| Peptide transporters participate in the absorption of peptidomimetic drugs from the intestine and their reabsorption in the kidney. The focus of this research was to examine how peptide transporters interact with peptide-like ACE inhibitor drugs via (1) inhibition studies of radiolabeled GlySar uptake and (2) electrogenic studies using a two-microelectrode voltage-clamp technique in Xenopus oocytes expressing rat PEPT1 and PEPT2. It was found that ACE inhibitors displayed low-affinity interactions with both transporters. Kinetic analyses also demonstrated that quinapril inhibited the uptake of GlySar in a noncompetitive manner, while enalapril inhibited GlySar uptake in a competitive manner. Captopril and enalapril, but not quinapril, induced inwardly-directed currents in oocytes expressing rat PEPT2 as well as PEPT1. Quinapril was not a viable substrate but an inhibitor capable of reducing the captopril-evoked currents in oocytes. This finding is novel in that it argues against a free cc-amino group as being an absolute requirement for substrate recognition by PEPT2. Our findings also point to differences between PEPT1 and PEPT2 in their affinity to, rather than in their specificity for, ACE inhibitors. Additionally, the transport of captopril, enalapril and GlySar was studied in rat, rabbit and human PEPT1 and PEPT2 (expressed in oocytes) via an electrophysiological approach. An electrogenic and concentration-dependent transport of GlySar, enalapril and captopril was detected in all isoforms tested, which points to a unified peptide transport mechanism in mammals. Finally, using simultaneous radioactive uptake and current measurements (under voltage-clamp conditions), the stoichiometry of PEPT2 was determined to be 2:1 for protons:substrate. |
