Potassium-activated phosphatase from human red blood cells : The asymmetrical effects of K(+), Na (+), Mg (++) and adenosine triphosphate.

The cell membrane K(+)-activated phosphatase activity was measured in reconstituted ghosts of human red cells having different ionic contents and incubated in solutions of varying ionic composition. When K(+)-free ghosts are suspended in K(+)-rich media, full activation of the phosphatase is obtained. Conversely, very little ouabainsensitive activity is detected in K(+)-rich ghosts suspended in K(+)-free media. These results, together with the fact that Na(+) competitively inhibits the effects of K(+) only when present externally, show that the K(+) site of the membrane phosphatase is located at the outer surface of the cell membrane. The Mg(++) requirements for K(+) activation of the membrane phosphatase are fulfilled by internal Mg(++). Addition of intracellular Na(+) to ATP-containing ghosts raises the apparent affinity of the enzyme for K(+), suggesting that the sites where ATP and Na(+) produce this effect are located at the inner surface of the cell membrane. The asymmetrical features of the membrane phosphatase are those expected from the proposed role of this enzyme in the Na(+)-K(+)-ATPase system.