Membrane-bound Na+,K+-ATPase as the cardiac glycoside receptor

DSC studies are carried out to characterize Na⁺,K⁺-ATPase isolated from pig kidney in its natural membrane environment as well as in its purified state upon detergent treatment. The transition temperatures of the investigated thermal protein unfolding process, observed between 43 and 64.5° C, depend on the local membrane environment as well as onpH. In addition, the transition temperature is significantly increased upon binding of different cations and ligands which are known to interact with the enzyme. Evidence for a lipid phase transition around 23 °C in the original biological membrane is reported.The application of a calorimeter equipped with removable cells appears to be more suitable for the investigation of this type of membrane sample than an instrument with fixed capillary cells. Filling the sample capillary cell with an usual syringe, consisting of a long and thin needle, can influence the experimental results.Na⁺, K⁺ -ATPase acts as the receptor for cardiac glycoside binding. The thermodynamic parameters of this binding process are determined by titration calorimetry. The binding of ouabain, as a typical example, is unusually slow and is enthalpy driven. The enthalpy change upon binding enthalpy is –75 kJ mol^–1 at 25 °C. The surprisingly low stoichiometric coefficient, resulting from an evaluation based on a simple one step binding model, is interpreted in terms of a dimeric receptor unit.The authors are grateful to E. Lewitzki, H. Ruf, E. Schick and F. Thommen for stimulating discussions, to A. Schacht for skilfull enzyme preparations and to the German Research Foundation for financial support (SFB 169).