ION CHANNELS, RECEPTORS AND TRANSPORTERS Functional expression of the oligopeptide transporter PepT1 from the sea bass (Dicentrarchus labrax) Rachele Sangaletti & Genciana Terova & Antonio Peres & Elena Bossi & Samuela Corà & Marco Saroglia Received: 20 May 2009 / Revised: 22 June 2009 / Accepted: 7 July 2009 / Published online: 18 July 2009 # Springer-Verlag 2009 Abstract Complementary RNA, derived from the intestine of the sea bass Dicentrarchus labrax and putatively coding for a pH-dependent oligopeptide transporter PepT1 (SLC15 family), was injected in Xenopus oocytes that were subsequently tested with electrophysiological techniques. Transport-associated currents were observed when various di- or tripeptides were applied at concentrations ranging between 0.1 and 10 mM. No currents were generated by histidine nor by other single amino acids. Sea bass PepT1 also exhibited presteady-state currents in the absence of substrates. Acidic pH slowed down the relaxation time constant of these currents and shifted both Q/V and τ/V relationships toward more positive voltages. Michaelis– Menten analysis of the transport currents showed an increase in apparent substrate affinity at acidic pH, which was very similar to that exhibited by the related transporter from zebrafish (Danio rerio), but in contrast, did not demonstrate a significant effect of pH on the maximal transport current. Keywords PepT1 . Cotransporter . Dicentrarchus labrax . Aquaculture . Electrophysiology . Xenopus oocyte Introduction In the course of the past few years, the study of intestinal peptide transport has rapidly evolved into a field of exciting nutritional and biomedical applications. In particular, the effect of dietary protein on oligopeptide transporter 1 (PepT1) expression and activity has been an area of active research in recent years. Results from these studies (for a review, see [3, 7]) demonstrated that the expression level of PepT1 and its function is very responsive to dietary treatments. The corresponding information for fish is completely unknown; however, this would be of great importance in particular for the farmed species raised in feed-based aquaculture systems. From the commercial perspective, the relevance of dipeptides in fish nutrition arises since fish meal substitutes are utilized as protein sources, therefore originating the need for limiting amino acids to be added to the feed, and even fractional improve- ments in this area have the potential to save the industry millions of euros and also to reduce the amount of nitrogen excreted into the environment. PepT1 is an integral plasma membrane protein respon- sible for the uptake of dietary di- and tripeptides in cells. It transports peptides against a concentration gradient in a pH-dependent manner. PepT1 transporter is electrogenic; therefore, its activity can be suitably monitored using electrophysiological techniques, which offer the important advantage of keeping the membrane voltage under control. Two-electrode voltage-clamp experiments on Xenopus oocytes heterologously expressing exogenous cRNA have provided a detailed functional and biophysical description of how many transporters operate. These studies have identified various common features in ion-coupled trans- porters such as the existence, in addition to the transport- associated current, of two other types of currents: the capacitive-like presteady-state current, arising from the first steps of the transport cycle before substrate binding, and a leak current, uncoupled from the substrate transport. In particular, the presteady-state current has been observed in R. Sangaletti : G. Terova : A. Peres (*) : E. Bossi : S. Corà : M. Saroglia Department of Biotechnology and Molecular Sciences (DBSM), University of Insubria, Via J.H. Dunant, 3-21100 Varese, Italy e-mail: antonio.peres@uninsubria.it Pflugers Arch - Eur J Physiol (2009) 459:47–54 DOI 10.1007/s00424-009-0700-0