Environ. Sci. Technol. 2001, 35, 2448-2454
Enantiomeric Composition of Chiral Polychlorinated Biphenyl Atropisomers in Aquatic and Riparian Biota C H A R L E S S . W O N G , †,‡ A R T H U R W . G A R R I S O N , * ,† PAUL D. SMITH,† AND WILLIAM T. FOREMAN§ Ecosystems Research Division, National Exposure Research Laboratory, U.S. Environmental Protection Agency, Athens Georgia 30605, and National Water Quality Laboratory, U.S. Geological Survey, P.O. Box 25046, MS 407, Denver Colorado 80225
The enantiomeric composition of polychlorinated biphenyl (PCB) atropisomers was measured in river and riparian biota (fish, bivalves, crayfish, water snakes, barn swallows) from selected sites throughout the United States by using chiral gas chromatography/mass spectrometry. Nonracemic enantiomeric fractions (EFs) were observed for PCBs 91, 95, 136, and 149 for aquatic and riparian biota from Lake Hartwell, SC, a reservoir heavily contaminated with PCBs, and for these congeners and PCBs 132, 174, 176, and 183 in river fish and bivalves nationwide. Fish and bivalves showed marked differences in EFs as compared to sediment found at the same sampling sites, thus suggesting that PCBs are bioprocessed in biota in a different manner from those found in sediment (e.g., reductive dechlorination). Species-dependent patterns in PCB EFs were observed, which suggest differences in the ability of different species to bioprocess PCBs enantioselectively, most likely by metabolism. The presence of nonracemic PCBs in fish and bivalves suggests greater metabolic degradation of PCBs in these organisms than indicated from previous achiral studies and underscores the powerful potential of chiral analysis as a tracer of environmental bioprocesses.
Introduction Polychlorinated biphenyls (PCBs) were extensively used in North America for more than 40 yr and are ubiquitous environmental contaminants. PCBs are of concern despite bans on their use because of their propensity to accumulate in food webs and their potential deleterious effects on organisms. Nineteen of the 209 possible PCB congeners are chiral (1) due to asymmetric chlorine substitution about the long axis of the molecule and restricted rotation (atropisomerism) around the central C-C biphenyl bond from the presence of three or four ortho chlorine atoms. These atropisomers were released into the environment as racemates but may be present in nonracemic proportions because of enantioselective environmental bioprocessing. * Corresponding author e-mail:
[email protected]; phone: (706)355-8219; fax: (706)355-8202. † U.S. Environmental Protection Agency. ‡ Present address: Department of Chemistry, University of Toronto, Toronto, ON, M5S 3H6 Canada. § U.S. Geological Survey. 2448 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 12, 2001
To assess the potential risk of a chiral compound, it is important to understand the environmental behavior of the individual enantiomers, which may have different biological and toxicological properties from each other and from the racemate (2-6), making current environmental assessments based on the racemate inaccurate. Enantiomers are also important markers of biological activity and can be used as a tool to interpret the environmental fate of organic pollutants. Only bioprocesses (e.g., enzymatic biotransformation, uptake, depuration) can affect the enantiomeric composition of a chiral compound. There have been some measurements of chiral PCBs in biota (7-9) and sediment (10-12); however, little is known about the distribution and fate of chiral PCBs in the environment. This lack of information is particularly true for sediment, fish, and bivalves in rivers and riparian ecosystems, where only a handful of studies have reported any measurements of chiral PCBs (10-12). A wide survey of environmental chirality is helpful to assess the extent of enantioselectivity in the environment and the ability of aquatic and riparian biota to biotransform xenobiotic compounds that they bioaccumulate. In this paper, we discuss the enantiomeric composition of chiral PCBs in river and riparian biota from different parts of the United States to determine if these compounds have been bioprocessed in the environment and to understand if the enantioselectivity observed (if any) agrees with what is known about the biodegradation of these compounds by aquatic and riparian organisms. Some comparisons are also made between chiral signatures measured in biota and that measured in sediment (11) to determine similarities and differences in PCB bioprocessing in the two environmental media.
Experimental Section Environmental extracts from two groups of biological samples that contain PCBs were analyzed for chiral PCB enantiomers. The first of these groups is from Lake Hartwell, an artificial reservoir straddling the Georgia-South Carolina state line (Figure 1). This lake is contaminated with an estimated 200 000 kg of PCBs, consisting of about 80% Aroclor 1016 and 20% Aroclor 1254, that were discharged into the Twelve Mile Creek arm of the lake from a capacitor manufacturing plant from 1955 to 1976 (13). Riparian biota were sampled in this arm of the lake and the Twelve Mile Creek tributary in 1994 and were prepared and analyzed by the U.S. Environmental Protection Agency (U.S. EPA) by using achiral techniques (14). Biota were collected from several sites in the region as shown in Figure 1. The sediment in the part of the lake between Madden and Maw Bridges contains the lake’s highest concentrations of PCBs (15, 16), several chiral congeners of which were present in nonracemic quantities (11). Briefly, largemouth bass (Micropterus salmoides) and bluegill sunfish (Lepomis macrochirus) were collected by electroshocking, crayfish (Procambarus sp.) and water snakes (Nerodea sipedon sipedon) were collected with baited minnow traps along the shoreline, and barn swallows (Hirundo rustica) were taken from nests under the bridges. PCBs were extracted from individual specimens if the wet weight was greater than 25 g; specimens were composited if the weight was lower. Sample extracts with high concentrations of the target compounds were selected for chiral analysis (14 bass, 14 bluegills, 5 crayfish, 7 snakes, and 15 barn swallows). The second sample group consisted of extracts of riverine biota (fish, freshwater bivalves) from the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) 10.1021/es0018872 Not subject to U.S. copyright. Publ. 2001 Am. Chem.Soc. Published on Web 05/17/2001
(Astec, Whippany, NJ) and Cyclosil-B (J&W, Folsom, CA) chiral columns (21). The NAWQA samples were analyzed for chiral PCB congeners 91, 95, 132, 136, 149, 174, 176, and 183 by using the same columns (Cyclosil-B and Chirasil-Dex, Chrompack, Raritan, NJ; B-PH, Astec) that were used for sediment extracts (11). Enantiomeric ratios (ERs) for PCBs 91, 95, and 183, for which pure enantiomers were not available, were quantified as the area ratio of the first-eluting peak/second-eluting peak (E1/E2) on Chirasil-Dex (Chrompack, Raritan, NJ) for 91 and 95 (elution order is the same on B-DM for PCB 95 and is reversed for PCB 91), and on B-PH (Astec) for 183 as described previously (11). Otherwise, ERs were area ratios of the (+)/(-) peaks for all other PCBs [elution order is (-)/(+) for PCBs 136 and 149 on B-DM]. EFs (22, 23) were calculated from measured ERs as follows:
EF )
FIGURE 1. Map of Lake Hartwell, Twelve Mile Creek, and biota sampling sites at 183 Bridge, 137 Bridge, Maw Bridge, Madden Bridge, and 133 Bridge. Program (17). These samples were taken from rivers throughout the United States between 1992 and 1995 (18) and prepared and analyzed achirally by the USGS (19, 20). Briefly, fish were collected by electroshocking or seining and then composited, with each sample consisting of 5-8 fish (19). Bivalves (Corbicula genus) were collected by hand or with a rake, rinsed and depurated with streamwater for 24 h, and then composited (about 50 organisms per sample) (19). Extracts were selected for chiral analysis from samples with high concentrations of the target compounds and at sites where sediment and biota samples were taken simultaneously to determine the enantiomeric composition of the target compounds in the different media (Table 1). Extracts from both groups of samples were stored at 4 °C or less in glass vials with Teflon-lined caps between the time of achiral analysis and this study. The enantiomeric composition of these samples is not expected to change during storage because only interactions with other chiral molecules (e.g., enzymes and other chiral biomolecules from or in living tissue) can be enantioselective, and interactions with these biomolecules and organisms will generally be disrupted by organic solvents. Details on chiral analytical procedures are described elsewhere (11, 21). PCB enantiomers were quantified by chiral capillary gas chromatography/mass spectrometry (GC/MS) on a Hewlett-Packard 6890/5973 mass selective detector in the electron impact (EI) mode with a suite of modified cyclodextrin columns. The Lake Hartwell samples were analyzed for chiral PCBs 91, 95, 136, and 149 by using B-DM
ER ) 1 + ER
1 1+
(1)
1 ER
Racemates (ER ) 1) therefore would have an EF of 0.5, and pure single enantiomers (ER ) 0 or ER ) ∞) would have an EF of 0 or 1, respectively. Replicate injections of analytical standards reflected racemic compositions (11), with EFs from 0.492 to 0.504 for all measured atropisomers. Variations in both replicate and duplicate sample measurements were small ( 0.05). There were no significant differences (Kruskal-Wallis test, all P > 0.05) in EFs from those organisms of the same species sampled at different locations (data not shown). VOL. 35, NO. 12, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. River Sites with Samples Analyzed for Chiral PCBs in This Study, Their Location, Sampling Date, and Speciesa latitude
longitude
date (m/d/yr)
species
∑PCB (µg/g)
Hudson River Basin 43°05′26′′ 43°02′37′′ 42°55′28′′ 41°43′18′′
75°09′27′′ 75°03′44′′ 73°20′54′′ 73°56′28′′
09/11/92 09/28/94 09/10/92 09/16/92
1 2 2 2
33. 1.7 1.5 3.5
5 6b 7 8b 9 10 11 12b 13 14b 15 16b 17 18b 19b 20
Connecticut River Basin Connecticut River, Montague City, MA 42°34′48′′ 72°34′30′′ East Branch Housatonic River, Pittsfield, MA 42°26′43′′ 73°14′40′′ Housatonic River, Woods Pond, Lenox, MA 42°21′02′′ 73°14′22′′ Mill River, Northampton, MA 42°19′05′′ 72°39′21′′ Swift River, West Ware, MA 42°16′04′′ 72°19′59′′ Green River, Great Barrington, MA 42°11′31′′ 73°23′28′′ Quaboag River, Palmer, MA 42°09′10′′ 72°19′53′′ Quinebaug River, Sandersdale, MA 42°04′18′′ 72°00′55′′ Connecticut River, Longmeadow, MA 42°01′53′′ 72°36′14′′ French River, North Grosvenordale, CT 41°58′41′′ 71°54′03′′ Still River, Nelsons Corner, CT 41°56′55′′ 73°02′58′′ Quinebaug River, Clayville, CT 41°37′18′′ 71°58′51′′ Mattabasset River, Little River, CT 41°36′09′′ 72°42′29′′ Housatonic River, Town Hill, CT 41°33′51′′ 73°24′33′′ Quinnipiac River, Stillmans Corner, CT 41°33′48′′ 72°52′54′′ Connecticut River, Portland, CT 41°33′46′′ 72°37′24′′
07/15/93 08/16/94 09/13/94 08/15/94 08/02/94 08/16/94 08/02/94 08/08/94 07/14/93 08/04/94 08/17/94 07/20/93 08/10/94 07/21/93 08/09/94 07/13/93
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
0.82 55. 72. 0.19 0.17 0.62 0.16 1.0 1.4 0.24 0.15 0.67 1.1 12. 1.4 0.94
21 22b 23b
Willamette River, Swan Island, OR Johnson Creek, Milwaukie, OR Fanno Creek, Durham, OR
Willamette River Basin 45°33′29′′ 122°42′52′′ 45°27′11′′ 122°38′31′′ 45°24′13′′ 122°45′13′′
09/01/92 09/01/92 09/01/92
1 3 3
1.2 0.40 0.20
24b
Codorus Creek, Pleasureville, PA
Lower Susquehanna River Basin 40°00′37′′ 76°42′37′′
09/24/92
2
1.5
08/18/93
2
0.95
site
site name
1b 2 3 4b
Mohawk River, Utica, NY Mohawk River, Frankfort, NY Hoosic River, Hoosick Falls, NY Hudson River, Poughkeepsie, NY
25
St. Vrain Creek (mouth), Platteville, CO
South Platte River Basin 40°15′29′′ 104°52′45′′
26 27 28
Teton River, St. Anthony, ID Snake River, Blackfoot, ID Portneuf River, Pocatello, ID
Upper Snake River Basin 43°55′38′′ 111°36′55′′ 43°07′31′′ 112°31′06′′ 42°52′20′′ 112°28′05′′
09/09/93 09/15/92 09/16/92
4 5 5
0.18 0.13 1.9
29b 30b 31b
White River, Indianapolis, IN White River, Indianapolis, IN Salt Creek, Oolitic, IN
White River Basin 39°44′13′′ 86°10′13′′ 39°44′13′′ 86°10′13′′ 38°53′17′′ 86°30′30′′
09/17/92 09/17/92 09/22/92
6 6 6
0.46 0.22 6.7
32
Chattahoochee River, Columbus, GA
Chattahoochee River Basin 32°27′45′′ 84°59′52′′
09/01/93
6
0.18
a
Species: 1, common carp (Cyprinus carpio); 2, white sucker (Catostomus commersoni); 3, sculpins (Cottus genus); 4, Paiute sculpin (Cottus beldingi); 5, Utah sucker (Catostomus ardens); 6, freshwater bivalves (Corbicula genus). Total PCB concentrations (∑PCB in micrograms per gram wet weight ) µg/g) are from ref 18. b Site at which sediment sample was simultaneously taken and analyzed for chiral PCBs (11).
NAWQA Biota. The chiral composition of PCB atropisomers in river fish and bivalves was in general more nonracemic than in sediments from the same site (11). In sediment, nonracemic amounts of chiral PCBs were found in bed-sediment samples of the Hudson River and Housatonic River (a subbasin of the Connecticut River basin), at sites where reductive dechlorination of PCBs has been shown to occur in situ and in laboratory microcosms. Racemic amounts of chiral PCBs were observed in most other sediment sites, suggesting little enantioselective bioprocessing in the sediment at these sites. In contrast, most of the fish samples, including those from the Hudson and Housatonic River sites, had nonracemic residues for all PCBs measured except for PCB 183, of which only six samples were significantly nonracemic (Figure 3). Sculpins (three sample sites) had significantly different EFs for PCBs 91 (average EF of 0.634, n ) 2), 95 (average EF of 0.096, n ) 3), and 136 (average EF of 0.259, n ) 3) than suckers, which consisted of most of the fish samples measured (average EFs of 0.444, 0.350, and 0.469, respectively; n ) 23 for each congener). Bivalves (samples 29-32, Table 1) had significantly nonracemic EFs (t-test, all P < 0.05) for all PCB congeners measured, except for PCBs 136 and 176. At the sites for which sediment (11) and biota samples were taken at the same time, there were significant 2450
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differences (sign test, all P < 0.05) in EFs between sediment and suckers for PCBs 95, 136, 149, and 174 as well as between sediment and all fish (regardless of species) for these same congeners. These results are shown in Figure 4A, where paired sediment and fish EFs fall off the 1:1 line for these congeners. If a normal distribution is assumed, there were also significant differences (t-test, P < 0.05) between sediment and bivalves for PCBs 91, 95, 132, 149, 174, and 183 (Figure 4B), although the low number of data points suggests that these results should be interpreted with caution. The enantioselective preference was reversed between fish and bivalves for PCB 95 (EF < 0.5 for fish, EF > 0.5 for bivalves) and PCB 149 (EF > 0.5 for fish; EF < 0.5 for bivalves). PCB 183 was nonracemic in bivalves (average EF ) 0.355, n ) 4) but racemic in fish (average EF ) 0.499 for all species, n ) 26) and sediment (average EF ) 0.495, n ) 13). No relations between total PCB concentration and EFs were apparent for any congener.
Discussion Atropisomeric PCBs were originally released into the environment as racemates. Therefore, a nonracemic composition of chiral PCBs in an environmental setting is strong evidence of PCB bioprocessing (i.e., metabolism, uptake, depuration). Although it is possible that nonbiological chiral interactions
FIGURE 2. Box plots of enantiomeric fractions (EFs) for PCBs 91, 95, 136, and 149 for Lake Hartwell biota samples from all sites. Circles are EF values for individual samples for each congener. Sampling sites are not distinguished in this figure. might exist, the most likely chiral compounds that might interact with chiral PCBs are enzymes, so any observed enantioselectivity is likely caused by interaction with biological agents. Nonracemic amounts of PCBs were found in the biota in this study, much as they were in other biota reported in the literature. Slightly nonracemic amounts of PCB 149 (EF ) first-peak/second-peak areas ) 0.50-0.55) were observed in blue mussels of the German Bight (7). The elution order on the chiral column used in that study is unknown, but if (-)PCB 149 eluted first, the EFs reported would be of the same magnitude and direction as those observed for NAWQA bivalves. Nonracemic amounts of PCBs 95, 132, 135, 149, and 176 were observed in dead striped dolphins found washed up on the shores of the Mediterranean Sea, whereas racemic amounts were found for PCBs 136 and 174 (9). The livers of sharks from the Atlantic had some nonracemic amounts of PCB 132 [EF ) 0.502-0.565, expressed as (+)/(-) based on (-)-PCB 132 eluting first on the Chirasil-Dex column used in the study (25)] but racemic amounts of PCBs 95 and 149 (8). Nonracemic PCB 132 was also observed in human milk (EF ) 0.541-0.689 expressed as (+)/(-)) (26). Highly nonracemic methylsulfonyl PCBs (PCB metabolites, many of which are themselves chiral) were found in polar bears and ring seals (27), which suggests that the parent chiral PCBs also might have been nonracemic in these two species; however, chiral PCBs were not directly measured in that study. The nonracemic residues of PCBs observed in biota are caused by uptake of nonracemic mixtures of PCBs from the water column and diet; by in vivo enantioselective uptake, metabolism, or depuration of PCBs; or by some combination of these processes. Enantioselective uptake or depuration of PCBs might be unlikely because the transfer of hydrophobic organic compounds from the gastrointestinal tract into the
body seems to be passive and does not involve selective transport of these molecules during food digestion (28, 29). This fact, therefore, suggests that organisms are not just accumulating nonracemic PCBs from the outside environment but are metabolizing them enantioselectively, at least to some degree. This observation is supported by the species trends in enantiomeric signatures (Figures 2 and 3), thus suggesting differences in PCB metabolism among species. The possibility of enantioselective PCB metabolism in biota is also suggested by the differences in enantiomeric signatures between biota and sediment, which are markedly different, both in the Lake Hartwell biota and in the NAWQA fluvial organisms (Figure 4). In Lake Hartwell sediment between the Maw and 133 Bridges, the EFs of PCBs 91, 95, and 149 were less than 0.5, while the EF of PCB 136 was greater [sediment cores G27 and G30 (11)]. On the other hand, it is clear (Figure 4) that the enantiomeric composition in biota may be different from that of the sediment. In the NAWQA samples, white suckers and Corbicula feed near sediment, the former consuming sediment-dwelling organisms and the latter consuming filter-feeding particles in the water column (including resuspended sediment) near the sediment-water interface. If there is no enantioselective bioprocessing of sediment-borne PCBs by organisms after ingestion, then the enantiomeric signatures in the sediment and the biota should be the same, which is not the case for many congeners (Figure 4). It is not likely, however, that the species in this study are feeding directly on sediment exclusively at the sites where they were found. The fish species move within the streams where they live and might be exposed to PCB concentrations different from those where they were caught in this study. They also feed on other organisms with nonracemic PCB residues. Similar arguments hold for the other species. These considerations could account for a sizable amount of the VOL. 35, NO. 12, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Box plots of National Water-Quality Assessment (NAWQA) Program river biota enantiomeric fractions (EFs). PCB chlorine substitution pattern (ring 1-ring 2) is beside PCB label in each plot as well as the vicinal hydrogen atoms present in the congener. Circles are EF values for individual samples for each congener. Sampling sites are not distinguished in this figure. sample-to-sample variability in the PCB EFs we measured. Thus, we cannot rule out the possibility that the nonracemic amounts of PCBs observed in organisms were caused by, at least in part, the uptake of previously bioprocessed PCBs, whether in the sediment, water column, or elsewhere in the food chain. Further research is needed to clarify the behavior of chiral PCBs in biota. The nonracemic PCB residues observed in biota are consistent with PCB metabolism by most organisms, which oxidatively biotransform PCBs with cytochrome P-450-type enzymes. In this discussion, we refer to individual PCB congeners by listing the chlorine-substituted position on each ring separated by a hyphen (e.g., PCB 149, or 2,2′,3,4′,5′,6hexachlorobiphenyl, is 236-245). PCBs that lack both meta, para and ortho, meta vicinal hydrogen atoms (30), including chiral PCB 183 (2346-245), are difficult to metabolize. This observation is consistent with the racemic compositions of PCB 183 in most of our biological samples, thus suggesting little enantioselective metabolism, although nonenantioselective metabolism is possible. However, bivalves had markedly nonracemic amounts of PCB 183, thus indicating that bivalves were bioprocessing PCB 183 to a greater extent than expected. Congeners with meta, para vicinal hydrogen atoms (30-33), which would include chiral PCBs 91 (23624), 95 (236-25), 132 (234-236), 136 (236-236), 149 (236-245), 174 (2345-236), and 176 (2346-236), can be metabolized by phenobarbital-type (PB) cytochrome P-450 2B enzymes. Congeners with ortho, meta vicinal hydrogen atoms may also be attacked by P-450 1A isozymes. PCB metabolism by P450 may be enantioselective for at least some of these congeners. In contrast, literature results suggest that fish and bivalves have only a limited ability to metabolize PCBs. Although P-450 activity has been observed in fish (34, 35), bivalves (36), and 2452
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FIGURE 4. PCB enantiomeric fractions (EFs) of National WaterQuality Assessment (NAWQA) Program samples from river sites at which (A) sediment and fish (white sucker) and (B) sediment and bivalve (Corbicula) samples were taken simultaneously. Vertical and horizontal lines indicate racemic PCB amounts (EF ) 0.5) for sediment and biota, respectively. Points along 1:1 diagonal line indicate sites at which PCB EFs have the same value in sediment and biota samples. crustaceans (37), these organisms generally have lower P-450 levels and activities than mammals. While some feeding experiments have measured biotransformation of PCBs (38),
others have not (39). Various field observations of PCB congener distribution in aquatic biota have suggested some degree of PCB biotransformation consistent with P-450 activity but to a lesser degree than in mammals (30, 32, 4042). Field metabolism was inferred from lower measured biomagnification factors (i.e., lower relative concentrations in predator body residues as compared to that in prey), lower concentrations relative to recalcitrant congeners such as PCB 153, or lower retention of congeners expected to be metabolized based on structure-activity relations. However, the field methods for inferring metabolism are all indirect and do not provide direct evidence either for or against aquatic organism PCB biotransformation. Our observations of nonracemic PCB atropisomer residues in fish and bivalves suggest that these organisms might biotransform PCBs to a greater extent than indicated in previous studies but do so very slowly over the course of their lifetimes. If these organisms slowly metabolize PCBs enantioselectively, they would build up nonracemic residues of chiral PCBs over time that are readily detected by chiral analysis, which measures for evidence of metabolism. Laboratory and field studies that use achiral analysis, however, might miss this metabolism. The trends in chiral PCB enantiomeric signatures in different species suggest that the enzymes responsible for biotransforming PCBs in these species also differ in their stereoselectivity. For example, the low EFs for PCB 95 in sculpin as compared to suckers may be caused by a greater ability for sculpins to biotransform this congener enantioselectively, assuming neither species is accumulating nonracemic PCBs significantly. Similar arguments may explain the observed presence of nonracemic PCB 183 in bivalves but not in fish. The reversals in enantioselective preference for a number of congeners in different species (e.g., PCBs 95 and 149 between fish and bivalves) also suggest that enzymes in the various organisms vary in the way they process these congeners. These observations imply that the active sites of enzymes that biotransform PCBs have greater affinities for one enantiomer over the other, which lead to enantioselective attack, and that these affinities differ (and may be reversed) among species. The observations in organisms are in agreement with the different enantioselective signatures and reversals in enantioselectivity observed for chiral PCBs in sediment known to bear microbial consortia that reductively dechlorinate PCBs in different ways (11). These differences in dechlorination may arise from different enzyme affinities for enantiomers. Some similarities in EFs among species can also be found. For instance, river suckers and the Lake Hartwell bass and bluegill all share general depletion of the first enantiomer of PCB 91 and the (+) enantiomer of PCB 136, thus suggesting similarities in the way these congeners are bioprocessed by these fish species. However, we should be careful in generalizing about species trends in PCB enantiomeric signatures, particularly for sculpins and Corbicula bivalves, because of the low number of samples analyzed in this study.
Implications of PCB Enantioselectivity in Biota The chiral analysis results of this study reveal a number of important implications on the bioprocessing of PCBs by organisms. The presence of nonracemic PCB residues in fish and bivalves suggests that there is greater metabolic degradation of PCBs in these organisms than reported in previous studies. Laboratory biotransformation experiments would be essential in understanding enantioselective breakdown of chiral contaminants. We observed no relation between biota PCB concentration and enantioselectivity. In fact, we have not been able to find a way to predict enantioselectivity in organisms nor to determine the extent to which specific species metabolize chiral PCBs enantioselectively as com-
pared to the extent to which they pass nonracemic residues with the same EF up the food chain. Chiral analysis thus can be a useful tool in understanding the behavior of PCBs in aquatic and riparian food webs. Finally, there is little knowledge of the biological and toxicological effects of enantiomers on biota, thus underscoring a need for more research on the presence of individual enantiomers of chiral pollutants in biota and on the relative toxicity of these enantiomers.
Acknowledgments We thank Don Brockway of the National Exposure Research Laboratory, Ecosystem Research Division (NERL/ERD) in Athens, GA, for the Lake Hartwell biota extracts and Jimmy Avants and John Evans, also of NERL/ERD, for assistance with sample analysis. We also thank Kathy Echols, Bob Eganhouse, and Jon Raese of USGS for valuable review comments. In addition, we would also like to thank the many individuals in the USGS National Water-Quality Assessment (NAWQA) Program and the National Water Quality Laboratory who contributed to the collection, processing, and analysis of NAWQA samples used in this study. This paper has been reviewed in accordance with the U.S. EPA and USGS peer and administrative review policies and approved for publication. Use of firm, brand, or trade names in this paper is for identification purposes only and does not constitute endorsement by the U.S. Government.
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Received for review November 20, 2000. Revised manuscript received April 16, 2001. Accepted April 16, 2001. ES0018872
