First insights into the operational mode of epithelial peptide transporters

Nutrient transporter function studied in heterologous expression systems

Although a large number of plasma cell nutrient transport proteins has been cloned in the last couple of years, much remains to be learned about their structure-function relationships, membrane topology, posttranslational regulation, and bioenergetics of transport. Major progress in the study of the human and animal transporters has come from heterologous expression systems, which offer the benefits of ease of genetic selection and manipulation, short generation time of the organisms in which transporters are expressed, and comparatively high levels of expression of the recombinant proteins. Because our main focus is mammalian peptide transporters, the intestinal peptide transporter, PEPT1, and its renal counterpart, PEPT2, will serve here as models for the analysis of their structure and function when they are heterologously expressed in different cell systems.

Oral administration of (14)C labeled gelatin hydrolysate leads to an accumulation of radioactivity in cartilage of mice (C57/BL)

Several investigations showed a positive influence of orally administered gelatin on degenerative diseases of the musculo-skeletal system. Both the therapeutic mechanism and the absorption dynamics, however, remain unclear. Therefore, this study investigated the time course of gelatin hydrolysate absorption and its subsequent distribution in various tissues in mice (C57/BL). Absorption of (14)C labeled gelatin hydrolysate was compared to control mice administered (14)C labeled proline following intragastric application. Plasma and tissue radioactivity was measured over 192 h. Additional "gut sac" experiments were conducted to quantify the MW distribution of the absorbed gelatin using SDS-electrophoresis and HPLC. Ninety-five percent of enterally applied gelatin hydrolysate was absorbed within the first 12 h. The distribution of the labeled gelatin in the various tissues was similar to that of labeled proline with the exception of cartilage, where a pronounced and long-lasting accumulation of gelatin hydrolysate was observed. In cartilage, measured radioactivity was more than twice as high following gelatin administration compared to the control group. The absorption of gelatin hydrolysate in its high molecular form, with peptides of 2.5-15kD, was detected following intestinal passage. These results demonstrate intestinal absorption and cartilage tissue accumulation of gelatin hydrolysate and suggest a potential mechanism for previously observed clinical benefits of orally administered gelatin.

4-aminomethylbenzoic acid is a non-translocated competitive inhibitor of the epithelial peptide transporter PepT1

1. 4-Aminomethylbenzoic acid, a molecule which mimics the special configuration of a dipeptide, competitively inhibits peptide influx in both Xenopus Laevis oocytes expressing rabbit PepT1 and through PepT1 in rat renal brush border membrane vesicles. 2. This molecule is not translocated through PepT1 as measured both by direct HPLC analysis in PepT1-exp ressing oocytes and indirectly by its failure to trans-stimulate labelle d peptide efflux through PepT1 in oocytes and in renal membrane vessicle s. 3. However 4-aminiomethylbenzoic acid does reverse trans-stimulation through expressed PepT1 of labelled peptid efflux induced by unlabelled peptide. Quantitatively this reversal is compatible with 4-aminomethyl benzoic acid competitively binding to the external surface of PepT1. 4. 4-Aminomethylbenzoic acid (the first molecule discovered to be a non-translocated competitive inhibitor of proton-coupled oligopeptide transport) and its derivatives may thus be particularly useful as experimental tools.

H(+)-coupled uphill transport of the dipeptide glycylsarcosine by bovine intestinal brush-border membrane vesicles

In monogastric species, a considerable portion of amino acid nitrogen is absorbed across the brush-border membrane of the small intestine as small peptides (e.g., tripeptides and dipeptides). In ruminants, however, this process is less clear. Therefore, we investigated the uptake of radioactively labeled glycylsarcosine as a model dipeptide across the intestinal brush-border membrane using brush-border membrane vesicles prepared from the bovine small intestine. Uphill transport of glycylsarcosine was energized by a transmembrane H+ gradient and was further stimulated by an electrical potential difference across the membrane. Transport mediated by a carrier contributes to total glycylsarcosine transport across the brush-border membrane. Comparison of the apparent kinetic constants between brush-border membranes prepared from the proximal jejunum or ileum revealed similar half-saturating substrate concentrations (1.28 and 0.93 mmol/L for proximal jejunum and ileum, respectively), but maximal transport rates appeared to be somewhat higher in the proximal small intestine (2.15 and 1.20 nmol/mg of protein per 3 s for proximal jejunum and ileum, respectively). Uptake of glycylsarcosine was strongly inhibited by other dipeptides, but the amino acids glycine and sarcosine did not affect transport. Inhibition of glycylsarcosine uptake by cephalexin indicated an affinity of the carrier for cephalosporin antibiotics. Transport of intact dipeptides across the brush-border membrane of the small intestine might be of physiological importance in ruminants because the microbial and dietary proteins resistant to rumen degradation are digested and absorbed in the small intestine.

Renal transport of organic anions

This review deals with the different transport mechanisms mediating the apical and basolateral transport of organic anions, all of which are restricted to the proximal tubule. Several transport mechanisms, such as the para-aminohippurate basolateral transporter and the apical proton coupled di- and tripeptide transporter have been cloned, and their role in renal transport has been well characterized. Other transport proteins have been cloned from the kidney, liver, or intestine, but their role in the renal transport of organic anions needs to be elucidated. This is the case with Mdr2, oatp1 and OAT-K1, which were identified in the apical membrane of the proximal tubule, and with MDR1, the precise localization of which is still uncertain. Other apical transport mechanisms, sodium coupled transports and anion exchangers are involved in organic anion reabsorption.