Environ. Sci. Technol. 1991,25,536-539
Bioconcentration Factors and Lipid Solubility Sujit Banerjee * Institute of Paper Science and Technology, 575 14th Street NW, Atlanta, Georgia 30318
George L. Baughman Environmental Research Laboratory, U S . Environmental Protection Agency, Athens, Georgia 3061 3
The log-log relationship between bioconcentration and hydrophobicity breaks down for several medium and high molecular weight solutes that bioconcentrate either to a small extent or not at all. Much of the failure is attributed to the relatively low solubility of these compounds in lipid. Inclusion of a term in octanol solubility (in place of lipid solubility, which is generally unavailable) considerably improves the quality of the relationship ( r = 0.95). It is speculated that the octanol solubility term compensates for the relatively low solubility of large compounds in lipid. Introduction
The bioconcentration factor (BCF) in fish is frequently related to the octanol-water partition coefficient KO, through log BCF = ~1 + ~2 log KO, (1) where c1 and c2 are constants. The relationship breaks down for high-KO, compounds such as octachloronaphthalene, which do not bioconcentrate ( I ) , and a similar trend has been reported for many dyes (2). Thus, while physical parameters such as water solubility can usually be calculated within the tolerance necessary for environmental work, BCF can only be estimated qualitatively. Equation 1 breaks down for strongly hydrophobic compounds (log KO, >6), and the maximum observed value for log BCF (log BCF,,,) seems to be -5.5 (1-15). One view is that since hydrophobic compounds tend to be large, they have difficulty in traversing the gill membrane (1,5,8,10, 15). Another suggestion is that large compounds of low lipid solubility have lower than expected BCFs (4)because of difficulty in cavity formation in lipid. Either view implicates molecular size rather than hydrophobicity as the root cause of the attenuated BCFs. For nonpolar compounds, the difference is indiscernible since size often correlates well with hydrophobicity (16). For polar compounds the distinction can be quite clear; e.g., dyes can be outliers from the log BCF-log KO, relationship even though their log Kowsare much less than 6 ( 2 ) . The essence of the lipid solubility hypothesis (4) is that ylipid increases more rapidly than yoctanolWe note, further, that for BCF (Ywater/y[jpid) to decrease, Ylipid must also increase more rapidly than ywater.This is reasonable since lipid is highly structured and should resist the incursion of large solutes more so than water. Octanol is much less structured than water, and yoctanol increases much less rapidly than ywaterfor nonpolar solutes (17). Thus, for compounds for which yoctanol >> 1,BCF could decrease even though KO, increases. Gobas et al. (18) studied partitioning between ~ - a phosphatidylcholine dimyristoyl membrane vesicles (MW) and water and found that plots of log KMW - log KO, leveled off a t log KO, = 6-7. The analogy to the log BCF log KO, plot suggests that membrane vesicles are a better surrogate for fish lipid than is octanol. Even so, the membrane vesicle model is not universal; e.g., it does not extend to large compounds such as hexabromobenzene and several dyes (to be considered later), which do not bio536
Environ. Sci. Technol., Vol. 25, No. 3, 1991
concentrate despite their appreciable log KMW values. Model Development. An alternative to searching for a better lipid substitute is to simulate lipid behavior by combining more than one octanol-related property. KO, alone is inadequate because it is dominated by water-solute interactions. We reasoned that inclusion of a term the solute activity coefficient in octanol, might in yoctanol, correct for the inadequacy. Since yoctanol can be expressed the molar solubility in octanol, and mp, in terms of Soctanol, the melting point (in "C) (19),eq 1 expands to log BCF = c3 + c4 log KO, + c5 log S o c t a n o l + c6 (mp - 25) (2) where c3-c6 are constants. The mp term is intended to allow octanol solubilities for both liquids and solids to be included in the same equation (16). For liquids, mp is taken as 25 to remove the entire term. Equation 2 implies that for solutes that are strongly nonideal in both octanol and lipid, log yoctanol will be linearly related to log ylipid. For small hydrophobic comand ylipid will be close to 1, the terms pounds, both yoctanol associated with c5 and c6 in eq 2 will tend to be constant, and eq 2 will be equivalent in form to eq 1. Hence, eq 2 should apply to small and large solutes alike. It might be argued that the octanol-related terms in eq 2 should be replaced with parameters associated with membrane vesicles since the latter may better represent lipid behavior. This would be desirable if KMW and associated values were widely available. However, given the paucity of KMW data, the wide availability of KO, values, and that algorithms exist for estimating KO,and octanol solubility from structure, the use of octanol is preferred, if only on practical grounds. BCF and associated values were collected for the compounds listed in Table I. Structures of the dyes in Table I are provided in Figure 1. Values for Y~~~~~~~were calvalues are culated by the UNIFAC method (23). Soctanol - yoctanol pairs could be acquired relatively scarce; Soctanol for only 21 compounds in Table I, and eq 3 log Soctanol = 0.793 - 0.0067 (mp - 25) 1.18 log Y~~~~~~~ ( n = 21, r = 0.97) (3) was obtained from these values. An analogous equation has been reported earlier (24)for water-saturated octanol. Equation 3 was used to calculate Soctanol for the remaining compounds in Table I. BCFs calculated from eq 1 (with c1 = -0.78; c2 = 0.75) are compared to measured values in Figure 2; as expected, the fit is poor (r = 0.73). The eq 2 relationship log BCF = -1.13 + 1.02 log KO, + 0.84 log Socbnol + 0.0004 (mp - 25) ( n = 36, r = 0.95) (4) is a marked improvement, as illustrated in Figure 3. The fit is good even though metabolic effects are not considered, and the BCF data in Table I are uncorrected for variations in lipid content of the different species used. The coefficient of the mp term is much lower than the expected value of 0.0084 (0.01 X 0.84). There are strong theoretical reasons and a wealth of evidence (25, 26) to support the importance of the mp term in solubility re-
0013-936X/91/0925-0536$02.50/0
0 1991 American Chemical Society
Br
IX
IV
v,
FN
0- -0
CYJ \ /
'C
N,C,n$n' 'CH,Cn~OCOCH,
N=N-
N
I
cn2cnc,n,
X
I
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Figure 1. Structures of dyes listed in Table I .
I
1
il -
I '
11
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*
0
d
'
e**
*
I
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0-i
I
~
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i 2 3 4 log BCF (meas.)
~
5
I
6
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Figure 2. Comparison of measured BCFs with estimates from eq 1 ( c , = -0.78; Cp = 0.75).
lationships. However, Arbuckle (27) and Isnard and Lambert (28) have also reported relationships where the significance of the mp term was much lower than expected. I t should be noted that only 13% of the compounds in Isnard and Lambert's data set melted above 125 "C;for lower melting solids, the mp term is expected to depress solubility by less than 1 order of magnitude. Furthermore,
I
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I
-1
I
0
I
I
1
I
I
2
I
I
3
1
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4
1
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5
1
1
6
log BCF (meas,) Figure 3. Comparison of measured BCFs with estimates from eq 4.
the high-melting solids were all nonpolar, and for these, Isnard and Lambert's melting point correlates with KO,,,. relationship does not apply to polar high-melting solids. Most of the compounds in Table I are solids, and there and is a strong correlation ( r = 0.82) between log Soctanol melting point within the series, which lowers the mp coefficient. We have noted a similar effect in another study (29). We expect the coefficient of the mp term in Environ. Sci. Technol., Voi. 25, No. 3, 1991
537
Table I. Comparison of Measured BCFs with Values Estimated from Equation 4
mp, "C 1,2-dichloroethane dye IX atrazine dye I1 dye VI11 dye IV isophorone dye X dye I carbon tetrachloride dve VI1 hkxachloroethane 1,4-dichlorobenzene 1in d a ne acenaphthene naphthalene biphenyl 4-chlorobiphenyl benzyl butyl phthalate fluorene 1,2,3,5-tetrachlorobenzene 1,2,4-trichlorobenzene 1,4-dichloronaphthalene phenanthrene pyrene 2-chloronaphthalene 1,2,4,5-tetrabromobenzene decachlorobiphenyl pentachlorobenzene hexachlorobenzene 2,2',3,3',4,4',5,5'-octachlorobiphenyl 1,3,7-trichloronaphthalene DDT 1,3,5,7-tetrachloronaphthalene 2,2',5,5'-tetrachlorobiphenyl 2,2',4,4',5,5'-hexachlorobiphenyl additional compounds used for developing eq 38 4,4'-dichlorobiphenyl 1-methyl fluorene 2,4,5-trichlorobiphenyl anthracene 2,3,4,5-tetrachlorobiphenyl 2,3-benzofluorene chrysene 2,3-benzanthracene perylene 1,2,5,6-dibenzanthracene coronene a
21.
liq 157 177 229 117 219 liq 173 193 liq 194 192 53 113 93 81 71 77 liq 114 51 liq 68 97 150
60 182 305 83 230 159 113 109 180 87 103
149 85 77 216 91 209 255 357 277 266 360
1%
log
KO,
Soctanolr
1.45b 4.00d 2.63e 3.40d 4.10e 4.40e 1.67b 4.50e 5.40d 2.64b 5.40d 3.93b 3.3ab 3.85b 3.97b 3.59h 3.8ah 4.26b 4.05b 4.188 4.65b 4.04b 4.88' 4.579 5.188 4.19' 6.0gb 8.26-J 5.1gb 5.66b 7.1W 5.59' 6.1gk 6.38' 6.10' 6.90'
log BCF
M
0.44' -2.25' -1.32e -3.56e -2.50' -3.34e 0.43' -2.23' -3.83e 0.59' -2.6se -0.66' 0.25' -1.03' -0.599 -0.029 -0.168 -0.229 -0.34' -0.659 0.15r 0.19' -0.28' -0.409 -0.85g
-0.12' -1.56' -2.779 -0.49' -1.8Ze -1.61' -0.70' -1.09f -1.27' -0.63g -1.04'
meas
calc
0.30b 0.30e 0.4ae 0.4ae 0.70' 0.70e 0.85b 0.90' 1.00e 1.24b 1.76' 2.14b 2.33h 2.5Ib 2.5ab 2.63b 2.64b 2.77b 2.8gb 3.11h 3.26b 3.32b 3.36' 3.42b 3.43h 3.63' 3.81b 4.021 4.11b 4.27e 4.35' 4.43' 4.47h 4.53' 4.921 5.321
0.72 1.11 0.50 -0.58 0.99 0.62 0.93 1.64 1.22 2.06 2.21 2.39 2.54 1.96 2.45 2.54 2.71 3.05 2.72 2.62 3.47 3.15 3.63 3.22 3.49 3.06 3.83 5.07 3.81 4.08 4.81 4.08 4.30 4.37 4.25 5.06
YoctanolD
5.11 3.57 4.58 4.55 6.21 4.12
12.6 27.0 5.29
13.3
5.46 6.23 6.78 7.38 8.43 12.4 14.6 14.6 24.8 27.8 79.4
-1.15 -0.56 -0.75 -1.93 -0.86 -1.75 -2.70 -2.28 -2.52 -3.03 -3.41
Used to develop eq 3. Reference 6. Calculated with eq 3. dReference 20. e Reference 2. 'Reference 18. "Reference Reference 1. Reference 4.
eq 4 to increase in magnitude as liquids and high-melting solids are added to the data set. The difference in the quality of fit between Figure 2 and term in Figure 3 emphasizes the importance of the Soctanol term reflects lipideq 4. We speculate that the Soctanol solute incompatibility. However, inasmuch as this conclusion is suggested by a correlation rather than by experimental evidence, it needs to be confirmed independently. At present, it should be regarded as only one of many factors that can affect bioconcentration, e.g., growth dilution, differences in solute bioavailability, and metabolism. Equation 4 is preliminary; it is based on only 21 Soctanol measurements, and the coefficient of the mp term is unexpectedly low, as discussed above. Finally, we applied eq 4 to a number of solutes whose BCFs are known to be overestimated by eq 1. The results are shown in Table 11, and except for octachloronaphthalene, the eq 4 estimates are reasonable. Opper538
Environ. Sci. Technol., Vol. 25, No. 3, 1991
17.
Reference
Table 11. Comparison of Measured BCFs of Large Solutes with Values Calculated from Equation 1 YP, C
1%
log
log BCF
KO, Soctanol, M meas calc
octachloronaphthalene 197 7.9" hexabromobenzene >300 >6d dye I11 225 3 . F dye V 148 4.09 184 4.49 dye VI
-1.81b
