Complementary role of surface hydrolysis and intact transport in the intestinal assimilation of di- and tripeptides

Abstract

The small intestine is known to possess mechanisms for intact transport and membrane hydrolysis of oligopeptides. To determine the relative role of these processes in peptide assimilation the fate of two model peptides known to be high-affinity substrates for the brush border aminooligopeptidase were studied in rat small intestine in vivo. Both 20 mM Gly- L-Pro, a potent inhibitor of peptide transport, and specific inhibitors of the aminopeptidase, 10 mM L-Ala-β-naphthylamide or the phthalimido derivative of 0.1 mM L-leucine bromomethyl ketone, reduced assimilation of L-Leu-Gly-Gly and L-Leu- L-Leu. Further inhibition was found when both transport and peptidase inhibitors were included in the intestinal perfusate suggesting that the model di- and tripeptides utilize both intact transport and surface hydrolysis for their assimilation. Although comparative kinetic parameters of intact transport ( K m = 22 mM; V = 1.9 · 10 −3 μmol · s −1 · cm −2) and surface hydrolysis ( K m = 8.7; V = 1.1 · 10 −3) for l-Leu- l-Leu differed markedly, the relationship of peptide concentration to assimilation rate was nearly identical for intact transport and surface hydrolysis in the physiological range of 1–10 mM substrate. Both intact peptide transport and surface hydrolysis appear to be efficient and complementary processes that promote efficient assimilation of dipeptides and tripeptides. The relative importance of each assimilation process appears to depend upon the amino acid composition of the peptide nutrient.