Salmo salar oligopeptide transporters PepT1a and PepT1b: a comparative electrophysiological characterization of the transport of lysine-containing peptides.

The H+-coupled peptide transporter 1 (PepT1), belonging to SoLute Carrier family (SLC15A1), is responsible for the absorption of di- and tri-peptides in enterocytes. In addition to the physiological nutrition role, for PepT1 has been hypothesized a role of transceptor, a peptides sensor/transporter, involved in the gut hormones release from enteroendocrine cell. The study in fish of the role of PepT1 in amino acid absorption is significant for the direct correlation between dietary protein availability and animal growth. In this contest because lysine is one of limiting amino acid substrate like Lys-cointaining peptide are of particularly interests in studying PepT1 function. Two intestine transporters of Salmo salar, ssPepT1a and ssPepT1b, heterologously expressed in Xenopus laevis oocytes were tested by Two Electrode Voltage Clamp and the transport-associate current generated by 1mM of different peptides (GK, KG, PG, MK, KM, KKK) were recorded. Lysine-containing substrates elicit transport-associated currents of different amplitude in the two transporters: ssPepT1b generated large currents when exposed to peptides carrying Lys in the N-terminus (KG, KM); conversely, ssPepT1a produced small currents independently from Lys position in the dipeptides tested. The Current-Voltage relationship (I/V), in the presence of Lys-containing substrates, shows small and similar currents at the two different pH (6.5, 7.6) in ssPepT1a. Conversely, in ssPepT1b the I/V curves were differed at the most negative potentials with larger currents recorded at pH 7.6. KG dose response experiments were also conducted, the fitting with logistic equation allowed to obtain the kinetic parameters (K0.5 and Imax) at each voltage for ssPepT1b, only an estimation was possible for ssPepT1a at -140 and -120mV. The different substrate specificity for Lys-containing peptides among salmon PepT1a and PepT1b transporters, support the idea of their distinct roles in peptide transport and sensing.