DIPEPTIDE TRANSPORT AND HYDROLYSIS IN RAT SMALL-INTESTINE, IN-VITRO

A range of natural and mixed D-/L-stereoisomer phenylalanine dipeptide s was used to investigate peptide uptake and hydrolysis by isolated ri ngs of rat jejunum. Characterisation of dipeptide hydrolysis by the br ush border fraction revealed apparent K-m values in the 0.1-1.0 mM ran ge which, except for the charged dipeptides, were significantly higher than those for hydrolysis by the cytosolic fraction. Uptake of L-/L- dipeptides into jejunal rings, which was followed by HPLC, was unaffec ted by the presence of peptidase inhibitors in the incubation medium a lthough the absorbed peptides were completely hydrolysed in the cytoso l; comparison of the effects of excess leucine on dipeptide uptake and on the uptake of the two constituent amino acids were also consistent with absorption of intact dipeptide followed by cytosolic hydrolysis. The uptake of hydrolysis-resistant mixed D-/L- dipeptides was also st udied and confirmed that peptide uptake preceded hydrolysis; D-alanyl- L-phenylalanine accumulated within the rings to twice the medium conce ntration.