The proton‐coupled amino acid transporter, SLC36A1 (hPAT1), transports Gly‐Gly, Gly‐Sar and other Gly‐Gly mimetics

BACKGROUND AND PURPOSE The intestinal proton‐coupled amino acid transporter, SLC36A1, transports zwitterionic α‐amino acids and drugs such as vigabatrin, gaboxadol and δ‐aminolevulinic acid. We hypothesize that SLC36A1 might also transport some dipeptides. The aim of the present study was to investigate SLC36A1‐mediated transport of Gly‐Gly and Gly‐Gly mimetics, and to investigate Gly‐Sar transport via SLC36A1 and the proton‐coupled dipeptide/tripeptide transporter, SLC15A1 in Caco‐2 cells. EXPERIMENTAL APPROACH Transport of a compound via SLC36A1 was determined by its ability to induce an increase in the inward current of two‐electrode voltage clamped SLC36A1 cRNA‐injected Xenopus laevis oocytes. SLC36A1‐mediated l‐[3H]Pro uptake in Caco‐2 cells was measured in the absence and presence of Gly‐Gly or Gly‐Sar. In addition, apical [14C]Gly‐Sar uptake was measured in the absence and presence of the SLC36A1 inhibitor 5‐hydroxy‐l‐tryptophan (5‐HTP) or the SLC15A1 inhibitor l‐4,4′‐biphenylalanyl‐l‐proline (Bip‐Pro). KEY RESULTS In SLC36A1‐expressing oocytes, an inward current was induced by Gly‐Sar, Gly‐Gly, δ‐aminolevulinic acid, β‐aminoethylglycine, δ‐aminopentanoic acid, GABA, Gly and Pro, whereas Val, Leu, mannitol, 5‐HTP and the dipeptides Gly‐Ala, Gly‐Pro and Gly‐Phe did not evoke currents. In Caco‐2 cell monolayers, the apical uptake of 30 mM Gly‐Sar was inhibited by 20 and 22% in the presence of 5‐HTP or Bip‐Pro, respectively, and by 48% in the presence of both. CONCLUSION AND IMPLICATIONS Our results suggest that whereas Gly‐Gly amid bond bioisosteres are widely accepted by the hPAT1 carrier, dipeptides in general are not; and therefore, Gly‐Sar might structurally define the size limit of dipeptide transport via SLC36A1.