Interactions Between Tryptophan, Phenylalanine and Sugar Transport in the Small Intestinal Mucosa

Summary

The kinetics of the influx of tryptophan and phenylalanine into guineapig intestinal rings have been examined. The transfer of these two amino acids can be described by a single transport system, each amino acid having an affinity constant, K_t, of about 4 mM for the influx mechanism. Mutual inhibition studies have shown that the inhibitory constant of each of the amino acids is also 4 mM.

Although fully competitive inhibition between the two amino acids occurs, the inhibition of the influx of the amino acids by sugars exhibits kinetics of the “pseudo-competitive” type. Such behaviour is compatible with an allosteric interaction between two different binding sites, one for each class of compounds. The lack of correlation between the inhibitory potency of a given sugar and its rate of transfer, as testified by a comparison of the effects of galactose and β-methyl-glucoside on phenylalanine influx, can be reconciled with the “allosteric-interaction hypothesis”, but specifically repudiates any theory that attempts to explain such interactions in a way that requires such a correlation. The fact that allosteric interactions are retained in cells preloaded with sodium also precludes a primary role for sodium in the mechanism of such interactions.