1. 4-Aminomethylbenzoic acid, a molecule which mimics the special configuration of a dipeptide, competitively inhibits peptide influx in both Xenopus Laevis oocytes expressing rabbit PepT1 and through PepT1 in rat renal brush border membrane vesicles. 2. This molecule is not translocated through PepT1 as measured both by direct HPLC analysis in PepT1-exp ressing oocytes and indirectly by its failure to trans-stimulate labelle d peptide efflux through PepT1 in oocytes and in renal membrane vessicle s. 3. However 4-aminiomethylbenzoic acid does reverse trans-stimulation through expressed PepT1 of labelled peptid efflux induced by unlabelled peptide. Quantitatively this reversal is compatible with 4-aminomethyl benzoic acid competitively binding to the external surface of PepT1. 4. 4-Aminomethylbenzoic acid (the first molecule discovered to be a non-translocated competitive inhibitor of proton-coupled oligopeptide transport) and its derivatives may thus be particularly useful as experimental tools.
