魚類筋原繊維のMg-ATPase活性およびそのCa-感受性の温度依存性

The effect of temperature on myofibrillar Mg-ATPase activity was studied among several fish species living at different environmental temperatures. The Mg-ATPase was assayed between 1-50°C in both presence and absence of Ca2+ and the velocity of ATP-splitting was obtained from an initial linear phase of the reaction betwwen 20-60s. For all fish species sutdied, the Arrhenius plot for Mg-ATPase activity in the presence of Ca2+ was non-linear with a break at around 16-18°C whereas in the absence of Ca2+ the plot was linear over the ssame temperature range. However, the temperature dependence of Mg-ATPase activity and its Ca-sensitivity were significantly different among fish species. In particular, the Ca-sensitivity was found to be fully highat around the environmental temperature at which fish live. From the corresponding Arrhenius plot for Mg-ATPase activity in the presence of Ca2+ over the temperature range of 1-18°C, thermodynamic activation parameters wre calculated. It was thus found that the enthalpy and the entropy of activation were considerably decreased from warm-water to cold-water habitates and the free energy of activation was also relatively low for cold-water fish. These observed changes are probably an evolutionary response to attain the high catalytic and regulatory efficencies of this enzyme to thermal environments.