On the bound state of alkali cations in subcellular preparations of rat liver.

Gel filtration and ion-specific electrodes were used together with atomic absorption spectrophotometry in a search for substances in rat liver which are capable of binding alkali cations. In the cytosol, a material which binds K specifically and reduces the ion activity of potassium can be detected. The binding material, which may be destroyed by alpha-chymotrypsin, has been purified about 100-fold. Its molecular weight is around 5 x 10(3). 1 muval K becomes bound per 20-40 amino acid residues; the total binding capacity may amount to 10-15% binding of the K in rat-liver cytoplasm. Washed nuclear residues, consisting mainly of chromatin, are capable of binding Na in a cation-specific mode. DNA and RNA are ruled out as binding material, so it is assumed to consist of protein, and would then contain about 20 amino acid residues per Na. The cation-binding processes are discussed with regard to nucleo-cytoplasmic sequestration of Na and K, with consequences for the cellular chemi-osmotic gradients, and for the regulation of gene activity in the nucleus.