Thyroid hormone-induced alterations in membrane structure-function relationships: studies on kinetic properties of rat kidney microsomal Na(+),K (+)-ATPase and lipid/phospholipid profiles.

The effects of thyroidectomy (Tx) and subsequent treatment with 3,5,3'-triiodothyronine (T(3)) or combined replacement therapy (T(R)) with T(3 )and thyroxine (T(4)) 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 T(3) treatment, while T(R) treatment caused a decrease. In the Tx and T(3) groups enzyme activity resolved in two kinetic components, while in the T(R) group the enzyme showed allosteric behavior up to 0.5 mM: ATP concentration. The K(m) and V(max) values of both the components decreased in Tx animals without affecting the catalytic efficiency. T(3) 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 T(R) 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.