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The H(+)/peptide cotransporter PEPT2 is expressed in a variety of organs including kidney, lung, brain, mammary gland, and eye. PEPT2 substrates are di- and tripeptides as well as peptidomimetics, such as beta-lactam antibiotics. Due to the presence of PEPT2 at the bronchial epithelium, the aerosolic administration of peptide-like drugs might play a major role in future treatment of various pulmonary and systemic diseases. Moreover, PEPT2 has a significant influence on the in vivo disposition and half-life time of peptide-like drugs within the body, particularly in kidney and brain. PEPT2 is known to have similar but not identical structural requirements for substrate recognition and transport compared to PEPT1, its intestinal counterpart. In this review we compiled available affinity constants of 352 compounds, measured at different mammalian tissues and expression systems and compare the data whenever possible with those of PEPT1.

Angiotensin-converting enzyme (ACE) inhibitors are often regarded as substrates for the H ϩ /peptide transporters (PEPT)1 and PEPT2. Even though the conclusions drawn from published data are quite inconsistent, in most review articles PEPT1 is claimed to mediate the intestinal absorption of ACE inhibitors and thus to determine their oral availability. We systematically investigated the interaction of a series of ACE inhibitors with PEPT1 and PEPT2. First, we studied the effect of 14 ACE inhibitors including new drugs on the uptake of the dipeptide [14 C]glycylsarcosine into human intestinal Caco-2 cells constitutively expressing PEPT1 and rat renal SKPT cells expressing PEPT2. In a second approach, the interaction of ACE inhibitors with heterologously expressed human PEPT1 and PEPT2 was determined. In both assay systems, zofenopril and fosinopril were found to have very high affinity for binding to peptide transporters. Medium to low affinity for transporter interaction was found for benazepril, quinapril, trandolapril, spirapril, cilazapril, ramipril, moexipril, quinaprilat, and perindopril. For enalapril, lisinopril, and captopril, very weak affinity or lack of interaction was found. Transport currents of PEPT1 and PEPT2 expressed in Xenopus laevis oocytes were recorded by the two-electrode voltage-clamp technique. Statistically significant, but very low currents were only observed for lisinopril, enalapril, quinapril, and benazepril at PEPT1 and for spirapril at PEPT2. For the other ACE inhibitors, electrogenic transport activity was extremely low or not measurable at all. The present results suggest that peptide transporters do not control intestinal absorption and renal reabsorption of ACE inhibitors.Angiotensin-converting enzyme (ACE) inhibitors are effective drugs for the treatment of hypertension, congestive heart failure, postmyocardial infarction, and diabetic nephropathy (Bertrand, 2004;Wong et al., 2004). The compounds inhibit the rate-limiting enzyme in the formation of angiotensin II, thereby reducing its capability for binding to its receptor. After oral administration as the primary route, most ACE inhibitors display absorption rates of 30 to 100% of a dose (Steinhilber et al., 2005). Because many ACE inhibitors sterically resemble AlaPro dipeptide or Xaa-Ala-Pro tripeptide structures, it was hypothesized that they share the same intestinal transport route as di-and tripeptides (for review, see Bai and Amidon, 1992;Amidon and Sadée, 1999). Di-and tripeptides are taken up into intestinal cells by the low-affinity H ϩ /peptide cotransporter PEPT1. In the kidney tubule, di-and tripeptides are reabsorbed by PEPT1 and by the high-affinity H ϩ /peptide cotransporter PEPT2 (for review, see Nielsen and Brodin, 2003;Daniel and Kottra, 2004;Terada and Inui, 2004;Biegel et al., 2006). ␤-Lactam antibiotics and antivirals such as valacyclovir were unequivocally demonstrated to use PEPT1 and PEPT2 for intestinal absorption or renal reabsorption, respectively (Bretschneider et al., 1999;Nielsen and Brod...

The proton-coupled amino acid transporter 1 (PAT1) represents a major route by which small neutral amino acids are absorbed after intestinal protein digestion. The system also serves as a novel route for oral drug delivery. Having shown that H+ affects affinity constants but not maximal velocity of transport, we investigated which histidine residues are obligatory for PAT1 function. Three histidine residues are conserved among the H+-coupled amino acid transporters PAT1 to 4 from different animal species. We individually mutated each of these histidine residues and compared the catalytic function of the mutants with that of the wild type transporter after expression in HRPE cells. His-55 was found to be essential for the catalytic activity of hPAT1 because the corresponding mutants H55A, H55N and H55E had no detectable l-proline transport activity. His-93 and His-135 are less important for transport function since H93N and H135N mutations did not impair transport function. The loss of transport function of His-55 mutants was not due to alterations in protein expression as shown both by cell surface biotinylation immunoblot analyses and by confocal microscopy. We conclude that His-55 might be responsible for binding and translocation of H+ in the course of cellular amino acid uptake by PAT1.

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