The K+-induced apparent heterogeneity of high-affinity nucleotide-binding sites in (Na+ + K+)-ATPase can only be due to the oligomeric structure of the enzyme

K⁺ induces an apparent heterogeneity among an otherwise homogeneous population of nucleotide-binding sites in (Na⁺ + K⁺)-ATPase preparations from pig kidney. With the help of ouabain we show that this heterogeneity cannot be due to a mixture of different and independent sites and conclude that each enzyme molecule must contain two nucleotide site-containing units that show interaction. Na⁺ induces an apparent heterogeneity among an otherwise homogeneous population of ouabain-binding sites. The argument is, therefore, extended to include one ouabain site on each of the structural units that bind nucleotide. All these structural units are shown to hydrolyse substrate at identical rates. Using the presently available molecular weight data, it is concluded that the enzyme is composed of two subunits each possessing one nucleotide-binding site, one ouabain-binding site, one α-peptide and the capacity for hydrolysing ATP and $\text{p-}\text{nitrophenyl}$ phosphate.