Understanding enantioselective processes: a laboratory rat model for alpha-hexachlorocyclohexane accumulation

Tracking the transformation of persistent organic pollutants in food webs using multi element isotope and enantiomer fractionation

In order to track the transformation of persistent organic pollutants (POPs) in food webs, field experiments were conducted at two sites using stable isotope and enantiomer fractionation concepts. The enantiomers of α-hexachlorocyclohexane (α-HCH) were selected as representative compounds for POPs. Isotope and enantiomer fractionation allowed the characterization of α-HCH enantiomer biotransformation processes along trophic levels of the food web - from soil and plants to animal livers, fat tissues and milk. The enrichment of heavy isotopes in soils, plants and sediments as well as the changes of enantiomer fractionation indicate that the biotransformation of α-HCH occurred in these compartments. Moreover, the increase of carbon and chlorine isotopic compositions as well as the changes of enantiomer fractionation of liver, fat tissues and milk demonstrated that the overall HCH exposure was much higher than estimates based on concentration levels, while the isotope and enantiomer fractionation revealed the enantiomer specific enantiomer uptake across the blood-brain barriers. Dual element isotope analysis suggested that complex transformation processes have occurred along the potential food web from the HCH sources over different environmental compartments to animal livers, fat tissues and milk. The results imply that the analyses of stable isotope compositions and concentrations has potential to reconstruct the exposure of higher organisms to POPs.

Enantioselective characteristics, bioaccumulation and toxicological effects of chlordane-related compounds in laying hens

Chlordane-related compounds are ubiquitously detected in the environment and can transfer and accumulate to animals through food chain to cause adverse effects. In this study, the dynamic distribution and the enantiomeric profile of chlordane-related compounds in laying hens over time were investigated. The effect of these compounds on immune-associated cells in the intestinal tract and histopathology in some tissues were also evaluated after long-term exposure. The chlordane-related compounds preferentially accumulated in fat, followed by the intestines, ovum, and egg yolk during long-term exposure. The metabolites heptachlor epoxide and oxychlordane were mainly formed in the liver of hens by epoxidation or hydroxylation. The high accumulation ratios of trans-nonachlor and MC5 were found in ovum and egg yolk after long-term exposure, implying a greater risk to the hens' offspring. Chlordane-related compounds may cause abnormal lipometabolism and glycometabolism in liver of hens. Additionally, (-)-Cis-chlordane was dominant in all tissues of laying hens and its dominance increased over time. Conversely, (+)-isomer of metabolite oxychlordane was overwhelmingly dominant during the experiment. These findings about enantioselectivity, metabolic processes and toxicological effects are crucial in understanding the exposure risk of chlordane-related compounds.

A critical review on the formation, fate and degradation of the persistent organic pollutant hexachlorocyclohexane in water systems and waste streams

The environmental impacts of persistent organic pollutants (POPs) is an increasingly prominent topic in the scientific community. POPs are stable chemicals that are accumulated in living beings and can act as endocrine disruptors or carcinogens on prolonged exposure. Although efforts have been taken to minimize or ban the use of certain POPs, their use is still widespread due to their importance in several industries. As a result, it is imperative that POPs in the ecosystem are degraded efficiently and safely in order to avoid long-lasting environmental damage. This review focuses on the degradation techniques of hexachlorocyclohexane (HCH), a pollutant that has strong adverse effects on a variety of organisms. Different technologies such as adsorption, bioremediation and advanced oxidation process have been critically analyzed in this study. All 3 techniques have exhibited near complete removal of HCH under ideal conditions, and the median removal efficiency values for adsorption, bioremediation and advanced oxidation process were found to be 80%, 93% and 82% respectively. However, it must be noted that there is no ideal HCH removal technique and the selection of removal method depends on several factors. Furthermore, the fates of HCH in the environment and challenges faced by HCH degradation have also been explained in this study. The future scope for research in this field has also received attention.

Enantioselectivity in biotransformation and bioaccumulation processes of typical chiral contaminants

Chirality is a critical topic in the medicinal and agrochemical fields. One quarter of all agrochemicals was chiral in 1996, and this proportion has increased remarkably with the introduction of new compounds over time. Despite scientists have made great efforts to probe the enantiomeric selectivity of chiral chemicals in the environment since early 1990s, the different behaviours of individual enantiomers in biologically mediated processes are still unclear. In the present review, we highlight state-of-the-knowledge on the stereoselective biotransformation and accumulation of chiral contaminants in organisms ranging from invertebrates to humans. Chiral insecticides, fungicides, and herbicides, polychlorinated biphenyls (PCBs), pharmaceuticals, flame retardants hexabromocyclododecane (HBCD), and perfluorooctane sulfonate (PFOS) are all included in the target compounds. Key findings included: a) Changes in the enantiomeric fractions in vitro and in vivo models revealed that enantioselectivity commonly occurs in biotransformation and bioaccumulation. b) Emerging contaminants have become more important in the field of enantioselectivity together with their metabolites in biological transformation process. c) Chiral signatures have also been regarded as powerful tools for tracking pollution sources when the contribution of precursor is unknown. Future studies are needed in order to understand not only preliminary enrichment results but also detailed molecular mechanisms in diverse models to comprehensively understand the behaviours of chiral compounds.

Enantiomer-specific measurements of current-use pesticides in aquatic systems

Some current-use pesticides are chiral and have nonsuperimposable mirror images called enantiomers that exhibit identical physical-chemical properties but can behave differently when in contact with other chiral molecules (e.g., regarding degradation and uptake). These differences can result in variations in enantiomer presence in the environment and potentially change the toxicity of pesticide residues. Several current-use chiral pesticides are applied in urban and agricultural areas, with increased potential to enter watersheds and adversely affect aquatic organisms. The present study describes a stereoselective analytical method for the current-use pesticides fipronil, cis-bifenthrin, cis-permethrin, cypermethrin, and cyfluthrin. We show use of the method by characterizing enantiomer fractions in environmental sample extracts (sediment and water), and laboratory-dosed fish and concrete extracts previously collected by California organizations. Enantiomer fractions for most environmental samples are the same as racemic standards (equal amounts of enantiomers, enantiomer fraction = 0.5) and therefore are not expected to differ in toxicity from racemic mixtures typically tested. In laboratory-derived samples, enantiomer fractions are more frequently nonracemic and favor the less toxic enantiomer; permethrin enantiomer fractions range from 0.094 to 0.391 in one type of concrete runoff and enantiomer fractions of bifenthrin in dosed fish range from 0.378 to 0.499. We use enantiomer fractions as a screening tool to understand environmental exposure and explore ways this uncommon measurement could be used to better understand toxicity and risk. Environ Toxicol Chem 2018;37:99-106. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Subacute oral toxicity assessment of benalaxyl in mice based on metabolomics methods

In this study, the metabolic responses of mice after 30 days of exposure to benalaxyl were assessed using NMR-based untargeted metabolomics and LC-MS-based targeted profiling of 20 amino acids. Urinary ¹H NMR analyses revealed alterations in energy metabolism, lipid metabolism, vitamin B metabolism, the urea cycle and amino acid metabolism, and targeted analyses indicated that the serum levels of asparagine, histidine, lysine and aspartic acid were significantly altered. Additionally, significant oxidative stress was observed in the liver and kidney, although no apparent histopathological injury was observed. The tissue distribution indicated a significant stereoselectivity in the brain, where (-)-R-benalaxyl was enriched. These data provide a comprehensive picture of the subacute toxic effects of benalaxyl in mice. The results of this study suggested that, for a toxicity evaluation, metabolomics analysis is much more sensitive than traditional toxicological methods. The results also highlight the combined use of untargeted and targeted metabolomics approaches in evaluating the health risks of xenobiotics.

Enantioselective Characteristics and Montmorillonite-Mediated Removal Effects of α-Hexachlorocyclohexane in Laying Hens

α-Hexachlorocyclohexane (α-HCH) is a chiral organochlorine pesticide that is often ubiquitously detected in various environmental matrices and may be absorbed by the human body via food consumption, with serious detriments to human health. In this study, enantioselective degradation kinetics and residues of α-HCH in laying hens were investigated after a single dose of exposure to the pesticide, whereas enantioselectivity and residues of α-HCH in eggs, droppings, and various tissues were investigated after long-term exposure. Meanwhile, montmorillonite (MMT), a feed additive with high capacity of adsorption, was investigated for its ability to remove α-HCH from laying hens. Most non-brain tissues enantioselectively accumulated (-)-α-HCH, while (+)-α-HCH was preferentially accumulated in the brain. The enantiomer fractions (EFs) in most tissues gradually decreased, implying continuous depletion of (+)-α-HCH in laying hens. After 30 days of exposure and 31 days of elimination, the concentration of α-HCH in eggs and tissues of laying hens with MMT-containing feed was lower than that with MMT-free feed, indicating the removal effects of MMT for α-HCH in laying hens. The findings presented herein suggest that modified MMT may potentially be useful in reducing the enrichment of α-HCH in laying hens and eggs, thus lowering the risk of human intake of α-HCH.

Effect of pregnancy on the disposition of 2,2',3,5',6-pentachlorobiphenyl (PCB 95) atropisomers and their hydroxylated metabolites in female mice

Chiral PCBs, such as PCB 95, are developmental neurotoxicants that undergo atropisomeric enrichment in nonpregnant adult mice. Because pregnancy is associated with changes in hepatic cytochrome P450 enzyme activity as well as lipid disposition and metabolism, this study investigates the effect of pregnancy on the maternal disposition of chiral PCBs. Female C57BL/6 mice (8 weeks old) were dosed daily beginning 2 weeks prior to conception and continuing throughout gestation and lactation (56 days total) with racemic PCB 95 (0, 0.1, 1.0, or 6.0 mg/kg body wt/day) in peanut butter. Levels and chiral signatures of PCB 95 and its hydroxylated metabolites (OH-PCBs) were determined in adipose, blood, brain, and liver. Tissue levels of PCB 95 increased 4- to 12-fold with increasing dose, with considerable enrichment of the second eluting atropisomer in all tissues (EF range 0.11 to 0.26). OH-PCBs displayed atropisomeric enrichment in blood and liver but were not detected in adipose and brain. Levels of PCB 95 and its metabolites were 2- to 11-fold lower in pregnant dams relative to those previously reported in nonpregnant age-matched female mice; however, PCB 95 and OH-PCB profiles and chiral signatures were similar between both studies. In contrast, human brain samples contained racemic PCB 95 residues (EF = 0.50). These results demonstrate that changes in cytochrome P450 enzyme activity and lipid disposition during pregnancy reduce the PCB body burden in dams but do not affect metabolite profiles or chiral signatures. The differences in chiral signatures between mice and humans suggest species-specific differences in atropisomeric disposition, the toxicological significance of which remains to be determined.

Enantioselective metabolism of quizalofop-ethyl in rat

The pharmacokinetic and distribution of the enantiomers of quizalofop-ethyl and its metabolite quizalofop-acid were studied in Sprague-Dawley male rats. The two pairs of enantiomers were determined using a validated chiral high-performance liquid chromatography method. Animals were administered quizalofop-ethyl at 10 mg kg(-1) orally and intravenously. It was found high concentration of quizalofop-acid in the blood and tissues by both intragastric and intravenous administration, and quizalofop-ethyl could not be detected through the whole study which indicated a quick metabolism of quizalofop-ethyl to quizalofop-acid in vivo. In almost all the samples, the concentrations of (+)-quizalofop-acid exceeded those of (-)-quizalofop-acid. Quizalofop-acid could still be detected in the samples even at 120 h except in brain due to the function of blood-brain barrier. Based on a rough calculation, about 8.77% and 2.16% of quizalofop-acid were excreted through urine and feces after intragastric administration. The oral bioavailability of (+)-quizalofop-acid and (-)-quizalofop-acid were 72.8% and 83.6%.

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.

Metabolism of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) atropisomers in tissue slices from phenobarbital or dexamethasone-induced rats is sex-dependent

1. Chiral polychlorinated biphenyls (PCBs) such as PCB 136 enantioselectively sensitize the ryanodine receptor (RyR). In light of recent evidence that PCBs cause developmental neurotoxicity via RyR-dependent mechanisms, this suggests that enantioselective PCB metabolism may influence the developmental neurotoxicity of chiral PCBs. However, enantioselective disposition of PCBs has not been fully characterized. 2. The effect of sex and cytochrome P450 (P450) enzyme induction on the enantioselective metabolism of PCB 136 was studied using liver tissue slices prepared from naïve control (CTL), phenobarbital (PB; CYP2B inducer) or dexamethasone (DEX; CYP3A inducer) pretreated adult Sprague-Dawley rats. PCB 136 metabolism was also examined in hippocampal slices derived from untreated rat pups. 3. In liver tissue slices, hydroxylated PCB (OH-PCB) profiles depended on sex and inducer pretreatment, and OH-PCB levels followed the rank orders male > female and PB > DEX > CTL. In contrast, the enantiomeric enrichment of PCB 136 and its metabolites was independent of sex and inducer pretreatment. Only small amounts of PCB 136 partitioned into hippocampal tissue slices and no OH-PCB metabolites were detected. 4. Our results suggest that enantioselective metabolism, sex and induction status of P450 enzymes in the liver may modulate the neurotoxic outcomes of developmental exposure to chiral PCBs.

Metabolism and distribution of p,p'-DDT during flight of the white-crowned sparrow, Zonotrichia leucophrys

This study evaluated the interactions of flight, fasting, and 1,1,1-trichloro-bis(4-chlorophenyl)ethane (p,p'-DDT) loading on residue metabolism and distribution in recently exposed white-crowned sparrows (Zonotrichia leucophrys). Female sparrows were dosed with 5 mg p,p'-DDT per kg body weight over 3 d. Following 1 d of recovery, sparrows were flown in a wind tunnel for up to 140 min, in 15-min blocks. Food was withheld from the start of the flight period until birds were euthanized. DDT, 1,1-dichloro-2,2-bis(4 chlorophenyl)ethane (DDD), and 1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene (DDE) were present in all tissues examined. 1-Chloro-2,2-bis(4-chlorophenyl)ethene (DDµ), 1,1-bis(4-chlorophenyl)ethane (p,p'-DDη), and 2,2-bis(4-chlorophenyl)ethanol (p,p'-DDOH) were not found. Fasting did not significantly affect the rate of residue increase over time in any of the tissues examined. When sparrows flew and fasted simultaneously, fasting seldom contributed to an increase in tissue residues. However, the length of time flown was significantly correlated with increasing toxicant concentrations in the brain, kidney, and liver, effectively demonstrating the potential for brief flights to enhance mobilization of DDT and its metabolites. Dose, flight, and fasting also increased residues in brain tissue. These contaminant redistributions may have important ramifications on the stresses experienced by migratory birds.

Accumulation dynamics of chlordanes and their enantiomers in cockerels (Gallus gallus) after oral exposure

After a single oral exposure of technical chlordane, levels of cis-chlordane (CC), trans-chlordane (TC), heptachlor (HEP), heptachlorepoxide (HEPX), and oxychlordane (OXY) were determined in gastrointestinal residues, droppings, and various tissues of cockerels at times of 60, 120, 160, 200, 300, 500, 1000, and 2000 min. Over 98% of CC and TC were found to be bioaccessible; only 1.1% of CC and TC were directly excreted through droppings without further biotransformation. According to the single-compartment toxicokinetic modeling, CC and TC shared similar absorption rates in the whole body while TC showed a slightly more rapid elimination rate, with a half-life of 13.4 h for CC and 12.5 h for TC. The metabolites HEPX and OXY appeared quickly in tissues 60 min after exposure and were mainly accumulated in fat and liver tissues. Concentrations of CC, TC, and HEP in cockerel tissues roughly followed the order as fat > intestine > skin > liver> brain > muscle > blood. Levels of CC, TC, and HEP in various tissues showed significant correlation with the lipid contents of the tissues (p < 0.05) for samples beginning 500 min after exposure. A multicompartment toxicokinetic model was developed to characterize the accumulation dynamics of CC and TC in the various tissues. All tissues of cockerels enantioselectively accumulated (-)-CC and (+)-TC, and fat, skin, and liver tissues showed a relatively stronger capacity of enantioenrichment. The enantiomer fractions (EFs) of droppings remained nearly racemic at first but gradually decreased to less than 0.5 for CC and increased to more than 0.5 for TC, which could rule out enantioselective absorption and excretion of CC and TC in cockerels. The one-compartment toxicokinetic model was applied to the individual enantiomers of CC and TC. Different elimination rates but similar absorption rates were observed between the enantiomers for both CC and TC.

Dynamic changes of α-hexachlorocyclohexane and its enantiomers in various tissues of Japanese Rabbits (Oyctolagus cuniculus) after oral or dermal exposure

Japanese Rabbits (Oyctolagus cuniculus) were exposed to α-hexachlorocyclohexane (α-HCH) either orally or dermally and concentrations of α-HCH and its two enantiomers were measured at different time intervals in the blood, intestine, liver, kidney, fat, brain, and muscle. The time trends were quantified using toxicokinetic models. It was found that absorption and elimination of α-HCH in blood were first-order processes which can be characterized by a single compartmental kinetic model. The absorption of dermally exposed α-HCH in blood was more than one order of magnitude faster than that of orally exposed α-HCH. The transport of α-HCH from the blood to other tissues was characterized using a two-compartment model. The accumulation rates were different among tissues depending on blood flow rate and fat content. Significant correlation was revealed between α-HCH concentration and fat content for various tissues either before or 1 d after the exposure. However, there was no such correlation at 10 min immediately after the exposure. The enantiomeric fraction (EF) of α-HCH in rabbit blood was nearly racemic before the exposure and increased to 0.73 and 0.82 after oral or dermal exposure, respectively. The result of a toxicokinetic modeling suggested that the strong enantioenrichment of (+)-α-HCH was primarily because the elimination rate of (-)-α-HCH was more than two times higher than that of (+)-α-HCH. The EFs for other tissues also increased dramatically after the exposure and the EFs in brain reached as high as 0.99 due to enantioselective transport across the blood brain barrier.

Enantioselective behavior of alpha-HCH in mouse and quail tissues

alpha-HCH (hexachlorocyclohexane) is chiral and can still be detected in almost all environmental media. In this study, the enantioselective behavior of alpha-HCH in mice (CD1) and quail (Coturnix japonica) was investigated and compared after a single dose of exposure. The primary nerve cell culture was conducted to evaluate the enantioselective metabolic capacity of nerve cells of mouse and quail for alpha-HCH. In various tissues of the mice and quail, the alpha-HCH concentrations showed a typical pattern of first-order dynamics after exposure. The enantiomeric fractions (EFs) in nonbrain tissues of mice decreased substantially, indicating continuous depletion of (+)-alpha-HCH in mice. Tissue-specific EF trends in quail and enantioselective degradation of (-)-alpha-HCH in quail liver were observed. These observations indicated that the dynamic changes of EFs in mice and quail were independent of concentration changes in the same tissues. In brain tissues, the enantioenrichment of (+)-enantiomer was totally independent of their concentrations in blood. The in vitro metabolism of alpha-HCH in the primary nerve cells were negligible, and the slight EF changes in primary nerve cells demonstrated that metabolism, uptake, and excretion in the brain cells would not lead to the observed dramatic enantioenrichment of (+)-alpha-HCH in the brain tissues of the two animals. The enantioselective transport across the blood-brain barrier was the primary cause for the enantioenrichment of (+)-alpha-HCH in the brain tissues.

Chiral polychlorinated biphenyls are biotransformed enantioselectively by mammalian cytochrome P-450 isozymes to form hydroxylated metabolites

In vitro incubations of purified rat cytochrome P-450 (CYP) 2B1 and human CYP 2B6 were performed to determine if CYP isozymes biotransform polychlorinated biphenyls (PCBs) enantioselectively. Enantioselective metabolism of chiral PCBs 45, 84, 91, 95, 132, and 136 and production of hydroxylated PCB metabolites (OH-PCBs) were observed, while no changes in PCB 183 atropisomer composition were observed for either isozyme. Enantiomer fractions (EFs) of parent PCBs, individually incubated as racemates at 25 ng/mL initial concentration, with rat CYP 2B1 ranged from 0.353 to 0.822. Enantioselectivity was also observed for PCBs 45 (EF = 0.437) and 132 (EF = 0.537) incubated at that concentration with human CYP 2B6. Both atropisomers of chiral PCBs appeared to be biotransformed simultaneously by rat CYP 2B1, except for (+)-PCB 132, but at different rates. Hydroxylated PCBs were identified using gas chromatography-high resolution mass spectrometry for all chiral PCBs enantioselectively transformed by CYPs. These metabolites did not correspond to any commercially available authentic standards, supporting the hypothesis that many unidentified OH-PCBs detected in wildlife may have arisen from in vivo biotransformation of chiral PCBs. A rough estimate suggested that more than half of the total congener metabolized by rat CYP 2B1 was converted to OH-PCBs. Similar concentration decreases were observed for congeners incubated with human CYP 2B6, but less OH-PCBs were formed. Formation of OH-PCBs via an enantioselective OH insertion mechanism was suggested, and may be a source of the unidentified OH-PCBs currently found in the environment.

Chiral organochlorine contaminants in blood and eggs of glaucous gulls (Larus hyperboreus) from the Norwegian Arctic

Glaucous gulls (Larus hyperboreus) and their eggs from Svalbard (Norwegian Arctic) have been used as biomonitors of contaminants in the marine environment. In this study, the enantiomer fractions (EFs) of chiral chlordanes and atropisomeric polychlorinated biphenyl (PCB) congeners were determined in the blood plasma of adult male and female glaucous gulls from three breeding colonies in Svalbard. Plasma EFs were similar in magnitude and direction to EFs previously reported in glaucous gulls from other arctic food webs, suggesting overall similarities in the biochemical processes influencing the EFs of bioaccumulated organochlorine (OC) contaminants within the food webs at those locations. Additionally, EFs in yolk of eggs collected concurrently from within the same nesting colonies varied with location, laying date, and OC concentrations, and may be influenced by changes in the local feeding ecology between those colonies. No differences were found between the EFs for any analyte in female gulls compared to those found in egg yolk, indicating that processes involved in the maternal transfer of chlordanes and PCBs to eggs do not modulate the stereochemical ratio between enantiomers. Therefore, the use of eggs as a valuable and noninvasive means of OC biomonitoring may also extend to enantiomer compositions in glaucous gulls, and perhaps also in other seabird species from arctic regions.

Hexabromocyclododecane in white-sided dolphins: temporal trend and stereoisomer distribution in tissues

Hexabromocyclododecane (HBCD) is a brominated flame retardant used primarily in expanded polystyrene foams and other styrene resins. Samples of blubber (n = 57) and liver (n = 16) from Atlantic white-sided dolphins, Lagenorhynchus acutus, that stranded on the eastern coast of United States between 1993 and 2004 were obtained from the National Marine Mammal Tissue Bank (NMMTB). Blubber samples from most of these animals (n = 47) were previously analyzed for polybrominated diphenyl ethers (PBDE), polychlorinated biphenyls (PCB), and several toxaphene congeners. The three most abundant diastereomers in the technical HBCD mixture (alpha-HBCD, beta-HBCD, and gamma-HBCD) and their enantiomers were determined using liquid chromatography-triple quadrupole mass spectrometry (LC-MS/MS). alpha-HBCD was found in all blubber and liver samples while beta-HBCD and gamma-HBCD were not detected in any samples. The alpha-HBCD concentration in blubber and liver ranged from 14 ng/g wet mass (19 ng/g lipid) to 280 ng/g wet mass (380 ng/g lipid) and 0.051 ng/g wet mass (2.9 ng/g lipid) to 3.6 ng/g wet mass (140 ng/g lipid), respectively. Concentrations of alpha-HBCD were 2 to 3 orders of magnitude lower than previously reported PBDE, PCB, and toxaphene concentrations in these same animals. There was not a significant temporal trend for these compounds in white-sided dolphin blubber. The enantiomeric fractions (EF) measured in blubber and liver were not statistically different and ranged from 0.34 to 0.53. Blubber EFs were significantly correlated with both alpha-HBCD concentrations and white-sided dolphin body length. In general, concentrations of HBCDs were lower in these white-sided dolphins than in cetaceans from Western Europe.

The freshwater invertebrate Mysis relicta can eliminate chiral organochlorine compounds enantioselectively

Accumulation and elimination of chiral polychlorinated biphenyls (PCBs) and organochlorine (OC) pesticides by the opossum shrimp, Mysis relicta, was investigated to determine if zooplankton can stereoselectively process chiral OC contaminants. Concentrations and enantiomer fractions were measured within mysids over a 10-day exposure followed by a 45-day depuration period. Rapid accumulation occurred within mysids exposed to sediment contaminated with racemic chiral OC compounds at microg/g levels. Enantiomer enrichment was observed within mysids for the second-eluting enantiomer and the (-)-enantiomer of PCB 95 and trans-chlordane, respectively, after 7 days of exposure to spiked sediment, and for the second-eluting enantiomers of PCBs 91 and 183 and (-)-PCB 149 over longer time periods. Enantiomer fractions decreased with time during the depuration phase of the experiment for these compounds, showing that their elimination from mysids was stereoselective. Oxychlordane was detected in nonracemic proportions after exposure, indicating that mysids can metabolize trans-chlordane enantioselectively. Minimum elimination rates calculated were higher than biotransformation rates calculated for fish in previous studies, which have been shown to metabolize OC contaminants. This study is the first to show stereoselective processing of chiral OC contaminants by aquatic invertebrates.

Environmental fate processes and biochemical transformations of chiral emerging organic pollutants

This review highlights the analytical chemistry, environmental occurrence, and environmental fate of individual stereoisomers of chiral emerging pollutants, which are modern current-use chemicals of growing environmental concern due to their presence in the environment and potential for deleterious effects. Comparatively little is known about individual stereoisomers of pollutants, which can have differential toxicological effects and can be tracers of biochemical weathering in the environment. Stereoisomers are resolved by gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis (CE). Separation techniques in environmental analysis are typically coupled to mass spectrometry (MS) and tandem mass spectrometry (MS/MS), as these provide the sensitivity and selectivity needed. The enantiomer composition of phenoxyalkanoic and acetamide herbicides, organophosphorus and pyrethroid pesticides, chiral polychlorinated biphenyl metabolites, synthetic musks, hexabromocyclododecane, and pharmaceuticals in the environment show species-dependent enantioselectivity from biotransformation and other biologically mediated processes affecting enantiomers differentially. These enantiomer compositions are useful in detecting biologically mediated environmental reactions, apportioning sources of pollutants, and gaining insight into the biochemical fate of chiral pollutants in the environment, which are needed for accurate risk assessment of such chemicals.

Enantiomeric fractions and congener specific determination of polychlorinated biphenyls in eggs of predatory birds from Doñana National Park (Spain)

The content of 30 polychlorinated biphenyls (PCBs) and the enantiomeric fractions of 10 chiral PCBs were determined in 17 infertile eggs from three different predatory bird species collected in Doñana National Park (DNP, Spain) in the period 1999-2000. The highest PCB concentration was found in eggs from red kites (0.52-110 microg/g on a fresh weight basis, f.w.) followed by buzzard (0.08-13 microg/g f.w.) and booted eagle (0.10-1.5 microg/g f.w.). Seventy-five percent of the red kite eggs had PCB levels above 4.7 microg/g f.w., which is associated in the literature with reproductive failure. This could be related to the fact that red kite populations have decreased by more than 50% in the last five years in DNP. PCBs # 138, 153, and 180 were the most abundant in all cases. This is the first time that atropisomers of 10 chiral PCBs (PCBs # 45, 84, 91, 95, 132, 135, 136, 149, 174, and 176) in predatory bird eggs have been performed. The enantiomeric fractions (EFs) for most PCBs investigated were non-racemic (EF not = 0.5), ranging from 0.05 to 0.95. The results suggested that predatory birds, mainly red kite species, are highly polluted by PCBs, and PCBs # 95, 132, 135, 136, and 174 strongly deviate from the racemic-mixture values.

Stereoselective degradation kinetics of tebuconazole in rabbits

Tebuconazole[(RS)-1-p-chlorophenyl-4,4-dimethyl-3-(1H-1,2,4-triazol-1-ylmethyl)pentan-3-ol] is a potent triazole fungicide and consists of a pair of enantiomers. The enantioselective degradation kinetics of tebuconazole was investigated in rabbits by intravenous (iv) injection. The concentrations of (-)-(R)-tebuconazole and (+)-(S)-tebuconazole in plasma and tissues were determined by HPLC with a cellulose tris(3,5-dimethylphenylcarbamate)-based chiral stationary phase. Enantioselective analysis methods for this fungicide in plasma and tissues were developed and validated. Good linearities were obtained over the concentration range of 0.25-25 mg/l for both enantiomers. The degradation followed pseudo-first-order kinetics and the degradation of the (+)-(S)-tebuconazole was much faster than that of the (-)-(R)-tebuconazole in plasma after administration of racemic tebuconazole. This study also indicated that environmental assessment of enantiomeric degradation may be needed to fully evaluate risks of tebuconazole use.

Stereoselective kinetic study of hexaconazole enantiomers in the rabbit

Hexaconazole [(RS)-2-(2,4-dichlorophenyl)-1-(1H-1,2,4-triazol-1-yl)hexan-2-ol] is a potent triazole fungicide. The (-) isomer accounts for most of the fungicidal activity. The stereo- and/or enantioselective kinetics of hexaconazole were investigated in rabbits by intravenous injection. The concentrations of (-)- and (+)-hexaconazole in plasma, liver, and kidney tissue were determined by HPLC with a cellulose tris(3,5-dimethylphenylcarbamate)-based chiral stationary phase and by gas chromatography-mass spectrometry. After intravenous administration of racemic hexaconazole (rac-hexaconazole) at 30 mg/kg, plasma, liver, and kidney levels of the (+)-enantiomer decreased more rapidly than those of the (-)-enantiomer. The (-)-/(+)-enantiomer ratio of the area under the concentration-time curve (AUC(0-infinity)) was 1.35. The total plasma clearance value (CL) of (+)-enantiomer was more than 1.3-fold higher than that of the (-)-hexaconazole. The enantiomeric ratio (ER) increased with time in plasma, liver, and kidney. Other pharmacokinetic parameters of the enantiomers were also different. These results indicate substantial stereoselectivity in the kinetics of hexaconazole enantiomers in rabbits.

Evidence of enantioselective degradation of alpha-hexachlorocyclohexane in groundwater

In the fall of 2000, 34 groundwater samples were collected from beneath an active pesticide reformulating and packaging facility in coastal northeastern Florida to measure the enantiomer fractions (EFs) of alpha-hexachlorocyclohexane (alpha-HCH) as an indicator of biodegradation of this chlorinated pesticide in groundwater. Concentrations of alpha-HCH as high as 500 microg/L were observed beneath the historical source area and decreased with distance downgradient. Seventy-eight percent of the EF values were greater than 0.504 and ranged up to 0.890, indicating that the (-)-alpha-HCH enantiomer is preferentially degraded relative to the (+)-alpha-HCH enantiomer at this site. Samples taken from the groundwater that flows north from the historical disposal facility to a local discharge point at a creek did not indicate enantioselective degradation (EF values ranged from 0.495 to 0.512). The acidity (pH 3.7-4.6) and short flow path to the creek for this lobe of the groundwater plume likely preclude biodegradation of alpha-HCH. In contrast, the neutral lobe of the groundwater plume, which flows eastward from the historical source area, demonstrated enantioselective degradation (EF values ranged from 0.500 to 0.890 and increased with distance from the source area). Groundwater conditions beneath this portion of the site are conducive to biodegradation of HCH owing to anaerobic reducing conditions and lengthy travel times, and the chiral signatures for alpha-HCH provide evidence that biological degradation is occurring beneath this portion of the site.

Profile of persistent chlorinated contaminants, including selected chiral compounds, in wolverine (Gulo gulo) livers from the Canadian Arctic

Wolverines (Gulo gulo) are circumpolar omnivores that live throughout the alpine and arctic tundra ecosystem. Wolverine livers were collected at Kugluktuk (Coppermine), NU (n=12) in the western Canadian Arctic to report, for the first time, the residue patterns of persistent organochlorine contaminants (OCs) in this species. The enantiomer fractions (EFs) of several chiral OCs, including PCB atropisomers, in wolverines were also determined. Results were compared to OC concentrations and EFs of chiral contaminants in arctic fox (Alopex lagopus) from Ulukhaqtuuq (Holman), NT (n=20); a closely related species that scavenges the marine and terrestrial arctic environment. The rank order of hepatic concentrations for sum ( summation operator ) OC groups in wolverines were polychlorinated biphenyls ( summation operator PCB)>chlordane-related components ( summation operator CHLOR)>DDT-related compounds ( summation operator DDT)>hexachlorocyclohexane isomers ( summation operator HCHs). The most abundant OC analytes detected in wolverine liver were PCB-153, PCB-180, and oxychlordane (OXY). Wolverine age and gender did not influence OC concentrations, which were comparable to lipid-normalized values in arctic fox. The EFs of several chiral OCs (alpha-HCH, cis- and trans-chlordane, OXY, heptachlor exo-epoxide) and PCB atropisomers (PCB-136, 149) were nonracemic in arctic fox and wolverine liver and similar to those previously calculated in arctic fox and polar bears from Iceland and the Canadian Arctic. Results suggest that these species have similar ability to biotransform OCs. As well, contaminant profiles suggest that terrestrial mammals do not represent the major source of OC exposure to wolverines and that wolverines are scavenging more contaminated prey items, such as marine mammals. While summation operator PCB did not exceed the concentrations associated with mammalian reproductive impairment, future research is required to properly evaluate the potential affect of other OCs on the overall health of wolverines.

Complications with using ratios for environmental data: comparing enantiomeric ratios (ERs) and enantiomer fractions (EFs)

Complications arise when ratios are used to present environmental data because ratios are an unbounded, multiplicative scale that can lead to asymmetrical (skewed) data distributions. Enantiomeric ratios (ERs), historically used in discussions of chiral signatures, often are published as mean ER+/-single-value standard deviation. Application of statistical summaries, such as the widely used sample mean and standard deviation, to skewed ratio data is misleading and often inappropriate. Comparison of statistically summarized ER and enantiomer fraction (EF) data (which are based on a bounded, additive scale) for a range of hypothetical values reveals substantial discrepancies when conversion between ER and EF formats is used. These discrepancies are largest when the ratio data are greater than one and have large variability, because the data are more skewed. In many cases, the use of fractions instead of ratios can help to minimize misrepresentation of environmental data, including chiral data. The use of nonparametric statistical summaries, e.g., median and percentiles, provides a more robust indicator of the typical value and spread for both ER and EF data.

Enantiomer fractions of chiral organochlorine pesticides and polychlorinated biphenyls in standard and certified reference materials

Enantiomeric ratios (ERs) and enantiomeric fractions (EFs) of a number of chiral organochlorine pesticides and PCB atropisomers were measured by chiral gas chromatography/mass spectrometry (GC/MS) in five standard (SRM) and certified (CRM) reference materials: SRM 1588a (organics in cod liver oil), SRM 1945 (organics in whale blubber), Marine Mammal Quality Assurance Exercise Control Material IV (NIST IV, organics in whale blubber), CRM trout, and CRM EC-5 (sediment). Target analytes were cis- and trans-chlordane, heptachlor exo-epoxide, oxychlordane, U82, MC5, MC6, MC7, o,p'-DDT, and PCB congeners 91, 95, 136, 149, 174, 176, and 183. Measured ERs and EFs are in close agreement with the few literature values reported for some of these analytes in SRMs and CRMs. Chiral PCB ERs and EFs measured by one-dimensional chiral GC/MS were similar to values measured using multidimensional chiral GC/MS. Non-racemic chiral compositions are in agreement with known uptake and biotransformation in the respective environmental matrices. These values should aid in the quality assurance/quality control methodologies for chiral environmental chemistry using standardized reference materials.

Enantiomer-specific accumulation of PCB atropisomers in the bowhead whale (Balaena mysticetus)

Blubber (n = 40) and liver (n = 20) samples from the bowhead whale (Balaena mysticetus) were collected during the 1997-1998 Native (Inuit) subsistence harvests in Barrow, AK. Bowhead tissues and zooplankton were analyzed for polychlorinated biphenyl (PCB) concentrations and the enantiomeric fractions (EFs) of eight chiral PCB congeners (PCB-91, 95, 135, 136, 149, 174, 176, and 183) to quantify the enantiomer-specific accumulation of PCBs in this cetacean. PCB concentrations in bowhead blubber were low (mean +/- 1 SE: 610 +/- 54 ng g(-1) lipid) relative to other cetaceans. The accumulation of several chiral PCBs (PCB-91, 135, 149, 174, 176, and 183) in bowhead blubber was enantiomer-specific relative to bowhead liver and zooplankton, suggesting that biotransformation processes within the bowhead whale are enantioselective. The EFs for PCB-95 and 149 were significantly correlated with body length in male and female whales, while EFs for PCB-91 correlated with length in males only. Despite evidence for enantioselective biotransformation, all three congeners bioaccumulated in the bowhead relative to PCB-153. Results suggest that enantioselective accumulation of PCB-91, 95, and 149 is influenced by PCB concentrations, age, and/or the modification of an uncharacterized stereoselective process (or processes) during sexual maturity.

Rainbow trout (Oncorhynchus mykiss) can eliminate chiral organochlorine compounds enantioselectively

Dietary accumulation of four chiral organochlorine compounds--alpha-hexachlorocyclohexane (alpha-HCH), trans-chlordane, and chlorobiphenyls (CBs) 95 and 136-by immature rainbow trout (Oncorhynchus mykiss) was studied to determine if fish can accumulate and eliminate these compounds enantioselectively. Fish rapidly accumulated all four compounds from food spiked at micrograms per gram concentrations during a 40-d feeding period. Depuration half-lives were from 13 d for (+/-)-alpha-HCH to 375 d for (+/-)-CB 136. Fish preferentially eliminated (-)-trans-chlordane and (+)-CB 136, with significant nonracemic residues observed after 20 d. These results are consistent with field measurements of these compounds in fish as well as known metabolic pathways. Enantiomeric fractions (EFs) for these two compounds changed significantly over the course of the experiment, suggesting that trout were enantioselectively biotransforming the compounds during the 238-d depuration phase. CB 95 and alpha-HCH residues were racemic throughout the experiment. High biomagnification factors for CB 95 suggest that it was not metabolized. Minimum values for metabolic elimination rates calculated from EF suggest that at least 58% of the trans-chlordane depuration rate can be attributed to metabolism, and all of the CB 136 depuration rate can be attributed to it. This study highlights the potential of chiral analysis as a tracer of in vivo biotransformation processes of xenobiotic compounds.

Enantiomer-specific activity of o,p'-DDT with the human estrogen receptor

There is a growing concern that environmental xenobiotics may be affecting human and wildlife health by disrupting normal endocrine function via interaction with steroid hormone receptors. Several of these persistent contaminants are chiral and may have enantiomer-specific biological properties. Previous experiments have demonstrated that (-)-o,p'-DDT enantiomer is a more active estrogen-mimic than the (+)-enantiomer in rats. However, these results have not been extrapolated to other biological systems. This study used a yeast-based assay to assess the enantiomer-specific transcriptional activity of DDT with the human estrogen receptor (hER). (+)-17beta-estradiol, racemic DDT and individual DDT enantiomers were added to yeast cultures and hER activity was measured by quantification of beta-galactosidase. The relative activity of o,p'-DDT was weak compared to estradiol. For o,p'-DDT, the (-)-enantiomer was the active estrogen mimic whereas the hER activity of (+)-o,p'-DDT was negligible. The presence of the (+)-enantiomer at relatively greater concentration decreased the transcriptional activity of (-)-o,p'-DDT. This data demonstrates the need to consider stereochemistry of environmental contaminants and their potential influence on biological responses.