Intestinal transport of l-tryptophan in vitro: inhibition by high concentrations

¹ Cancer Research Institute, New England Deaconess Hospital; Peter Bent Brigham Hospital; and Department of Biological Chemistry, Harvard Medical School, Boston, Massachusetts

At a concentration of 5 x 10^–3 m, everted hamster intestinal sacs showed net transport of l-tryptophan from the mucosal to the serosal side, when the concentration was initially equal on the two sides. Thus, l-tryptophan is not an exception to the generalization that such intestinal preparations transport monoamino-monocarboxylic amino acids against a concentration gradient. High concentrations of l-tryptophan inhibit such transport. This may account for previous failure by others to observe l-tryptophan transport against a gradient at an initial concentration of 20 x 10^–3 m. Transport of l-tryptophan was greater by sacs from the middle of the small intestine than by those from the ends (this has been previously reported for l-phenylalanine and l-tyrosine). On the basis of this observation and those of mutual inhibition of transport, it is likely that the four aromatic amino acids share at least one common step in their intestinal absorption. Segments of hamster small intestine have been shown to accumulate l-tryptophan, developing a concentration greater than that of the bathing fluid.