Interaction between ACE inhibitors and renal oligopeptide transporters

This dissertation examines the interaction between angiotensin converting enzyme (ACE) inhibitors and renal oligopeptide transporters. Three types of experiments were performed using brush border membrane vesicles (BBMV) isolated from different sections of rabbit kidney. They include: (i) interaction of ACE inhibitors with high-affinity peptide transporter in kidney, and the relationship between inhibition potency and lipophilicity, (ii) competitive inhibition of GlySar uptake by enalapril in rabbit renal BBMV, and the effect on high-affinity peptide transporter, and (iii) GlySar uptake in rabbit renal BBMV isolated from outer-cortex or outer-medulla, and the evidence for heterogeneous distribution of oligopeptide transporters. Overall, ACE inhibitors showed different inhibition potencies toward the uptake of GlySar (15 {dollar}mu{dollar}M). Only fosinopril and zofenopril showed IC50 values of less than 100 {dollar}mu{dollar}M (55 {dollar}mu{dollar}M and 81 {dollar}mu{dollar}M, respectively). Other ACE inhibitors showed low-affinity interactions with the renal peptide transporter of which enalaprilat's interaction was the weakest (IC50 52 mM) toward GlySar uptake. With respect to structure-function relationships, the log IC50 and log distribution coefficient are significantly and linearly correlated. The IC50 decreases as the distribution coefficient increases. Subsequent studies regarding the interaction mechanism of ACE inhibitors with the renal high-affinity peptide transporter were performed using enalapril as a model ACE inhibitor. It was found that enalapril competitively inhibited the uptake of GlySar, a dipeptide substrate for peptide transporters, suggesting that enalapril might be a substrate for the high-affinity renal peptide transporter. In order to have a better understanding of the physiological and phannacological significance of the interaction between peptides or peptidomimetics (e.g., ACE inhibitors) and the renal peptide transporter, the intrarenal localization of PEPT1 and PEPT2 was assessed by determining GlySar uptake in BBMV isolated from outer-cortex (OC) or outer-medulla (OM) of rabbit kidneys. In contrast to earlier predictions, the high-affinity/low-capacity carrier system was found in both OC and OM membrane preparations (i.e., proximal convoluted and proximal straight tubules) while the low-affinity/high-capacity carrier system was found only in OC preparations (i.e., proximal convoluted tubules). In conclusion, although most ACE inhibitors appear to be weak inhibitors of GlySar uptake by the renal high-affinity peptide transporter, their inhibition potency increased as their lipophilicity increased. In addition, the competitive inhibition data suggest that enalapril may be a transportable substrate of the renal high-affinity peptide transporter. Finally, it appears that peptides and peptidomimetics are handled in a sequential manner in the kidney, first mainly by the low-affinity/high-capacity system (PEPT1) and then by the high-affinity/low-capacity transport system (PEPT2).