The peptide transporter PEPT1, expressed in the brush border membrane of enterocytes, mediates the uptake of di- and tripeptides from luminal protein digestion in the small intestine. PEPT1 was proposed not to be expressed in normal colonic mucosa but may become detectable in inflammatory states such as Crohn's disease or ulcerative colitis. We reassessed colonic expression of PEPT1 by performing a systematic analysis of PEPT1 mRNA and protein levels in healthy colonic tissues in mice, rats, and humans. Immunofluorescence analysis of different mouse strains (C57BL/6N, 129/Sv, BALB/c) demonstrated the presence of PEPT1 in the distal part of the colon but not in proximal colon. Rat and human intestines display a similar distribution of PEPT1 as found in mice. However, localization in human sigmoid colon revealed immunoreactivity present at low levels in apical membranes but substantial staining in distinct intracellular compartments. Functional activity of PEPT1 in colonic tissues from mice was assessed in everted sac preparations using [¹⁴C]Gly-Sar and found to be 5.7-fold higher in distal compared with proximal colon. In intestinal tissues from Pept1-/- mice, no [¹⁴C]Gly-Sar transport was detectable but feces samples revealed significantly higher water content than in wild-type mice, suggesting that PEPT1 contributes to colonic water absorption. In conclusion, our studies unequivocally demonstrate the presence of PEPT1 protein in healthy distal colonic epithelium in mice, rats, and humans and proved that the protein is functional and contributes to electrolyte and water handling in mice.

Vegetarian diets have gained in popularity, especially among highly educated women, and are considered beneficial to health. Comparative studies assessing the diet of vegetarians against omnivores are rather limited and often provide ambivalent results. Therefore, this study examined the nutrient intake and nutritional quality of vegetarian and omnivorous diets in a group of 61 female students in Germany. Habitual dietary intake was evaluated using a validated graphical online food frequency questionnaire (FFQ). Differences in nutrient intakes were analyzed by Mann–Whitney-U-Tests. Odds Ratios (OR) were calculated for vegetarians exceeding dietary reference values (DRV) compared to omnivores. The overall nutritional quality was assessed using the Healthy-Eating-Index-2015 (HEI-2015). In omnivores, intakes of total energy from saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), long-chain omega-3 polyunsaturated fatty acids (LC-n3-PUFA), cholesterol, sucrose, lactose, retinol, and cobalamin were significantly higher than in vegetarians. Significantly lower intakes were observed for fiber, magnesium, and beta-carotene. Significant OR were detected for total fat (OR = 0.29), SFA (OR = 0.04), beta-carotene (OR = 4.55), and cobalamin (OR = 0.32). HEI-2015 scores were higher for vegetarians than for omnivores (79 points versus 74 points) and significant differences were recorded for the HEI-2015 components dairy, seafood & plant proteins, fatty acids, added sugars, and saturated fatty acids.

The intestinal peptide transporter PEPT1 provides bulk quantities of amino acids to epithelial cells. PEPT1 is a high-capacity and low-affinity solute carrier of the SLC15 family found in apical membranes of enterocytes in small intestine and distal colon. Surprisingly, murine PEPT1 (mPEPT1) has an apparent molecular mass of ∼95 kDa in the small intestine but ∼105 kDa in the large intestine. Here we describe studies on mPEPT1 protein glycosylation and how glycans affect transport function. Putative N-glycosylation sites of mPEPT1 were altered by site-directed mutagenesis followed by expression in Xenopus laevis oocytes. Replacement of six asparagine residues (N) at positions N50, N406, N439, N510, N515, and N532 by glutamine (Q) resulted in a decrease of the mPEPT1 mass by around 35 kDa. Electrophysiology revealed all glycosylation-deficient transporters to be functional with comparable expression levels in oocyte membranes. Strikingly, the mutant protein with N50Q exhibited a twofold decreased affinity for Gly-Sar but a 2.5-fold rise in the maximal inward currents compared with the wild-type protein. Elevated maximal transport currents were also recorded for cefadroxil and tri-l-alanine. Tracer flux studies performed with [(14)C]-Gly-Sar confirmed the reduction in substrate affinity and showed twofold increased maximal transport rates for the N50Q transporter. Elimination of individual N-glycosylation sites did not alter membrane expression in oocytes or overall transport characteristics except for the mutant protein N50Q. Because transporter surface density was not altered in N50Q, removal of the glycan at this location appears to accelerate the substrate turnover rate.

The intestinal transporter PEPT1 mediates the absorption of di- and tripeptides originating from breakdown of dietary proteins. Whereas mice lacking PEPT1 did not display any obvious changes in phenotype on a high-carbohydrate control diet (HCD), Pept1(-/-) mice fed a high-fat diet (HFD) showed a markedly reduced weight gain and reduced body fat stores. They were additionally protected from hyperglycemia and hyperinsulinemia. Energy balance studies revealed that Pept1(-/-) mice on HFD have a reduced caloric intake, no changes in energy expenditure, but increased energy content in feces. Cecal biomass in Pept1(-/-) mice was as well increased twofold on both diets, suggesting a limited capacity in digesting and/or absorbing the dietary constituents in the small intestine. GC-MS-based metabolite profiling of cecal contents revealed high levels and a broad spectrum of sugars in PEPT1-deficient mice on HCD, whereas animals fed HFD were characterized by high levels of free fatty acids and absence of sugars. In search of the origin of the impaired digestion/absorption, we observed that Pept1(-/-) mice lack the adaptation of the upper small intestinal mucosa to the trophic effects of the diet. Whereas wild-type mice on HFD adapt to diet with increased villus length and surface area, Pept1(-/-) mice failed to show this response. In search for the origin of this, we recorded markedly reduced systemic IL-6 levels in all Pept1(-/-) mice, suggesting that IL-6 could contribute to the lack of adaptation of the mucosal architecture to the diets.