Functional characterization of a cloned pig intestinal peptide transporter (pPepT1)

Absorption of dietary protein can be mediated through the uptake of AA as free AA or small peptides. A H₊-coupled, peptide transport protein, PepT1, is responsible for the absorption of small peptides arising from digestion of dietary proteins in the small intestine. The magnitude of peptide absorption and the nutritional significance of PepT1 are unknown for many food-producing animals; thus, the objective of this study was to clone and determine the functional characteristics of the pig PepT1 (pPepT1). Two cDNA-encoding pPepT1 were isolated, which contain alternative polyadenylation sites. The predicted pPepT1 is a 708-AA protein, which shows 82.8, 85.7, and 64.7% AA identity to human, sheep, and chicken PepT1, respectively. On northern blots, two pPepT1 mRNA of approximately 2.9 and 3.5 kb were detected in the duodenum, jejunum, and ileum of the small intestine and are presumed to result from alternative polyadenylation. Uptake of [³H]-Gly-Sar was measured in Chinese hamster ovary cells transiently transfected with a pPepT1 expression vector to study the functional expression of pPepT1. Peptide transport was H⁺-dependent, with an optimal pH of 6.0 to 6.5. The ability ofpPepT1 to transport various peptides was assayed by calculating the concentration of unlabeled peptide that inhibited 50% of [³H]-Gly-Sar uptake (IC ₅₀) in transfected cells. Eleven dipeptides and two tripeptides had IC₅₀ values that ranged from 0.004 to 0.53 mM. Three peptides, Lys-Lys, Arg-Lys, and Lys-Trp-Lys, had IC₅₀ values greater than 1.38 mM and seem to be poor substrates for pPepT1. For all three tetrapeptides examined, uptake of Gly-Sar was too small to measure, even at a concentration of 10 mM tetrapeptide; therefore, IC₅₀ values could not be calculated. These results demonstrate that pPepT1 can transport a variety of dipeptides and tripeptides but not tetrapeptides.