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The review briefly addresses the mechanisms underlying micro- and macromolecular transport of proteins, fats and carbohydrates in the fish intestines. A special focus is on the differences between some mechanisms of micromolecular transport of peptides, amino acids and hexoses, as well as craniocaudal transport gradients, in fish compared to mammals. Transport proteins emerge at the earliest stages of fish ontogenesis, before the transition of fish larvae to exogenous feeding. The reasons behind the differences in the intestinal transport systems are analyzed. [ABSTRACT FROM AUTHOR]

The review reports the data on the presence in fish and mammals of the identical elementary or universal functional blocks (FBs)-molecules and supramolecular complexes providing exotrophic processes, with a special focus on those involved in the regulation of feeding behavior, digestion and transport of nutrients. When describing FBs implementing symbiotic digestion and induced autolysis, the data on the bacterial and mammalian enzymes have been analyzed. It is assumed that the structural and functional similarity of FBs in different organisms, which maintains the existence of trophic networks, is based on the need to perform similar functions and hence to develop similar mechanisms of FB adaptation in organisms phylogenetically distant from each other (bacteria, invertebrates, fish and mammals). [ABSTRACT FROM AUTHOR]

Turbot ( Scophthalmus maximus L.), a carnivorous fish species with high dietary protein requirement, was chosen to examine the expression pattern of peptide and amino acid transporter genes along its digestive tract which was divided into six segments including stomach, pyloric caeca, rectum, and three equal parts of the remainder of the intestine. The results showed that the expression of two peptide and eleven amino acid transporters genes exhibited distinct patterns. Peptide transporter 1 (PepT1) was rich in proximal intestine while peptide transporter 2 (PepT2) was abundant in distal intestine. A number of neutral and cationic amino acid transporters expressed richly in whole intestine including B-type amino acid transporter 1 (BAT1), L-type amino acid transporter 2 (LAT2), T-type amino acid transporter 1 (TAT1), proton-coupled amino acid transporter 1 (PAT1), yL-type amino acid transporter 1 (yLAT1), and cationic amino acid transporter 2 (CAT2) while ASC amino acid transporter 2 (ASCT2), sodium-coupled neutral amino acid transporter 2 (SNAT2), and yL-type amino acid transporter 2 (yLAT2) abundantly expressed in stomach. In addition, system b transporters (rBAT and bAT) existed richly in distal intestine. These findings comprehensively characterized the distribution of solute carrier family proteins, which revealed the relative importance of peptide and amino acid absorption through luminal membrane. Our findings are helpful to understand the mechanism of the utilization of dietary protein in fish with a short digestive tract. [ABSTRACT FROM AUTHOR]