Group translocation of the ribose moiety of inosine by vesicles of plasma membrane from T(3 cells transformed by Simian virus 40

P-glycoprotein in the catfish intestine: inducibility by xenobiotics and functional properties

The p-glycoprotein (pgp)-mediated multixenobiotic resistance (MXR) mechanism of aquatic animals has been associated with protection against pollution. Recent studies in mammals suggest that intestinal pgp may modulate intestinal bioavailability of dietary xenobiotics. In order to further delineate this mechanism in the catfish, these studies: (1) examined the pgp-related distribution in the intestine and liver of catfish, (2) evaluated the MXR response following exposure to various dietary xenobiotics and a prototypic pgp inducer and (3) evaluated pgp functional activity in membrane vesicles, using prototypic substrates and inhibitors. For this purpose, catfish were exposed in vivo to the pgp inducer vincristine (VIN), and the xenobiotics beta-naphthoflavone (BNF), benzo[a]pyrene (BaP), and 3,4,3',4'-tetrachlorobiphenyl (TCB). Membrane vesicles, prepared from liver and intestine (proximal and distal sections) of control and exposed catfish, were subjected to SDS PAGE, Western Blot, and detection with the pgp C219 monoclonal antibody. Transport activity was evaluated in vitro using the pgp substrate [3H]vinblastine (VBL), and the pgp inhibitor verapamil (VP). Immunoblot studies demonstrated a pgp-related protein of approximately 170 kDa in the intestine and liver of catfish. This protein appears to be very susceptible to degradation, and was present in higher levels in the liver, in comparison to the intestine, where regional differences were not observed. Dietary exposure to the pgp substrate VIN, or the xenobiotics BNF, BaP, and TCB, did not appear to affect pgp-related reactivity. Transport studies with VBL indicate that the pgp-related protein of the catfish intestine displays classic pgp-mediated multidrug resistance (MDR) characteristics, such as energy-dependency, and sensitivity to VP. These studies suggest that the pgp-related protein in the catfish intestine and liver is not only immunochemically, but also functionally related to the mammalian MDR. Moreover, the results presented indicate that pgp-related reactivity and transport in intestinal vesicles of catfish may be influenced by factors including method sensitivity, sample collection, sample preparation, and immunoblot conditions.

Sodium ion uptake into isolated plasma membrane vesicles: indirect effects of other ions

Vesicles derived from plasma membrane of corneal endothelium were agitated to their minimum size distribution. When isotonic salt solutions surrounding the vesicles were changed there were alterations to the vesicle size distribution: the modal point of the logarithmic distribution did not change but the log variance did, indicating that substantial fission and fusion of vesicles occurred depending upon the nature of the surrounding solute. Orientation and total membrane area was conserved in the transformed population of vesicles. Although the ions added to the external isotonic salt solutions in the present series of experiments have no direct effect upon sodium membrane transporters in these membranes, kinetics of sodium accumulation into the vesicles were affected in a way that correlated with changes to the vesicle size distribution. Early-saturating (<1 min) intravesicular concentrations of sodium corresponded with apparently stable populations. Late-saturating (>1 min) intravesicular concentrations of sodium corresponded with significant vesicle distribution shifts and included a few seconds of delay. During the linear accumulation phase, both populations showed similar magnitudes of sodium transport. The significance of these data is discussed.

Two membrane-bound proteins associated with alanine resistance and increased A-system amino acid transport in mutants of CHO-K1

Growth of CHO-K1, a proline auxotroph, is inhibited by amino acids that prevent proline transport. From a hydroxyurea-treated, alanine-resistant, constitutive mutant, alar4, we isolated, in a stepwise fashion, mutants, resistant to higher concentrations of alanine, that have increased velocity of amino acid transport through the A system. Two such mutants, alar4-H2.1 and alar4-H3.9, isolated as resistant to 50 mM and 125 mM alanine, respectively, showed increases in Vmax of proline transport through the A system that are directly proportional to their resistance to alanine. Alar4-H3.9, as compared to alar4 and CHO-K1, has six and 29 times the Vmax of proline transport through the A system and two and five times the velocity of transport through the combined ASC and P systems, respectively, and no change in system L. No double-minute or homologous staining regions were detectable in alar4-H3.9. A-system activity of alar4-H2.1 and alar4-H3.9, when grown under nonselective conditions, was stable for 20 generations and then declined. The phenotype of alar4-H3.9 is codominant with that of alar4 and partially recessive to that of CHO-K1. Membrane vesicles prepared from alar4-H3.9 show increases mainly in A-system transport. In sodium dodecylsulfate-polyacrylamide gel electrophoresis analysis of A-system active membrane vesicles and endoplasmic reticulum, two bands of molecular weight of approximately 62-66 kd and 29 kd are present in higher concentrations in alar4-H3.9 than in CHO-K1. These results are compatible with the hypothesis that the phenotype of alar4-H3.9 is the result of gene amplification of an A-system transporter structural gene and that the two bands may represent this transporter.

The human erythrocyte ghost: a new experimental model for studying adenosine transport

Previous work on adenosine transport has always had problems with the interference of adenosine metabolism, due to its high metabolic rate and because the enzymes involved are consistently present in most tissues. A new experimental model for studying adenosine transport in human erythrocyte ghosts is presented in this work: Human erythrocyte ghosts were sealed in the presence of erythro-3(2-hydroxynonyl)adenine and P1-P5-di(adenosine)5'-pentaphosphate, inhibitors of adenosine deaminase and adenosine kinase, respectively. These ghosts proved to lack adenosine metabolism when incubated in [U-14C]adenosine at 10 microM concentration at lack 37 degrees C for 60 min. Ghosts were 99.4% sealed in the correct orientation and had constant intracellular water volume. With these characteristics, the erythrocyte ghost preparation has many advantages for studying adenosine transport without adenosine metabolism interference. Adenosine transport was studied following the technique of W. R. Lieb and W. D. Stein [(1974) Biochim. Biophys. Acta 373, 165-177, 178-196.] Experiments to study Zero-trans influx and efflux, equilibrium exchange, and infinite-trans influx and efflux are presented. Adenosine transport did not behave linearly in any of these experimental procedures. Adenosine basic kinetic constants, calculated according to the procedure of Lieb and Stein, were R1----2 = 4.1 X 10(-4), R2----1 = 3.97 X 10(-4), Ree = 1.94 X 10(-4), Roo = 6.08 X 10(-4), K1----2 = 125.67 microM, and K2----1 = 84.36 microM. Lieb and Stein rejection criteria were used to distinguish a simple pore from a simple carrier. The data accumulated indicate that adenosine transport is carried out by a system that satisfies the criteria used for the simple carrier model. Asymmetric behavior was observed indicating lower affinity of the carrier for adenosine influx, although Vmax values for influx and efflux were similar.

Enzymological aspects of disorders in purine metabolism

Congenital enzyme defects of purine synthesis de novo and the salvage pathway are responsible for excessive uric acid production and are often associated with hyperuricemia and gout. On the other hand, defects of enzymes essential for the purine nucleotide cycles are the biochemical basis of dysfunction of the immune system. The influence of several congenital enzyme deficiencies on the regulation of biosynthesis de novo, on the regulation of purine nucleotide concentrations, and on adenosine concentration, as well as the effect on purine transport through cell membranes are discussed. The determination of enzymes involved in purine metabolism in noncongenital diseases seems to be of diagnostic importance. As examples, enzyme activities in lymphocytes of leukemic patients, and the determination of serum guanase activity in patients with liver dysfunction are described.

Enhanced labelling of RNA synthesized in nuclei and reduced labelling of RNA synthesized in the cytoplasm by [32P]orthophosphate in the presence of nucleosides

Labelling of cellular RNA by [32P]orthophosphate can be enhanced up to a factor of three by adding 2 to 10 X 10(-5)M of non-labelled nucleosides to the culture medium. The lag period of labelling can be reduced by a factor of two. Adenosine has the highest effect, while uridine is without effect. Labelling of viral RNA, which occurs in the cytoplasm, is reduced by nucleosides, suggesting that two precursor pools of nucleoside triphosphates are differently affected. The base composition of the labelled RNA cannot be influenced by incubation with non-labelled nucleosides. As a possible explanation for this observation a group translocation transport mechanism is discussed.

Distinct mechanisms of hypoxanthine and inosine transport in membrane vesicles isolated from Chinese hamster ovary and Balb 3T3 cells

Both enzyme-mediated group translocation and facilitated diffusion have been proposed as mechanisms by which mammalian cells take up purine bases and nucleosides. We have investigated the mechanisms for hypoxanthine and inosine transport by using membrane vesicles from Chinese hamster ovary cells (CHO), Balb/c 3T3 and SV3T3 cells prepared by identical procedures. Uptake mechanisms were characterized by analyzing intravesicular contents, determining which substrates could exchange with the transport products, assaying for hypoxanthine phosphoribosyltransferase activity, and measuring the stimulation of uptake of hypoxanthine by phosphoribosyl pyrophosphate (PRib-PP). We found that the uptake of hypoxanthine in Balb 3T3 vesicles was stimulated 3--4-fold by PRib-PP. The intravesicular product was predominantly IMP. The hypoxanthine phosphoribosyltransferase activity copurified with the vesicle preparation. These results suggest the possible involvement of this enzyme in hypoxanthine uptake in 3T3 vesicles. In contrast to the 3T3 vesicles, CHO vesicles prepared under identical procedures did not retain hypoxanthine phosphoribosyltransferase activity and did not demonstrate PRib-PP-stimulated hypoxanthine uptake. The intravesicular product of hypoxanthine uptake in CHO vesicles was hypoxanthine. These results and data from our kinetic and exchange studies indicated that CHO vesicles transport hypoxanthine via facilitated diffusion. An analogous situation was observed for inosine uptake; CHO vesicles accumulated inosine via a facilitated diffusion mechanism, while in the same experiments SV3T3 vesicles exhibited a purine nucleoside phosphorylase-dependent translocation of the ribose moiety of inosine. Vesicles prepared from a CHO cell line temperature-sensitive for hypoxanthine uptake (Azarts) showed a temperature-sensitivity in Km for uptake parallel to that of the intact cells. This suggests that the defect in Azarts may be caused by a missense mutation in the gene coding for the hypoxanthine transport carrier.

Nutrient transport in a bovine lens epithelial cell line

A bovine calf lens epithelial cell line (CLE-1) that synthesizes crystallin has been established in culture and some of its transport properties have been characterized using both cells and membrane vesicles derived from them. The membrane vesicles fractionate with high recovery of plasma membrane markers, showing a 40-fold purification of 5'-AMPase and a 20-fold decrease in the specific activity of the mitochondrial marker enzyme succinic dehydrogenase relative to a cell homogenate. Transport sites demonstrated higher specific activity than has been seen in vesicles from cell lines studied previously. The uptake of alpha-amino isobutyric acid (AIB) (an alanine analog) by CLE-1 cells is stimulated four- to fivefold by Na+ and exhibits a Km of 5.4 mM with a Vmax of 50 pmoles/min.microgram of cell protein. The uptake of leucine was not Na+ stimulatable. The uptake of AIB by the cells was reduced by 43% at confluence. Thus, the cell density dependent behavior of the uptake of the alanine amino acid family in CLE-1 is similar to that of various fibroblast cells. The Na+ caused a threefold stimulation of AIB uptake in the membrane vesicles, while vesicular uptake of leucine was unaffected by Na+. The uptake of adenine, guanine, uridine, and guanosine was also tested in these vesicles. The substrates were rapidly accumulated, came to a steady state distribution within 1-2 minutes, and were recovered as the unaltered compounds after uptake.

Thiamine transport in the brush border membrane vesicles of the guinea-pig jejunum

Transport of [35S]thiamine was studied with membrane vesicles prepared from the brush border of guinea-pig jejunum, in which thiamine pyrophosphokinase (EC 2.7.6.2) was not detected. The presence of an Na+ gradient from outside to inside of the vesicles did not affect thiamine transport, whereas L-proline uptake into the vesicles of the same preparation was stimulated under identical conditions. The equilibrium level of thiamine uptake decreased with increasing osmolarity of the medium, which indicates that thiamine is transported into the membrane vesicles. The initial rate (30 s) of thiamine uptake increased linearly with increasing thiamine concentration throughout the range from 0.06 to 10 microM in the medium, in the presence and absence of an Na+ gradient. No effect of other monovalent cations, including K+, Li+ and choline+, was observed on thiamine transport. Pyrithiamine, an antimetabolite of thiamine, and unlabeled thiamine, both added in very excessive amounts, did not inhibit labeled thiamine transport into the membrane vesicles. These results confirm the assumption that thiamine passes through the brush border membrane of guinea-pig jejunum by simple diffusion.

Facilitated transport of inosine and uridine in cultured mammalian cells is independent of nucleoside phosphorylases

The zero-trans uptake of uniformly and base-labeled inosine and uridine was measured a 25 degrees C in suspensions of Novikoff rat hepatoma cells, Chinese hamster ovary cells, mouse L cells, mouse S49 lymphoma cells and a purine-nucleoside phosphorylase-deficient subline thereof (NSU-1), and in monolayer culture of mouse 3T3 and L cells. The initial velocities of uptake of both nucleosides were about the same in all cell lines investigated, regardless of the position of the label or of the substrate concentration between 3 and 300 microM or whether or not the cells possessed uridine or purine-nucleoside phosphorylase activity. The kinetic parameters for the facilitated transport of uridine and inosine were also similar in phosphorylase positive and negative cell lines (K = 120--260 microM and V = 6--40 pmol/microliters cell water per s) and the transport activities of the cells exceeded their total phosphorylase activities by at least 10-fold for uridine and 1--2-fold for inosine. Chromatographic fractionation of the intracellular contents and of the culture fluid showed that the free nucleosides appeared intracellularly prior to and more rapidly than their phosphorolysis products. During the initial 20--60 s of uptake of U-14C-labeled nucleosides the rates of intracellular appearance of ribose-1-P and base were about the same. After several minutes of incubation, on the other hand, the main intracellular component was ribose-1-P whereas the base attained a low intracellular steady-state concentration and accumulated in the medium due to exit transport. Other nucleosides, dipyridamole and nitrobenzylthioinosine, specifically inhibited the transport of uridine and inosine, and depressed the intracellular accumulation of ribose-1-P and the formation of base commensurate with that inhibition. The data indicate that the metabolism of inosine and uridine by the various cell lines can be entirely accounted for by the facilitated transport of unmodified nucleoside into the cell followed by intracellular phosphorolysis.

Stereospecific D-glucose transport in mixed membrane and plasma membrane vesicles derived from cultured chick embryo fibroblasts

Mixed membrane vesicles prepared from cultured chick embryo fibroblasts possess a stereospecific D-glucose transport system, the properties of which are identical to those of the system in intact cells. Uptake of D-glucose proceeds without chemical alteration. The rate of stereospecific uptake of D-glucose into the mixed vesicles is 70% greater than that of the homogenate and uptake is directly proportional to membrane protein concentration. Stereospecific D-glucose uptake appears linear for 0.3 min, reaches a maximum at 2--5 min, and declines to zero by 5 h as L-glucose enters the vesicles. Uptake is osmotically sensitive and inhibited by cytochalasin B (Ki = 0.13 microM) and the structural analogues of D-glucose : D-mannose, 2-deoxy-D-glucose, 3-O-methyl-D-glucose, D-galactose and maltose, but not by sucrose of L-glucose. Uphill counterflow can be demonstrated and the apparent activation energy displays a transition from 47.7 kcal/mol below 11 degrees C to 18.1 kcal/mol above 11 degrees C. Stereospecific uptake rates of mixed vesicles prepared from Rous sarcoma virus-transformed cells are increased 30% over control values, and are increased 66% in vesicles derived from cells incubated for 24 h in glucose-free medium. Plasma membrane vesicles prepared from these cells by a dextran cushion centrifugation procedure display a 9-fold increase in the specific activity of stereospecific D-glucose uptake relative to the homogenate. Extraction of these membranes with dimethylmaleic anhydride (5 mg/mg protein) results in substantial or complete removal of major polypeptides of molecular weight 40 000, 55 000, 75 000, 78 000 and 200 000 with no loss in total uptake activity. Following extraction, major polypeptides of molecular weight 28 000, 33 000 and 68 000 remain in the membrane residue.

Hexose and amino acid transport by chicken embryo fibroblasts infected with temperature-sensitive mutant of Rous sarcoma virus. Comparison of transport properties of whole cells and membrane vesicles

The effect of transformation on hexose and amino acid transport has been studied using whole cells and membrane vesicles of chicken embryo fibroblasts infected with the temperature-sensitive mutant of the Rous sarcoma virus, TS-68. In whole cells, TS-68-infected chicken embryo fibroblasts cultured at the permissive temperature (37 degrees C) had a 2-fold higher rate of 2-deoxy-D-glucose uptake than the same cells cultured at the non-permissive temperature (41 degrees C). However, both the non-transformed and transformed cells had comparable rates of alpha-aminoisobutyric acid transport. Membrane vesicles, isolated from TS-68-infected chicken embryo fibroblasts cultured at 41 degrees C or 37 degrees C, displayed carrier-mediated, intravesicular uptake of D-glucose and alpha-aminoisobutyric acid. Membrane vesicles from TS-68-infected chicken embryo fibroblasts cultured at 37 degrees C had an approx. 50% greater initial rate of stereospecific hexose uptake than the membrane vesicles from fibroblasts cultured at 41 degrees C. The two types of membrane vesicle had similar uptake rates of alpha-aminoisobutyric acid. The results of hexose and amino acid uptake by the membrane vesicles correlated well with those observed with the whole cells. Km values for stereospecific D-glucose uptake by the membrane vesicles from TS-68-infected chicken embryo fibroblasts cultured at 41 and 37 degrees C were similar, but the V value was greater for the membrane vesicles from TS-68-infected cells cultured at 37 degrees C. Cytochalasin B competitively inhibited stereospecific hexose uptake in both types of membrane vesicle. These findings suggest that the membrane vesicles retained many of the features of hexose and amino acid transport observed in whole cells, and that the increased rate of hexose transport seen in the virally-transformed chicken embryo fibroblasts was due to an increase in the number or availability of hexose carriers.

Is inosine the physiological energy source of pig erythrocytes?

Pig erythrocytes are unable to metabolize glucose and their physiological energy source is unknown. These cells have a high-capacity nucleoside transport system with similar properties to that responsible for nucleoside transport in other species. Nucleoside transport is sufficiently rapid to allow the possibility that inosine and/or adenosine may represent major energy substrates for pig erythrocytes in vivo. Normal and adenosine deaminase-deficient pig erythrocytes have similar ATP levels, suggesting that adenosine is not important in this respect. However, it was calculated that an extracellular inosine concentration of only 40 nM could support the cells' entire energy requirement, a value 40-fold lower than plasma levels of this nucleoside.

Concanavalin A induces an intraluminal alkalinization of thymocyte membrane vesicles

Weak acid distribution methods demonstrate that mitogenic levels of concanavalin A induce an intravesicular alkalinization of isolated thymocyte membrane vesicles. Experiments with chemical reagents that crosslink the high affinity concanavalin A receptor and extensive correlation with known cellular events suggest that a "membrane Bohr effect" may participate in the initiation of mitogenesis.

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.

Ultrastructure of blebbing phenomenon and phagocytosis of blebs in laryngeal carcinoma

The epithelial-connective tissue junction of invasive squamous cell laryngeal carcinoma was examined electron microscopically. The pleomorphic cytoplasmic protrusions known as blebs were observed on the lateral and basal surfaces of malignant keratinocytes. These blebs were pinched off from the malignant epithelial cells and were then observed in the connective tissue or in the intercellular spaces. After the pinching-off process some blebs were seen to be closely surrounded by pseudopods of phagocytizing histiocytic cells in the lamina propria or by adjacent malignant keratinocytes in the intercellular spaces. Since blebs are believed to be intact parts of viable cells this engulfment can be interpreted as cytophagocytosis. During cytophagocytosis the zeiotic blebs exhibited varying degrees of enzymatic digestion. The process of blebbing was discussed in respect to cellular locomotion of malignant keratinocytes during tumor invasion. This phenomenon seems to occur only in the preliminary stages of malignant tumor growth in the larynx.

Thymidine transport in cultured mammalian cells. Kinetic analysis, temperature dependence and specificity of the transport system

The transport of thymidine has been characterized kinetically and thermodynamically in Novikoff rat hepatoma cells grown in culture and, less extensively, in mouse L cells, Chinese hamster ovary cells, P388 murine leukemia cells and HeLa cells. That the characterizations pertained to the transport system per se was ensured, (i) by employing recently developed methods for rapid sampling of cell/substrate mixtures in order to follow isotope movements within a few seconds after initial exposure of cells to substrate; (ii) by utilizing cells rendered, by genetic or chemical means, incapable of metabolizing thymidine; and (iii) by demonstrating conformity of the transport data to an integrated rate equation derived for a simple, carrier-mediated system. The results indicate that thymidine is transported into mammalian cells by a functionally symmetrical, non-concentrative system for which the carrier : substrate dissociation constant ranges from about 100 microM in Chinese hamster ovary cells, to 230 microM in Novikoff hepatoma cells. In all cell lines investigated, the velocity of transport was sufficient to nearly completely equilibrate low concentration of thymidine across the membrane membrane within 15 s. Temperature dependence of transport velocity and substrate : carrier dissociation were continuous (EA = 18.3 kcal/mol, delta H0' = 9.3 kcal/mol, respectively), and showed no evidence of abrupt transitions. Several natural and artificial nucleosides and nucleic acid bases inhibited influx of radiolabeled thymidine, apparently by competing with thymidine for the transport carrier.

Plasma membrane vesicles isolated from epimastigote forms of Trypanosoma cruzi

Membrane vesicles can be obtained from epimastigote forms of Trypansoma cruzi by incubating cells with either cross-linking reagents or acid pH. Acetate, phtalate or citrate, at pH 4.0, but not at higher pH values, were able to induce plasma membrane vesiculation. Vesicles have been purified by sucrose density centrifugation and their membrane origin was demonstrated by the following criteria: (a) Vesicles are 5--10 times richer in protein-bound iodine when they are prepared from cells previously labeled with 131I by the lactoperoxidase catalyzed reaction. (b) Electron microscopy of vesiculating cells shows physical continuity between cell plasma membrane and vesicle membrane. (c) Antibodies prepared against purified vesicles are able to agglutinate epimastigote forms of T. cruzi with sera dilutions up to 1 : 256 to 1 : 512. (d) Freeze-fracture studies of the purified vesicles have shown images of faces P and E compatible with known images of the intact cell plasma membrane. Typical preparations of acetate vesicles present the following characteristics: total carbohydrate : protein=1.5--2.0; orcinol : protein-0.07 and absence of diphenylamine reaction. Vesicles contain 0.2--0.5% and 0.3--1.0% of the total homogenate protein and carbohydrate, respectively. The presence of 10 major protein bands and 30--50-fold enrichment of the four sugar-containing macromolecules present in epimastigote forms of T. cruzi have been demonstrated in these preparations.

Cholesterol levels and plasma membrane fluidity in 3T3 and SV101-3T3 cells

Polyene antibiotics such as filipin selectively inhibit wheat germ agglutinin-induced agglutination of transformed and malignant cells compared to normal cells (Hatten ME, Burger MM: Biochemistry 18: 739, 1979). Since filipin binds specifically to cholesterol, we measured cholesterol levels in 3T3 cells and SV101-3T3 cells. SV101-3T3 cells contained 50-100% more cholesterol per cell than 3T3 cells. Both cell types were starved for cholesterol by growth in lipid-depleted medium plus 25-hydroxycholesterol. The cholesterol level of SV101-3T3 cells decreased by 30-50%, while the level in 3T3 cells remained constant. Filipin-stained SV101-3T3 cells revealed bright patches of filipin under fluorescence microscopy. These patches were absent in 3T3 cells and in SV101-3T3 and 3T3 cells starved for cholesterol. We selectively labeled plasma membranes of these cells with a spin label analog of phosphatidylcholine. The spin label indicated differences in plasma membrane fluidity that may be related to the different cholesterol levels in 3T3 and SV101-3T3 cells.

Nucleoside transport in mammalian cell membranes. III. Kinetic and chemical modification studies of cytosine-arabinoside and uridine transport in hamster cells in culture

Transport of the nucleoside analog cytosine-arabinoside (CAR) in transformed hamster cells in culture has been studied in conditions of minimal metabolic conversion. Uptake (zero-trans in) properties at 20 degrees C over a limited range of CAR concentrations were characterized by a Km of 350 micrometer and a maximal velocity (V) of 780 micrometer.min-1 (V/Km = 2.28 min-1). Equilibrium exhcange at 20 degrees C over a wider range of concentrations was best described by a saturable component with a Km of 500 micrometer and a v of 1230 micrometer.min-1 (V/Km = 2.26 min-1) and either a saturable component of high Km or a nonsaturable component of k = 0.3 min-1. For the saturable component, the v/Km values were similar in both procedures. CAR transport was inhibited by various metabolizable nucleosides. Uptake of some of these nucleosides was inhibited by CAR. CAR transport and uridine uptake were inhibited in a reversible but partially competitive fashion by high affinity probes like S-(p-nitrobenzyl-6-mercaptoinosine (NBMI) (Ki less than 0.5 nM) and in an irreversible fashion by SH reagents such as N-ethylmaleiimide (NEM). The organomercurial p-hydroxymercuribenzene sulfonate (pMBS) markedly stimulated transport of these nucleosides, but also markedly potentiated the inhibitory effects of either NBMI or NEM. The effects are interpreted either in terms of models which invoke allosteric properties or in terms of two transport systems which display distinct chemical susceptibilities to externally added probes.

Reconstitution of neutral amino acid transport systems from Ehrlich ascites tumor cells

Amino acid transport systems for alanine and leucine were reconstituted into artificial lipid vesicles. Purified plasma membrane vesicles from Ehrlich ascites cells were dissolved in 2% sodium cholate, 1mM dithiothreitol, and 0.5 mM EDTA a mixture that solubilized approximately 50% of the membrane protein. This solubilized protein fraction was further purified by a combination of ammonium sulfate precipitations, gel filtration, and DEAE-cellulose chromatography. A fraction containing approximately 15 Coomassie blue-staining bands on sodim dodecyl sulfate gels was obtained. This material was reconstituted into liposomes, and preliminary results demonstrated transport of alanine and leucine dependent on a sodium gradient. In addition, an electrogenic gradient mediated by valinomycin-induced potassium diffusion seemed to stimulate alanine uptake further.

Reconstitution of neutral amino acid transport system from Ehrlich ascites tumor cells

Amino acid transport systems for alanine and leucine have been reconstituted into artificial lipid vesicles. Purified plasma membrane vesicles from Ehrlich ascites cells were dissolved in 2% sodium cholate, 1 mM dithiothreitol, 0.5 mM EDTA, a mixture which solubilized approximately 50% of the membrane protein. This solubilized protein fraction was further purified by a combination of ammonium sulfate precipitations, gel filtration, and DEAE-cellulose chromatography. A fraction containing approximately 15 Coomassie blue staining bands on sodium dodecyl sulfate gels was obtained. This material was reconstituted into liposomes, and preliminary results demonstrated transport of alanine and leucine dependent on a sodium gradient. In addition, an electrogenic gradient mediated by valinomycin-induced potassium diffusion seemed to stimulate alanine uptake further.

Pathways of nucleotide biosynthesis in Mycoplasma mycoides subsp. mycoides

By measuring the specific activity of nucleotides isolated from ribonucleic acid after the incorporation of (14)C-labeled precursors under various conditions of growth, we have defined the major pathways of ribonucleotide synthesis in Mycoplasma mycoides subsp. mycoides. M. mycoides did not possess pathways for the de novo synthesis of nucleotides but was capable of interconversion of nucleotides. Thus, uracil provided the requirement for both pyrimidine ribonucleotides. Thymine is also required, suggesting that the methylation step is unavailable. No use was made of cytosine. Uridine was rapidly degraded to uracil. Cytidine competed effectively with uracil to provide most of the cytidine nucleotide and also provided an appreciable proportion of uridine nucleotide. In keeping with these results, there was a slow deamination of cytidine to uridine with further degradation to uracil in cultures of M. mycoides. Guanine was capable of meeting the full requirement of the organism for purine nucleotide, presumably by conversion of guanosine 5'-monophosphate to adenosine 5'-monophosphate via the intermediate inosine 5'-monophosphate. When available with guanine, adenine effectively gave a complete provision of adenine nucleotide, whereas hypoxanthine gave a partial provision. Neither adenine nor hypoxanthine was able to act as a precursor for the synthesis of guanine nucleotide. Exogenous guanosine, inosine, and adenosine underwent rapid cleavage to the corresponding bases and so show a pattern of utilization similar to that of the latter.

Reconstitution of neutral amino acid transport from partially purified membrane components from Ehrlich ascites tumor cells

Solubilized protein fractions have been obtained from plasma membranes of Ehrlich ascites cells either by extraction with 0.5% Triton X-100 or by extraction with 2% cholate. Partial purification of the solubilized protein fraction has been obtained by utilizing a combination of ammonium sulfate precipitation and column chromatography. Leucine-binding activity has been detected in the Triton X-100 solubilized membrane fraction. The leucine-binding activity was measured by equilibrium dialysis and was saturable with high levels of leucine or phenylalanine and is not strongly effected by alanine. These properties are similar to those previously identified as System L. In addition, the cholate extracted protein fraction was partially purified and reconstituted into liposomes. Sodium dependent uptake of alanine and leucine could be demonstrated in the reconstituted vesicles. Concentrative uptake was dependent upon a sodium gradient. A membrane potential produced by valinomycin mediated potassium diffusion in the presence of sodium also stimulated amino acid transport in reconstituted liposomes.

Nucleoside transport systems in Escherichia coli K12: specificity and regulation

Two nucleoside transport systems have been verified and separated by mating and recombination experiments. The recipient strain was a mutant which is negative for transport of all nucleosides. The two systems differ in specificity and in regulation. One system transports pyrimidine and adenine nucleosides. It is regulated by the cytR gene. The other system transports all nucleosides and is regulated by the cytR as well as by the deoR genes. Enzyme assays performed on whole cells of strains, able or unable to transport nucleosides, indicate that the nucleoside catabolizing enzymes are located inside the permeability barrier of the cell.

[Amino acid transport by membrane vesicles of virally transformed and nontransformed cells: effects of sodium gradient and cell density]

Mixed membrane vesicle populations composed of plasma membrane and endoplasmic reticulum were prepared from Balb/c 3T3 and simian virus 40-transformed Balb/c 3T3 mouse fibroblasts. The initial rates of uptake of L-leucine and alpha-aminoisobutyric acid by these vesicles were stimulated by a NaCl gradient (external greater than internal). Cation specificity for stimulation of L-leucine uptake was Na+ greater than Li+ greater than K+. NaSCN was as effective as NaCl. Stimulation of uptake of both amino acids by a NaCl gradient was twice as great in vesicles from transformed as compared to non-transformed cells. The NaCl gradient produced transient accumulation of both L-leucine and alpha-aminoisobutyric acid to twice the equilibrium level in vesicles from transformed cells. No such "overshoot" was observed in vesicles from nontransformed cells. In vesicles from the contact-inhibitable Balb/c 3T3 cells, transport of alpha-aminoisobutyric acid, but non L-leucine, exhibited a density-dependent decrease in Na+ gradient induced stimulation, from 248% for sub-confluent to 109% with confluent cells. No density-related changes in uptake were noted with vesicles from the transformed cells. These studies suggest that variation in amino acid uptake associated with viral transformation may be related, at least in part, to alterations in Na+ permeability of the surface membrane.

Regulation of amino acid and glucose transport activity expressed in isolated membranes from untransformed and SV 40-transformed mouse fibroblasts

Membrane vesicles isolated from untransformed Balb/c and Swiss mouse fibroblasts and their SV 40-transformed derivatives were shown to catalyze carrier-mediated, intravesicular uptake of alpha-aminoisobutyric acid and D-glucose. Concentrative uptake of alpha-aminoisobutyric acid required the presence of a Na+-gradient (external greater than internal) and could occur independently of endogenous (Na+ + K+)ATPase activity. A K+ diffusion gradient (internal greater than external) in the presence of valinomycin, or the addition of the Na+ salt of a highly permeant anion, conditions expected to create an interior-negative membrane potential stimulated Na+-gradient-dependent uptake, suggesting this process is electrogenic. D-Glucose uptake was nonconcentrative and did not require ion gradients or metabolic conversion. Na+ gradient-dependent transport of alpha-aminoisobutyric acid was reduced both in initial rate and extent of uptake in vesicles from confluent untransformed cells and increased in those from SV 40-transformed cells, compared with activities observed in vesicles from proliferating untransformed cells. No changes in D-glucose carrier activity were observed when assayed at low glucose concentrations.

The function and activity of certain membrane enzymes when localized on- and off- the membrane

A group of enzymes known to be involved in group translocation-type transport mechanisms for the uptake of a variety of nucleotide precursors are enzymatically active both in their natural membrane milieu and in aqueous solution. The activity in aqueous solution markedly differ, however, from the enzymatic activity when the enzyme is membrane localized. The adenine phosphoribosyltransferase (PRT) of E. coli (Hochstadt-Ozer and Stadtman, 71a) is capable of carrying out an exchange reaction between the base moieties of adenine and AMP without requiring P-ribose-PP as an intermediate; the enzyme in aqueous solution requires P-ribose-PP, indicating a different reaction mechanism in the two environments. Like the adenine PRT of E. coli, the hypoxanthine PRT of Salmonella typhimurium (Jackman and Hochstadt, '76) also carried out an exchange reaction on the membrane only and also is more sensitive to a number of inhibitors in aqueous solution relative to the sensitivity when embedded in the membrane. In addition, however, the hypoxanthine PRT, while restricted to hypoxanthine as a substrate in the membrane, also accepts guanine as substrate in its soluble form. The membrane capacities reas determined in a guanine PRT deletion strain (Jackman and Hochstadt, '76). Finally, in mammalian cell lines purine nucleoside phosphorylase, which translocates the ribose moiety of inosine across the plasma membrane of mouse fibroblasts undergoes a 30-fold increase in substrate turnover number upon liberation from the membrane. These data raise two important caveats with respect to study of membrane enzymes and transport. Firstly, an enzyme once solubilized and found to differ kinetically from substrate transport in situ cannot be excluded from participating in translocations in the membrane on the basis of its activity in aqueous solution. Secondly, an enzyme which "appears" largely soluble upon cell rupture cannot be assumed to be a cycloplasmic enzyme because of majority of the solubilized activity may represent only a small fraction of the enzyme molecules highly activated concomitant to their solubilization. In this latter case the ability to activate enzyme still residing on the membrane (e.g., with detergents) would be necessary in order to estimate total membrane associated activity after cell rupture.

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.

Regulation of purine utilization in bacteria. VII. Involvement of membrane-associated nucleoside phosphorylase in the uptake and the base-mediated loss of the ribose moiety of nucleosides by Salmonella typhimurium membrane vesicles

Although uridine and adenosine are converted by membrane-associated nucleoside phosphorylases to ribose-1-phosphate (ribose-1-P) and the corresponding bases (uracil and adenine), only ribose -1-P is accumulated within Salmonella typhimurium LT2 membrane vesicles. In accordance with these observations, no uptake is observed when the vesicles are incubated with the bases or nucleosides labeled in their base moieties. The vesicles lack a transport system for ribos-1-P, since excess ribose-1-P does not inhibit the uptake of the ribose moiety of uridine. In addition, there is no exchange with preaccumulatedribose-1-P. Thus, uridine, rather than ribose-1-P, must serve as the initially transported substrate. The uptake of the ribose portion of uridine is coupled to electron transport, and the levels to which ribose-1-P are accumulated may be reduced by adding various bases to the reaction mixtures. The bases appear to inhibit the uridine phosphorylase reaction and/or cause an efflux of ribose-1-P from the vesicles. This loss of ribose-1-P reflects the accumulation of nucleosides in the external medium after being synthesized within the membranes. Synthesis of the nucleosides from intravesicular ribose-1-P and exogenous base proceeds even though the bases are not accumulated by the vesicles. Furthermore, ribose-1-P cannot significantly inhibit uridine phosphorylase activity unless the membranes are disrupted. These observations indicate that the membrane-associated nucleoside phosphorylases may have a transmembranal orientation with their base and ribose-1-P binding sites on opposite sides of the membranes. Such an asymmetric arrangement of these enzymes may facilitate the uptake of the ribosyl moiety of nucleosides by a group translocation mechanism. Thus, nucleosides may be cleaved during the membrane transport process, with the resultant bases delivered to the external environment while ribose-1-P is shunted to the intravesicular space.