Altered kinetic properties of rat heart mitochondrial enzymes following experimental-thyrotoxicosis.

Effects of T3- and T4-induced thyrotoxicosis on temperature-dependent Arrhenius kinetics of succinate oxidase and Mg2+- and Mg2+ + 2,4-dinitrophenoldependent ATPase activities in rat heart mitochondria were examined, For succinate oxidase system, treatment with T3 and T4 caused increase in the energy of activation in high temperature range in a dose-dependent manner. For low temperature range, increase in energy of activation was apparent only with higher doses of the hormones; with low doses a small but reproducible decrease was evinced. The phase transition temperature decreased significantly under these conditions. For the Mg2+- and Mg2++2,4-dintrophenol- dependent-ATPase activities, the activation energy values in high temperature range decreased in general. Activation energy values in low temperature range recorded a generalized increase in the Mg2+-ATPase enzyme system while the value did not change significantly for the Mg2+ + 2,4-dinitrophenol-ATPase; phase transition temperature registered a small but reproducible decrease under these conditions. The results are suggestive of increased membrane fluidization possibly through increased proportion of unsaturated fatty acids. The differential effects seen for succinate oxidase and ATPase systems are consistent with different lipid protein domains of these enzyme systems.