Role, Relevance and Regulation of PEPT1 in Peptide Intestinal Absorption.

Abstract

Peptide transporter 1 (PEPT1), a high capacity and low affinity transporter, is mainly expressed on apical membranes of epithelial cells in the small intestine. To explore the relevance of PEPT1 in small peptide intestinal absorption, everted jejunal uptake studies from wild-type and PEPT1 null mice were performed on glycylsarcosine (GlySar). GlySar uptake from wild-type mice displayed a bell-shape curve with maximal activities at pH 6.0 to 7.0. The uptake of GlySar was saturable with Km = 9.96 mM and Vmax = 233 pmol mg-1 20 sec-1. Furthermore, PEPT1 substrates significantly inhibited GlySar uptake. In contrast, transport activities of GlySar were relatively low and insensitive to changes in pH for PEPT1 null mice. GlySar uptake was proportionally increased as the concentration of GlySar increased in the buffer (i.e., linear uptake). Little inhibition was observed by PEPT1 substrates. These findings demonstrated that, under in vitro conditions, PEPT1 was the major transporter responsible for small peptide intestinal absorption. Moreover, the PEPT1 null mouse colony is a valid model to study PEPT1 relevance. To examine the regulation of PEPT1 during food deprivation, wild-type mice were fasted overnight (16 hr). Although the mRNA of PEPT1 did not change during fasting conditions, PEPT1 protein was significantly induced in the duodenum, jejunum and ileum compared to fed controls. To test functional activity, GlySar was administered intravenously to wild-type and PEPT1 null mice during fed-fasted conditions. The clearance of GlySar was comparable in the four groups. After oral administration of GlySar, the wild-type mice during fasted conditions showed the highest AUC and Cmax, followed by wild-type mice in the fed state. In contrast, GlySar pharmacokinetics were unchanged in the fed-fasted conditions of PEPT1 null mice. These findings demonstrated that, under in vivo conditions, changes in PEPT1 expression during the fasting condition correlated to changes in the functional response to GlySar. The intestinal absorption of GlySar was independent of feeding conditions in PEPT1 null mice. Therefore, food itself has little effect on small peptide intestinal absorption. Rather, the presence or absence of food can affect protein expression levels of PEPT1, which served to alter the extent of GlySar absorption.