214
p.F. Landrum and S.w. Fisher
comparisons among species and the role of lipid content on residue effects concentrations have yet to be fully explored. However, based on the intraspecies role
of lipid and the general understanding of the storage of toxins and their impact on
the toxic response (see above), it is clear that variation in the storage lipid capacity
among organisms will help explain the variance among species when comparing
residue-based effects.
9.5.2. Lipids and Membrane Narcosis
The direct interactions of contaminants with membrane lipids can result in
narcosis. Two reviews on this subject (van Wezel and Opperhuizen, 1995;
Mullins, 1954) suggest that the function of the lipid membrane becomes impaired
when a sufficient molar volume of contaminant becomes dissolved in the membrane. The in vivo membrane burden of toxicant that produces narcosis is 40-160
mmol . kg-I lipid, and this range corresponds to approximately 3 ml . kg-Ion a
volume basis (Mullins, 1954). The change in membrane function is thought to
occur because the ion permeability of the membranes increases due to an increase
in fluidity of the lipids with the solubilization of contaminants (van Wezel and
Opperhuizen, 1995). Such changes in membrane fluidity have been demonstrated
in the presence of the narcotic benzyl alcohol through the use of nuclear magnetic
resonance (NMR) (Ma et aI., 1992). These NMR studies demonstrated that the
thermodynamic character of lipid membranes changes with the introduction of a
narcotic.
9.5.3. Effect of Toxins on Lipid Metabolism and Function
The incorporation of modified fatty acid molecules, such as chlorinated fatty
acids, into the lipid biomolecules and subsequently into membrane structures is a
newly recognized problem. The presence of chlorine in the fatty acid moiety
causes a bend in the molecule similar to that produced by a double bond. Changes
in chemical character and molecular conformation mayor may not be recognized
by the biochemistry of the organism. When these chlorinated fatty acids are
incorporated into lipid bilayers, the membrane character may be altered. Further,
incorporation of lipids containing chlorinated fatty acids into storage lipids should
not initially affect the organism, but when these are mobilized for energy, there
may be less energy available (Ewald, 1996). In highly polluted areas, up to I % of
the total fatty acids in an organism may be chlorinated (Hakansson et a!., 1991).
The exposure to contaminants can also directly affect the lipid content, either
through effects on lipid metabolism or by increasing the overall stress, thus
increasing catabolism with resultant reductions in lipid. However, the exposure to
chemicals need not always result in reductions in lipids. Rainbow trout (Oncorhynchus mykiss) exposed to heptadecane not only sequester the contaminant in
the lipid stores but metabolize the heptadecane to fatty acids that become incorporated into the neutral and phospholipid fractions of the organism (Cravedi and
Tulliez, 1986).
p.F. Landrum and S.w. Fisher
comparisons among species and the role of lipid content on residue effects concentrations have yet to be fully explored. However, based on the intraspecies role
of lipid and the general understanding of the storage of toxins and their impact on
the toxic response (see above), it is clear that variation in the storage lipid capacity
among organisms will help explain the variance among species when comparing
residue-based effects.
9.5.2. Lipids and Membrane Narcosis
The direct interactions of contaminants with membrane lipids can result in
narcosis. Two reviews on this subject (van Wezel and Opperhuizen, 1995;
Mullins, 1954) suggest that the function of the lipid membrane becomes impaired
when a sufficient molar volume of contaminant becomes dissolved in the membrane. The in vivo membrane burden of toxicant that produces narcosis is 40-160
mmol . kg-I lipid, and this range corresponds to approximately 3 ml . kg-Ion a
volume basis (Mullins, 1954). The change in membrane function is thought to
occur because the ion permeability of the membranes increases due to an increase
in fluidity of the lipids with the solubilization of contaminants (van Wezel and
Opperhuizen, 1995). Such changes in membrane fluidity have been demonstrated
in the presence of the narcotic benzyl alcohol through the use of nuclear magnetic
resonance (NMR) (Ma et aI., 1992). These NMR studies demonstrated that the
thermodynamic character of lipid membranes changes with the introduction of a
narcotic.
9.5.3. Effect of Toxins on Lipid Metabolism and Function
The incorporation of modified fatty acid molecules, such as chlorinated fatty
acids, into the lipid biomolecules and subsequently into membrane structures is a
newly recognized problem. The presence of chlorine in the fatty acid moiety
causes a bend in the molecule similar to that produced by a double bond. Changes
in chemical character and molecular conformation mayor may not be recognized
by the biochemistry of the organism. When these chlorinated fatty acids are
incorporated into lipid bilayers, the membrane character may be altered. Further,
incorporation of lipids containing chlorinated fatty acids into storage lipids should
not initially affect the organism, but when these are mobilized for energy, there
may be less energy available (Ewald, 1996). In highly polluted areas, up to I % of
the total fatty acids in an organism may be chlorinated (Hakansson et a!., 1991).
The exposure to contaminants can also directly affect the lipid content, either
through effects on lipid metabolism or by increasing the overall stress, thus
increasing catabolism with resultant reductions in lipid. However, the exposure to
chemicals need not always result in reductions in lipids. Rainbow trout (Oncorhynchus mykiss) exposed to heptadecane not only sequester the contaminant in
the lipid stores but metabolize the heptadecane to fatty acids that become incorporated into the neutral and phospholipid fractions of the organism (Cravedi and
Tulliez, 1986).