Sidedness of (sodium, potassium)-adenosine triphosphate of inside-out red cell membrane vesicles. Interactions with potassium.

Inside-out membrane vesicles of human red cells, prepared according to the method of Steck et al. (1970) Science 168, 255-257) have sufficiently low cation permeability to allow the examination of the side-specific interactions of ligands with the asymmetric sodium pump complex. In accordance with the known properties of the pump in intact cells the following results were observed: (a) ATP-dependent sodium influx and (b) maximal (sodium, potassium)-ATPase with K+ present inside the vesicles with larger than or equal to 20 micronM ATP. With much lower [ATP], K+ inhibited sodium-activated ATPase. K+ was inhibitory at either surface. Inhibition was different on the two sides since cytoplasmic (extravesicular) Na+ counteracted inhibition by cytoplasmic (extravesicular) K+ but not inhibition by K+ at the plasma or external membrane surface, i.e. intravesicular K+. A decrease in the steady state level of the phosphenzyme intermediate of sodium-activated ATPase was caused also by K+ at either surface. The effect of cytoplasmic K+ is compatible with its competitive inhibition of activation of phosphorylation of the enzyme by cytoplasmic Na+. At 37 degrees, the inhibitory effect of external K+ is due to interaction with the phosphoenzyme to form a stable complex of K+ with the dephosphenzyme resulting in a decreased overall reaction rate but increased turnover of the phosphoenzyme (E-P + K leads to EK + Pi). At 0 degree, external K+ inhibits by interacting with the unphosphorylated enzyme to form an occluded enzyme-K complex. This results in a decreased overall rate but relatively small change in apparent turnover of the phosphoenzyme. At 0 degree, but not at 37 degrees, external Na+ counteracted the inhibitory effects of external K+.