Amino Acid Transport

The gradient of amino acids within and outside cells is maintained by active transport. Simple inspection of T.ab!e 2.4 shows that different transport mechanisms must exist for different amino acids to produce the range of concentration gradients observed. A variety of different transporters exist for different types and groups of amino acids (13, 14, 15 and 16). Amino acid transport is probably one of the more difficult areas of amino acid metabolism to quantify and characterize. The affinities of the transporters and their mechanisms of transport determine the intracellular levels of the amino acids. Generally, the essential amino acids have lower intracellular/extracellular gradients than do the nonessential amino acids (Iable.2.4), and they are transported by different carriers. Amino acid transporters are membrane-bound proteins that recognize different amino acid shapes and chemical properties (e.g., neutral, basic, or anionic). Transport occurs both into and out of cells. Transport may be thought of as a process that sets the intracellular/extracellular gradient, or the transporters may be thought of as processes that set the rates of amino acid cellular influx and efflux, which then define the intracellular/extracellular gradients ( 13). Perhaps the more dynamic concept of transport defining flows of amino acids is more appropriate, but the gradient (e.g., intracellular muscle amino acid levels) is measurable, not the rates.

The transporters fall into two classes: sodium-independent and sodium-dependent carriers. The sodium-dependent carriers cotransport a sodium atom into the cell with the amino acid. The high extracellular/intracellular sodium gradient (140 mEq outside and 10 mEq inside) facilitates inward transport of amino acids by the sodium-dependent carriers. These transporters generally produce larger gradients and accumulations of amino acids inside cells than outside. The sodium entering the cell may be transported out via the sodium-potassium pump, which transports a potassium ion in for the removal of a sodium ion.

Few transporter proteins have been identified; most information concerning transport results from kinetic studies of membranes using amino acids and competitive inhibitors or amino acid analogues to define and characterize individual systems. Table.2.5 lists the amino acid transporters characterized to date and the amino acids they transport. The neutral and bulky amino acids (the BCAAs, phenylalanine, methionine, and histidine) are transported by system L. System L is sodium independent, operates with a high rate of exchange, and produces small gradients. Other important transporters are systems ASC and A, which use the energy available from the sodium ion gradient as a driving force to maintain a steep gradient for the various amino acids transported (e.g., glycine, alanine, threonine, serine, and proline) (13, 14). The anionic transporter (XAG-) also produces a steep gradient for the dicarboxylic amino acids, glutamate and aspartate. Other important carriers are systems N and Nm for glutamine, asparagine, and histidine. System y+ handles much of the transport of the basic amino acids. Some overall generalizations can be made in terms of the type of amino acid transported by a given carrier, but the system is not readily simplified because individual carrier systems transport several different amino acids, and individual amino acids are often transported by several different carriers with different efficiencies. Thus, amino acid gradients are formed and amino acids are transported into and out of cells via a complex system of overlapping carriers.

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Table 2.5 Amino Acid Transporters

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