1. This paper describes the uptake by rings of everted hamster jejunum in vitro of three peptides with structural features making them resistant to hydrolysis, glycylsarcosylsarcosine, glycylsarcosylsarcosylsarcosine and β-alanylglycylglycine.

2. Glycylsarcosylsarcosine was taken up by a saturable mechanism and accumulated intact in the intracellular compartment of the intestinal wall, apparently against an electrochemical gradient. Its uptake was reduced by Na+-replacement, anoxia and metabolic inhibitors. It was concluded that uptake of this peptide was the result of Na+-dependent active transport.

3. Glycylsarcosylsarcosylsarcosine was very poorly taken up and its uptake did not appear to be the result of active transport.

4. β-Alanylglycylglycine appeared intact in the intracellular compartment of the intestinal wall on a substantial scale though it was not concentrated. No satisfactory evidence of uptake by a saturable mechanism was obtained. Uptake was, however, inhibited by anoxia, 2,4-dinitrophenol and Na+-replacement. Reasons are given for supposing that uptake of this peptide may be the result of Na+-dependent active transport by the same carrier as that utilized by glycylsarcosylsarcosine.

5. The results suggest that provided that they escape brush-border hydrolysis, tripeptides, like dipeptides, are actively transported into the absorptive cells of the intestinal mucosa, but that the ability of these cells to take up peptides by an active mechanism is unlikely to extend to tetrapeptides.

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