Characteristics of dipeptide transport in normal and papain-treated brush border membrane vesicles from mouse intestine. I. Uptake of glycyl-L-phenylalanine.

Papain treatment of isolated brush border membrane vesicles was carried out to study peptide transport in the absence of hydrolytic events associated with the brush border membrane. Such a treatment allowed a 70% decrease in the activity of membrane-associated oligopeptidases and the study of peptide transport in the complete absence of free amino acids up to 1 min of incubation. A comparison between the time course curves of glycyl-L-phenylalanine uptake by normal and papain-treated vesicles showed that the overshoots seen in the presence of Na+ and K+ gradients (extravesicular greater than intravesicular) when using normal vesicles were no longer evident after papain treatment. This result, together with the demonstration of uptake into an osmotically reactive intravesicular space and the analysis of uptake of free phenylalanine, allowed the conclusion that peptide transport was the result of two complementary mechanisms, uptake of free amino acids following hydrolysis by the membrane-bound oligopeptidases, and intact peptide transport down a concentration gradient by a non-Na+ (and non-K+)-dependent process. These results also showed the non-involvement of gamma-glutamyltransferase and the gamma-glutamyl cycle in peptide absorption. A linear relationship has been established between initial dipeptide uptake and glycyl-L-phenylalanine concentration for the intact peptide transport process. However, this process can be inhibited to various extents by other di- and tripeptides but the inhibition never exceeded 43%. These results are consistent with both passive and facilitated diffusion mechanisms of intact peptide transport, the latter occurring by either a low affinity-high capacity or a high affinity-low capacity system.