Environ. Sci. Technol. 2002, 36, 1257-1262
Rainbow Trout (Oncorhynchus mykiss) Can Eliminate Chiral Organochlorine Compounds Enantioselectively CHARLES S. WONG,† FIONA LAU,† MATTHEW CLARK,† S C O T T A . M A B U R Y , * ,† A N D DEREK C. G. MUIR‡ Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada, and National Water Research Institute, Environment Canada, Burlington, Ontario L7R 4A6, Canada
Dietary accumulation of four chiral organochlorine compoundssR-hexachlorocyclohexane (R-HCH), transchlordane, and chlorobiphenyls (CBs) 95 and 136sby 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 halflives were from 13 d for (()-R-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 R-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.
Introduction Fish readily accumulate persistent organochlorine compounds (OCs) from water and from food. Despite bans on production and use, bioaccumulation of OCs by fish remains a serious problem, resulting in fish advisories and closing of fisheries. Literature results suggest that fish have only a limited ability to metabolize many OCs, such as polychlorinated biphenyls (PCBs) and organochlorine pesticides. This lack of biotransformation capacity is inferred from the lower cytochrome P-450 activities in fish as compared to mammals or birds (1, 2), from observations that fish accumulate metabolizable OCs to a greater extent than mammals, and * Corresponding author phone: (416)978-1780; fax: (416)978-3596; e-mail:
[email protected]. † University of Toronto. ‡ Environment Canada. 10.1021/es0156791 CCC: $22.00 Published on Web 02/08/2002
2002 American Chemical Society
from the limited metabolism observed in laboratory experiments (3, 4). The lack of OC metabolism by fish is also reflected in modeling of OCs in aquatic food webs where biotransformation is implicitly or explicitly set to zero (5, 6). In recent years, there has been increasing attention paid to the behavior of chiral OCs in the environment. Approximately 25% of all pesticides used are chiral (7) as well as 19 PCB congeners (8). The enantiomers of a chiral compound have the same physical and chemical properties but may have different biological and toxicological properties from each other and from the racemate (7, 9, 10), the form in which many chiral compounds are released into the environment. As such, chiral OCs are powerful tracers of biotransformation because only environmental processes that are symmetry-dependent (e.g., binding to enzyme receptors, enzyme-catalyzed transformations) can affect one enantiomer preferentially over another (enantioselectivity) and thereby change the relative composition of the enantiomers. Nonracemic residues of OCs such as trans-chlordane (11-14), R-hexachlorocyclohexane (R-HCH) (15), and PCB atropisomers (16) have been measured in fish and other aquatic biota. However, it is not entirely clear from these field measurements if the nonracemic OCs present in the biota are due to uptake of nonracemic OCs or from in vivo enantioselective processes. If the latter process is the case, then the commonly held assumption that fish have minimal OC biotransformation activity is incorrect, with direct implications to the modeling of OCs in aquatic environments. The paper details a novel study to see if fish exposed to four common racemic OCs via dietary uptake can accumulate and eliminate chiral organochlorine pesticides and PCBs enantioselectively in a controlled environment. Rainbow trout were selected in this experiment as the target fish species because much is known about concentrations and fate of contaminants in this species in field and laboratory studies. To our knowledge, this study is the first to examine experimentally both the uptake and elimination of individual OC enantiomers in fish.
Experimental Section Chemicals and Food Preparation. trans-Chlordane, 2,2′,3,5′,6pentachlorobiphenyl (CB 95), and 2,2′,3,3′,6,6′-hexachlorobiphenyl (CB 136) were purchased from Accustandard (New Haven, CT), while R-HCH was purchased from Chem Services (West Chester, PA). Commercial trout chow (Purina Mills, St. Louis, MO) was treated by mixing it with known quantities of these four compounds dissolved in hexane and slowly evaporating the solvent to dryness in a rotary evaporator. The chow consisted of 50% protein, 17% fat, and 3% fiber. Control food was treated in a similar manner but without the addition of target compounds. Concentrations of target compounds were determined in control and treated food using the same analytical techniques as for fish tissue (Table 1). Experimental Setup. Immature rainbow trout (initial weights 30-60 g) were obtained from a nearby trout hatchery and kept in two 475-L flow-through fiber glass aquaria (54 fish each) using Toronto city water (e9 °C) treated with ultraviolet light and carbon dechlorinated. Fish were maintained on a 12 h light:12 h dark schedule. One tank of fish was exposed to treated food for 40 d followed by 238 d of depuration with untreated food, while the other tank was given untreated food throughout the experiment. The daily rate of feeding (lipid-normalized) was equal to 1.7% of the mean lipid weight of the fish, corrected after determining fish weight after each sampling period. All food was consumed VOL. 36, NO. 6, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1257
TABLE 1. Concentrations (Cfood, µg/g wet wt) and Enantiomeric Fractions (EFs) of Chiral Compounds (mean ( SD) for Control (n ) 2) and Treated Food (n ) 3 each treatment)a control compound
Cfood
R-HCH trans-chlordane CB 95 CB 136
