Enzymes catalyze essential chemical reactions needed for living processes. (Na+ + K+)-ATPase (NKA) is one of the key enzymes that control intracellular ion homeostasis and regulate cardiac function. Little is known about activation of NKA and its biological impact. Here we show that native activity of NKA is markedly elevated when protem-protein interaction occurs at the extracellular DVEDSYGQQWTYEQR (D-R) region in the (alpha-subunit of the enzyme. The apparent catalytic turnover of NKA is approximately twice as fast as the controls for both ouabain-resistant and ouabain-sensitive enzymes. Activation of NKA not only markedly protects enzyme function against denaturing, but also directly affects cellular activities by regulating intracellular Ca2+ transients and inducing a positive inotropic effect in isolated rat cardiac myocytes. Immunolluorescent labeling indicates that the D-R. region of NKA is not a conventional digital is-binding site. Our findings uncover a novel activation site of NKA that is capable of promoting the catalytic function of the enzyme and establish a new concept that activating NKA mediates cardiac contraction. (c) 2005 Elsevier Inc. All rights reserved.