Abstract

The changes in dissociation rate constants (k_d) of (Na⁺ + K⁺)-ATPase complexes with digitoxin and digitoxigenin bisdigitoxide brought about by acetylation of their sugar moieties were examined in the hope of elucidating the role of each sugar hydroxyl group in binding to the enzyme. With every acetyl derivative the k_d value of the drug-enzyme complex formed in the presence of Na⁺, Mg²⁺, and ATP (type I complex) exceeded that of the corresponding complex formed in the presence of Mg²⁺ and P_[unknown] (type II complex). A remarkable increase in k_d values for both types of complexes was produced by acetylation of the 3'-hydroxyl group in the first sugar moiety. Binding by the 3'-hydroxyl group of the first sugar dominates binding by the second and third digitoxose moieties in both types and occurs via hydrogen bonding, as is the case for the monoglycoside. The axial 3[unknown]-hydroxyl group of the third digitoxose moiety may also bind to the enzyme by hydrogen bonding in both types of complexes, but the equatorial 4[unknown]-hydroxyl group is not involved. The second digitoxose group does not bind to the enzyme in the same manner as the others. In the type II complex the 3''-hydroxyl group is not involved in binding, but the hydrophobic component of the sugar opposite to the 3''-hydroxyl group or the pyranoside oxygen may bind to the extended area of the first sugar binding site. On the other hand, in the type I complex, the 3''-hydroxyl group may bind to another specific site of the enzyme, opposite to the first sugar binding site.