Evidence for the Absolute Conformational Specificity of the Intestinal H+/Peptide Symporter, PEPT1

This study was initiated to determine whether the intestinal H + /peptide symporter PEPT1 differentiates between the peptide bond conformers of substrates. We synthesized a modified dipeptide where the peptide bond is replaced by the isosteric thioxo peptide bond. The Ala-Pro derivative Ala-ψ[CS-N]-Pro exists as a mixture of cis and trans conformation in aqueous solution and is characterized by a low cis/trans isomerization rate. The compound was recognized by PEPT1 with high affinity. The K i value of Ala-ψ[CS-N]-Pro for the inhibition of the uptake of radiolabeled glycylsarcosine in Caco-2 cells was 0.30 ± 0.02 m m , determined in solution with 96% trans conformation. In contrast, the K i value was 0.51 ± 0.02 m m when uptake media with 62% trans conformer were used. We conclude that only the trans conformer interacts with the transport system. From our data, a significant affinity of the cis conformer at PEPT1 cannot be derived. In a second approach, conformer-specific uptake of Ala-ψ[CS-N]-Pro was studied by analyzing the intracellular content of Caco-2 cells following transport as well as the composition of the extracellular medium using capillary electrophoresis. The percentage of trans conformer that was 62% in the uptake medium increased to 92% inside the cells. This is the first direct evidence that an H + /peptide cotransport system selectively binds and transports the trans conformer of a peptide derivative.