Neutral amino acid carrier

Greig NH, Momma S, Sweeney DJ, et al. Facilitated transport of melphalan at the rat blood-brain barrier by the large neutral amino acid carrier system. Cancer Res 1987 47 1571-1576. 

The ionization state of the substrate can affect BBB transport of carrier-mediated substrates. For example, histidine is an imidazole amino acid that is highly charged under acidic conditions and crosses the BBB via the basic amino acid carrier. However, under neutral conditions, histidine is 90% neutral and traverses the BBB via the neutral amino acid carrier. 

Hexose carrier Neutral amino acid carrier Monocarboxylic acid (MCA) carrier Adenosine carrier D-glucose (but not L-glucose), mannose, galactose phenylalanine (and 13 other neutral amino acids) lactate, pyruvate, and the ketone bodies /Fhydroxybutyrate and acetoacetate adenosine... 

Drags which have structures similar to that of endogenous nutrients may be taken up by a specialized transport system (carrier-mediated transport, receptor-mediated transcytosis) existing in the brain endothelium for nutrients. For example, drags having a molecular structure similar to a large neutral amino acid may cross the BBB via the neutral amino acid carrier such drags include melphalan (phenylalanine mustard), L-dopa, a-methyldopa, and p-chlorophenylalanine. 

As discussed above, certain nutrients are taken up into the brain by carrier-mediated systems. If a dmg possesses a molecular structure similar to that of a nutrient which is a substrate for carrier-mediated transport (Table 13.1), the pseudo-nutrient dmg may be transported across the BBB by the appropriate carrier-mediated system. For example, the dmg L-dopa crosses the BBB via the neutral amino acid carrier system. Other neutral amino acid dmgs that are transported through the BBB on this transport system are a-methyldopa, a-methylparatyrosine, and phenylalanine mustard. 

Peptides and small molecules may use specific transporters expressed on the luminal and basolateral sides of the endothelial cells to cross into the brain. So far, at least eight different nutrient transporters have been identified to transport a group of nutrients with similar structures. Drugs can be modified to closely mimic the endogenous carrier substrates of these transporters and be transported through the specific transporter-mediated transcytosis. Dopamine can be used to treat Parkinson s disease, but itself is nonbrain penetrant. Instead, dopamine s metabolic precursor, L-Dopa, if delivered by a neutral amino acid carrier through its transporter at the BBB, shows a clear clinical benefit on patients with Parkinson s disease [6,121]. To use a BBB transporter for drug delivery, several important factors must... 

Like the glucose carrier, the carriers for large neutral amino acids, the so-called L-system - now designated LAT - are present at both sides of the endothelial cell membranes and transport at least 10 essential amino acids. The L-transporter at the BBB has a much higher transport capacity than those in other tissues. Its marked preference for phenylalanine analogs explains why the anticancer drugs melphalan and d,l-NAM-7 are transported by the L-system, as is the L-Dopa used to treat Parkinson s disease [42]. 

The amino acid L-tryptophan is the precursor for the synthesis of 5-HT. The synthesis and primary metabolic pathways of 5-HT are shown in Figure 13-5. The initial step in the synthesis of serotonin is the facilitated transport of the amino acid L-tryptophan from blood into brain. The primary source of tryptophan is dietary protein. Other neutral amino acids, such as phenylalanine, leucine and methionine, are transported by the same carrier into the brain. Therefore, the entry of tryptophan into brain is not only related to its concentration in blood but is also a function of its concentration in relation to the concentrations of other neutral amino acids. Consequently, lowering the dietary intake of tryptophan while raising the intake of the amino acids with which it competes for transport into brain lowers the content of 5-HT in brain and changes certain behaviors associated with 5-HT function. This strategy for lowering the brain content of 5-HT has been used clinically to evaluate the importance of brain 5-HT in the mechanism of action of psychotherapeutic drugs. 

Amino acids and some small peptides are absorbed into the enterocytes in the jejnnnm. The transport of amino acids from the lumen into the ceU is an active process, coupled to the transport of Na ions down a concentration gradient. There are at least six carrier systems with different amino acid specificities neutral amino acids (i.e. those with no net charge, e.g. branched-chain amino acids) neutral plus basic amino acids imino acids (proline, hydroxyproline) and glycine basic amino acids (e.g. arginine and lysine) P-amino acids and taurine acidic amino acids (glutamic and aspartic acids). 

The transport of amino acids at the BBB differs depending on their chemical class and the dual function of some amino acids as nutrients and neurotransmitters. Essential large neutral amino acids are shuttled into the brain by facilitated transport via the large neutral amino acid transporter (LAT) system [29] and display rapid equilibration between plasma and brain concentrations on a minute time scale. The LAT-system at the BBB shows a much lower Km for its substrates compared to the analogous L-system of peripheral tissues and its mRNA is highly expressed in brain endothelial cells (100-fold abundance compared to other tissues). Cationic amino acids are taken up into the brain by a different facilitative transporter, designated as the y system, which is present on the luminal and abluminal endothelial membrane. In contrast, active Na -dependent transporters for small neutral amino acids (A-system ASC-system) and cationic amino acids (B° system), appear to be confined to the abluminal surface and may be involved in removal of amino acids from brain extracellular fluid [30]. Carrier-mediated BBB transport includes monocarboxylic acids (pyruvate), amines (choline), nucleosides (adenosine), purine bases (adenine), panthotenate, thiamine, and thyroid hormones (T3), with a representative substrate given in parentheses [31]. 

An alternative delivery strategy for small molecules is based on the presence of the nutrient transporters. Drugs that are structurally similar to substrates of a carrier system can undergo facilitated brain uptake as pseudoneutrients. The best example of this is the therapeutic use of L-DOPA in Parkinson s disease. Unlike the neurotransmitter dopamine itself, which cannot cross the BBB in significant amounts, its precursor L-DOPA is a substrate for LAT, the transporter of large neutral amino acids [56]. Its uptake by the brain is saturable, and subject to competition by the other substrates of the carrier present in plasma. 

Carrier proteins (e.g., large neutral amino acid transporter, NaVglucose-cotransport protein)... 

The other major class of transporter protein is the carrier protein. A prototypic example of a carrier protein is the large neutral amino acid transporter. An important function of the LNAA transporter is to transport molecules across the blood-brain barrier. As discussed previously, most compounds cross the BBB by passive diffusion. However, the brain requires certain compounds that are incapable of freely diffusing across the BBB phenylalanine and glucose are two major examples of such compounds. The LNAA serves to carry phenylalanine across the BBB and into the central nervous system. Carrier proteins, such as the LNAA transporter, can be exploited in drug design. For example, highly polar molecules will not diffuse across the BBB. However, if the pharmacophore of this polar molecule is covalently bonded to another molecule which is a substrate for the LNAA, then it is possible that the pharmacophore will be delivered across the BBB by hitching a ride on the transported molecule. 

The transporters have overlapping specificities. Thus, there is one transporter (called system L) for leucine and neutral amino acids with branched or aromatic side chains, another for basic amino acids (the Ly system), and a low-activity carrier (the dicarboxylate system) for dicarboxylic amino acids. 

A third classification includes those transporters which catalyze transport of metabolites which are neutral at physiological pH. These include the transporters for glutamine [31], and for the other neutral amino acids [32]. It also includes the one required for the oxidation of fatty acids which catalyze the exchange of carnitine for acyl carnitine [33]. This carrier also catalyzes net uptake of carnitine or acyl carnitines into the mitochondrial membrane, but at a rate only 1-2% of the exchange rate [34-36]. 

Free amino acids are transported into enterocytes by four active, carrier-mediated, Na+-dependent transport systems remarkably similar to the system for glucose. These systems transport, respectively, neutral amino acids basic amino acids (Lys, Arg, His) and cystine aspartic and glutamic acids and glycine and imino acids. Some amino acids (e.g., glycine) have affinities for more... 

Intracellular metabolism of amino acids requires their transport across the cell membrane. Transport of L-amino acids occurs against a concentration gradient and is an active process usually coupled to Na -dependent carrier systems as for transport of glucose across the intestinal mucosa (Chapter 12). At least five transport systems for amino acids (with overlapping specificities) have been identified in kidney and intestine. They transport neutral amino acids, acidic amino acids, basic amino acids, ornithine and cystine, and glycine and proline, respectively. Within a given carrier system, amino acids may compete for transport (e.g., phenylalanine with tryptophan). Na+-independent transport carriers for neutral and lipophilic amino acids have also been described, d-Amino acids are transported by simple diffusion favored by a concentration gradient. 

Inherited defects in amino acid transport affect epithelial cells of the gastrointestinal tract and renal tubules. Some affect transport of neutral amino acids Hartnup disease), others that of basic amino acids and ornithine and cystine (cystinuria), or of glycine and proline (Chapter 12). Cystinosis is an intracellular transport defect characterized by high intralysosomal content of free cystine in the reticuloendothelial system, bone marrow, kidney, and eye. After degradation of endocytosed protein to amino acids within lysosomes, the amino acids normally are transported to the cytosol. The defect in cystinosis may reside in the ATP-dependent efflux system for cystine transport, and particularly in the carrier protein. 

Three major factors are considered as important in determining the supply of tryptophan to the brain leading to serotonin synthesis (1) the extent of binding of tryptophan to serum albumin, which influences the pool of free (unbound) tryptophan that interacts with the amino acid carrier mechanism located at the blood-brain barrier, (2) the plasma tryptophan concentrations, and (3) the plasma concentration of other large neutral amino acids (LNAA), which compete with tryptophan for uptake into brain. Each factor can be influenced by the nutritional or hormonal status of the host and also by interorgan relationships in the metabolism of amino acids. 

The urea cycle converts NH4 to urea, a less toxic molecule. The sources of the atoms in urea are shown in color. Cit-rulline is transported across the inner membrane by a carrier for neutral amino acids. Ornithine is transported in exchange for H+ or citrulline. Fumarate is transported back into the mitochondrial matrix (for reconversion to malate) by carriers for a-ketoglutarate or tricarboxylic acids. 

Orally administered amines do not cross the blood-brainbarrier, but neutral amino acids such as a-methyldopa are transported into the brain by a specific carrier system. a-Meth-yldopa is subsequently concentrated in neuronal cells, where it becomes a substrate in the catecholamine biosynthesis and is transformed into a-methylnoradrenaline (180). 

Amino acid absorption in adult cestodes occurs through multiple carriers that are saturable and sensitive to temperature and to a variety of inhibitors (2,13). Amino acid accumulation typically occurs against steep concentration gradients (16), and the amino acid carrier systems in cestodes seem to be similar to those in mammals. There are separate carriers for acidic and basic amino acids and multiple carriers for the neutral amino acids, which overlap in their specificities. Unlike mammals, however, which show higher affinities for L-amino acids, cestode transporters are not stereoselective. Also,... 

At least six different Na -dependent amino acid carriers are located in the apical brush border membrane of the epithelial cells. These carriers have an overlapping specificity for different amino acids. One carrier preferentially transports neutral amino acids, another transports proline and hydroxyproline, a third preferentially transports acidic amino acids, and a fourth ffansports basic amino acids (lysine, arginine, the urea cycle intermediate ornithine) and cystine (two cysteine residues linked by a disulfide bond). In addition to these Na -dependent carriers, some amino acids are transported across the luminal membrane by facilitated tiansport carriers. Most amino acids are transported by more than one tiansport system. 

The entry of small neutral amino acids, such as alanine, glycine, proline, and 7-aminobutyrate (GABA), is markedly restricted because their influx could dramatically change the content of neurotransmitters (see section 111). They are synthesized in the brain, and some are transported out of the CNS and into the blood via the A-(alanine-preferring)-system carrier. Vitamins have specific transporters through the blood-brain barrier as they do in most tissues. 

Owing to observations on certain competitive and reciprocal antiport relations between sugar and amino acid transport, Alvarado and Crane and Alvarado have recently postulated a new polyfunctional, mobile carrier system. involved in the uphill transport of sugars, neutral amino acids and basic amino acids in the small intestine that consists of a mosaic of fixed, specific membrane sites which acquire mobility as a result of deformations of the mobile membrane resulting in local, transient engagements of the two protein surfaces, thus allowing bound substrates to be alternately exposed to the extra- and intercellular fluids. ... 

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