Preparation and properties of the (Na+ + K+)-ATPase of plasma membranes from Ehrlich ascites cells

Identification of the integrin alpha 3 beta 1 as a component of a partially purified A-system amino acid transporter from Ehrlich cell plasma membranes

We have previously reported [McCormick and Johnstone (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 7877-7881] the partial purification of the Na(+)-dependent A-system amino acid transporter from Ehrlich cell plasma membranes and have suggested that a 120-130 kDa peptide, a major component of the purified fraction [octyl glucoside (OG) extract], is involved in Na(+)-dependent amino acid transport. In the present study, N-terminal sequence analysis of the 120-130 kDa peptide revealed a sequence similar to that of the alpha 3 subunit of the integrin alpha 3 beta 1. The presence of alpha 3 beta 1 was confirmed by Western blots of the OG extract probed with anti-alpha 3 or -beta 1 antibodies. Western blots also showed that an antibody originally raised against the 120-130 kDa peptide crossreacts with both the alpha 3 and beta 1 integrin subunits. Co-purification of alpha 3 beta 1 and Na(+)-dependent transport activity suggested that the two activities might be associated. Evidence that alpha 3 plays a role in transport is shown by the fact that an antibody against human alpha 3, but not beta 1, removed transport activity (approximately 25% loss) from cholate-solubilized Ehrlich membranes. Further purification of OG extracts using concanavalin A and wheat-germ lectin columns resulted in the separation of transport activity from the bulk (but not all) of alpha 3 beta 1 integrin without loss of the transport activity. These results indicate that the integrin itself is not essential for amino acid transport. Reconstitution of a purified alpha 3 beta 1-depleted protein fraction showed high levels of Na(+)-dependent, alpha-methylaminoisobutyric-acid-inhibitable amino acid transport in proteoliposomes, whereas reconstituted integrin alone showed little transport activity. However, in the integrin-depleted fractions, high amino acid uptake occurred in K+ which compromised the accurate measurement of the Na(+)-dependent component of uptake. The data suggest that alpha 3 may be associated with the A-system transporter and may modulate the activity of this carrier. Moreover, transfection of K562 and RD cells with human alpha 3 and alpha 2 cDNA showed that the former but not the latter increased A-system transport, thus providing more direct evidence that alpha 3 may modulate A-system transport activity.

Involvement of essential histidine residue(s) in the activity of Ehrlich cell plasma membrane NADH-ferricyanide oxidoreductase

The existence of histidine residue(s) implicated in the active site of NADH-ferricyanide oxidoreductase in plasma membrane vesicles isolated from Ehrlich ascites tumour cells is investigated. The shape of the pH-dependence curve of the enzyme activity suggests that one or more histidine residues are located at (or near) the active site of the enzyme. This hypothesis is supported by the following experimental data: the loss of activity after treatment with diethyl pyrocarbonate (DEPC) or photooxidation by using Rose bengal, and the strong inhibition caused by Zn2+ ions at micromolar concentrations. The combined arguments support the statement that histidine plays an essential role in the catalytic activity of NADH-ferricyanide oxidoreductase from Ehrlich ascites tumour cells.

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.

Solubilization and reconstitution of a nucleoside-transport system from Ehrlich ascites-tumour cells

Uptake of [3H]uridine by Ehrlich cells was mediated by both nitrobenzylthioinosine (NBMPR)-sensitive (75%) and NBMPR-insensitive (25%) mechanisms. Each cell contained approx. 26,000 high-affinity (KD = 0.19 nM) recognition sites for [3H]NBMPR, and binding was inhibited by dipyridamole and adenosine at concentrations similar to those required for inhibition of [3H]uridine uptake. Calculations show that each cell contains a total of about 35,000 nucleoside transporters. Photoaffinity labelling of a partially purified preparation of plasma membranes with [3H]NBMPR resulted in a single broad 3H-labelled band on SDS/polyacrylamide gels, with an apparent molecular-mass peak of 42 kDa. This is in contrast with human erythrocyte membranes, where [3H]NBMPR photolabelled two broad bands with peaks at 55 and 80 kDa. Treatment of photoaffinity-labelled membranes with endoglycosidase F decreased the apparent molecular masses of both the Ehrlich-cell and erythrocyte [3H]NBMPR-labelled proteins to approx. 40 kDa. These results suggest that the human erythrocyte [3H]NBMPR-binding polypeptides are more extensively glycosylated than the corresponding Ehrlich-cell polypeptides. Octyl beta-D-glucopyranoside [1.0% (w/v) + asolectin] solubilized over 90% of the [3H]NBMPR-binding sites, with near-complete retention of [3H]NBMPR-binding characteristics. The only major change was a 65-fold decrease in affinity for dipyridamole, which was partly reversed upon incorporation of the solubilized proteins into asolectin membranes. Proteoliposomes, prepared by using asolectin and the octyl glucoside-solubilized plasma membranes, were capable of accumulating [3H]uridine via a protein-dependent dipyridamole/nitrobenzylthioguanosine/dilazep-sensitive mechanism. We have thus demonstrated the efficient solubilization and functional reconstitution of a nucleoside-transport system from Ehrlich ascites-tumour cells.

Asymmetric reconstitution of the glucose transporter from Ehrlich ascites cell plasma membrane: role of alkali cations

Gel chromatography of solubilized Ehrlich cell plasma membranes and preformed asolectin vesicles coupled to a freeze-thaw cycle results in the reconstitution of 3-O-methyl-D-glucose transport. The transport activity of the liposomes formed is critically dependent on the cation present during reconstitution. Liposomes formed in K+ show high levels of carrier-mediated 3-O-methyl-D-glucose uptake (495 pmol/min/mg protein) while those formed in Na+ do not (33 pmol/min/mg protein). The inactivity in Na+ is not due to a diminished incorporation of glucose transporter nor is it due to carrier molecules reconstituted with a different orientation from those in K+ liposomes. Instead, the low glucose transport level in Na+ liposomes is related to the small size of vesicles formed with Na+. A second freeze-thaw cycle in K+ causes a two- to threefold increase in the available intravesicular volume of Na+ liposomes and results in an eightfold increase in carrier-mediated 3-O-methyl-D-glucose uptake. K+ liposomes, treated in an identical manner, show only a twofold increase in uptake. The glucose transporter was identified as a protein with a molecular mass range of 44.7 to 66.8 kDa, by the D-glucose-inhibitable photoincorporation of [3H]cytochalasin B. The carrier protein is inserted in reconstituted vesicles in a nonrandom manner with at least 80% of the molecules oriented with the cytoplasmic domain accessible to the external medium. In contrast, the neutral Na+-dependent amino acid transport system appears to be randomly reconstituted.

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.

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.

Plasma membrane heterogeneity in ascites tumor cells. Isolation of a light and a heavy membrane fraction of the glycogen-free Ehrlich-Lettré substrain

In this work we report on the isolation of two plasma membrane fractions of a glycogen-free substrain of Ehrlich-Lettré ascites cells, a light fraction sedimenting in a sucrose gradient at 1.10 g/ml, and a heavy fraction sedimenting at nuclei by a combination of short-term swelling and mild Dounce homogenization. A 12 000 X g postnuclear pellet (PII) containing major portions of the plasma membrane marker enymes, 5'-nucleotidase, ouabain-sensitive (Na+ + K+)-ATPase and the alkaline phosphatase, was prepared by differential centrifugation. The two plasma membrane fractions were obtained by centrifugation on a discontinuous sucrose gradient, from which they were further purified on a linear sucrose gradient applying sedimentation velocity conditions only. Enrichment factors for the three marker enzymes were between 5- and 14-fold for the light fraction and between 3- and 7-fold for the heavy fraction with an overall yield of 1--4% and 0.5--1.7%, respectively, of cellular protein. Contamination of both fractions with nuclear material was minor. Mitochondrial contamination was about 8% for the light material and somewhat higher for the heavy material. In the light fraction, co-sedimentation of lysosomal and Golgi marker enzymes was detected. The presence of membrane structures of these organelles could not be confirmed definitely by electron microscopy. Differences in sialic acid content and phospholipid composition within the two fractions, especially in the relative proportion of lecithin to sphingomyelin, suggests differences in membrane fluidity. The light material showed mostly unit membrane vesicles in thin-section and freeze-etch electron microscopy, whereas the heavy fraction mainly consisted of sheet-like membrane fragments.

Na+-linked cotransport of glycine in vesicles of Ehrlich cells

Vesicles have been prepared from Ehrlich cells by using a method based on the early experiments of Forte et al. (Forte, J.G., Forte, T.M. and Heinz, E. (1973) Biochim. Biophys. Acta 298, 827-841) with some minor modifications, using the filter technique. From electron micrographs and from the sensitivity of these vesicles towards osmotic pressure changes in the medium induced by (nonpermeant) sucrose, it is concluded that the vesicles are closed. The counterflow phenomenon with glycine and Na+-linked contransport of glycine appears to indicate that these vesicles are still functioning. The observation of the overshoot phenomenon interpreted in terms of theoretical predictions confirms that the active accumulation of glycine is energized by the Na+ electrochemical potential gradient. In particular, the contribution of the electrical components of these gradients is evidenced by the effects of the anion of the added sodium salt or of the addition of valinomycin. In contrrast to observations by others we found that ouabain does not directly affect Na+-linked cotransport of glycine whereas HgCl2 does so. Nor could any significant overshoot be demonstrated in the absence of an Na gradient. Since these vesicles were not metabolically active, these experiments do not exclude the possibility that in intact cells glycine is in addition transported primarily or partially actively.

The use of membrane vesicles in transport studies

Transport-competent plasma membrane vesicles isolated from mammalian cells provide a system to investigate mechanisms and regulation of nutrient and ion transport systems. The characteristics of membrane vesicle systems to study transport in erythrocytes, renal and epithelial membranes, Ehrlich ascites cells, and mouse fibroblasts are discussed. Studies of Na+-stimulated and Na+-independent amino acid and glucose transport in these systems are evaluated, with emphasis on experimental verification of concepts stated in the Na+ gradient hypothesis. Nucleoside, phosphate, and calcium transport systems in plasma membrane vesicles from mouse fibroblast cultures are discussed. Also, current biochemical approaches to investigate mechanisms of regulation of nutrient transport systems by hormones or cellular proliferative state are described.

Nature of lectin-induced alteration of potassium transfer in Ehrlich ascites tumor cells

The way in which the lectins concanavalin A (Con A) and Ricinus communis agglutinin (Ricin) alter the K+ content of Ehrlich ascites tumor cells was investigated. Unidrectional and net fluxes were determined in unwashed cells during a time course following lectin addition. Total influx, ouabain sensitive influx, Mg++- and Na+-K+-ATPase activity were all unaffected. Cell ATP content was normal for at least 19 minutes after exposure to Con A. Early after contact with Ricin or Con A efflux was stimulated 2-3-fold, resulting in net K+ loss, but after 20 minutes efflux had returned to normal. Ricin and Con A acted similarly although Ricin was present at only 1/50 the concentration of Con A. When the findings are evaluated together with previous work it is suggested that a particular membrane glycoprotein may be concerned in the efflux alteration observed.

Uptake of amino acids in reconstituted vesicles derived from plasma membranes of Ehrlich ascites cells

To obtain a clearer concept of the mechanism of organic solute transport in mammalian cells, we have attempted to reconstitute a functional transport system for amino acids from plasma membranes of Ehrlich ascites cells. Purified plasma membranes were dissolved in 2% Na cholate--4 M urea, a mixture which brought over 85% of the membrane proteins into solution. After centrifugation of the solubilized material for 2 hrs at 100,000 x g, the supernatant was dialyzed in the cold for 20 hrs with additional lipid. The reformed vesicles were tested for the ability to transport amino acids. The preliminary results obtained show that the uptake of alpha-aminoisobutyric acid can be inhibited by L-methionine and much less by L-leucine as would be predicted from the known properties of alpha-aminoisobutyric transport in the intact cells. In addition, it has been possible to show accelerated efflux of intravesicular phenylalanine when phenylalanine is added to the trans side (medium side). The data are consistent with the conclusion that there is carrier mediated transport in the reconstituted vesicles.

Modification of the fatty acid composition of Ehrlich ascites tumor cell plasma membranes

The fatty acyl group composition of Ehrlich ascites tumor cell plasma membranes was modified by feeding the tumor-bearing mice diets rich in either coconut or sunflower oil. When coconut oil was fed, the oleate content of the membrane phospholipids was elevated and the linoleate content reduced. The opposite occurred when sunflower oil was fed. Qualitatively similar changes were observed in the plasma membrane phosphatidylethanolamine, phosphatidylcholine and mixed phosphatidylserine plus phosphatidylinositol fractions. These diets also produced differences in the sphingomyelin fraction, particularly in the palmitic and nervonic acid contents. Unexpectedly, the saturated fatty acid content of the plasma membrane phospholipids was somewhat greater when the highly polyunsaturated sunflower oil was fed. The small quantities of neutral lipids contained in the plasma membrane exhibited changes in acyl group composition similar to those observed in the phospholipids. These fatty acyl group changes were not accompanied by any alteration in the cholesterol or phospholipid contents of the plasma membranes. Therefore, the lipid alterations produced in this experimental model system are confined to the membrane acyl groups.

Interactions of lectins with plasma membrane glycoproteins of the Ehrlich ascites carcinoma cell

Several aspects of the interaction of various lectins with the surface of Ehrlich ascites carcinoma cells are described. The order of agglutinating activity for various lectins is Ricinus communis greater than wheat germ greater than or equal to concanavalin A greater than or equal to soybean greater than Limulus polyphemus. No agglutination was noted for Ulex europaeus. Using 125I-labeled lectins it was determined that there are 1.6 and 7 times as many Ricinus communis lectin binding sites for concanavalin A and soybean lectins. Sodium deoxycholate-solubilized plasma membrane material was subjected to lectin affinity chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The lectin receptors of the plasma membrane appeared to be heterogeneous and some qualitative differences could be discerned among the electrophoretically analyzed material, which bound to and was specifically eluted from the various lectin affinity columns. The characteristics of elution of bound material from individual lectin columns indicated secondary hydrophobic interactions between concanavalin A or wheat germ agglutinin and their respective lectin receptor molecules.

Ionophore-mediated coupling between ion fluxes and amino acid absorption in mouse ascites-tumour cells. Restoration of the physiological gradients of methionine by valinomycin in the absence of adenosine triphosphate

1. Preparations of mouse ascites-tumour cells depleted of ATP and Na(+) ions accumulated l-methionine, in the presence of cyanide and deoxyglucose, from a 1mm solution containing 80mequiv. of Na(+)/l and about 5mequiv. of K(+)/l. Valinomycin increased, from about 4 to 16, the maximum value of the ratio of the cellular to extracellular concentrations of methionine formed under these conditions without markedly affecting the distributions of Na(+) and of K(+). Similar observations were made with 2-aminoisobutyrate, glycine and l-leucine. Increasing the extracellular concentration of K(+) progressively decreased the accumulation of methionine in the presence of valinomycin. Over the physiological range of ionic gradients, the system behaved as though the absorption of methionine with Na(+) was closely coupled to the electrogenic efflux of K(+) through the ionophore. The process was insensitive to ouabain and so the sodium pump was probably not involved. 2. The amount of methionine accumulated during energy metabolism was similar to the optimal accumulation in the presence of valinomycin when ATP was lacking. It was also similarly affected by increasing the methionine concentration. 3. A mixture of nigericin and tetrachlorosalicylanilide mimicked the action of valinomycin. The anilide derivative inhibited the absorption of 2-aminoisobutyrate in the presence of valinomycin, but not in its absence. 4. Gramicidin inhibited methionine absorption and caused the preparations to absorb Na(+) and lose K(+). 5. The observations appear to verify the principle underlying the gradient hypothesis by showing that the tumour cells can efficiently couple the electrochemical gradient of Na(+) to the amino acid gradient.