Dipeptide transport and hydrolysis in rat small intestine, in vitro.

A range of natural and mixed D-/L-stereoisomer phenylalanine dipeptides was used to investigate peptide uptake and hydrolysis by isolated rings of rat jejunum. Characterisation of dipeptide hydrolysis by the brush border fraction revealed apparent Km values in the 0.1-1.0 mM range 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 unaffected by the presence of peptidase inhibitors in the incubation medium although the absorbed peptides were completely hydrolysed in the cytosol; 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 studied and confirmed that peptide uptake preceded hydrolysis; D-alanyl-L-phenylalanine accumulated within the rings to twice the medium concentration.