Stabilisation of Na,K-ATPase structure by the cardiotonic steroid ouabain

• Ouabain binding to pig and shark Na,K-ATPase enhances thermal stability.
• Ouabain stabilises both membrane-bound and solubilised Na,K-ATPase.
• Synchrotron radiation circular dichroism is used for structure determination.
• Secondary structure in general is not affected by ouabain binding.
• Stabilisation is due to re-arrangement of tertiary structure.

Cardiotonic steroids such as ouabain bind with high affinity to the membrane-bound cation-transporting P-type Na,K-ATPase, leading to complete inhibition of the enzyme. Using synchrotron radiation circular dichroism spectroscopy we show that the enzyme-ouabain complex is less susceptible to thermal denaturation (unfolding) than the ouabain-free enzyme, and this protection is observed with Na,K-ATPase purified from pig kidney as well as from shark rectal glands. It is also shown that detergent-solubilised preparations of Na,K-ATPase are stabilised by ouabain, which could account for the successful crystallisation of Na,K-ATPase in the ouabain-bound form. The secondary structure is not significantly affected by the binding of ouabain. Ouabain appears however, to induce a reorganization of the tertiary structure towards a more compact protein structure which is less prone to unfolding; recent crystal structures of the two enzymes are consistent with this interpretation. These circular dichroism spectroscopic studies in solution therefore provide complementary information to that provided by crystallography.