No abstract

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many beta-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

In glycation reactions, the side chains of protein-bound nucleophilic amino acids such as lysine and arginine are post-translationally modified to a variety of derivatives also known as Maillard reaction products (MRPs). Considerable amounts of MRPs are taken up in food. Here we have studied the interactions of free and dipeptide-bound MRPs with intestinal transport systems. Free and dipeptide-bound derivatives of N(6)-(1-fructosyl)lysine (FL), N(6)-(carboxymethyl)lysine (CML), N(6)-(1-carboxyethyl)lysine (CEL), formyline, argpyrimidine, and methylglyoxal-derived hydroimidazolone 1 (MG-H1) were synthesized. The inhibition of L-[(3)H]lysine and [(14) C]glycylsarcosine uptakes was measured in Caco-2 cells which express the H(+)/peptide transporter PEPT1 and lysine transport system(s). Glycated amino acids always displayed lower affinities than their unmodified analogues towards the L-[(3)H]lysine transporter(s). In contrast, all glycated dipeptides except Ala-FL were medium- to high-affinity inhibitors of [(14)C]Gly-Sar uptake. The transepithelial flux of the derivatives across Caco-2 cell monolayers was determined. Free amino acids and intact peptides derived from CML and CEL were translocated to very small extents. Application of peptide-bound MRPs, however, led to elevation (up to 80-fold) of the net flux and intracellular accumulation of glycated amino acids, which were hydrolyzed from the dipeptides inside the cells. We conclude 1) that free MRPs are not substrates for the intestinal lysine transporter(s), and 2) that dietary MRPs are absorbed into intestinal cells in the form of dipeptides, most likely by the peptide transporter PEPT1. After hydrolysis, hydrophobic glycated amino acids such as pyrraline, formyline, maltosine, and argpyrimidine undergo basolateral efflux, most likely by simple diffusion down their concentration gradients.

The great potential for medical applications of inorganic nanoparticles in living organisms is severely restricted by the concern that nanoparticles can harmfully interact with biological systems, such as lipid membranes or cell proteins. To enable an uptake of such nanoparticles by cells without harming their membranes, platinum nanoparticles were synthesized within cavities of hollow protein nanospheres (apoferritin). In vitro, the protein-platinum nanoparticles show good catalytic efficiency and long-term stability. Subsequently the particles were tested after ferritin-receptor-mediated incorporation in human intestinal Caco-2 cells. Upon externally induced stress, for example, with hydrogen peroxide, the oxygen species in the cells decreased and the viability of the cells increased.

This study was initiated to determine if there are differences in the recognition of ␤-lactam antibiotics as substrates between intestinal and renal peptide transporters, PEPT 1 and PEPT 2. Reverse transcription-coupled polymerase chain reaction and/or Northern blot analysis have established that the human intestinal cell line Caco-2 expresses PEPT 1 but not PEPT 2, whereas the rat proximal tubule cell line SKPT expresses PEPT 2 but not PEPT 1. Detailed kinetic analysis has provided unequivocal evidence for participation of PEPT 2 in SKPT cells in the transport of the dipeptide glycylsarcosine and the aminocephalosporin cephalexin. The substrate recognition pattern of PEPT 1 and PEPT 2 was studied with cefadroxil (a cephalosporin) and cyclacillin (a penicillin) as model substrates for the peptide transporters constitutively expressed in Caco-2 cells (PEPT 1) and SKPT cells (PEPT 2). Cyclacillin was 9-fold more potent than cefadroxil in competing with glycylsarcosine for uptake via PEPT 1. In contrast, cefadroxil was 13-fold more potent than cyclacillin in competing with the dipeptide for uptake via PEPT 2. The substrate recognition pattern of PEPT 1 and PEPT 2 was also investigated using cloned human peptide transporters functionally expressed in HeLa cells. Expression of PEPT 1 or PEPT 2 in HeLa cells was found to induce H ؉ -coupled cephalexin uptake in these cells. As was the case with Caco-2 cells and SKPT cells, the uptake of glycylsarcosine induced in HeLa cells by PEPT 1 cDNA and PEPT 2 cDNA was inhibitable by cyclacillin and cefadroxil. Again, the PEPT 1 cDNA-induced dipeptide uptake was inhibited more potently by cyclacillin than by cefadroxil, and the PEPT 2 cDNA-induced dipeptide uptake was inhibited more potently by cefadroxil than by cyclacillin. It is concluded that there are marked differences between the intestinal and renal peptide transporters in the recognition of ␤-lactam antibiotics as substrates.