Modifications induced by acylphosphatase in the functional properties of heart sarcolemma Na⁺,K⁺ pump

Acylphosphatase purified from cardiac muscle actively hydrolyzes the phosphoenzyme intermediate of heart sarcolemma Na⁺,K⁺‐ATPase. This effect occurred with acylphosphatase amounts (up to 800 membrane protein) that fall within the physiological range and the low value of the apparent K _m (0.69 × 10⁻⁷ M) indicates a considerable affinity of the enzyme towards this specific substrate. Acylphosphatase addition to purified sarcolemmal vesicles significantly increased the rate of Na⁺,K⁺‐dependent ATP hydrolysis. Maximal stimulation, observed with 800 protein, resulted in an ATPase activity which was about 2‐fold over basal value. The same acylphosphatase amounts significantly stimulated, in a similar and to an even greater extent, the rate of ATP driven Na⁺ transport into sarcolemmal vesicles. These findings lead to suppose that an accelerated hydrolysis of the phosphoenzyme may result in an enhanced activity of heart sarcolemmal Na⁺,K⁺ pump, therefore suggesting a potential role of acylphosphatase in the control of this active transport system.