| Proton-coupled oligopeptide transporters (POTs) (i.e., PEPT1, PEPT2, PHT1 and PHT2) translocate various small peptide/mimetic across the biological membrane. The first part of this dissertation focuses on investigating the transport properties of carnosine in kidney using SKPT cell cultures as a model of proximal tubular transport. Results demonstrated that carnosine is expected to have a substantial cellular accumulation in kidney due to its high influx clearance across apical membranes by PEPT2, but minimal tubular reabsorption into blood because of very low efflux clearance across basolateral membranes.;Although the role of PEPT1 in intestinal absorption of small peptide/mimetics has been demonstrated previously by in vitro models, its relative importance during in vivo intestinal absorption is unknown. Therefore, the objective of the second part of this dissertation is to delineate the relative importance of PEPT1 in intestinal absorption and disposition of small peptides/mimetics using wild-type and PEPT1 deficient mice, and glycylsarcosine (GlySar) as a model dipeptide substrate. In situ intestinal perfusions and in vivo absorption models in mice were used in our investigations. The results from our in situ studies show that PEPT1 is responsible for at least 90% of GlySar uptake in the small intestine and the transport protein exhibits low-affinity kinetics. However, during in vivo conditions, the extent of reduction in absorption, due to the absence of PEPT1, was lower than that of the in situ model. Specifically, the extent of GlySar absorption was reduced by about 50% due to the absence of PEPT1 transporter during in vivo condition. When partial AUC0--120 min was used as an indicator of the rate of absorption, there was a 60% reduction in the rate of GlySar absorption in PEPT1 deficient mice compared to the wild-type animals. With the exception of small intestine, PEPT1 had little effect on the tissue distribution of GlySar. In conclusion, the present studies demonstrate, using both in situ and in vivo models, that PEPT1 ablation significantly reduces both the rate and extent of oral absorption of small peptide/mimetic substrates (i.e., GlySar). These studies suggest that variability in intestinal PEPT1 (expression and/or activity) should exert a similar fate on peptide-like drugs. |
