AMINO ACID ACCUMULATION IN EHRLICH ASCITES CARCINOMA CELLS

Abstract

Measurements of the transport of amino acids into Ehrlich ascites carcinoma cells have shown that the following relationship exists between the intracellular steady-state concentration of the amino acid (Cx) and the extracellular concentration (Co): [image] where Cm is the maximum intracellular concentration (formed when Co is large) and Em is a constant. It is shown that Em is identical with Km, the Michaelis constant, if a carrier enzyme is involved in the process of active transport and is, therefore, a measure of the affinity of the amino acid for the effective agent involved in the transport phenomenon. The ratio of the steady-state intracellular and extracellular concentrations of amino acids exceeds unity with all amino acids examined. The responses of L-S-ethylcysteine transport to changes of potassium ion concentration and to changes of temperature differ from those of glycine transport and indicate that different carriers are involved in the active transport of these amino acids into Ehrlich ascites cells. This conclusion is supported by the fact that, whereas glycine and L-serine compete with each other for concentrative uptake, such mutual competition does not occur between S-ethylcysteine and glycine or L-serine or L-leucine. Effects of the metabolic inhibitors, 2,4-dinitrophenol, iodoacetate, and stilbestrol show that these substances exercise inhibitory effects on active transport of amino acids by suppression of respiratory or glycolytic energy. Stilbestrol, which is a particularly potent inhibitor, is more effective under aerobic conditions (in the absence of glucose) than under anaerobic conditions (in the presence of glucose). It is reasonable to account for these results on the hypothesis that the carrier responsible for amino acid transport is adenosine triphosphate (ATP) dependent and that the carrier breaks down, and is no longer available for amino acid transfer, if the ATP content of the cell is depleted.