The kinetics of sodium-dependent phenylalanine influx in the intestine of the dog: a comparison between ileum and colon.

The kinetics of sodium-dependent phenylalanine influx into samples of dog ileal and colonic mucosa in vitro have been compared and contrasted. The data were evaluated by non-linear regression analysis, on the assumption that the total influx of the amino-acid is made up of the sum of a single saturable and a diffusive component. In both mucosae, the principal effect of changing the sodium concentration in the incubation medium was to alter the Kt for phenylalanine influx. The results were compatible with a general non-compulsory model for the formation of a ternary complex between the carrier, a phenylalanine molecule and a sodium ion which can be formed from either binary complex, i.e., either species can combine first with the carrier. Within the context of this model, it was shown that the Vmax for phenylalanine influx was substantially smaller in the colon than in the ileum. In addition, the constants governing the dissociation of the ternary complex into either binary complex were both significantly smaller in the colon, whilst the dissociation constants of the binary complexes were similar in the two regions of the intestine.