314
Marine Mammal Physiology: Requisites for Ocean Living
The evolution and diversification of enzymes associated with detoxification and xenobiotic biotransformation (i.e., cytochrome p450s; CYP) has also been attributed to the production of secondary metabolites by plant species and subsequent herbivory, an example
of chemical warfare (Gonzalez and Nebert 1990). Cytochrome P450 is a superfamily of
enzymes which are responsible for biotransformation of endogenous compounds and
xenobiotics in the liver and other tissues. There have been a number of isozymes identified within the CYP superfamily in nearly all mammalian lineages. Consequently, species
and even individuals have high diversity of p450 genes (Gonzalez and Nebert 1990). CYPs
are involved in the biotransformation of complex organic poisons such as PCBs via the
conjugation with oxygen to form a hydroxylated metabolite (Robertson and Hansen 2001).
There are 209 congeners of PCBs based on the number and position of chlorine atoms on
the biphenyl ring. While the congeners can be grouped according to the number of chlorine substitutions, physical structure (i.e., coplanar), or substitution pattern (para, meta,
ortho), individual congeners can have vastly different physical properties and toxicities,
as well as various mechanisms of toxicity. PCBs are known endocrine disrupters, and
relatively high concentrations of PCBs are reported in some marine mammals and their
associated prey species (Tanabe 2002). Relatively high concentrations of PCB congeners
of concern have been associated with adverse reproductive outcomes in ringed seals and
harbor seals (Reijnders 1986). Partial expressed gene sequences have been obtained via
RT-PCR for CYP1A from the livers of minke whale (Balaenoptera acutorostrata), Dall’s porpoise (Phocoenoides dalli), Steller sea lion (Eumetopias jubatus), spotted seal (Phoca largha), and
ribbon seal (Phoca fasciata) (Goksøyr 1995; Teramitsu et al. 2000). Perhaps unsurprisingly,
given the large diversity of p450 enzymes seen in other species, Goksøyr (1995) found differences when comparing the p450 system of marine mammals to terrestrial mammals.
There was also a difference in concentrations of CYP2B protein between seals (harp seal
and hooded seal) and whales (minke whale) (Goksøyr 1995). It has been suggested that
small cetaceans have a limited capacity to biotransform certain PCB congeners compared
to terrestrial mammals, perhaps due to limited activity of phenobarbital or methylcholanthrene-dependent microsomal systems (Tanabe et al. 1988). It is conceivable that, due to
their ancient divergence from terrestrial herbivorous ancestors, some cetaceans have lost
certain p450 enzymes as compared to terrestrial mammals. In humans, the mechanism of
PCB biotransformation appears to be dependent on the formation of an arene oxide intermediate (Tanabe et al. 1988; Robertson and Hansen 2001) and Tanabe et al. (1988) suggested
the lack of observed biotransformation of certain isomers of PCBs in small cetaceans was
a result of the inability to form this hydroxylated intermediate.
What follows is a discussion of several driving forces of physiologic adaptation of
marine mammals to poisons as well as discussion of individual organism acclimation.
Attention will be given to traditional poisons (chemicals), and physical forms of contamination such as debris, external oiling and sound pollution will be only briefly discussed.
Specific examples will be provided by order in subsequent sections based on the specific
process of interest.
14.2 Physiology and bioaccumulation
Some marine mammals have been advocated as ecosystem sentinels for environmental
poisons due to their unique physiology and ecology (Ross 2000) that integrates multiple
components of an ecosystem. Unfortunately, many of the characteristics that cause them
to be effective sentinels also result in higher toxicant concentrations than relevant prey
species (biomagnification). Marine mammals are generally long-lived, and most occupy
Marine Mammal Physiology: Requisites for Ocean Living
The evolution and diversification of enzymes associated with detoxification and xenobiotic biotransformation (i.e., cytochrome p450s; CYP) has also been attributed to the production of secondary metabolites by plant species and subsequent herbivory, an example
of chemical warfare (Gonzalez and Nebert 1990). Cytochrome P450 is a superfamily of
enzymes which are responsible for biotransformation of endogenous compounds and
xenobiotics in the liver and other tissues. There have been a number of isozymes identified within the CYP superfamily in nearly all mammalian lineages. Consequently, species
and even individuals have high diversity of p450 genes (Gonzalez and Nebert 1990). CYPs
are involved in the biotransformation of complex organic poisons such as PCBs via the
conjugation with oxygen to form a hydroxylated metabolite (Robertson and Hansen 2001).
There are 209 congeners of PCBs based on the number and position of chlorine atoms on
the biphenyl ring. While the congeners can be grouped according to the number of chlorine substitutions, physical structure (i.e., coplanar), or substitution pattern (para, meta,
ortho), individual congeners can have vastly different physical properties and toxicities,
as well as various mechanisms of toxicity. PCBs are known endocrine disrupters, and
relatively high concentrations of PCBs are reported in some marine mammals and their
associated prey species (Tanabe 2002). Relatively high concentrations of PCB congeners
of concern have been associated with adverse reproductive outcomes in ringed seals and
harbor seals (Reijnders 1986). Partial expressed gene sequences have been obtained via
RT-PCR for CYP1A from the livers of minke whale (Balaenoptera acutorostrata), Dall’s porpoise (Phocoenoides dalli), Steller sea lion (Eumetopias jubatus), spotted seal (Phoca largha), and
ribbon seal (Phoca fasciata) (Goksøyr 1995; Teramitsu et al. 2000). Perhaps unsurprisingly,
given the large diversity of p450 enzymes seen in other species, Goksøyr (1995) found differences when comparing the p450 system of marine mammals to terrestrial mammals.
There was also a difference in concentrations of CYP2B protein between seals (harp seal
and hooded seal) and whales (minke whale) (Goksøyr 1995). It has been suggested that
small cetaceans have a limited capacity to biotransform certain PCB congeners compared
to terrestrial mammals, perhaps due to limited activity of phenobarbital or methylcholanthrene-dependent microsomal systems (Tanabe et al. 1988). It is conceivable that, due to
their ancient divergence from terrestrial herbivorous ancestors, some cetaceans have lost
certain p450 enzymes as compared to terrestrial mammals. In humans, the mechanism of
PCB biotransformation appears to be dependent on the formation of an arene oxide intermediate (Tanabe et al. 1988; Robertson and Hansen 2001) and Tanabe et al. (1988) suggested
the lack of observed biotransformation of certain isomers of PCBs in small cetaceans was
a result of the inability to form this hydroxylated intermediate.
What follows is a discussion of several driving forces of physiologic adaptation of
marine mammals to poisons as well as discussion of individual organism acclimation.
Attention will be given to traditional poisons (chemicals), and physical forms of contamination such as debris, external oiling and sound pollution will be only briefly discussed.
Specific examples will be provided by order in subsequent sections based on the specific
process of interest.
14.2 Physiology and bioaccumulation
Some marine mammals have been advocated as ecosystem sentinels for environmental
poisons due to their unique physiology and ecology (Ross 2000) that integrates multiple
components of an ecosystem. Unfortunately, many of the characteristics that cause them
to be effective sentinels also result in higher toxicant concentrations than relevant prey
species (biomagnification). Marine mammals are generally long-lived, and most occupy