Tryptophan transport

Lin, Z Wang, W and Uhl, G. R. (2000) Dopamine transporter tryptophan mutants highlight candidate dopamine- and cocaine-selective domains. Mol. Pharmacol. 58, 1581-1592. 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

Some investigations have tested the ability of reversed micelles to act as efficient carriers of molecular species. Solutions of water-containing AOT-reversed micelles have been employed for the selective transport and the efficient separation of the two amino acids tryptophane and j9-iodophenylalanine [160]. 

A number of genetic diseases that result in defects of tryptophan metabolism are associated with the development of pellagra despite an apparently adequate intake of both tryptophan and niacin. Hartnup disease is a rare genetic condition in which there is a defect of the membrane transport mechanism for tryptophan, resulting in large losses due to intestinal malabsorption and failure of the renal resorption mechanism. In carcinoid syndrome there is metastasis of a primary liver tumor of enterochromaffin cells which synthesize 5-hydroxy-tryptamine. Overproduction of 5-hydroxytryptamine may account for as much as 60% of the body s tryptophan metabolism, causing pellagra because of the diversion away from NAD synthesis. 

Another important function of albumin is its ability to bind various ligands. These include free fatty acids (FFA), calcium, certain steroid hormones, bilirubin, and some of the plasma tryptophan. In addition, albumin appears to play an important role in transport of copper in the human body (see below). A vatiety of drugs, including sulfonamides, penicilhn G, dicumarol, and aspirin, are bound to albumin this finding has important pharmacologic implications. 

Additional evidence for conformational changes in the transporter has come from measurement of the intrinsic fluorescence of the protein tryptophan residues, of which there are six, in the presence of substrates and inhibitors of transport. The fluorescence emission spectrum of the transporter has a maximum at about 336 nm, indicating the presence of tryptophan residues in both non-polar environments (which would emit maximally at about 330 nm) and in polar environments (which would emit at 340-350 nm) [154], The extent of quenching by the hydrophilic quencher KI indicates that more than 75% of the fluorescence is not available for quenching, and so probably stems from tryptophan residues buried within the hydrophobic interior of the protein or lipid bilayer [155]. Fluorescence is quenched... 

Serretti, A., Zanardi, R., Rossini, D. etal. (2001b). Influence of tryptophan hydroxylase and serotonin transporter genes on fluvoxamine antidepressants activity. Mol. Psychiatry, 6, 586-92. 

Hanson, G.R., Gibb, J.W., Metzger, R.R., Kokoshka, J.M., Fleckenstein, A.E. Methamphetamine-induced rapid and reversible reduction in the activities of tryptophan hydroxylase and dopamine transporters oxidative consequences Ann. N.Y.Acad. Sci. 844 103, 1998. 

DHT may occur over different tryptophan forms in proteins as they quite often have inhomogeneously broadened electronic spectra [31]. A very interesting case of DHT is described between two indole rings in bichromophoric solutes tryptophan dipeptide [32]. Such directed transport allows to correctly interpret spectral properties of dipeptide and other multichromophoric solutes. The theory of inductive-RET in solutions with inhomogeneous spectral broadening is given in Ref. [33]. In more detail, DHT mechanism will be explained in Sect. 2.2 (vide infra). 

Derrick studied the interaction of L-tryptophan and ibuprofen with human serum albumin (HSA),74 which is an abundant transport blood protein capable of binding efficiently several species.75 They acquired 1H NMR spectra of L-Tryptophan-HSA system for different ligand protein molar ratios, that is 3 1, 5 1, 7 1 and 10 1. The aromatic resonances of L-Tryptophan are difficult to be observed due to the overlap with HSA signals, even at 10 1 molar ratio, so that the spectral subtraction was performed. D values of L-Tryptophan were calculated by integration of the subtracted spectra and were in good agreement with those predicted by computer simulations. In the case of ibuprofen, only for 140 1 molar ratio, the resonances of ibuprofen are clearly visible also in this case, the... 

The organization of lipids around the plasma membrane Ca2+-transport ATPase of erythrocytes has been also determined by FRET. Taking advantage of the intrinsic fluorescence of the ATPase due to tryptophan residues and labeling different types of lipids with pyrene, it was demonstrated that the transporter is preferentially surrounded by negatively charged lipids such as phosphoinositides [167],... 

Subsequent anatomical techniques (e.g. immunohisto-chemistry of 5-HT or tryptophan hydroxylase, an enzyme unique to the synthesis of 5-HT retrograde and anterograde axonal transport studies) have allowed a more complete and accurate characterization of the serotonergic innervation of forebrain areas. 

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. 

In biological systems, therefore, the behavior of Li+ is predicted to be similar to that of Na+ and K+ in some cases, and to that of Mg2+ and Ca2+ in others [12]. Indeed, research has demonstrated numerous systems in which one or more of these cations is normally intrinsically involved, including ion transport pathways and enzyme activities, in which Li+ has mimicked the actions of these cations, sometimes producing inhibitory or stimulatory effects. For example, Li+ can replace Na+ in the ATP-dependent system which controls the transport of Na+ through the endoplasmic reticulum Li+ inhibits the activity of some Mg2+-dependent enzymes in vitro, such as pyruvate kinase and inositol monophosphate phosphatase Li+ affects the activity of some Ca2+-dependent enzymes— it increases the levels of activated Ca2+-ATPase in human erythrocyte membranes ex vivo and inhibits tryptophan hydroxylase. 

Kleinberger-Doron, N. and Kanner, B. I. (1994) Identification of tryptophan residues critical for the function and targeting of the gamma-aminobutyric acid transporter (subtype A)../. Biol. Chem. 269, 3063-3067. 

Chen, N., Vaughan, R. A., and Reith, M. E. (2001) The role of conserved tryptophan and acidic residues in the human dopamine transporter as characterized by site-directed mutagenesis. J. Neuro chem. 77,1116-1127. 

TnaT, a member of this family from Symbiobacterium thermophilum, has recently been discovered to be a sodium-dependent tryptophan transporter (Androutsellis-Theotokis, A., Goldberg, N. R., Ueda, K., et al. (2003) Characterization of a functional bacterial homologue of sodium-dependent neurotransmitter transporters. J. Biol. Chem. 278,12703-12709). 

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