Obligate Function of Brush Border Aminopeptidase

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 ,umol/s per cm2 in vitro; 17 ,tmol/s per cm2 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-LeuGly-Gly disappearance suggesting that the tetrapeptide does not utilize the known intact transport mechanism. Hypoxic conditions (N2 atmosphere) in vitro markedly inhibited transport of glucose, leucine, and This work was presented in part at the Annual Meeting of the American Federation for Clinical Research-American Society for Clinical Investigation, Atlantic City, N. J., 4 May 1975, and published as an abstract: 1975. Clin. Res. 23: 257A. Dr. Smithson is a trainee under U. S. Public Health Service National Institutes of Health Training Grant in Gastroenterology AM 05418. Dr. Gray is recipient of U. S. Public Health Service National Institutes of Health Research Career Development Award AM 47443. Received for ptublication 13 January 1977 and in revised form 31 March 1977. 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