Mechanistic implications of the potassium-potassium exchange carried out by the sodium-potassium pump.

1. The magnitude of the K-K exchange carried out by the Na pump in human red cells was measured as a function of the external K concentration in cells with high and low intracellular K. The apparent K1/2 for external K and the Vm increased by the same proportion when intracellular K was raised. The result suggests that the K-K exchange is part of a ping-pong reaction mechanism. 2. The velocity of the exchange increases monotonically with intracellular phosphate and ATP concentration; neither substrate inhibits at very high concentration. 3. These results seem to require that the enzyme species which carries out the exchange is both phosphorylated and combined with ATP.