9. Bioaccumulation and Trophic Transfer of Organic Contaminants
205
9.1.2. Organism Adiposity and Internal Distribution
of Contaminants
Although contaminants can be accumulated from several environmental compartments, once the contaminants are bioaccumulated, organism lipids become the
dominant force controlling the dynamics of distribution and the manifestation of
toxic effects. Lipids in organisms make up a significant portion of cell membranes, are particularly concentrated around neurons, and are the fundamental
energy stores for organisms. Thus, lipids serve as membrane barriers to accumulation and distribution of contaminants, the sites of toxic action where changes in
membrane function are disturbed by contaminant interaction, and the ultimate
storage site for contaminants that have high lipid solubility. The interaction of
contaminants with lipids depends, in large part, on the ability of the contaminant
to dissolve in the lipid, and contaminants that have low lipid solubility will not
cross membranes except through facilitated or active transport sites (Hunn and
Allen, 1974). By contrast, highly hydrophobic contaminants will be found more
strongly associated with lipid membranes and storage lipids of the organism.
Initial efforts to understand the interactions of organic contaminants and lipids
come from the pharmacological literature and are focused on understanding the
distribution of drugs within an organism (Bickel, 1984). Contaminants that are
freely dissolved in the blood are available for transport among tissues whereas
those that are sequestered by blood proteins and lipids are less mobile. Further,
contaminants that are highly lipophilic eventually end up in organism lipid stores
away from their receptor sites for toxic action However, lipid mobilization by an
organism under stress can release contaminants back to the general circulation. In
addition, contaminants of differing lipophilicity differ in their ability to be assimilated because crossing the cell membranes into an organism requires the contaminant to pass through lipid membranes. Thus, differing lipid solubilities along with
other molecular features such as molecular size produce differences in the rates
and extent of bioaccumulation. Because of the interest in developing better drugs
with more improved transfer efficiencies and rates, the partitioning of contaminants between water and representatives of lipid materials has been studied. The
best small molecule found to be representative of lipid materials was n-octane-l01 (usually referenced as octanol). The partitioning behavior between octanol and
water provides a standardized reference source for partitioning that has proved
very useful for evaluating the lipophilicity of contaminants (reviewed in Leo et
aI.,1971).
The kinetics and efficiency of transport into organisms are determined by the
rate of presentation of contaminants to the membrane, the chemical activity
(chemical potential) difference between the source compartment and the organism
tissue, and the contaminant's resistance for crossing membranes. Because membranes are composed primarily of lipid bilayers, the ability of contaminants to
dissolve into, and move through, membranes dictates part of the kinetics of
transport (Hunn and Allen, 1974). The rate of exit from the membrane into the
circulating fluid and transport to the site of action or storage can also significantly
205
9.1.2. Organism Adiposity and Internal Distribution
of Contaminants
Although contaminants can be accumulated from several environmental compartments, once the contaminants are bioaccumulated, organism lipids become the
dominant force controlling the dynamics of distribution and the manifestation of
toxic effects. Lipids in organisms make up a significant portion of cell membranes, are particularly concentrated around neurons, and are the fundamental
energy stores for organisms. Thus, lipids serve as membrane barriers to accumulation and distribution of contaminants, the sites of toxic action where changes in
membrane function are disturbed by contaminant interaction, and the ultimate
storage site for contaminants that have high lipid solubility. The interaction of
contaminants with lipids depends, in large part, on the ability of the contaminant
to dissolve in the lipid, and contaminants that have low lipid solubility will not
cross membranes except through facilitated or active transport sites (Hunn and
Allen, 1974). By contrast, highly hydrophobic contaminants will be found more
strongly associated with lipid membranes and storage lipids of the organism.
Initial efforts to understand the interactions of organic contaminants and lipids
come from the pharmacological literature and are focused on understanding the
distribution of drugs within an organism (Bickel, 1984). Contaminants that are
freely dissolved in the blood are available for transport among tissues whereas
those that are sequestered by blood proteins and lipids are less mobile. Further,
contaminants that are highly lipophilic eventually end up in organism lipid stores
away from their receptor sites for toxic action However, lipid mobilization by an
organism under stress can release contaminants back to the general circulation. In
addition, contaminants of differing lipophilicity differ in their ability to be assimilated because crossing the cell membranes into an organism requires the contaminant to pass through lipid membranes. Thus, differing lipid solubilities along with
other molecular features such as molecular size produce differences in the rates
and extent of bioaccumulation. Because of the interest in developing better drugs
with more improved transfer efficiencies and rates, the partitioning of contaminants between water and representatives of lipid materials has been studied. The
best small molecule found to be representative of lipid materials was n-octane-l01 (usually referenced as octanol). The partitioning behavior between octanol and
water provides a standardized reference source for partitioning that has proved
very useful for evaluating the lipophilicity of contaminants (reviewed in Leo et
aI.,1971).
The kinetics and efficiency of transport into organisms are determined by the
rate of presentation of contaminants to the membrane, the chemical activity
(chemical potential) difference between the source compartment and the organism
tissue, and the contaminant's resistance for crossing membranes. Because membranes are composed primarily of lipid bilayers, the ability of contaminants to
dissolve into, and move through, membranes dictates part of the kinetics of
transport (Hunn and Allen, 1974). The rate of exit from the membrane into the
circulating fluid and transport to the site of action or storage can also significantly