Abstract
The sodium and potassium ion pump is an intrinsic enzyme of plasma membranes. In these experiments it was driven backward in a transient twoβstep operation involving, first, phosphorylation of the enzyme from inorganic phosphate, and second, transfer of the phosphate group from the enzyme to ADP upon addition of a high concentration of Na^+^. There was no evidence of a significant concentration gradient across the membranes. Na^+^ presumably reached the solutions on both faces of the membrane simultaneously and provided the energy for synthesis simply as a consequence of ligand binding. An interaction free energy between the free energy of the binding of Na^+^ presumably reached the solutions on both faces of the membrane simultaneously and provided the energy for synthesis simply as a consequence of ligand binding. An interaction free energy between the free energy of the binding of Na^+^ and the free energy of hydrolysis of the phosphate group on the enzyme was estimated. The experiments also suggested a feature of the transport mechanism. This is control by phosphorylation of access pathways from the solutions in contact with the faces of the membrane to an active center for cation binding. In the dephosphoenzyme access would be to the intracellular solution and in the phosphoenzyme access would be to the extracellular solution.