Emerging Concepts for the Role of Amino Acid Transporters

Potential for absorption of D-amino acids by intestinal epithelia

Given the fundamental differences in the amount of d-amino acids likely to be seen by animals that have a large pregastric fermentation capacity, and hence, a relatively greater production of bacterially derived d-amino acids, it is reasonable to suggest that the intestinal epithelia of ruminants may possess a larger capacity to absorb d-amino acids than postgastric fermenters. In this regard, several amino acid transport systems and proteins capable of d-amino acid absorption have been identified. Of particular importance is those systems capable of absorbing d-amino acids which are especially abundant in bacterial cell walls (d-glutamate, d-alanine and d-serine). Biochemically characterized system x c activity has been defined as the Na+-independent obligate exchange of l-glutamate and l-cys-teine, which may be inhibited by d-glutamate (Dantzig et al., 1978). Recently a cRNA has been generated (xCT) that apparently encodes for system x c—like activity, when co-expressed with 4F2hc (Sato et al., 1999). However, expression of x c activity by enterocytes is unknown to these authors and investigation of the ability of xCT to recognize and transport D-glutamate was not reported.

Although asc-like activity has been reported in the basolateral membranes of enterocytes (Mailliard et al., 1995), the localization of Asc-1 has not been confirmed. In contrast, ASCT2 is localized to the apical membrane of intestinal epithelia. With regard to aromatic D-amino acids, both LAT1 and TAT1 are localized to the basolateral membrane and transport D-pheny-lalanine. In addition, the absorption of at least D-phenylalanine-containing peptides by PepTl has been documented (Meredith and Boyd,

2000). The differential expression of ASCT2 and Asc-1 activity on both membranes of enterocytes indicates the capacity to absorb significant amounts of bacterial-derived D-amino acids from the intestinal lumen into the blood. These understandings, and that the flux of specific amino acids into the blood depends on their ability to compete for transport, suggest that the indiscriminant use of D-, L-isomer combinations as a supplemental source for one lamino acid may perturb the flux of others into blood.

Other emerging concepts not explicitly covered in this review regarding the physiological consequences of transporter expression and function, include the potential pathological consequences that system y+ (CAT1; Kim et al., 1991; Wang et al., 1991), ASC (ASCT1; Marin et al., 2000), and B° (ASCT2; Rasko et al., 1999; Tailor et al.,

2001) transporters serve as recognition molecules for various retroviruses and, in terms of ion fluxes, that system X AG (EAAT5; Fairman et ai, 1995) and N (SN1, Chaudhry et ai, 1999) 'amino acid' transporters may be more accurately considered to function as amino acid-gated ion channels. Respectively, these understandings suggest that the ability to transport amino acids is associated with a health 'risk/cost' and indicate that amino acids can serve as 'signalling' molecules.

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