Monovalent cations, transport and exchange diffusion of neutral amino acids in Ehrlich ascites cells.

1. Methionine exchange diffusion is insensitive to Na+, K+, their gradients and prior depletion of cellular ATP. 2. In the absence of extracellular Na+, methionine uptake can he divided into two components; an initial rapid exchange between methionine and several endogenous amino acids and a slower net uptake inhibitable by alpha-aminoisobutyric acid and L-alanine. 3. After depletion of endogenous ATP and amino acids, carrier mediated methionine uptake from Na+-free medium was barely detectable. This and the evidence in 2 above indicates that the exchange carrier cannot catalyze net methionine uptake. 4. Cellular ATP and extracellular Na+, rather than transmembrane Na+, K+ or Na+ plus K+ gradients, are required for optimal amino acid accumulation. Cellular ATP and extracellular Na+ both decrease the Km values for amine acid influx. 5. Cation gradient-induced amino acid movement in ATP-depleted cells was insufficient to account for accumulation under more physiological conditions. 6. The Na+ gradient hypothesis is not generally applicable to amino acid transport in these cells