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1. A study has been made of ATP splitting and ouabain binding to the sodium pump reconstituted from protein and phosphatidylserine.

2. Ouabain was bound to protein alone, but when phosphatidylserine was added, binding was increased threefold. The stimulation resembled the course of activation of sodium-dependent ATPase activity.

3. EGTA partly simulated the activation of ATPase by phosphatidylserine but did not enhance binding.

4. The dissociation constant for the enzyme-ouabain complex was 3·5 x 10^-8 M. The turnover number (2,000 molecules of ATP per minute) and the number of receptor sites (3·8 x 10¹³ per mg protein) were calculated.

5. The results provide further evidence of the involvement of phosphatidylserine in the action of the sodium pump.