The structural specificity of the glycine transport system of Ehrlich carcinoma cells.

In previous studies the relative uptake of glycine-l-Cl4 in Ehrlich mouse ascites tumor cells was diminished with increased extracellular levels of unlabeled glycine or related compounds, but was enhanced by the presence of these same compounds inside the cell at high levels (2). Kinetic studies indicated that in the former case, the depressed influx was due to competition for a saturable active transport mechanism. The increased influx upon preloading, conforming to a mathematical relationship verified experimentally, could be related to an intensified exchange between labeled and unlabeled compounds across the cell membrane (3). The occurrence of both competition and exchange has been interpreted as strong evidence for a specific carrier mechanism in the transport of glycine and its analogues. The capacity of any substance either to compete or to exchange with glycine should therefore indicate a special affinity of this substance for the glycine transport carrier. Direct proof of the existence of a carrier in any system remains to be advanced. However, it was felt that certain new inferences regarding the glycine carrier in Ehrlich cells might be made after examining the structural specificity of the glycine transport system. Such specificity follows from comparison among various derivatives of glycine with respect to their aflinity for the transport system. This affinity, herein designated as “transport affinity,” was measured either by competition or by exchange of the tested substance with glycine-Cl* during uptake. Either of these methods should, ideally, yield the same measure of the affinity, since a single carrier mechanism appears to be responsible for both active transport and exchange diffusion in this system. In accordance with the model describing the simultaneous operation of these processes, free active carrier is assumed to occur essentially in the same form whether at the interface between the membrane and the external medium or at the interface between the membrane and the cell interior; i.e. a given amino acid should have the same affinity for active carrier at either interface (3). In the present report, some of the earlier observations of Chris