Uptake of cyclic dipeptide by PEPT1 in Caco-2 cells: phenolic hydroxyl group of substrate enhances affinity for PEPT1

Uptake of cyclic dipeptides by H+/oligopeptide cotransporter (PEPT1) was studied in monolayers of the human intestinal cell line, Caco-2. The cyclic dipeptides studied were cyclic glycylphenylalanine (cyclo(Gly-Phe)), cyclic phenylalanylserine (cyclo(Phe-Ser)), cyclic seryltyrosine (cyclo(Ser-Tyr)) and cyclic glycyltyrosine (cyclo(Gly-Tyr)). These molecules have both peptide bonds and aromatic rings, and are similar in structure to cephalexin and cephadroxil, which are transported by PEPT1. Cellular uptake of these cyclic dipeptides was pH dependent, and was inhibited by the addition of PEPT1 substrates such as glycylsarcosine, indicating PEPT1-mediated transport. Michaelis constants (Km) for these cyclic dipeptides were cyclo(Ser-Tyr) < cyclo(Phe-Ser), and cyclo(Gly-Tyr) < cyclo(Gly-Phe), indicating that tyrosine possessing phenol moiety has higher affinity for PEPT1 than phenylalanine possessing benzen moiety. The Km for cephadroxil possessing phenol moiety was reportedly lower than that for cephalexin possessing benzen moiety. Therefore, it was concluded that the phenolic hydroxyl group of the substrate may enhance affinity for PEPT1.