Neutral amino acids, transport systems

Audus KL, Borchardt RT. Characterization of the large neutral amino acid transport system of bovine brain micro vessel endothelial cell monolayers. J Neurochem 1986 47 484-488. 

Saier, M.H. Jr., Daniels, G.A., Boemer, P., Lin, J. (1988). Neutral amino acid transport systems in animal cells. Potential targets of oncogene action and regulators of cellular growth. J. Membr. Biol. 104,1-20. 

R. L. Preston,). E Schaeffer, and P E Curran, Structure-affinity relationships of substrates for the neutral amino acid transport system in rabbit ileum, /. Gen. Physid., 64 443-467 (1974). 

Methylmercury transport across the blood-brain barrier appears to be mediated by the large neutral amino acid transport system (system L) on the luminal surface of brain capillary endothelial cells (Kerper et al. 1992). Previous in vivo studies had shown that the amino acid, L-cysteine, accelerates methylmercury uptake into brain in vivo, but the mechanism was not identified. Because the methylmercury-L-cysteine complex has close struc-... 

Specific nutrient transport systems in brain capillaries can be used to facilitate drug entry into the brain. L-dopa (L-3,4-dfiiydroxyphenylalanine), a metabolic precursor of dopamine, is transported across endothelial cells by the neutral amino acid transport system [5], r-dopa permeates through capillaries into the striatal tissue, where it is decarboxylated to form dopamine. Therefore, systemic administration of L-dopa is often beneficial to patients with Parkinson s disease. Certain protein modifications, such as cationization [6] and anionization [7], produce enhanced uptake in the brain. Modification of drugs [8,9] by linkage to an antitransferrin receptor antibody also appears to enhance transport into the brain. This approach depends on receptor-mediated transcytosis of transferrin-receptor complexes by brain endothehal cells substantial uptake also occurs in the Hver. 

Monomethylation of the a-amino group has been shown to restrict transport of an amino acid to System A (16) this additional group is not well tolerated by other neutral amino acid transport systems as evidenced by the lack of inhibition of any system other than System A by N-monomethylated amino acids. Thus, N-methylated amino acids oflfer the investigator a specific substrate and inhibitor as a probe for System A activity. The beneficial properties of AIB and N-methylation have been combined in the nonmetabolizable. System A-specific analog, 2-(meth-ylamino)-isobutyric acid (MeAIB). Transport of MeAIB or inhibition of other test substrates by an excess of MeAIB represents the best available method for distinguishing System A activity without interference by other transport systems. 

Aschner, M., Eberle, N., Goderie, S., et al., 1990. Methylmercury uptake in rat primary astrocyte cultures the role of the neutral amino acid transport system. Brain Res. 521, 221—228. 

Oxender, D. L., Lee, M., Moore, P. A., and Cecchini, G., 1977, Neutral amino acid transport systems of tissue culture cells, /. Biol. Chem. 252 2675. 

Utsunomiya-Tate, N., Endo, H., and Kanai, Y. (1996) Cloning and functional characterization of a system ASC-like Na+-dependent neutral amino acid transporter. J. Biol. Chem. 271, 14883-14890. 

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]. 

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. 

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. 

A direct K+ requirement for translocation has, however, been reported for glutamic acid transport in brain (Kanner and Schuldiner, 1987 Carlson et al., 1989). The dicarboxylic amino acids appear to be transported largely by specific transporters which do not participate in neutral amino acid transport. Recent studies, both in reconstituted systems and the expression of the cloned transporter, have confirmed the K+ requirement (see below). 

Le Cam, A. Freychet, P. (1976). Glucagon stimulates the A system for neutral amino acid transport in isolated hepatocytes of adult rat. Biochem. Biophys. Res. Commun. 72, 893-901. 

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. 

Hartnup disorder is an autosomal recessive impairment of neutral amino acid transport affecting the kidney tubules and small intestine. It is believed that the defect is in a specific system responsible for neutral amino acid transport across the brush-border membrane of renal and intestinal epithelium, but the defect has not yet been characterised. 

SLC38 System A and N, Na -coupled neutral amino acid transporter 6 ... 

The neutral amino acid transporter is responsible for movement of neutral amino acids from the blood into the brain these compounds are not soluble in membranes and, therefore, would not diffuse into the brain in the absence of a transport system. This transporter is very efficient in brain capillaries, but it is found in other tissues as well (Table 5.5). The concentrations of amino acids in the blood are close to the values of for brain capillaries, suggesting that the transporter is near saturation under normal conditions. Because of this, and because the transporter is equally efficient with a number of amino acids, changes in the blood concentration of one neutral amino acid can influence the rate of transport of all the other neutral amino acids. 

The classic example of this approach involves the use of levodopa (l-3,4-dihydroxyphenylalanine, Figure 8.13) to treat Parkinson s disease [58]. Parkinson s disease is distinguished by the marked depletion of dopamine— an essential neurotransmitter—in the basal ganglia. Direct dopamine replacement is not possible, because dopamine does not permeate through the blood-brain barrier. However, the metabolic precursor of dopamine, levodopa, is transported across brain capillaries by the neutral amino acid transporter (see Table 5.5 and the related discussion). Peripheral administration of levodopa, therefore, produces an increase in levodopa concentration within the central nervous system some of these molecules are converted into dopamine due to the presence of decarboxylate enzymes in the brain tissue, but decar-boxylate activity is also present in the intestines and blood. To prevent conversion of levodopa into dopamine before entry to the brain, levodopa is usually administered with decarboxylase inhibitors. 

The following review will not cover, in detail, interorgan amino acid flows, the impact of transport on metabolism, or the description of the various hepatic amino acid transport systems. All of these subjects have been discussed in recent reviews (2, 3, 8). Instead, the purpose of the present discussion is to focus on the characteristics and hormonal regulation of a specific transport system for neutral amino acids, namely System A. System A-mediated transport has been the subject of a considerable... 

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