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The effects of thyroidectomy (Tx) and subsequent treatment with 3,5,3′-triiodothyronine (T3) or combined replacement therapy (TR) with T3 and thyroxine (T4) on the substrate and temperature kinetics properties of Na+,K+-ATPase and lipid/phospholipid makeup of rat kidney microsomes were examined. Enzyme activity was somewhat high in the hypothyroid (Tx) animals and increased significantly following T3 treatment, while TR treatment caused a decrease. In the Tx and T3 groups enzyme activity resolved in two kinetic components, while in the TR group the enzyme showed allosteric behavior up to 0.5 m m ATP concentration. The K m and V max values of both the components decreased in Tx animals without affecting the catalytic efficiency. T3 treatment caused a significant increase in the V max of both the components, with a significant increase in the catalytic efficiency, while the K m values were not upregulated. The TR regimen lowered the K m and V max of component II but improved the catalytic efficiency. Thyroid status-dependent changes were also noted in the temperature kinetics of the enzyme. Regression analysis revealed that changes in the substrate and temperature kinetics parameters correlated with specific phospholipid components. [ABSTRACT FROM AUTHOR]

Effect of thyroidectomy (Tx) and subsequent treatment with 3,5,3′-triiodo- l-thyronine (T3) or replacement therapy (TR) with T3 + l-thyroxine (T4) on the temperature kinetics properties of FoF1 adenosine triphosphatase (ATPase, ATP synthase, H+-translocating ATP synthase EC 3.6.3.14) and succinate oxidase (SO) and on the lipid/phospholipid makeup of rat kidney mitochondria were examined. Tx lowered ATPase activity, which T3 treatment restored. SO activity was unchanged in Tx but decreased further by T3 treatment. TR restored both activities. The energies of ATPase activation in the high and low temperature ranges ( E H and E L) increased in the Tx and T3 animals with decrease in phase transition temperature (Tt). TR restored E H and E L but not Tt to euthyroid levels. E H and E L of SO decreased in Tx animals. T3 and TR restored E H whereas E L was restored only in the TR group; Tt increased in both groups. Total phospholipid and cholesterol contents decreased significantly in Tx and T3-treated animals. In Tx animals, sphingomyelin (SPM) and phosphatidylcholine (PC) components decreased, while phosphatidylserine (PS) and diphosphatidylglycerol components increased. T3 and TR treatments caused decreases in SPM, phosphatidylinositol and PS. PC and phosphatidylethanolamine (PE) increased in the T3 group. TR resulted in increased lysophospolipids and PE. Changes in kinetic parameters of the two enzymes were differently correlated with specific phospholipid components. Both T3 and TR regimens were unable to restore normal membrane structure-function relationships. [ABSTRACT FROM AUTHOR]