Evidence that a major portion of cellular potassium is ''bound''

In this report we briefly review recent evidence which shows that a substantial proportion of intracellular K⁺ is ''bound'' or perturbed from the physicochemical properties expected in dilute aqueous solutions. In addition, we present evidence from electron probe x-ray microanalysis of thin cryosections of cells which indicates that the binding of K⁺ to anionic groups either carboxyl groups (HCO₂) on proteins or to phosphate groups in creatine phosphate (CrP), in adenosine triphosphate, (ATP), in protein and in nucleic acids, are the main determinants of the maintenance of (as differentiated from the generated of) the well known intra- to extracellular K⁺ concentration difference. The collective evidence suggests that much of cellular K⁺ is reduced in its mobility and in its chemical activity due to association with negative charge groups (e.g. carboxyl and phosphates). This fact forces abandonment of the misleading assumption that the majority of intracellular K⁺ and other inorganic ions are as free as would be expected under ideal solution conditions. This realization should have far reaching consequences toward understanding transmembrane movement of water and solutes in cells.