Peptidebound and Free Amino Acid Transport Activities and Proteins

Paralleling tissue flux studies, which have described the relative flux and absorption potential of peptides and amino acids, has been research conducted to characterize specific biochemical activities (ion coupling, relative substrate affinities) and the molecular identity of specific gene products (proteins) capable of absorbing peptide-bound and free amino acids (transporters). The biochemical characterization of mammalian peptide-bound (peptide) or free (amino acid) amino acid transport systems has revealed the presence of at least two H+-dependent and one H+-independent peptide transport activities, one H+-dependent amino acid transporter, and at least eight free amino acid systems in intestinal tissue (Fig. 3.2). The recent cloning and biochemical characterization of the cloned proteins responsible for these specific activities has revealed that, typically, several different proteins are capable of a biochemical 'system' activity, and that several Na+-inde-pendent activities result from the function of two separate proteins. Although the data are limited compared with that for humans and laboratory animals, the intestinal epithelia of pigs, cattle and sheep appear to express a similar complement of transport activities and specific transport proteins as observed for other species. For all species, however, relatively little is known regarding how expression and function of individual transporters are coordinated with that of other transporters to account for the quantity and relative ratio of amino acids that are absorbed across the intestinal epithelium. Ultimately, to design truly 'efficient' diets to support a desired level of animal production, knowledge regarding the intestinal luminal amino acid load must be matched with digestive and absorptive capacities for peptide-bound and free amino acids. The goal of this section is to identify and/or describe current information about peptide-bound and free amino acid transport.

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