Uptake of a Series of Neutral Dipeptides Including l-Alanyl-l-Alanine, Glycylglycine and Glycylsarcosine by Hamster Jejunum in Vitro

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Abstract
This is the last of a set reporting an investigation of the kinetics of jejunal uptake and inhibitory ability of a series of neutral dipeptides, Gly-Gly, L-Ala-L-Ala, L-Val-L-Val and L-Leu-L-Leu, with progressively longer and more lipophilic side chains. At pH 5, apparently uptake of L-Ala-L-Ala, like that of L-Val-L-Val and L-Leu-L-Leu, was the result of 2 processes, uptake of intact peptide and uptake of free amino acid released extracellularly. Gly-Gly uptake was entirely in the form of intact peptide. In contrast to L-Val-L-Val and L-Leu-L-Leu uptake, the proportion of intact L-Ala-L-Ala taken up by mediated transport was greatest at the lowest concentration studied and smallest at the highest concentration. Although the corresponding sries of amino acids, Gly, L-Ala, L-Val and L-Leu, showed a progressive increase in apparent affinity for uptake and a decrease in Vmax, no such regular progression with the peptides was found. The results of work on inhibition of uptake of 1 dipeptide by another were complex. Examples were the very powerful inhibitory effect of L-Val-L-Val on uptake of glycylsarcosine, not suggested by the Kt inhibition constant of the former peptide, and the failure of glycylsarcosine to cause complete inhibition of uptake of L-Ala-L-Ala and L-Leu-L-Leu, though it could completely inhibit uptake of L-Val-L-Val. There may be more than 1 uptake system for intact peptides, but an explanation for all the results on inhibitions of uptake cannot be suggested.

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