The Oxidative Metabolism of Antarctic Fish: Some Peculiar Aspects of Cold Adaptation
133
Some Peculiar Features of the Lipid Metabolic Pathway
A peculiar feature of Antarctic fishes is their preferential utilization of the
lipid metabolic pathway [25]. Hence, "Antarctic" enzymes of central
pathways of aerobic energy metabolism, such as citrate synthase and
cytochrome oxidase, do show maximal activities which are 1.5- to 5-fold
higher with respect to their temperate zone counterparts. Along the same
line, fatty acid oxidation as indicated by specific enzyme markers,
palmitoyltransferase and I3-hydroxyacil CoA dehydrogenase, is 1.3- to 27fold higher in tissues from polar species while those for both aerobic and
anaerobic metabolism of carbohydrates are generally lower with respect to
temperate-zone fish.
All these data indicate that Antarctic fish display a significant cold
adaptation of aerobic energy metabolism and suggest that this metabolism
is reliant primarily upon oxidation oflipid rather than carbohydrate fuels.
It is worthwhile to mention that tissues from Antarctic fishes have high
concentrations of polyunsaturated fatty acids such as eicosapentenoic acid
and docosahexaenoic acid. These polyunsaturated fatty acids range from 8
to 26% of total fatty acid pools from skeletal muscle and serum and occur
as result of diet. Thus many Antarctic marine invertebrates, prominent
prey of Antarctic fish, are rich in these polyunsaturated fatty acids.
The very high corporeal lipid content of Antarctic fish appears therefore
not only linked to buoyancy regulation [26] but also to specific
requirements of energy metabolism. However, besides these two
fundamental roles, other aspects, not minor, have to be considered. These
are based upon the observation that some temperate-zone fish display
dramatic accumulation of intracellular lipid during cold acclimation at the
level of their oxidative muscles. Hence, on the basis of this observation
and of the combined solubility and diffusivity characteristics of oxygen in
lipids two other additional roles of intracellular lipid inclusions in the
oxygen economy of oxidative tissues of Antarctic fish have been
hypothesized. These may be summarized as follows:
1 The presence of lipid droplets may accelerate the rate of oxygen
diffusion between capillaries and mitochondria.
2 Intracellular lipids may display a role of oxygen store, damping out
otherwise large variations in oxygen supply to mitochondria during
changes in muscular activities.
These peculiar characteristics, together with the major activation of the
oxidative metabolism could be at the basis of an increased production of
lipoperoxides and oxygen derived free radicals. Therefore, it has been
hypothesized that Antarctic fish should be confronted by an increased
oxidative stress which could be very harmful for some of their biological
133
Some Peculiar Features of the Lipid Metabolic Pathway
A peculiar feature of Antarctic fishes is their preferential utilization of the
lipid metabolic pathway [25]. Hence, "Antarctic" enzymes of central
pathways of aerobic energy metabolism, such as citrate synthase and
cytochrome oxidase, do show maximal activities which are 1.5- to 5-fold
higher with respect to their temperate zone counterparts. Along the same
line, fatty acid oxidation as indicated by specific enzyme markers,
palmitoyltransferase and I3-hydroxyacil CoA dehydrogenase, is 1.3- to 27fold higher in tissues from polar species while those for both aerobic and
anaerobic metabolism of carbohydrates are generally lower with respect to
temperate-zone fish.
All these data indicate that Antarctic fish display a significant cold
adaptation of aerobic energy metabolism and suggest that this metabolism
is reliant primarily upon oxidation oflipid rather than carbohydrate fuels.
It is worthwhile to mention that tissues from Antarctic fishes have high
concentrations of polyunsaturated fatty acids such as eicosapentenoic acid
and docosahexaenoic acid. These polyunsaturated fatty acids range from 8
to 26% of total fatty acid pools from skeletal muscle and serum and occur
as result of diet. Thus many Antarctic marine invertebrates, prominent
prey of Antarctic fish, are rich in these polyunsaturated fatty acids.
The very high corporeal lipid content of Antarctic fish appears therefore
not only linked to buoyancy regulation [26] but also to specific
requirements of energy metabolism. However, besides these two
fundamental roles, other aspects, not minor, have to be considered. These
are based upon the observation that some temperate-zone fish display
dramatic accumulation of intracellular lipid during cold acclimation at the
level of their oxidative muscles. Hence, on the basis of this observation
and of the combined solubility and diffusivity characteristics of oxygen in
lipids two other additional roles of intracellular lipid inclusions in the
oxygen economy of oxidative tissues of Antarctic fish have been
hypothesized. These may be summarized as follows:
1 The presence of lipid droplets may accelerate the rate of oxygen
diffusion between capillaries and mitochondria.
2 Intracellular lipids may display a role of oxygen store, damping out
otherwise large variations in oxygen supply to mitochondria during
changes in muscular activities.
These peculiar characteristics, together with the major activation of the
oxidative metabolism could be at the basis of an increased production of
lipoperoxides and oxygen derived free radicals. Therefore, it has been
hypothesized that Antarctic fish should be confronted by an increased
oxidative stress which could be very harmful for some of their biological