Exercise in the Cold: High Energy Turnover in Antarctic Fish
229
A compensatory increase of mitochondrial enzyme activities after cold
acclimation has been reported for many temperate fish like goldfish, carp
and green sunfish [27-29] but may be absent in salmonid species like trout
and lake whitefish [30,31]. Higher activities of oxidative enzymes per
gram of tissue may be caused by an increased number of mitochondria, a
higher amount of enzyme per mitochondrion or modulation of the specific
activity of the individual enzyme molecule. Several studies of Wodtke
[28,32] provide evidence that the activity of membrane bound
mitochondrial enzymes - like cytochrome c oxidase (COX) and succinate
dehydrogenase (SDH) - is enhanced by changes of the fluidity and lipid
composition of the membranes during cold acclimation. While the
increases of COX activity in seasonally cold acclimated fish range
between 30 and 100% [27-29], Gobionothoten gibberifrons displays 5
times higher COX activities in the white muscle than a temperate zone
fish with similar lifestyle [33]. Oxidative enzymes, located in the
mitochondrial matrix as opposed to membrane bound enzymes, generally
show only a moderate increase in cold acclimated or adapted animals.
While COX and SDH are significantly enhanced in cold acclimated green
sunfish, malate dehydrogenase remains unchanged [29]. Crockett and
Sidell [33] found only 1.4 - 2.8 times increased citrate synthase activities
in Trematomus newnesi and G. gibberifrons when compared to temperate
species. Thus, a rise in mitochondrial density during cold acclimation or
adaptation leads to increased activities of oxidative enzymes. The activity
of membrane bound enzymes, such as COX and SDH, may be further
increased by an activating effect of homeoviscous response of
mitochondrial membranes.
Shifts in the saturation and fluidity of mitochondrial membranes during
thermal acclimation have been documented for several species, such as
goldfish and carp [32,34]. No data are available, however, on the
membrane composition of mitochondria from Antarctic fish. Cytoplasmic
membranes of Antarctic fish contain high percentages of polyunsaturated
fatty acids and show up to 100% homeoviscous response [35]. Cold
adapted organisms commonly show higher efficiencies of homeoviscous
adaptation than eurythermal animals after cold acclimation [35].
Therefore, it can be assumed that mitochondrial membranes of Antarctic
fish show at least the same degree of homeoviscous response as
mitochondria from cold acclimated animals. At a given temperature a
higher fraction of unsaturated fatty acids increases the permeability of
membranes to ions especially to protons [36]. High rates of proton leakage
through the inner mitochondrial membrane increase the oxygen
consumption of mitochondria without a concomitant rise in ATP
229
A compensatory increase of mitochondrial enzyme activities after cold
acclimation has been reported for many temperate fish like goldfish, carp
and green sunfish [27-29] but may be absent in salmonid species like trout
and lake whitefish [30,31]. Higher activities of oxidative enzymes per
gram of tissue may be caused by an increased number of mitochondria, a
higher amount of enzyme per mitochondrion or modulation of the specific
activity of the individual enzyme molecule. Several studies of Wodtke
[28,32] provide evidence that the activity of membrane bound
mitochondrial enzymes - like cytochrome c oxidase (COX) and succinate
dehydrogenase (SDH) - is enhanced by changes of the fluidity and lipid
composition of the membranes during cold acclimation. While the
increases of COX activity in seasonally cold acclimated fish range
between 30 and 100% [27-29], Gobionothoten gibberifrons displays 5
times higher COX activities in the white muscle than a temperate zone
fish with similar lifestyle [33]. Oxidative enzymes, located in the
mitochondrial matrix as opposed to membrane bound enzymes, generally
show only a moderate increase in cold acclimated or adapted animals.
While COX and SDH are significantly enhanced in cold acclimated green
sunfish, malate dehydrogenase remains unchanged [29]. Crockett and
Sidell [33] found only 1.4 - 2.8 times increased citrate synthase activities
in Trematomus newnesi and G. gibberifrons when compared to temperate
species. Thus, a rise in mitochondrial density during cold acclimation or
adaptation leads to increased activities of oxidative enzymes. The activity
of membrane bound enzymes, such as COX and SDH, may be further
increased by an activating effect of homeoviscous response of
mitochondrial membranes.
Shifts in the saturation and fluidity of mitochondrial membranes during
thermal acclimation have been documented for several species, such as
goldfish and carp [32,34]. No data are available, however, on the
membrane composition of mitochondria from Antarctic fish. Cytoplasmic
membranes of Antarctic fish contain high percentages of polyunsaturated
fatty acids and show up to 100% homeoviscous response [35]. Cold
adapted organisms commonly show higher efficiencies of homeoviscous
adaptation than eurythermal animals after cold acclimation [35].
Therefore, it can be assumed that mitochondrial membranes of Antarctic
fish show at least the same degree of homeoviscous response as
mitochondria from cold acclimated animals. At a given temperature a
higher fraction of unsaturated fatty acids increases the permeability of
membranes to ions especially to protons [36]. High rates of proton leakage
through the inner mitochondrial membrane increase the oxygen
consumption of mitochondria without a concomitant rise in ATP