A new isoform of peptide transporter PepT1 (sbPepT1) and ion channels in touch sensation.

PepT1 is an electrogenic transporter expressed at the small intestinal brush-border membrane. It transports di- and tripeptides derived from diet using a H+-electrochemical gradient as driving force. The characterization of the oligopeptide transporters is important not only in nutrition science but also in drug delivery because these proteins are also able to transport a large number of peptido-mimetic drugs and therefore they are responsible for the high levels of oral bioavailability of many pharmaceutical compounds. Examples of PepT1 substrates include angiotensin-converting enzyme (ACE) inhibitors, anticancer drugs, penicillin and cephalosporin antibiotics and the antiviral agents Acyclovir. The research of my PhD was focused on an sea bass isoform PepT1 cloned at the University of Insubria from the intestine of sea bass (Dicentrarcus labbrax). The injection of cRNA into Xenopus laevis oocytes allows to obtain a high functional membrane expression of the protein that can be investigated with the use of electrophysiological techniques. Biophysical and kinetics features of the sea bass oligopeptides transporter 1 were characterized with the two electrode voltage clamp. Exposure of the oocytes to external solutions at different pHs, to substrate concentrations (between 0.03 and 10 mM) and at different membrane voltages allowed to investigate kinetic values as the maximal current (Imax) and the substrate apparent affinity (K0.5). sbPepT1, as other peptide transporters, exhibits transport-associated currents when different di- and tripeptides are applied to the external solution and presteady-state currents in the absence of substrate. To better understand the structure-function relation of sbPepT1, the research was extended to the other oligopeptide transporters classes (zebrafish and mammalian). The comparison of the PepT1 transport cycles among the three considered species showed a similar effect of external pH. In fact the presteady-state currents analysis presented a positive shift in the Q/V and τ/V curves and a slowing down of the decay time constants induced by acidity in all the three tested isoforms, especially in the rabbit transporter. These data were used to propose a new kinetic model. The simulation curves of the presteady-state and the transport currents calculated with the new model that includes the effect of the acidification of external environment were in good agreement with the experimental data.