The 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.