Membrane adenosine triphosphatase as a participant in the active transport of sodium and potassium in the human erythrocyte.

The identification of any enzyme in broken cells as a participant in membrane transport has not been reported. In this paper an enzymatic activity has been so identified through a demonstration that an unusual constellation of characteristics is common to it and to a transport system. Evidence is presented that an adenosine triphosphatase in broken human erythrocyte membranes is a part of the system for the active transport of sodium and potassium in intact erythrocytes. The unusual constellation of characteristics is the following. Both the ATPasel activity and transport require sodium and potassium ions together, not separately; both are inhibited by ouabain; both accept ammonium ion in place of potassium ion; and both show a competitive inhibition of potassium ion activation by high concentrations of sodium ion. Furthermore, the concentration at which each substance exerts a half-maximal effect is the same for both enzyme activity and transport. In intact human erythrocytes it is well established that the extrusion of sodium ion and accumulation of potassium ion proceed by active transport across the cell membrane (1). The energy comes from glycolysis, almost certainly by way of the high energy phosphate bonds of ATP (2-5). In particular, Hoffman2 has recently observed that intact membranes containing ATP, which was introduced by “reversible hemolysis,” transnort sodium actively whereas control membranes filled with -inosine triphosphate do not. It is also established that the active transports of sodium and potassium occur only together, not separately, and are therefore linked (6, 7). Now, if the hydrolysis of ATP is stoichiometrically coupled to the linked transport of sodium and potassium across the membrane, then in broken membranes there will be an ATPase which requires both sodium and potassium ions together, not separately, as cofactors. When Skou (8) reported such an ATPase in crab nerve, we were stimulated to investigate human erythrocyte membrane ATPase for features already known to be characteristic of active sodium and potassium transport in in-