The Oxidative Metabolism of Antarctic Fish: Some Peculiar Aspects of Cold Adaptation
131
should represent a key system to study evolutionary adaptations. In this
perspective, the increase in mitochondrial volume density observed during
cold acclimation implies that mitochondrial function becomes limiting
during activity at low temperatures [18].
Previous studies have shown that mitochondrial oxygen consumption is
an excellent indicator of temperature adaptation [19-21]. In addition, all
lipid bilayers are extremely sensitive to temperature changes and
temperature-induced structural changes at the level of the membrane may
seriously affect physiological function [22-24]. This is probably due to the
sophisticated interactions which are established at the level of the
membrane between lipids and proteins. Hence in order to study the
characteristic stenothermality of a given organism an integrated system
such as that represented by mitochondria is certainly more sensitive with
respect to isolated components such as proteins. In this respect it is
worthwhile to mention the case of the LDH (lactate dehydrogenase, Atype) of Pagothenia borchgrevinki, whose denaturation temperature
(40°C), although significantly lower than that of its homologues from
thermophilic species, is well above the lethal temperature of the animal.
The effect of temperature changes on mitochondrial oxygen
consumption exhibits a common pattern in all species: the rate of
respiration increases, as a function of temperature, up to a certain
temperature whereupon any further increase leads to a sharp reduction in
respiration rate. This pattern is shown most clearly when data are
presented in Arrhenius plots. The temperature at which the sharp change in
slope is observed is termed the Arrhenius break temperature (ABT) and
appears to be linked to the habitat temperature of the species. Although the
biochemical mechanisms underlying the ABT phenom~non per se are not
fully understood, disruption of lipid-protein interactions may be partially
responsible for ABTs.
The strong stenothermic character of Antarctic fishes has been strongly
outlined by recent experiments on mitochondria from the icefish
Chaenocephalus aceratus. As evident from the data reported in Figs. 1 and
2 in the case of C. aceratus the Arrhenius plot obtained for the activity of
NADH oxidase as well as ofNADH coenzyme Q reductase is reversed up
to 2 °e indicating that, within the range of temperature examined, we are
to the left of the ABT of the systems under investigation. Even if on the
basis of the present data we do not know the precise temperature of the
break point, it seems in the proximity of 2 °e outlining the fragility of
these systems with respect to temperature increases. In any case a
comparison with the data obtained in the case of trout is astonishing. In
addition, at 2 °e, the V max values of the enzymatic systems investigated
appear significantly higher in the case of the icefish with respect to those
131
should represent a key system to study evolutionary adaptations. In this
perspective, the increase in mitochondrial volume density observed during
cold acclimation implies that mitochondrial function becomes limiting
during activity at low temperatures [18].
Previous studies have shown that mitochondrial oxygen consumption is
an excellent indicator of temperature adaptation [19-21]. In addition, all
lipid bilayers are extremely sensitive to temperature changes and
temperature-induced structural changes at the level of the membrane may
seriously affect physiological function [22-24]. This is probably due to the
sophisticated interactions which are established at the level of the
membrane between lipids and proteins. Hence in order to study the
characteristic stenothermality of a given organism an integrated system
such as that represented by mitochondria is certainly more sensitive with
respect to isolated components such as proteins. In this respect it is
worthwhile to mention the case of the LDH (lactate dehydrogenase, Atype) of Pagothenia borchgrevinki, whose denaturation temperature
(40°C), although significantly lower than that of its homologues from
thermophilic species, is well above the lethal temperature of the animal.
The effect of temperature changes on mitochondrial oxygen
consumption exhibits a common pattern in all species: the rate of
respiration increases, as a function of temperature, up to a certain
temperature whereupon any further increase leads to a sharp reduction in
respiration rate. This pattern is shown most clearly when data are
presented in Arrhenius plots. The temperature at which the sharp change in
slope is observed is termed the Arrhenius break temperature (ABT) and
appears to be linked to the habitat temperature of the species. Although the
biochemical mechanisms underlying the ABT phenom~non per se are not
fully understood, disruption of lipid-protein interactions may be partially
responsible for ABTs.
The strong stenothermic character of Antarctic fishes has been strongly
outlined by recent experiments on mitochondria from the icefish
Chaenocephalus aceratus. As evident from the data reported in Figs. 1 and
2 in the case of C. aceratus the Arrhenius plot obtained for the activity of
NADH oxidase as well as ofNADH coenzyme Q reductase is reversed up
to 2 °e indicating that, within the range of temperature examined, we are
to the left of the ABT of the systems under investigation. Even if on the
basis of the present data we do not know the precise temperature of the
break point, it seems in the proximity of 2 °e outlining the fragility of
these systems with respect to temperature increases. In any case a
comparison with the data obtained in the case of trout is astonishing. In
addition, at 2 °e, the V max values of the enzymatic systems investigated
appear significantly higher in the case of the icefish with respect to those