All cells require a continuous supply of amino acids to meet their metabolic demands. A primary concern of animal nutritionists is the need to understand what the capacity for -amino acid absorption is, in order that diets can be formulated to provide adequate, but not excessive, amino acids for a given production state.
The literature is replete with the characterization of free and peptide-bound amino acid transport systems that are expressed by laboratory animals and humans. By comparison, little research has been conducted to identify the presumably analogous transport systems in farm animal species.
Given the economic importance of these species, and the high rates of growth and protein production currently demanded by producers, the lack of knowledge regarding specific farm animal transporter physiology may be limiting our ability to formulate the diets and to design feeding strategies that optimize protein synthesis and retention.
Due to the wealth of information regarding amino acid and peptide transport systems and proteins being generated by biomedical research, and the similarities that appear to exist among animals species, a unique opportunity exists to identify and characterize the function of livestock species-specific free and peptidebound amino acid absorption mechanisms
considerable variation exists in the amino acid requirements of these cells and in the complement of ‘transport systems’ that are expressed to meet these requirements. Transport systems (or activities) generally are defined as that protein which recognizes and transfers a selective group of substrates across cellular membranes, whether acting singly or in combination with other proteins.
Although best characterized in the plasma membranes of non-polarized cells (e.g. muscle and endothelial), and the apical and basolateral membranes of polarized epithelial cells (e.g. enterocytes and hepatocytes), free (amino acid) and peptide-bound (peptide) amino acid transport systems also mediate the passage of substrates across the membranes of cell organelles
Transport proteins allow the cell (or organelle) selectively to bind and acquire compounds from a milieu of other substrates. The physiological importance of transporters is usually discussed in terms of their relative ability to recognize and bind a substrate molecule (affinity), and the amount and rate of substrate translocation through the membranes.
Typically, transport systems that demonstrate relatively low affinities for substrates have large capacities for transport, whereas those that display high affinities have low capacities. The general process of transporter-mediated passage through membranes, however, is the same for all transporters: (i) the substrate(s) binds to the recognition domain of the transporter the substrate(s) is translocated through the membrane interior into the cell interior (cytoplasm); (iii) the substrate(s) dissociates into the cytosol; and (iv) the substratebinding and translocation domain(s) of the transporter is reoriented for future substrate binding