They are located in cell membranes They have an affinity for Na that is higher than for K on the inside of membranes. They have an affininty for K that is higher than for Na on the outside of membranes. They are capable of hydrolysing ATP at rates dependent on the concentrations of Na inside the cell and of K outside the cell. They are found in all cell in which active linked transport of Na and potassium occurs. The enzyme systems have the same quantitative relations to Na and K as do the transport systems of intact cells.
Lets look at some of the physiological evidence about the characteristics of the sodium pump.
All excitable tissues examined have a Na-K pump which moves Na out of the cell against its concentration gradient, and K into the cell against its concentration gradient. When the source of metabolic energy is interrupted using metabolic inhibitors or lowering the temperature, K leaks out and Na leaks into the cell. As a consequence it is electrogenic, adding negativity to the resting membrane potential when the pump is active. While the concentration gradients will take several hours to run down, the electrogenic contribution of the pump disappears almost immediately when the Na-K ATPase is inhibited using cardiac glycosides. The sodium pump represents what is referred to as direct or primary active transport, a process in which the hydrolysis of ATP is directly coupled to the movement of a molecule against its concentration gradient.