Intestinal transport of a tetrapeptide, L-leucylglycylglycylglycine, in rat small intestine in vivo.

1. The intestinal transport mechanism for the tetrapeptide L-leucylglycylglycylglycine, Leu-GlyGly-Gly, and its relation to the transport of free Leu, Leu-Gly and Leu-Gly-Gly were investigated Lt vivo by means of jejunal perfusion in rats. 2. The rates of net Leu absorption from peptides (Leu-Gly and Leu-Gly-Gly-Gly) were significantly greater than those from the free amino acid mixtures when the test solutions were perfused at a concentration of 15 mmol/l. 3. Net Leu absorption rates from Leu-Gly (10 ymol/ml) and Leu-Gly-Gly (10 ,vmol/ml) were extensively inhibited (84% and 68% respectively) by Gly-Pro at 100 mmol/l, whereas Gly-Pro had no effect on Leu absorption from Leu-GlyGly-Gly. L-Alanine (Ala, 100 ymol/ml), on the other hand, which completely inhibited Leu absorption during perfusion of free Leu, inhibited Leu uptake from Leu-Gly-Gly-Gly only about 50% at all concentrations studied. Ala had no effect on Leu absorption from Leu-Gly and Leu-Gly-Gly (10 y mollml). 4. Neither Ala at 100 pmol/ml nor Gly-Pro at 100 ymol/ml had any effect on brush-border aminopeptidase activity in vitro, suggesting that the hydrolytic capacity of the intestinal mucosal brush border was unaltered when Ala or Gly-Pro was included in the perfusion mixture. L-Alanyl-fi naphthylamide (20 ymollml), which inhibited