Molecular cloning and functional expression of Atlantic salmon Peptide Transporter 1 in Xenopus oocytes reveals efficient intestinal uptake of lysine-containing and other bioactive di- and tripeptides in teleost fish

Atlantic salmon (Salmo salar L.) is one of the most economically important cultured fish and also a key model species in fish nutrition. During digestion, dietary proteins are enzymatically cleaved and a fraction of degradation products in the form of di- and tripeptides translocates from the intestinal lumen into the enterocyte via the Peptide Transporter 1 (PepT1). With this in mind, a full-length cDNA encoding the Atlantic salmon PepT1 (asPepT1) was cloned and functionally characterized. When overexpressed in Xenopus laevis oocytes, asPepT1 operated as a low-affinity/high-capacity transport system, and its maximal transport activity slightly increased as external proton concentration decreased (varying extracellular pH from 6.5 to 8.5). A total of 19 tested di- and tripeptides, some with acknowledged bioactive properties, some containing lysine, which is conditionally growth limiting in fish, were identified as well transported substrates, with affinities ranging between approximately 0.5 and approximately 1.5 mmol/L. Analysis of body tissue distribution showed the highest levels of asPepT1 mRNA in the digestive tract. In particular, asPepT1 mRNA was present in all segments after the stomach, with higher levels in the pyloric caeca and midgut region and lower levels in the hindgut. Depriving salmon of food for 6 d resulted in a approximately 70% reduction of intestinal PepT1 mRNA levels. asPepT1 will allow systematic in vitro analysis of transport of selected di- and tripeptides that may be generated in Atlantic salmon intestine during gastrointestinal transit. Also, asPepT1 will be useful as a marker to estimate protein absorption function along the intestine under various physiological and pathological conditions.