Transport of di- and tripeptides in teleost fish intestine

The initial observation of peptide absorption in fish intestine dates back to 1981, when, in rainbow trout (Oncorhynchus mykiss), the rate of intestinal absorption of the dipeptide glycylglycine (Gly-Gly) was compared in vivo with the rate of absorption of its component amino acid glycine (Gly). The description of the identification of the underlying mechanisms that allow di- and tripeptide transport across the plasma membranes in fish was provided in 1991, when the first evidence of peptide transport activity was reported in brush-border membrane vesicles of intestinal epithelial cells of Mozambique tilapia (Oreochromis mossambicus) by monitoring uptake of radiolabelled glycyl-l-phenylalanine (Gly-l-Phe). Since then, the existence of a carrier-mediated, H+-dependent transport of di- and tripeptides (H+/peptide cotransport) in the brush-border membrane of fish enterocytes has been confirmed in many teleost species by a variety of biochemical approaches, providing basic kinetics and substrate specificities of the transport activity. In 2003, the first peptide transporter from a teleost fish, i.e. the zebrafish (Danio rerio) PEPtide transporter 1 (PEPT1), was cloned and functionally characterized in the Xenopus laevis oocyte expression system as a low-affinity/high-capacity system. PEPT1 is the protein in brush-border membranes responsible for translocation of intact di- and tripeptides released from dietary protein by luminal and membrane-bound proteases and peptidases. The transporter possesses affinities for the peptide substrates in the 0.1-10 mM range, depending on the structure and physicochemical nature of the substrates. After the molecular and functional characterization of the zebrafish transporter, the interest in PEPT1 in teleost fish has increased and approaches for cloning and functional characterization of PEPT1 orthologues from other fish species, some of them of the highest commercial value, are now underway. In this paper, we provide a brief overview of the transport of di- and tripeptides in teleost fish intestine by recalling the bulk of biochemical, biophysical and physiological observations collected in the pre-cloning era and by recapitulating the more recent molecular and functional data.