Kinetics of the co-transport of phenylalanine and sodium ions in the guinea-pig small intestine. 1. Phenylalanine fluxes.

The kinetics of phenylalanine influx into guinea-pig small intestinal rings at different external sodium-ion concentrations have been examined. A change in the sodium concentration alters the value of Kt for phenylalanine influx without affecting the Jmax for the process. By studying the relationship between the value of Kt and the sodium-ion concentration, the applicability of various models to the overall process of sodium-coupled phenylalanine influx has been considered. The results are most consistent with a general non-compulsory model for the formation of a ternary complex between carrier, phenylalanine molecule and a sodium ion, which can be formed from either binary complex, i.e., either species can combine first to the carrier. The consistency of the model was tested by determining phenylalanine influx as a function of the sodium-ion concentraion at different amino-acid concentrations, and a good fit was obtained with the predictions of the non-compulsory model.