5-fluorotryptophan resistant mutants affecting the A and L transport systems in the mouse L cell line A9.

Abstract

AbstractTryptophan transport has been examined in A9 and in mutants resistant to 5‐fluorotryptophan (5‐FT). Evidence indicates that in A9 cells two systems are present for tryptophan transport, which are analogous to the A and L systems found in Ehrlich ascites cells differing, however, in terms of amino acid specificity. Tryptophan uptake via the L system, a high affinity, low capacity system, is Na+ independent and occurs by a counter transport mechanism, while uptake via the A system, a low affinity, high capacity system, is Na+ dependent. Alanine, arginine, lysine, proline, asparagine, and aspartate (listed in order of decreasing inhibitory effect) inhibit tryptophan uptake via the A system from approximately 80‐50% while having no inhibitory effect on the L system. In addition, glutamine which inhibits tryptophan uptake by 80% via the L system only inhibits to the extent of 20% via the A system. Previous kinetic studies of 5FT resistant clone FTr37 indicated system A was altered while the analysis of the effects of the mutation on system L was inconclusive. However, in these studies Na+ independent uptake was not altered in FTr 37 indicating system L was not affected. Amino acid competition studies confirmed this observation and suggested that a change in the specificity of system A had occurred in FTr 37. The amino acid competition studies in FTr 23, indicated that the specificities of both systems differed from A9. The possibility that this change may be due to a single mutational event is discussed.