Identification of the segment of the catalytic subunit of (Na+,K+)ATPase containing the digitalis binding site.

Digitalis compounds that are extensively used in the treatment of cardiovascular disorders are known to bind specifically at the extracellular side of (Na+,K+)ATPase. We have recently reported the synthesis of [3H]p‐ nitrophenyltriazene ‐ouabain, a derivative of ouabain, which specifically alkylates the catalytic chain of the (Na+,K+)ATPase at a defined region of the sequence. The peptidic segment involved in the binding of digitalis to (Na+,K+)ATPase has been located after mild trypsin treatment of the labeled enzyme. In the presence of 100 mM KCl, tryptic fragmentation results in two peptide fragments of mol. wt. 58 000 and 41 000, respectively. The radioactive probe labeled only the 41 000 fragment indicating that the digitalis binding site is located on the 41 000 domain situated at the N‐terminal part of the sequence of the alpha‐subunit.