Sodium-dependent neutral amino acid transport in native and reconstituted membrane vesicles from Ehrlich cells

Liver distribution of 99mTc-DL-homocysteine in experimental hepatitis rats

Liver accumulation of 99mTc-DL-homocysteine (99mTc-Hcy), which had been found to accumulate in several experimental tumors, was studied using experimental hepatitis models of rats. The distribution of this compound in liver of rats treated with CCl4 (25-100 microL/100 g body weight) was more than five times higher than that of non-treated rats, corresponding to the accumulation of Evans Blue, a marker dye for capillary permeability. Furthermore, in vitro experiments indicated that the liver accumulation of 99mTc-Hcy was due to some changes in the plasma membranes caused by CCl4 administration. These results indicate that lesional accumulation of 99mTc-Hcy is due to, at least in part, an enlarged capillary permeability similar to the case of malignant tissues reported previously.

Solubilization and reconstitution of high- and low-affinity Na(+)-dependent neutral L-alpha-amino acid transporters from rabbit small intestine

High- and low-affinity Na(+)-dependent neutral L-alpha-amino acid transporters were solubilized with 0.25% octaethylene glycol dodecyl ether (C12E8) after removal of the proteins from the brush-border membrane vesicles with 2% CHAPS and 4 M urea. When the CHAPS-insoluble protein was treated with papain before its solubilization with C12E8, a substantial amount of protein was removed without any decrease of the transport activities. The solubilized transporters were reconstituted into proteoliposomes after removal of C12E8 with Bio-Beads SM2. Several parameters proved to be important for optimal reconstitution efficiency: (a) the type of detergent, and (b) the phospholipid/protein and detergent/protein ratio during reconstitution, and (c) the salt concentration during reconstitution. Reconstituted proteoliposomes showed rapid uptake of neutral L-alpha-amino acids but not imino acid, basic or acidic amino acids driven by an electrochemical potential of Na+ (out > in). The uptakes under low- and high-substrate condition were further augmented by an artificial membrane potential introduced by K+ diffusion via valinomycin (negative interior). Kinetic analysis revealed that both the brush-border membranes and the solubilized fraction involved two carrier-mediated pathways for alanine transport. The kinetic parameters were determined by curve fitting with a computer to be Kt1 = 0.28 mM (0.21 mM) and Kt2 = 43.2 mM (28.4 mM), respectively (those with brush-border membrane vesicles in parentheses). Studies on the specific activities for transport of individual amino acids under low or high substrate concentration and the cross-inhibitory effects of various amino acids on alanine uptake (low concentration) revealed that these transporters possess broad specificity for neutral L-alpha-amino acids.

Sodium-dependent L-serine transport in plasma membrane vesicles isolated from Ehrlich cells by two-phase compartmentation

Plasma membrane vesicles were prepared from Ehrlich cells using two-phase system compartmentation. The highly pure plasma membrane vesicles obtained presented a negligible mitochondrial contamination and were suitable for studies of amino acid transport. L-Serine transport showed a clear ionic specificity, maximum incorporation being observed when an inwardly directed NaSCN gradient was used. Na(+)-dependent L-serine transport was dependent on assay temperature and membrane potential, and it seemed to be carried out by two different transport systems. An essential sulfhydryl group seemed to be involved in the transport process.

L-glutamine transport in native vesicles isolated from Ehrlich ascites tumor cell membranes

Native vesicles isolated from Ehrlich ascites tumor cells accumulate glutamine by means of Na(+)-dependent transport systems; thiocyanate seems to be the more effective anion. The apparent affinity constant for the process was 0.38 mM. The Arrhenius plot gave an apparent activation energy of 12.3 kJ/mol. The structural analogs of glutamine, acivicin (2.5 mM) and azaserine (2.5 mM), inhibited the net uptake by 67 and 70%, respectively. The sulfhydryl reagents mersalyl, PCMBS, NEM, and DTNB also inhibited net uptake, suggesting that sulfhydryl groups may be involved in the activity of the carrier protein. A strong inhibition was detected when the vesicles were incubated in the presence of alanine, cysteine, or serine; in addition, histidine, but not glutamate or leucine, had a negative effect on glutamine transport.

Solubilization and reconstitution of the sodium-and-chloride-coupled glycine transporter from rat spinal cord

Synaptic membranes from rat spinal cord were solubilized in the presence of 2% sodium cholate, phospholipids and 15% ammonium sulphate. The soluble extract was incorporated into liposomes consisting of asolectin and crude rat brain lipids. Reconstitution of the functional transporter protein was achieved by removal of detergent by gel filtration. Several parameters proved to be important for optimal reconstitution efficiency: (a) the lipid composition of the liposomes, (b) the type of detergent, and (c) the phospholipid/protein and detergent/protein ratio during reconstitution. In the reconstituted system, the transport of glycine showed a specific activity about twice that of native vesicles. The ionic dependence of the transport, the inhibitory effect of nigericin in the presence of external sodium and the stimulatory effect of valinomycin in the presence of internal potassium on glycine transport were preserved and more clearly observed in the reconstituted system. These results indicate that, in this preparation, the glycine transporter protein retains the same features displayed in the synaptic plasma membrane vesicles, namely dependence on sodium and chloride, electrogenicity and inhibitor sensitivity.

Partial purification of alanine carrier from rabbit small intestine brush border membrane and its functional reconstitution into proteoliposomes

An alanine transport carrier was partially purified from brush border membranes of rabbit small intestine. The alanine carrier activity was not solubilized with 0.4% deoxycholate but recovered in the detergent-insoluble fraction. The detergent-insoluble proteins were reconstituted into proteoliposomes with soybean phospholipids. The reconstituted proteoliposomes were capable of uptake of alanine driven by an electrochemical potential of Na+. The initial rate of alanine uptake into the proteoliposomes was 90 pmoles/mg protein/sec, which was 15-fold higher than that observed with the native membrane vesicles. The uptake of alanine was effectively suppressed by various neutral amino acids but not by either cationic or anionic amino acids.

Solubilization and reconstitution of hepatic System A-mediated amino acid transport. Preparation of proteoliposomes containing glucagon-stimulated transport activity

System A-mediated amino acid transport activity from rat liver plasma membrane vesicles has been solubilized and reconstituted into proteoliposomes using a freeze-thaw-dilution technique. The presence of cholate, at a cholate to protein ratio of 1:1, during the freeze-thaw step resulted in an enhancement in recoverable transport activity. The carrier required both phosphatidylcholine and phosphatidylethanolamine for optimal activity, but the addition of cholesterol to the reconstitution procedure appeared to have no significant effect on the resulting activity. A lipid to protein ratio of 20:1 yielded maximal transport activity. Sonication of the proteoliposomes provided some improvement in the accuracy of replicate assays for a given proteoliposome preparation. Isolated liver plasma membrane vesicles prepared from rats treated in vivo with glucagon in combination with dexamethasone contained stimulated System A activity. This enhanced transport activity could be solubilized and recovered in proteoliposomes generated from these plasma membranes. The data support the proposal that hormone regulation of the hepatic System A gene results in the de novo synthesis and plasma membrane insertion of the carrier protein itself.

A simple and efficient method for reconstitution of amino acid and glucose transport systems from Ehrlich ascites cells

Solubilized Ehrlich cell plasma membrane proteins were incorporated into lipid vesicles in the presence of added phospholipid, using Sephadex G-50 chromatography combined with a freeze-thaw step. Liposomes formed in K+ exhibited high levels of Na+-dependent, alpha-aminoisobutyric acid uptake which was electrogenic and inhibited by other amino acids. The transport activity reconstituted was similar to that observed in native plasma membrane vesicles. In addition to transport by system A, leucine exchange activity (system L), Na+-dependent serine exchange activity (system ASC), and stereospecific glucose transport activity were also reconstituted. The latter was inhibited by D-glucose, D-galactose, cytochalasin B, and mercuric chloride. The medium used for reconstitution was critical for the recovery of Na+-dependent amino acid transport. The use of Na+ in the reconstitution procedure led to formation of liposomes which displayed little Na+-dependent and gradient-stimulated amino acid uptake. In contrast, all transport activities studied were efficiently reconstituted in K+ medium.

Inhibition of tumor growth by an alkylation of the plasma membrane

The effect of nitrogen mustard (2-chloro-N-2-chloroethyl-N-methylethanamine), Trenimon (2,3,5-trisethyleneiminobenzoquinone-1,4), chlorambucil (4-[p-(bis[2-chloroethyl]amino)-phenyl]butyric acid) and phosphamide mustard (N,N-bis(2-chloroethyl)-diamidophosphoric acid) on Na+/K+-ATPase, membrane fluidity and cell multiplication was studied. With the exception of chlorambucil which does not affect Na+/K+-ATPase all concentrations of the other alkylating agents which inhibit cell multiplication of Ehrlich ascites tumor cells depress the activity of the Na+/K+-ATPase. All alkylating agents--including chlorambucil--caused an increase in the apparent degree of fluorescence polarization after labelling of the plasma membrane with 1,6-diphenyl-1,3,5-hexatriene (DPH). This effect is interpreted as a decrease in membrane fluidity caused by the alkylating drugs. The decrease in membrane fluidity is due to a direct interaction of the alkylating agent with the plasma membrane and is expressed at all concentrations of the drug which inhibit cell proliferation. No effect on membrane fluidity is observed after treatment of cells resistant to nitrogen mustard. The biological consequence of a decrease in membrane fluidity was investigated by growing Friend erythroleukemia cells in the presence of 10 mM cholesterol hemisuccinate. This procedure raises the microviscosity of the plasma membrane and depresses cell proliferation.

Partial purification of amino acid transport systems in Ehrlich ascites tumor cell plasma membranes

Ehrlich ascites tumor cell plasma membranes were subjected to sequential selective protein extraction to identify protein components associated with amino acid transport. These membranes were extracted with Triton X-100 followed by 2,3-dimethylmaleic anhydride. Approximately 80% of the membrane proteins were extracted by these procedures while the original lipids were largely retained (approximately 70%). The quantity of carbohydrate per milligram protein in the residue increased on extraction, consistent with an enrichment of glycoprotein in the residue. The residual vesicles display the characteristic properties of Na+-coupled amino acid transport. These properties include Na+-stimulated uptake and Na+-gradient-stimulated uptake leading to an accumulation of the solute against its chemical gradient as well as inhibition of uptake by a competitive amino acid, L-methionine. The extracted vesicles exhibit a peak level of alpha-aminoisobutyrate uptake six times greater than that expected from equilibration of alpha-aminoisobutyrate. This accumulation is greater than that obtained with native vesicles, albeit slower. The accelerated exchange diffusion of L-leucine is not measurable in the residual vesicles after dimethylmaleic acid anhydride treatment, although it can be measured after Triton extraction. These results are consistent with the conclusion that the amino acid transport systems "A" (Na+-coupled) and "L" (Na+-independent) in Ehrlich cells, though having overlapping specificities for amino acids, and distinct physical entities.