Cardiac glycoside interaction with solubilized myocardial sodium and potassium dependent adenosine triphosphatase

Canine myocardial microsomal membranes were exposed under optimal binding conditions to 8 nM [³H]ouabain, a concentration producing only partial inhibition of the (Na⁺ + K⁺) ATPase activity in this preparation. Microsomal membrane components were then solubilized with the nonionic surfactant 'Lubrol WX'. After centrifugation at 100,000 x g for 1 hr and gel permeation chromatography on Sepharose 6B, [³H]ouabain was found exclusively in fractions containing (Na⁺ + K⁺) ATPase activity, and closely paralleled the enzyme activity profile. In Lubrol solubilized preparations, bound [³H]ouabain penetrated the gel with a component of apparent molecular weight 600,000. Sucrose density gradient centrifugation and liquid isoelectric focusing of Lubrol solubilized preparations also resulted in close correspondence between the presence of [³H]ouabain and (Na⁺ + K⁺) ATPase activity. Lubrol solubilized (Na⁺ + K⁺) ATPase interacted with ouabain in a manner similar to the membrano bound enzyme, as judged by identical half maximal inhibitory concentrations of 60 nM. Thus solubilization of myocardial microsomal membrane components resulted in preservation of ouabain binding and did not disclose any high affinity receptor separable from (Na⁺ + K⁺) ATPase by these techniques.