... FEBS 5907 Synthesis and characterization of a new and radiolabeled high-affinity substrate for H+/peptide cotransporters Ilka Knuătter1, Bianka Hartrodt2,Geza Toth3, Attila Keresztes3, ... Gly-Sar,Ala-Ala, Lys-Lys, Ala-Asp, d-Phe-Ala, Ala-Ala-Ala,d-aminolevulinic acid, cefadroxil and Ala-4-nitroanilide(all 100 lm, Table 1). Glycine, which is not a substrate of peptide transporters, ... the design, rational synthesis and functional characterization of a novel radiolabeled hydrolysis-resistant high-affinity substrate for H+⁄ peptide cotransporters. l-4,4Â-Biphenylalanyll-Proline(Bip-Pro)...