Dual activity of the H1-H2 domain of the (Na(+)+K+)-ATPase.
(Na(+)+K(+))-ATPase is a target receptor of digitalis (cardiac glycoside) drugs. It has been demonstrated that the H1-H2 domain of the alpha-subunit of the (Na(+)+K(+))-ATPase is one of the digitalis drug interaction sites of the enzyme. Despite the extensive studies of the inhibitory effect of digitalis on the (Na(+)+K(+))-ATPase, the functional property of the H1-H2 domain of the enzyme and its role in regulating enzyme activity is not completely understood. Here we report a surprise finding: instead of inhibiting the enzyme, binding of a specific monoclonal antibody SSA78 to the H1-H2 domain of the (Na(+)+K(+))-ATPase elevates the catalytic activity of the enzyme. In the presence of low concentration of ouabain, monoclonal antibody SSA78 significantly protects enzyme function against ouabain-induced inhibition. However, higher concentration of ouabain completely inactivates the (Na(+)+K(+))-ATPase even in the presence of SSA78. These results suggest that the H1-H2 domain of the (Na(+)+K(+))-ATPase is capable of regulating enzyme function in two distinct ways for both ouabain-sensitive and -resistant forms of the enzyme: it increases the activity of the (Na(+)+K(+))-ATPase during its interaction with an activator; it also participates in the mechanism of digitalis or ouabain-induced inhibition of the enzyme. Understanding the dual activity of the H1-H2 domain will help better understand the structure-function relationships of the (Na(+)+K(+))-ATPase and the biological processes mediated by the enzyme.
Department of Surgery, Division of Cardiac Surgery, University of Maryland School of Medicine, MSTF-434E, 10 South Pine Street, Baltimore, MD 21201, USA. Electronic address: kxu002@umaryland.edu.
Kai Y
KY
Xu
D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D000941:Antigens / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D004285:Dogs; D004791:Enzyme Inhibitors / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D017434:Protein Structure, Tertiary; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism*
D000911:Antibodies, Monoclonal; D000941:Antigens; D004791:Enzyme Inhibitors; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase