Intestinal assimilation of a tetrapeptide in the rat. Obligate function of brush border aminopeptidase

The small intestine is capable of taking up peptide nutrients of two or three amino acid residues, but the mechanism of intestinal assimilation of larger oligopeptides has not been established. The amino-oligopeptidase of the intestinal brush border possesses high specificity for oligopeptides having bulky side chains and is a candidate for a crucial role in the overall assimilation of dietary protein. Rat jejunum was used for in vitro gut sac and in vivo perfusion experiments with Gly-l-Leu-Gly-Gly (2 mM) as the test substrate with analysis of parent peptide and products by automatic ion-exchange chromatography. In these experiments, the tetrapeptide disappeared rapidly from the test solution (20 mumol/s per cm(2) in vitro; 17 mumol/s per cm(2) in vivo) by sequential removal of amino acid residues from the N-terminus to yield amino acids and the C-terminal dipeptide. In gut sac experiments, 61-100% of these products of hydrolysis appeared in the incubation medium and the remainder in the tissue. In contrast, only small amounts of hydrolytic products were found within intestinal lumen in vivo.Gly-l-Pro (10 mM), a peptide known to be transported intact but not to be hydrolyzed by the brush border aminopeptidase, failed to inhibit Gly-l-Leu-Gly-Gly disappearance suggesting that the tetrapeptide does not utilize the known intact transport mechanism. Hypoxic conditions (N(2) atmosphere) in vitro markedly inhibited transport of glucose, leucine, and Gly-Gly but failed to impair Gly-l-Leu-Gly-Gly disappearance suggesting that the first step in assimilation of the tetrapeptide does not involve a transport process. Disappearance of the tetrapeptide was completely blocked by l-leucyl-beta-naphthylamide (10 mM), a specific substrate for brush border aminopeptidase and by the phthalimido derivative of l-leucine bromomethyl ketone, a potent peptidase inhibitor. Hence, the amino-oligopeptidase at the intestinal surface appears to be essential for the initial stages of assimilation of this model tetrapeptide.