206
p.p. Landrum and S. W. Fisher
dictate the rate of accumulation of lipophilic contaminants. The same factors
affect the transfer of the contaminants out of a tissue. In summary, when the lipid
solubility of the contaminant is high relative to its solubility in the source compartment, the contaminant will tend to pass into and accumulate in the organism.
The impact of lipids on these kinetic processes is most easily observed in their
effect on the rate of elimination. Organisms with higher lipid contents exhibit
elimination rates that are substantially slower than observed for leaner organisms
(Van den Huevel et a!., 1991; Landrum, 1988). Differences in the overall elimination rates among organisms reflect the capacity of the organisms relative to the
source compartment, and decreases in elimination rate are reflected in increasing
bioaccumulation. In some cases, the rate of accumulation can increase with increasing lipid content as was observed for the zebra mussel (Bruner et a!., 1994a).
The mechanism for this increase is tied to maintenance of the concentration
gradient between the source compartment and the site of uptake. This presumes
that the rate processes involved in the distribution within the organism are not
rate-limiting. If distribution limitations become a significant portion of the rate
process, then distribution to the storage lipid will become disconnected from the
uptake rate process and lipid content will no longer be tightly coupled to uptake
rate. This was clearly demonstrated with the accumulation of trifluralin in rainbow trout, in which the uptake clearance rates declined in proportion to the
intercompartmental transfer rates with increasing organism size despite the increased lipid content for large fish (Schultz and Hayton, 1994).
9.1.3. Nonlipid Factors Affecting Internal Distributions
In addition to the lipophilicity of the contaminant, the size of the molecule may
preclude its dissolution into the membrane and, therefore, accumulation by the
organism. In the extreme case of polymers, it is clear that even hydrophobic
(highly lipophilic) molecules such as polymers of polydimethylsiloxane are not
accumulated and are only found on the surface of organisms (Kukkonen and
Landrum, 1995; Opperhuizen et a!., 1987). The failure of molecules to accumulate
can also be due to binding to extracellular materials such as dissolved organic
carbon (Bruggeman et a!., 1984) or decreased permeability of the membrane
resulting from molecular size limitations, >9.5 A (Saito et a!., 1990; Opperhuizen
et a!.. 1985; Zitko, 1980). Both mechanisms result in reduced bioavailabilities.
The effective molecular size range for interaction with lipids not only has an upper
limit, at which the lipophilic membranes act as essentially impermeable barriers,
but there is also a lower molecular size cutoff. At the lower end. the membranes
are permeable to small un-ionized molecules. This size cutoff is <50 a.m.u.
(Walter and Gutknecht, 1986). The extra permeability is not related to the lipophilicity of the contaminant but rather inversely related to the molecular size. The
molecular volume dependence was attributed to the membrane properties, with
the lipid behaving more like a polymer than a liquid hydrocarbon (Walter and
Gutknecht, 1986).
p.p. Landrum and S. W. Fisher
dictate the rate of accumulation of lipophilic contaminants. The same factors
affect the transfer of the contaminants out of a tissue. In summary, when the lipid
solubility of the contaminant is high relative to its solubility in the source compartment, the contaminant will tend to pass into and accumulate in the organism.
The impact of lipids on these kinetic processes is most easily observed in their
effect on the rate of elimination. Organisms with higher lipid contents exhibit
elimination rates that are substantially slower than observed for leaner organisms
(Van den Huevel et a!., 1991; Landrum, 1988). Differences in the overall elimination rates among organisms reflect the capacity of the organisms relative to the
source compartment, and decreases in elimination rate are reflected in increasing
bioaccumulation. In some cases, the rate of accumulation can increase with increasing lipid content as was observed for the zebra mussel (Bruner et a!., 1994a).
The mechanism for this increase is tied to maintenance of the concentration
gradient between the source compartment and the site of uptake. This presumes
that the rate processes involved in the distribution within the organism are not
rate-limiting. If distribution limitations become a significant portion of the rate
process, then distribution to the storage lipid will become disconnected from the
uptake rate process and lipid content will no longer be tightly coupled to uptake
rate. This was clearly demonstrated with the accumulation of trifluralin in rainbow trout, in which the uptake clearance rates declined in proportion to the
intercompartmental transfer rates with increasing organism size despite the increased lipid content for large fish (Schultz and Hayton, 1994).
9.1.3. Nonlipid Factors Affecting Internal Distributions
In addition to the lipophilicity of the contaminant, the size of the molecule may
preclude its dissolution into the membrane and, therefore, accumulation by the
organism. In the extreme case of polymers, it is clear that even hydrophobic
(highly lipophilic) molecules such as polymers of polydimethylsiloxane are not
accumulated and are only found on the surface of organisms (Kukkonen and
Landrum, 1995; Opperhuizen et a!., 1987). The failure of molecules to accumulate
can also be due to binding to extracellular materials such as dissolved organic
carbon (Bruggeman et a!., 1984) or decreased permeability of the membrane
resulting from molecular size limitations, >9.5 A (Saito et a!., 1990; Opperhuizen
et a!.. 1985; Zitko, 1980). Both mechanisms result in reduced bioavailabilities.
The effective molecular size range for interaction with lipids not only has an upper
limit, at which the lipophilic membranes act as essentially impermeable barriers,
but there is also a lower molecular size cutoff. At the lower end. the membranes
are permeable to small un-ionized molecules. This size cutoff is <50 a.m.u.
(Walter and Gutknecht, 1986). The extra permeability is not related to the lipophilicity of the contaminant but rather inversely related to the molecular size. The
molecular volume dependence was attributed to the membrane properties, with
the lipid behaving more like a polymer than a liquid hydrocarbon (Walter and
Gutknecht, 1986).