Characteristics and mechanism of glutamine-dipeptide absorption in human intestine.

Using in vivo and in vitro techniques, the mechanism by which intestinal mucosa obtains glutamine from luminal oligopeptides was investigated in humans. The rate of hydrolysis by mucosal brush border membrane was more than threefold greater for alanylglutamine than for glycylglutamine. Despite this difference, rates of dipeptide and amino acid disappearance during intestinal perfusion were greater from test solutions containing glycylglutamine than alanylglutamine. Furthermore, rates of intraluminal appearance of products of hydrolysis during the infusion of two dipeptides were similar and less than 5% of the disappearance rate of the parent dipeptide. In contrast to free glutamine, uptake of peptide-bound glutamine by brush border membrane vesicles was not inhibited by deletion of sodium or addition of free amino acids to the incubation medium but was inhibited by other oligopeptides and stimulated by a proton gradient. Inhibition constants for the saturable uptake of glycylglutamine and alanylglutamine by vesicles were not significantly different, suggesting similar affinities for the peptide transporter. It is concluded that in human intestine the predominant mechanism for assimilation of glutamine-dipeptides is absorption as intact dipeptide rather than hydrolysis.