Chlordane and Heptachlor Are Metabolized Enantioselectively by Rat Liver Microsomes

Chlordane, heptachlor, and their metabolites are chiral persistent organic pollutants that undergo enantiomeric enrichment in the environment. This study investigated the enantioselective metabolism of both chlordane isomers and heptachlor, major components of technical chlordane, by liver microsomes prepared from male rats treated with corn oil (CO) or inducers of CYP2B (PB; phenobarbital) and CYP3A enzymes (DX; dexamethasone), isoforms induced by chlordane treatment. The extent of the metabolism of all three parent compounds was dependent on the microsomal preparation used and followed the rank order PB > DX > CO. The mass balances ranged from 49 to 130% of the parent compound added to the microsomal incubations. Both <i>cis</i>- and <i>trans-</i>chlordane were enantioselectively metabolized to oxychlordane (EF = 0.45–0.89) and 1,2-dichlorochlordene (EF = 0.42–0.90). Heptachlor was metabolized enantioselectively, with heptachlor epoxide B (EF = 0.44–0.54) being the only metabolite. Interestingly, the direction on the enrichment for oxychlordane, 1,2-dichlorochlordene, and heptachlor epoxide differed depending on the microsomal preparation. These findings demonstrate that the direction and extent of the enantioselective metabolism of both chlordane isomers and heptachlor is P450 isoform-dependent and can be modulated by the induction of P450 enzymes

Microsomal Oxidation of 2,2′,3,3′,6,6′-Hexachlorobiphenyl (PCB 136) Results in Species-Dependent Chiral Signatures of the Hydroxylated Metabolites

Chiral polychlorinated biphenyls (PCBs) display variable atropisomeric enrichment in wildlife and animal models, especially at higher trophic levels. These differences in PCBs’ chiral signatures are, at least in part, due to species-dependent oxidation of PCBs to hydroxylated PCB metabolites (OH-PCBs). Here, we investigate the hypothesis that the cytochrome P450 (P450) enzyme-mediated oxidation of chiral PCBs results in species-dependent differences in the chiral signatures of OH-PCBs (i.e., the direction and extent of OH-PCBs’ atropisomeric enrichment). To investigate this hypothesis, we incubated PCB 136, a representative chiral PCB, with pooled human liver microsomes (HLMs) or liver microsomes from male guinea pig, hamster, monkey, mouse, and rabbit or female dog and determined average profiles and chiral signatures of the OH-PCBs. 2,2′,3,3′,6,6′-Hexachlorobiphenyl-4-ol (4–136) was the major metabolite in incubations with HLMs and monkey and rabbit microsomes. 2,2′,3,3′,6,6′-Hexachlorobiphenyl-5-ol (5–136) was the major metabolite formed by microsomes from all other species. Both 4–136 and 5–136 were formed atropselectively in all microsomal incubations; however, the direction and extent of the atropisomeric enrichment of both OH-PCB metabolites showed considerable differences across microsomal preparations obtained from different species. These differences in OH-PCBs’ atropisomeric enrichment may not only be toxicologically relevant but may also be useful to study sources and transport of OH-PCBs in the environment

Oxidation of Polychlorinated Biphenyls by Liver Tissue Slices from Phenobarbital-Pretreated Mice Is Congener-Specific and Atropselective

Mouse models are powerful tools to study the developmental neurotoxicity of polychlorinated biphenyls (PCBs); however, studies of the oxidation of chiral PCB congeners to potentially neurotoxic hydroxylated metabolites (OH-PCBs) in mice have not been reported. Here, we investigate the atropselective oxidation of chiral PCB 91 (2,2′,3,4′,6-pentachlorobiphenyl), PCB 95 (2,2′,3,5′,6-pentachlorobiphenyl), PCB 132 (2,2′,3,3′,4,6′-hexachlorobiphenyl), PCB 136 (2,2′,3,3′,6,6′-hexachlorobiphenyl), and PCB 149 (2,2′,3,4′,5′,6-hexachlorobiphenyl) to OH-PCBs in liver tissue slices prepared from female mice. The metabolite profile of PCB 136 typically followed the rank order 5-OH-PCB > 4-OH-PCB > 4,5-OH-PCB, and metabolite levels increased with PCB concentration and incubation time. A similar OH-PCB profile was observed with the other PCB congeners, with 5-OH-PCB/4-OH-PCB ratios ranging from 2 to 12. More 5-OH-PCB 136 was formed in liver tissue slices obtained from animals pretreated with phenobarbital (P450 2B inducer) or, to a lesser extent, dexamethasone (P450 2B and 3A enzyme inducer) compared to tissue slices prepared from vehicle-pretreated animals. The apparent rate of 5-OH-PCBs formation followed the approximate rank order PCB 149 > PCB 91 > PCB 132 ∼ PCB 136 > PCB 95. Atropselective gas chromatography revealed a congener-specific atropisomeric enrichment of major OH-PCB metabolites. Comparison of our results with published OH-PCB patterns and chiral signatures (i.e., the direction and extent of the atropisomeric enrichment) from rat liver microsomal revealed drastic differences between both species, especially following the induction of P450 2B enzymes. These species differences in the metabolism of chiral PCBs should be considered in developmental neurotoxicity studies of PCBs

2,2′,3,5′,6-Pentachlorobiphenyl (PCB 95) Is Atropselectively Metabolized to para-Hydroxylated Metabolites by Human Liver Microsomes

Exposure to neurotoxic, chiral PCBs has been associated with neurodevelopmental disorders, but their metabolism in humans remains unexplored. We investigated the enantioselective metabolism of PCB 95 by human liver microsomes (HLMs) to potentially neurotoxic, hydroxylated metabolites (OH-PCBs). OH-PCB profiles formed in experiments with HLMs differed from metabolite profiles reported for rodent species. The second eluting atropisomer of 2,2′,3,5′,6-pentachlorobiphenyl-4′-ol, the major metabolite, was preferentially formed by all HLM preparations investigated. Differences in metabolite formation rates were observed with single donor HLMs. The metabolism of PCBs and its role in PCB-mediated neurodevelopmental disorders need to be further characterized

Human Liver Microsomes Atropselectively Metabolize 2,2′,3,4′,6-Pentachlorobiphenyl (PCB 91) to a 1,2-Shift Product as the Major Metabolite

Polychlorinated biphenlys (PCBs) and their hydroxylated metabolites (OH-PCBs) have been implicated in neurodevelopmental disorders. Several neurotoxic PCBs, such as PCB 91, are chiral because they form stable rotational isomers, or atropisomers, that are nonsuperimposable mirror images of each other. Because only limited information about the metabolism of these PCBs by human cytochrome P450 (P450) enzymes is available, we investigated the biotransformation of PCB 91 to OH-PCBs by human liver microsomes (HLMs). Racemic PCB 91 was incubated with pooled or individual donor HLMs at 37 °C, and levels and chiral signatures of PCB 91 and its metabolites were determined. Several OH-PCBs were formed in the order 2,2′,4,4′,6-pentachlorobiphenyl-3-ol (3–100; 1,2 shift product) > 2,2′,3,4′,6-pentachlorobiphenyl-5-ol (5–91) ≫ 2,2′,3,4′,6-pentachlorobiphenyl-4-ol (4–91) ≫ 4,5-dihydroxy-2,2′,3,4′,6-pentachlorobiphenyl (4,5–91). Metabolite formation rates displayed interindividual variability. The first eluting atropisomers of PCB 91, 3–100 and 4–91, and the second eluting atropisomer of 5–91 were enriched in most metabolism studies. The unexpected, preferential formation of a 1,2-shift product and the variability of the OH-PCBs profiles in experiments with individual donor HLMs underline the need for further systematic studies of the atropselective metabolism of PCBs in humans

Exposure to Bisphenol A, Bisphenol F, and Bisphenol S in U.S. Adults and Children: The National Health and Nutrition Examination Survey 2013–2014

Bisphenol F (BPF) and bisphenol S (BPS) are replacing bisphenol A (BPA) in the manufacturing of products containing polycarbonates and epoxy resins. Data on current human exposure levels of these substitutes are needed to aid in the assessment of their human health risks. This study analyzed urinary bisphenol levels in adults (<i>N</i> = 1808) and children (<i>N</i> = 868) participating in the National Health and Nutrition Examination Survey (NHANES) 2013–2014 and investigated demographic and lifestyle factors associated with urinary levels of bisphenols. BPA, BPS, and BPF were detected in 95.7, 89.4, and 66.5% of randomly selected urine samples analyzed as part of NHANES 2013–2014, respectively. Median levels of BPA in U.S. adult were higher (1.24 μg/L) than BPF and BPS levels (0.35 and 0.37 μg/L, respectively). For children, median BPA levels were also higher (1.25 μg/L) than BPF and BPS levels (0.32 and 0.29 μg/L, respectively). The limits of detection for BPA, BPF, and BPS were 0.2, 0.2, and 0.1 μg/L, respectively. Urinary levels showed associations with gender, race/ethnicity, family income, physical activity, smoking, and/or alcohol intake that depended on the specific bisphenol. The results of this study indicate that exposure of the general U.S. population to BPA substitutes is almost ubiquitous. Because exposures differ across the U.S. population, further studies of environmental, consumer, and lifestyle factors affecting BPF and BPS exposures are warranted

Microsomal Metabolism of Prochiral Polychlorinated Biphenyls Results in the Enantioselective Formation of Chiral Metabolites

Polychlorinated biphenyl (PCB) congeners with multiple <i>ortho</i> chlorine substituents and their metabolites exist as stable rotational isomers, or atropisomers, that are nonsuperimposable mirror images of each other. Additionally, the oxidation of certain axially prochiral PCBs, such as 2,2′,4,6′-tetrachlorobiphenyl (PCB 51) and 2,2′,4,5,6′-pentachlorobiphenyl (PCB 102), in the <i>meta</i> position of the symmetrically substituted phenyl ring is expected to form axially chiral hydroxylated metabolites (OH-PCBs); however, the formation of chiral OH-PCBs from prochiral PCBs has not been demonstrated experimentally. Here, we investigate if the oxidation of PCB 51 and PCB 102 by different microsomal preparations results in the formation of chiral OH-PCBs. Gas chromatographic analysis revealed that PCB 51 and PCB 102 were metabolized to 2,2′,4,6′-tetrachlorobiphenyl-3′-ol (OH-PCB 51) and 2,2′,4,5,6′-pentachlorobiphenyl-3′-ol (OH-PCB 102), respectively, by liver microsomes from male rats pretreated with different inducers; untreated male monkeys, guinea pigs, rabbits, and hamsters; and female dogs. The formation of both metabolites was inducer- and species-dependent. Both OH-PCB 51 and OH-PCB 102 were chiral and formed enantioselectively by all microsomal preparations investigated. These findings demonstrate that axially chiral PCB metabolites are formed from axially prochiral PCB congeners, a fact that should be considered when studying the environmental fate, transport, and toxicity of OH-PCBs

Density Functional Theory Study of Semiquinone Radical Anions of Polychlorinated Biphenyls in the Syn- and Anti-like Conformation

Polychlorinated biphenyls (PCBs) can be metabolized to reactive metabolites, such as PCB semiquinone radical anions (SQ^•–), whose structure and role in PCB-induced toxicity are difficult to investigate due to their relative instability. The unrestricted UB3LYP/6-311G** method was used to investigate several molecular descriptors of the syn- and anti-like conformation of SQs^•–. The bond lengths and angles of the quinone moiety of the SQs^•– were in between the values reported for PCB quinones and hydroquinones, which is consistent with the distribution of the α highest occupied molecular orbital (α-HOMO). The dihedral angles between the two ring systems increased in the presence of <i>ortho</i> chlorine substituents and were smaller compared to the corresponding PCB quinones. The ground-state energies indicate that the anti-like conformation of the SQs^•– is more favorable than the syn-like conformation. Molecular descriptor used for modeling of quantitative structure–activity relationships displayed some dependence on the conformation. These findings suggest that SQs^•– in both the syn- and antilike conformation may interact differently with target molecules, which may have implications for the toxicity of PCBs

Elimination of Inhaled 3,3′-Dichlorobiphenyl and the Formation of the 4‑Hydroxylated Metabolite

The recent discovery of 3,3′-dichlorobiphenyl (CB11) as a byproduct of pigment manufacturing underscores the urgency to investigate its biological fate. The high level and ubiquity of atmospheric CB11 indicates that inhalation is the major route of exposure. However, few data on its uptake and elimination exist. A time course study was performed exposing male Sprague–Dawley rats to CB11 via nose-only inhalation with necropsy at 0, 4, and 8 h post exposure. An analytical method for CB11 and monohydroxylated metabolites employing pressurized liquid extraction and gas chromatography–mass spectrometry yielded efficient recovery of CB11 (73 ± 9%) and its metabolite 3,3′-dichlorobiphenyl-4-ol (4-OH–CB11) (82 ± 12%). Each rat was exposed to 106 μg/m³ vapor-phase CB11 for 2 h and received an estimated dose of 1.8 μg. Rapid apparent first-order elimination of CB11 was found in lung, serum, and liver with half-lives of 1.9, 1.8, and 2.1 h, respectively. 4-OH–CB11 was detected in the liver but not the lung or serum of exposed animals and displayed apparent first-order elimination with a 2.4 h half-life. This study demonstrates rapid metabolism of CB11 and elimination of 4-OH–CB11 and suggests that the metabolite is not retained in the body but is susceptible to further biotransformation

Metabolism and Photolysis of 2,4-Dinitroanisole in <i>Arabidopsis</i>

New insensitive munitions explosives, including 2,4-dinitroanisole (DNAN), are replacing traditional explosive compounds to protect soldiers and simplify transport logistics. Despite the occupational safety benefits of these new explosives, feasible strategies for cleaning up DNAN from soil and water have not been developed. Here, we evaluate the metabolism of DNAN by the model plant <i>Arabidopsis</i> to determine whether phytoremediation can be used to clean up contaminated sites. Furthermore, we evaluate the role of photodegradation of DNAN and its plant metabolites within <i>Arabidopsis</i> leaves to determine the potential impact of photolysis on the phytoremediation of contaminants. When exposed to DNAN for three days, <i>Arabidopsis</i> took up and metabolized 67% of the DNAN in hydroponic solution. We used high resolution and tandem mass spectrometry in combination with stable-isotope labeled DNAN to confirm ten phase II DNAN metabolites in <i>Arabidopsis</i>. The plants separately reduced both the <i>para</i>- and <i>ortho</i>-nitro groups and produced glycosylated products that accumulated within plant tissues. Both DNAN and a glycosylated metabolite were subsequently photolyzed within leaf tissue under simulated sunlight, and [¹⁵N₂]­DNAN yielded ¹⁵NO₂^– in leaves. Therefore, photolysis inside leaves may be an important, yet under-explored, phytoremediation mechanism