Bioavailability digestion and absorption of amino acids

Bioavailability of proteins (or amino acids) is a popularized term for an important factor of determining protein quality. It is usually considered as a level, up to which amino acids or small peptides from a sample protein consumed by a living organism are finally transported into the body. Therefore, it includes digestibility and absorption mechanisms. Some showings of bioavailability of food proteins should be given by the results of valid in vitro digestibility methods (Kies, 1981). Normally, protein digestion starts in the stomach by the activity of HCl, which denatures quaternary, tertiary, and secondary structures of proteins and causes the activation of pepsinogen to pepsin. Final products of gastric protein digestion include especially large polypeptides, oligopeptides, and free amino acids. Protein digestion is completed by intestinal enzymes in the lumen of the small intestine (Gropper et al., 2008). Amino acids are absorbed along the entire small intestine, most of them particularly in the proximal small intestine. Amino acids are transported by specific carriers.

Transport of amino acids across the apical membrane is not only via sodium-dependent symporters but also due to the proton-motive force and the gradient of other amino acids providing to absorb amino acids from the lumen efficiently. In the basolateral membrane, antiporters work together with facilitators to release amino acids without depleting cells of valuable nutrients. Individual amino acids are mostly transported by more than one transporter, affording a backup capacity for the absorption during the mutational inactivation of a transport system (Broer, 2008).

Briefly, neutral and anionic amino acids are transferred by Na+ sym-porters during the secondary active transport from the lumen into mucosal cells and then they are transported into the blood with carriers or by diffuse. If Na+ was absent, no accumulation was observed. There is recent evidence that methionine transport is made separately of other neutral amino acids transport. Cationic amino acids (arginine, lysine, and ornithine) are partly taken up into the enterocytes by Na+-indepen-dent mechanisms, as the membrane potential is a driving force for their uptake. Anionic amino acids are already resolved in the mucosal cells. They also have their own (Na+ and K+ dependent) carrier systems, and finally neutral amino acids apply several different transporters (Despopoulos and Silbernagl, 2009; Preston et al., 1974). In addition, diacidic amino acids (glutamic and aspartic acids) are largely transaminated to alanine during the absorption. Dipeptides and tripeptides are usually absorbed more rapidly than free amino acids. Inside enterocytes, peptides are hydrolyzed, and amino acids are released together with those absorbed by amino acid transporters. A number of specific amino acid absorption disorders are congenital and combined with similar defects of renal tubular reabsorption. A lot of inborn disorders influence amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic amino-aciduria, and some other less well-described disturbances of amino acid transport (Broer, 2008).

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