynthetic processes in marine organisms, and several OH-BDEs have been shown to be toxic. OH-BDEs are expected to form sulfate and glucuronide conjugates that are readily excreted, however there is little information on these pathways. We examined the human hepatic glucuronidation and sulfonation of 6-OH-BDE47, 2-OH-BDE68, 4-OH-BDE68 and 2-OH-6'methoxy-BDE68. Human liver microsomes and cytosol were from de-identified female and male donors aged 31 to 75 under an exempt protocol. Recombinant human SULT1A1, 1B1, 1E1 and 2A1 enzymes were prepared from bacterial expression systems. Sulfonation and glucuronidation of each OH-BDE were studied using radiolabeled co-substrates, 3'phosphoadenosine-5'phospho-³⁵S-sulfate or uridine diphospho-β-D-¹⁴C-glucuronic acid in order to quantify the sulfated or glucuronidated products. The OH-BDEs studied were more efficiently glucuronidated than sulfonated. Of the compounds studied, 2-OH-BDE68 was the most readily conjugated, and exhibited an efficiency (V_max/K_M) of glucuronidation of 0.274 ± 0.125 mL/min/mg protein, mean ± S.D., n = 3, while that for sulfonation was 0.179 ± 0.030 mL/min/mg protein. For both pathways, all K_m values were in the low μM range. Studies with human SULT enzymes showed that sulfonation of these four substrates was readily catalyzed by SULT1B1 and SULT1E1. Much lower activity was found with SULT1A1 and SULT2A1. Assuming that the glucuronide and sulfate conjugates are non-toxic and readily excreted, as is the case for most such conjugates, these studies suggest that OH-BDEs should not accumulate in people to the same extent as the parent BDEs.

Bioaccessibility of BDE 47 in a simulated gastrointestinal system and its metabolic transformation mechanisms in Caco-2 cells

Polybrominated diphenyl ethers (PBDEs) have been regarded as ubiquitous environmental pollutants. However, the absorption and transformation of these compounds after ingestion are not well understood yet. In this study, the bioaccessibility and metabolic pathway of 2,2',4,4'- tetrabromodiphenyl ether (BDE47) was investigated in an in vitro digestion/Caco-2 cell. Gastric and intestinal bioaccessibilities of BDE47 in 5 kinds of spiked soil samples were ranging from 11.39 ± 0.83% to 36.02 ± 4.34%, and 48.24 ± 3.24% to 81.52 ± 6.43%, respectively. Upon exposure to differentiated Caco-2 cells for 6 h, it was found that only a small amount of BDE47 in the gastrointestinal (GI) solution could pass through Caco-2 cells, and might enter the body. Moreover, BDE47 was found to be metabolized or transformed into BDE28, BDE75, BDE37, BDE32, BDE15 and BDE8 in Caco-2 cells. The metabolic pathway could be explained by using the Becke three-parameter hybrid functional (B3Lucifer yellow CHP) in the Density Functional Theory (DFT), denoted as the values of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at the atoms of BDE47 and its metabolic products. The obtained results suggest that oral intake of PBDEs is associated with low bioaccessibility, but also emphasize the risks associated with oral ingestion, namely toxicity resulting from the debromination of highly brominated diphenyl ethers. Although highly brominated diphenyl ethers are known to be the least toxic PBDEs, the debrominated products in human intestinal epithelia may elicit greater than expected toxicity.

Persistent halogenated organic pollutants in follicular fluid of women undergoing in vitro fertilization from China: Occurrence, congener profiles, and possible sources

Analysis of persistent halogenated organic pollutants (HOPs) in human follicular fluid is important given previous reports of their adverse effects on the reproductivity of women. In the present work, HOPs, including polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs), were analyzed in 127 follicular fluid samples collected from patients who were undergoing in vitro fertilization in Central China. The concentrations of ∑₇BDEs (sum of BDE-28, -47, -99, -100, -153, -154, and -183) in follicular fluid ranged from not detected (n.d.) to 110 ng/g lipid weight (lw), with an average of 50 ± 24 ng/g lw. BDE-100 was suggested to be an indicator of BDE congeners in follicular fluid, with the highest concentrations and showing a significantly high correlation (p < 0.01) with ∑₇BDEs. Penta-BDE products were the principal source of PBDEs in follicular fluid samples. The concentrations of ∑₇CBs (CB-28, -52, -101, -118, -138, -153, and -180) in follicular fluids ranged from n.d. to 250 ng/g lw, with an average of 77 ± 69 ng/g lw. CB-28 and CB-52 were considered to be indicator CB congeners, with tri-CBs and tetra-CBs dominating in follicular fluid. No significant correlation was observed between patient age and PBDE or PCB concentrations in follicular fluid, indicating that age was not the controlling factor influencing the bioaccumulation of most HOPs in this study.

Use of Phenoxyaniline Analogues To Generate Biochemical Insights into the Interactio n of Polybrominated Diphenyl Ether with CYP2B Enzymes

Human hepatic cytochromes P450 (CYP) are integral to xenobiotic metabolism. CYP2B6 is a major catalyst of biotransformation of environmental toxicants, including polybrominated diphenyl ethers (PBDEs). CYP2B substrates tend to contain halogen atoms, but the biochemical basis for this selectivity and for species specific determinants of metabolism has not been identified. Spectral binding titrations and inhibition studies were performed to investigate interactions of rat CYP2B1, rabbit CYP2B4, and CYP2B6 with a series of phenoxyaniline (POA) congeners that are analogues of PBDEs. For most congeners, there was a <3-fold difference between the spectral binding constants (K_S) and IC₅₀ values. In contrast, large discrepancies between these values were observed for POA and 3-chloro-4-phenoxyaniline. CYP2B1 was the enzyme most sensitive to POA congeners, so the Val-363 residue from that enzyme was introduced into CYP2B4 or CYP2B6. This substitution partially altered the protein-ligand interaction profiles to make them more similar to that of CYP2B1. Addition of cytochrome P450 oxidoreductase (POR) to titrations of CYP2B6 with POA or 2'4'5'TCPOA decreased the affinity of both ligands for the enzyme. Addition of cytochrome b₅ to a recombinant enzyme system containing POR and CYP2B6 increased the POA IC₅₀ value and decreased the 2'4'5'TCPOA IC₅₀ value. Overall, the inconsistency between K_S and IC₅₀ values for POA versus 2'4'5'TCPOA is largely due to the effects of redox partner binding. These results provide insight into the biochemical basis of binding of diphenyl ethers to human CYP2B6 and changes in CYP2B6-mediated metabolism that are dependent on POA congener and redox partner identity.

Biodegradation of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by Phanerochaete chrysosporium in the presence of Cd2

Aerobic biodegradation of 2,2',4,4'-tetrabrominated diphenyl ether (BDE-47) by Phanerochaete chrysosporium in the presence of Cd²⁺ was investigated in this study. The results showed that P. chrysosporium could effectively degrade BDE-47, and its extracellular enzyme played an important role in the process of decomposition. BDE-47 biodegradation by fungi was more tolerant than extracellular enzyme in the presence of Cd²⁺. Also, both of the activity of two typical enzymes, MnP and LiP, descended with ascended Cd²⁺ concentration. Based on the four mono-hydroxylated PBDEs (5-OH-BDE-47, 4'-OH-BDE-17, 6-OH-BDE-47, and 2'-OH-BDE-28) and two bromophenols (2,4-DBP, 4-BP) detected, three possible degradation pathways were proposed, inferring that BDE-47 was more easily to transform via hydroxylation. With addition of Cd²⁺, the types of degradation products did not change, merely a variation of the content of these products observed. Meanwhile, the major metabolites of BDE-47, bromophenol compounds, have been found to be transformed or even mineralized by P. chrysosporium quickly, which also helped better explain why the amounts of BDE-47 decomposed did not match with that of the metabolites detected.

RNA Sequencing Reveals Age and Species Differences of Constitutive Androstane Receptor-Targeted Drug-Processing Genes in the Liver

The constitutive androstane receptor (CAR/Nr1i3) is an important xenobiotic-sensing nuclear receptor that is highly expressed in the liver and is well known to have species differences. During development, age-specific activation of CAR may lead to modified pharmacokinetics and toxicokinetics of drugs and environmental chemicals, leading to higher risks for adverse drug reactions in newborns and children. The goal of this study was to systematically investigate the age- and species-specific regulation of various drug-processing genes (DPGs) after neonatal or adult CAR activation in the livers of wild-type, CAR-null, and humanized CAR transgenic mice. At either 5 or 60 days of age, the three genotypes of mice were administered a species-appropriate CAR ligand or vehicle once daily for 4 days (i.p.). The majority of DPGs were differentially regulated by age and/or CAR activation. Thirty-six DPGs were commonly upregulated by CAR activation regardless of age or species of CAR. Although the cumulative mRNAs of uptake transporters were not readily altered by CAR, the cumulative phase I and phase II enzymes as well as efflux transporters were all increased after CAR activation in both species. In general, mouse CAR activation produced comparable or even greater fold increases of many DPGs in newborns than in adults; conversely, humanized CAR activation produced weaker induction in newborns than in adults. Western blotting and enzyme activity assays confirmed the age and species specificities of selected CAR-targeted DPGs. In conclusion, this study systematically compared the effect of age and species of CAR proteins on the regulation of DPGs in the liver and demonstrated that the regulation of xenobiotic biotransformation by CAR is profoundly modified by age and species.

Integrated Framework for Identifying Toxic Transformation Products in Complex Environmental Mixtures

Complex environmental mixtures consist of hundreds to thousands of unknown and unregulated organic compounds that may have toxicological relevance, including transformation products (TPs) of anthropogenic organic pollutants. Non-targeted analysis and suspect screening analysis offer analytical approaches for potentially identifying these toxic transformation products. However, additional tools and strategies are needed in order to reduce the number of chemicals of interest and focus analytical efforts on chemicals that may pose risks to humans and the environment. This brief review highlights recent developments in this field and suggests an integrated framework that incorporates complementary instrumental techniques, computational chemistry, and toxicity analysis, for prioritizing and identifying toxic TPs in the environment.

Mechanisms of interaction between persistent organic pollutants (POPs) and CYP2B6: An in silico approach

Human Cytochrome P450s (CYP450) are a group of heme-containing metalloenzymes responsible for recognition and metabolism of numerous xenobiotics, including drugs and environmental contaminants. CYP2B6, a member of CYP450, is well known for being a highly inducible and polymorphic enzyme and for its important role in the oxidative metabolism of environmental pollutants, such as the Polybrominated Diphenyl Ethers (PBDEs) and Polychlorinated Biphenyls (PCBs). However the mechanisms of interaction of PBDEs and PCBs with CYP2B6 is not entirely known. In this work, a computational approach was carried out to study the interactions of 41 POPs (17 PBDEs, 17 PCBs, and 7 Dioxins) with four CYP2B6 protein structures downloaded from PDB data base (PDB: 3UA5, 3QOA, 3QU8 and 4I91) using molecular docking protocols with AutoDock Vina. The best binding affinity values (kcal/mol) were obtained for PBDE-99 (-8.5), PCB-187 (-9.6), and octachloro-dibenzo-dioxin (-9.8) that can be attributed to the hydrophobic interactions with important residues, such as Phe-363, in the catalytic site of CYP2B6. Molecular docking validation revealed the best values for PDB: 3UA5 (R = 0.622, p = 0.001) demonstrating the reliability of molecular docking predictions. The information obtained in this work can be useful in evaluating the modes of interaction of xenobiotic compounds with the catalytic site of CYP2B6 and provide insights on the important role of these enzymes in the metabolism of potentially toxic compounds in humans.

How PBDEs Are Transformed into Dihydroxylated and Dioxin Metabolites Catalyzed by the Active Center of Cytochrome P450s: A DFT Study

Predicting metabolism of chemicals and potential toxicities of relevant metabolites remains a vital and difficult task in risk assessment. Recent findings suggested that polybrominated diphenyl ethers (PBDEs) can be transformed into dihydroxylated and dioxin metabolites catalyzed by cytochrome P450 enzymes (CYPs), whereas the mechanisms pertinent to these transformations remain largely unknown. Here, by means of density functional theory (DFT) calculations, we probed the metabolic pathways of 2,2',4,4'-tetraBDE (BDE-47) using the active center model of CYPs (Compound I). Results show that BDE-47 is first oxidized to monohydroxylated products (HO-BDEs), wherein a keto-enol tautomerism is identified for rearrangement of the cyclohexenone intermediate. Dihydroxylation with HO-BDEs as precursors, has a unique phenolic H-abstraction and hydroxyl rebound pathway that is distinct from that for monohydroxylation, which accounts for the absence of epoxides in in vitro studies. Furthermore, we found only dihydroxylated PBDEs with heterophenyl -OH substituents ortho- and meta- to the ether bond serve as precursors for dioxins, which are evolved from aryl biradical coupling of diketone intermediates that are produced from dehydrogenation of the dihydroxylated PBDEs by Compound I. This study may enlighten the development of computational models that afford mechanism-based prediction of the xenobiotic biotransformation catalyzed by CYPs.

Monohydroxylated Polybrominated Diphenyl Ethers (OH-PBDEs) and Dihydroxylated Polybrominated Biphenyls (Di-OH-PBBs): Novel Photoproducts of 2,6-Dibromophenol

Hydroxylated polybromodiphenyl ethers (OH-PBDEs) are emerging aquatic pollutants, but their origins in the environment are not fully understood. There is evidence that OH-PBDEs are formed from bromophenols, but the underlying transformation processes remain unknown. Here, we investigate if the photoformation of OH-PBDEs from 2,6-dibromophenol in aqueous solution involves 2,6-bromophenoxyl radicals. After the UV irradiation of an aqueous 2,6-dibromophenol solution, HPLC-LTQ-Orbitrap MS and GC-MS analysis revealed the formation of a OH-PBDE and a dihydroxylated polybrominated biphenyl (di-OH-PBB). Both dimeric photoproducts were tentatively identified as 4'-OH-BDE73 and 4,4'-di-OH-PBB80. In addition, three debromination products (4-OH-BDE34, 4'-OH-BDE27, and 4,4'-di-OH-PBBs) were observed. Electron paramagnetic resonance spectroscopy revealed the presence of a 2,6-dibromophenoxyl radical with a six-line spectrum (a(H) (2 meta) = 3.45 G, a(H) (1 para) = 1.04 G, g = 2.0046) during irradiation of a 2,6-dibromophenol solution in water. The 2,6-dibromophenoxyl radical had a relatively long half-life (122 ± 5 μs) according to laser flash photolysis experiments. The para-para C-C and O-para-C couplings of these 2,6-dibromophenoxyl radicals are consistent with the observed formation of both dimeric OH-PBDE and di-OH-PBB photoproducts. These findings show that bromophenoxyl radical-mediated phototransformation of bromophenols is a source of OH-PBDEs and di-OH-PBBs in aqueous environments that requires further attention.

"One-shot" analysis of polybrominated diphenyl ethers and their hydroxylated and methoxylated analogs in human breast milk and serum using gas chromatography-tandem mass spectrometry

The presence of polybrominated diphenyl ethers (PBDEs) and their hydroxylated (OH-BDE) and methoxylated (MeO-BDE) analogs in humans is an area of high interest to scientists and the public due to their neurotoxic and endocrine disrupting effects. Consequently, there is a rise in the investigation of the occurrence of these three classes of compounds together in environmental matrices and in humans in order to understand their bioaccumulation patterns. Analysis of PBDEs, OH-BDEs, and MeO-BDEs using liquid chromatography-mass spectrometry (LC-MS) can be accomplished simultaneously, but detection limits for PBDEs and MeO-BDEs in LC-MS is insufficient for trace level quantification. Therefore, fractionation steps of the phenolic (OH-BDEs) and neutral (PBDEs and MeO-BDEs) compounds during sample preparation are typically performed so that different analytical techniques can be used to achieve the needed sensitivities. However, this approach involves multiple injections, ultimately increasing analysis time. In this study, an analytical method was developed for a "one-shot" analysis of 12 PBDEs, 12 OH-BDEs, and 13 MeO-BDEs using gas chromatography with tandem mass spectrometry (GC-MS/MS). This overall method includes simultaneous extraction of all analytes via pressurized liquid extraction followed by lipid removal steps to reduce matrix interferences. The OH-BDEs were derivatized using N-(t-butyldimethylsilyl)-N-methyltrifluoroacetamide (TBDMS-MTFA), producing OH-TBDMS derivatives that can be analyzed together with PBDEs and MeO-BDEs by GC-MS/MS in "one shot" within a 25-min run time. The overall recoveries were generally higher than 65%, and the limits of detection ranged from 2 to 14 pg in both breast milk and serum matrices. The applicability of the method was successfully validated on four paired human breast milk and serum samples. The mean concentrations of total PBDEs, OH-BDEs, and MeO-BDEs in breast milk were 59, 2.2, and 0.57 ng g(-1) lipid, respectively. In serum, the mean total concentrations were 79, 38, and 0.96 ng g(-1) lipid, respectively, exhibiting different distribution profiles from the levels detected in breast milk. This "one-shot" GC-MS/MS method will prove useful and cost-effective in large-scale studies needed to further understand the partitioning behavior, and ultimately the adverse health effects, of these important classes of brominated flame retardants in humans.