NA+ GRADIENT HYPOTHESIS IN CYTOPLASTS DERIVED FROM EHRLICH ASCITES TUMOR-CELLS

The concentration gradients of Na+ and the non-metabolizable amino acid, α-aminoisobutyric acid, and the membrane potential were measured in cytoplasts derived from Ehrlich ascites tumor cells in order to test the Na+ gradient hypothesis for the active transport of neutral amino acids in animal cells. According to this hypothesis, the Na+ electrochemical gradient and the amino acid activity gradient should be equal at the steady state. It has been difficult to measure the Na+ electrochemical gradient in intact Ehrlich cells because Na+ may be sequestered in the nuclei of these cells. This problem is avoided with cytoplasts derived from Ehrlich cells because they do not contain internal compartments where Na+ could be sequestered. Since these cytoplasts also maintain steady state concentrations of Na+, K+, and α-aminoisobutyric acid similar to those found in whole Ehrlich cells, they are uniquely suited for testing the Na+ gradient hypothesis. Assuming the activity coefficients of external and cytoplasmic Na+ are equal, the energy in the Na+ electrochemical gradient of cytoplasts was 90% of that in the α-aminoisobutyric acid concentration gradient at the steady state. If the Na+ gradient hypothesis is correct, the 10% difference between these two gradients cannot be explained in terms of the sequestration of Na+ in the nucleus because cytoplasts do not contain internal compartments. © 1979.