Specificity and mechanism of influence of amino acid residues on hepatic clearance of oligopeptides

We have investigated the influence of amino acid residues on hepatic clearance of oligopeptides by determining the rate of disappearance (nmol.(min.g liver)-1) of selective oligopeptides from the medium during isolated rat liver perfusion. (a) N terminus: the rate of disappearance of Ala-Leu was greater (p less than 0.01) than those of Gly-Leu, Phe-Leu, and Arg-Leu (208 +/- 13, 135 +/- 13, 116 +/- 12, and 127 +/- 12, respectively). (b) C terminus: the rate of disappearance of Leu-Ala (244 +/- 18) was significantly greater (p less than 0.01) than that of Leu-Gly (145 +/- 16). (c) Number of residues: with each increase in the number of alanine residues (2-4) there was a significant increase in the rate of peptide disappearance, and conversely, with each increase in the number of glycine residues (2-6) there was a significant decrease in the rate of peptide disappearance. Further studies showed no peptide transport by isolated liver plasma membrane vesicles and no significant correlation between the rates of peptide disappearance and hydrolase activities of the perfusion medium but highly significant correlation with hydrolase activity of plasma membrane. We conclude that certain amino acid residues, such as alanine, enhance hepatic clearance of oligopeptides by increasing their affinity as substrates for plasma membrane peptide hydrolases.