Leucine transport system

A number of identical cell suspensions are treated with different amounts of radioactively labeled leucine. The initial rates of leucine uptake are measured for each suspension (see the table below). What is the maximum possible rate of uptake of leucine by cells using the same transport system ... 

Is a leucine analogue and is transported both into the blood from the gut and also across the blood-brain barrier into the brain from the blood by the system L transport system... 

A. M. Lynch and J, D, McGivan, Evidence for a single common Na+-dependent transport system for alanine, glutamine, leucine and phenylalanine in brush-border membrane vesicles from bovine kidney, Biochim. Biophys. Acta, 899 176-184 (1987). 

The effects of CPH-treatment of rats (1200 mg/ kg/d for 3d) on the polypeptide composition of renal brush border from the proximal tubule cells enzymatic activities and transport systems of the brush border membrane vesicles (BBMV) were investigated [77]. The results of these studies showed that CPH-treatment induces a 20-30% decrease in the specific activities of renal brush border enzymes leucine aminopeptidase and D-glutamyltransferase. SDS-gel electrophoresis showed that CPH-treatment induced a decrease of the intensity of 3 brush border polypeptides of molecular weights of 72,000, 58,000 and 39,000 [77]. 

Proximal tubule cells in culture should have retained functional attributes such as (1) polar architecture and junctional assembly of epithelia and correct membrane distribution of enzymes and transport systems (2) vectorial transport of solutes and water, manifested by the formation of domes when cultured on solid supports [81] and the generation of transepithelial electrophysiological properties [82, 83] due to the expression of proximal tubule specific claudins 2- and 10 [84, 85] (3) cellular uptake of xenobiotics from either the apical or basolateral side, as observed in vivo and (4) expression of nephron segment-specific characteristics, i.e., distinct expression of differentiation markers, metabolic and transport properties, and hormone responsiveness. Such markers include the expression of the brush border enzymes alkaline phosphatase, leucine aminopeptidase, and y-glutamyl transferase [4, 86], In addition, proximal tubule cells should possess Na+,K+-ATPase activities, Na+-dependent glucose, and p-aminohippurate transport. Proximal tubule cells increase cAMP levels in response to parathyroid... 

After orally ingested, L-theanine is absorbed into the blood circulation through the small intestinal tract s brush-border membrane and then distributed to tissues." " It is easily transported into the brain through the blood-brain barrier s leucine-preferring amino acid transporter system L-Theanine does not appear to accumulate. The metabolic fate of theanine after its oral administration was verified to be enzymatically hydrolyzed to glutamic acid and ethylamine in the blood, kidney, liver, and brain then most of the ethylamine generated was immediately excreted into urine, with only a part circulated in plasma. It is completely absent 24 h after administration. 

Natural L-theanine is synthesized from glutamic acid and ethylamine in the root of the tea plant and transferred to young leaves. Theanine is absorbed by a common Na+-coupled cotransporter in the intestinal brush-border membrane and incorporated into the brain via the leucine-preferring transport system of the blood-brain barrier. L-Theanine does not appear to accumulate it is metabolized in the blood, liver, and brain, and then completely eliminated in the urine within 24 h. 

An individual has been shown to have a deficiency in an intestinal epithelial cell amino acid transport system for leucine. However,... 

Large neutral amino acids (LNAA, such as phenylalanine, leucine, tyrosine, isoleucine, valine, tryptophan, methionine, and histidine) rapidly enter the CSF via a single amino acid transporter. (L-[leucine preferring]-system amino acid transporter). Many of these compounds are essential in the diet and must be imported for protein synthesis or as precursors of neurotransmitters. Because a single transporter is involved, these amino acids compete with each other for transport into the brain. 

Studies on the uptake of amino acids and oligopeptides into embryos of Hordeum vulgare indicate independent transport systems (97) whilst uptake studies with chloroplasts from Pisum sativum have indicated the presence of two carrier systems, one transporting glycine and aliphatic amino acids (L-alanine, L-leucine, L-isoleucine, and l-valine) and the other transporting L-serine, L-threonine and L-methionine. The carrier systems may not perform active transport but merely facilitated diffussion (244). Other studies have also indicated the possibility of transport of amino acids into chloroplasts (285). 

We found that LUCK irreversibly inhibited the neutral and general transport systems of Neurospora whereas LCK had little effect on these systems. Leucine is a substrate for both systems inactivated, whereas lysine is a substrate for only the general system. Both TCK and PCK inactivated the specific tyrosine/phenylalanine transport system of Bacillus as well the transport system (s) for neutral, aliphatic amino acids. 

We determined the apparent first-order rate constant for the inactivation of the tyrosine/phenylalanine transport system by several concentrations of PCK. By plotting these data in double-reciprocal form we were able to demonstrate the intermediacy of an enzyme-PCK complex in the inactivation reaction and determine K, values of 194 and 177 itM for PCK inhibition of tyrosine and phenylalanine transport, respectively. In addition, this analysis gives the actual first-order rate constants for the inactivation of tyrosine and phenylalanine transport by the enzyme-PCK complex of 0.016 and 0.012 M", respectively. Given the level of imprecision in the assays using whole-cell systems, these numbers are in agreement. The rate of loss of leucine transport activity is comparable to that for tyrosine and phenylalanine transport activity. 

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. 

Six human sugar transporters with different tissue distributions, substrate kinetics, and specificities have been identified. A number of facilitated amino acid transporters have also been identified in mammalian cells. System L, which transports neutral amino acids, such as leucine and phenylalanine, is probably the best known of these. 

Proteasomes rather than cytosolic carboxy-peptidases act to trim the C-terminal amino acids to conform the peptide to the proper size for MHC class-I presentation. Presentation from N-extended precursors is inhibited by acetylation of the terminal a-amino group at the N-terminus [354], which prevents the peptide to be cleaved by aminopeptidases e.g. leucine aminopeptidase) but not by proteasomes or endopeptidases. The TAP system transports peptides to the ER including both mature epitopes and longer precursors. It seems then that the peptides to be presented by MHC class-I can arise from N-extended precursors both in the cytosol and in the endoplasmic reticulum (ER). This assertion has been experimentally confirmed [355,356]. 

Significant research on LM enzyme encapsulation systems has also been conducted at the University of Hannover, West Germany. Scheper et al. (19) proposed the use of LMs to resolve racemic D,L-phenylalanine methyl ester with encapsulated chymotrypsin. This enzyme cleaves the ester bond of the L-isomer only. The process employed Adogen 464 (TOMAC) as an anion carrier, but the pHs used were such that any L-phenylalanine formed would be zwitterionic LM transport of zwitterions would be expected to be poor. Further work has included development of an LM enzyme reactor for detoxification of blood (33), reductive amination of a-ketoisocaproate by L-leucine dehydrogenase with a coencapsulated... 

---