The separation of cells and organelles by partitioning in two-polymer aqueous phases

Experimental and Theoretical Validation of System Variables That Control the Position of Particles at the Interface of Immiscible Liquids

We construct a mathematical model describing the equilibrium flotation height of a spherical particle at the interface of immiscible liquids. The behavior of such a system depends on several experimentally measurable parameters, which include surface tensions, densities of all phases, and system scale. These parameters can be absorbed into three quantities that entirely determine the equilibrium position of the particle: the contact angle between the interface and particle, the Bond number, and the ratio of particle buoyant density to liquid phase densities-a new, dimensionless number that we introduce here. This experimentally convenient treatment allows us to make predictions that apply generally to the large parameter space of interesting systems. We find the model is in good agreement with experiments for particle size and interfacial tension spanning 3 orders of magnitude. We also consider the low interfacial tension case of aqueous two-phase systems (ATPSs) theoretically and experimentally. Such systems are more sensitive to changes in density than higher-tension aqueous/organic two-phase systems; we experimentally demonstrate that a millimeter-sized bead in an ATPS can be controllably positioned with between 5.9 and 95.1% of its surface area exposed to the bottom phase, whereas the same bead in an aqueous/organic system is limited to a range of 18.2-61.6%. Finally, we discuss the potential for wettability-based control for micron length-scale particles, which are not sensitive to changes in density. Our results can be used to simply define the experimentally controllable parameters that affect the equilibrium position and the length scales of a particle over which such parameters can be effectively tuned. A complete understanding of these properties is important for a number of applications including colloidal self-assembly and chemical patterning (e.g., formation of desymmetrized or Janus particles). By considering ATPSs, we broaden the potential uses to biological applications such as cell separation and interfacial tissue assembly.

A one-pot, isothermal DNA sample preparation and amplification platform utilizing aqueous two-phase systems

Infectious diseases remain one of the major causes of death worldwide in developing countries. While screening via conventional polymerase chain reaction (PCR) is the gold standard in laboratory testing, its limited applications at the point-of-care have prompted the development of more portable nucleic acid detection systems. These include isothermal DNA amplification techniques, which are less equipment-intensive than PCR. Unfortunately, these techniques still require extensive sample preparation, limiting user accessibility. In this study, we introduce a novel system that combines thermophilic helicase-dependent amplification (tHDA) with a Triton X-100 micellar aqueous two-phase system (ATPS) to achieve cell lysis, lysate processing, and enhanced nucleic acid amplification in a simple, one-step process. The combined one-pot system was able to amplify and detect a target gene from whole-cell samples containing as low as 10² cfu/mL, and is the first known application of ATPSs to isothermal DNA amplification. This system's ease-of-use and sensitivity underlie its potential as a point-of-care diagnostic platform to detect for infectious diseases. Graphical abstract ᅟ.

Enrichment and Recovery of Mammalian Cells from Contaminated Cultures Using Aqueous Two-Phase Systems

This Article describes a density-based method for removing contaminants, including microorganisms and nonviable cells, from mammalian cell cultures using an aqueous two-phase system (ATPS). The properties of a 7% w/w polyethylene glycol (PEG)-11% w/w Ficoll ATPS can be tuned to prepare a biocompatible system that removes contaminants with little to no adverse effects on the viability or growth of the cultured cells after treatment. This system can be used to enrich cell culture populations for viable cells and to reduce the number of microorganism contaminants in a culture, which increases the chances of subsequent antibiotic treatments being successful. We test the effectiveness of our method in model contaminated cultures of both adherent (HeLa) and suspension (HL-60 II) mammalian cells contaminated with bacteria (E. coli) and yeast (S. cerevisiae). An average of 70.2 ± 4.6% of HeLa cells added to the system are subsequently recovered, and 55.9 ± 2.1% of HL-60 II cells are recovered. After sedimenting to the interface of the ATPS, these cells have an average viability of 98.0 ± 0.2% and 95.3 ± 2.2%, respectively. By removing unwanted cells, desired cell populations can be recovered, and cultures that would otherwise need to be discarded can continue to be used.

Diagnosis of iron deficiency anemia using density-based fractionation of red blood cells

Iron deficiency anemia (IDA) is a nutritional disorder that impacts over one billion people worldwide; it may cause permanent cognitive impairment in children, fatigue in adults, and suboptimal outcomes in pregnancy. IDA can be diagnosed by detection of red blood cells (RBCs) that are characteristically small (microcytic) and deficient in hemoglobin (hypochromic), typically by examining the results of a complete blood count performed by a hematology analyzer. These instruments are expensive, not portable, and require trained personnel; they are, therefore, unavailable in many low-resource settings. This paper describes a low-cost and rapid method to diagnose IDA using aqueous multiphase systems (AMPS)-thermodynamically stable mixtures of biocompatible polymers and salt that spontaneously form discrete layers having sharp steps in density. AMPS are preloaded into a microhematocrit tube and used with a drop of blood from a fingerstick. After only two minutes in a low-cost centrifuge, the tests (n = 152) were read by eye with a sensitivity of 84% (72-93%) and a specificity of 78% (68-86%), corresponding to an area under the curve (AUC) of 0.89. The AMPS test outperforms diagnosis by hemoglobin alone (AUC = 0.73) and is comparable to methods used in clinics like reticulocyte hemoglobin concentration (AUC = 0.91). Standard machine learning tools were used to analyze images of the resulting tests captured by a standard desktop scanner to 1) slightly improve diagnosis of IDA-sensitivity of 90% (83-96%) and a specificity of 77% (64-87%), and 2) predict several important red blood cell parameters, such as mean corpuscular hemoglobin concentration. These results suggest that the use of AMPS combined with machine learning provides an approach to developing point-of-care hematology.

Separation of [(99m)Tc]pertechnetate and molybdate using polyethylene glycol coated C18 and C30 resins

Hydrophobic adsorbents such as C18 and C30 were coated with PEG and subsequently used for the separation of Mo/Tc. The most effective resin for adsorbing PEG was the C18-U resin, which demonstrated a coating capacity of 97.6±2.8mg PEG per g of resin. The ability to adsorb pertechnetate was proportional to the amount of PEG coated on the hydrophobic resin. The [(99m)Tc]pertechnetate recovery during the separation of cyclotron produced (99m)Tc from (100)Mo was 91.8±0.3% (n=2). The resultant product met relevant USP monograph specifications.

Enrichment of reticulocytes from whole blood using aqueous multiphase systems of polymers

This paper demonstrates the enrichment of reticulocytes by centrifuging whole blood through aqueous multiphase systems (AMPSs)-immiscible phases of solutions of polymers that form step-gradients in density. The interfaces of an AMPS concentrate cells; this concentration facilitates the extraction of blood enriched for reticulocytes. AMPS enrich reticulocytes from blood from both healthy and hemochromatosis donors. Varying the osmolality and density of the phases of AMPS provides different levels of enrichment and yield of reticulocytes. A maximum enrichment of reticulocytemia of 64 ± 3% was obtained from donors with hemochromatosis. When used on peripheral blood from normal donors, AMPS can provide a higher yield of enriched reticulocytes and a higher proportion of reticulocytes expressing CD71 than differential centrifugation followed by centrifugation over Percoll. Blood enriched for reticulocytes by AMPS could be useful for research on malaria. Several species of malaria parasites show a preference to invade young erythrocytes and reticulocytes; this preference complicates in vitro cultivation of these species in human blood. Plasmodium knowlesi malaria parasites invade normal human blood enriched for reticulocytes by AMPSs at a rate 2.2 times greater (P < 0.01) than they invade unenriched blood. Parasite invasion in normal blood enriched by AMPS was 1.8 times greater (P < 0.05) than in blood enriched to a similar reticulocytemia by differential centrifugation followed by centrifugation over Percoll. The enrichment of reticulocytes that are invaded by malaria parasites demonstrates that AMPSs can provide a label-free method to enrich cells for biological research.

Protein partitioning in two-phase aqueous polymer systems

Theories of protein partitioning in two-phase polymer systems which account for the effects of different aspects of system composition-such as the choice of materials, protein size, polymer molecular weight, polymer concentration, salt concentration, and affinity ligands-are reviewed. Although the present models provide some information about specific aspects of partitioning, a comprehensive and fundamental theory which can be used to predict protein partitioning behavior has not yet been developed. Some recommendations for future work are given.

Aqueous multiphase systems of polymers and surfactants provide self-assembling step-gradients in density

This Communication demonstrates the generation of over 300 phase-separated systems-ranging from two to six phases-from mixtures of aqueous solutions of polymers and surfactants. These aqueous multiphase systems (MuPSs) form self-assembling, thermodynamically stable step-gradients in density using a common solvent, water. The steps in density between phases of a MuPS can be very small (Δρ ≈ 0.001 g/cm(3)), do not change over time, and can be tuned by the addition of co-solutes. We use two sets of similar objects, glass beads and pellets of different formulations of Nylon, to demonstrate the ability of MuPSs to separate mixtures of objects by differences in density. The stable interfaces between phases facilitate the convenient collection of species after separation. These results suggest that the stable, sharp step-gradients in density provided by MuPSs can enable new classes of fractionations and separations based on density.

Effects of subinhibitory concentrations of nitroxoline on the surface properties of Escherichia coli

Nitroxoline (5-nitro-8-quinolinol; NIQ) at subinhibitory concentrations (sub-MIC) decreased the adherence of uropathogenic Escherichia coli to catheter surface and significantly enhanced cell surface hydrophobicity. The surface hydrophobicity increased in the presence of sub-MIC of NIQ and also in an excess of Mg2+. The effect of NIQ on the cell surface was not related to the bacteriostatic effect of this agent. The increase in nitrogen and decrease in phosphate content in the cell surface was found in the presence of NIQ. NIQ did not inhibit the expression of fimbriae.

In vivo antitumor activity of cis-bis-neodecanoato-trans-R,R-1, 2-diaminocyclohexane platinum(II) formulated in long-circulating liposomes

A lipophilic cisplatin derivative, cis-bis-neodecanoato-trans- R,R-1,2-diaminocyclohexane platinum (II) (NDDP), was formulated in liposomes composed of phosphatidylcholine (PC) and cholesterol (Chol) additionally containing monosialoganglioside (Gm1) or polyethyleneglycol conjugated to phosphatidylethanolamine (PEG-PE). These NDDP-containing long-circulating liposomes were examined for in vivo antitumor activity using the mouse RIF-1 solid tumor as a target residing outside the reticuloendothelial system (RES). Biodistribution studies, using C3H/HeJ mice and 111In-labelled DTPA-SA as a lipid marker, showed that the activity of GM1 and PEG-PE in prolonging the circulation times of liposomes was preserved in the presence of 3.0 mol% of NDDP in the liposome membranes. The high levels of liposomes remaining in the blood for PC/Chol/GM1 and PC/Chol/PEG3000-PE liposomes were associated with high levels of platinum in the blood as determined by atomic absorption spectrophotometry. These NDDP-containing long-circulating liposomes showed approximately a three-fold increase in tumor accumulation as compared to the conventional PC/Chol liposomes. In vitro cytotoxicity studies using RIF-1 tumor cells showed that the presence of PEG-PE, but not Gm1, significantly enhanced the cytotoxicity of liposomal NDDP. RIF-1 tumor-bearing C3H/HeJ mice were treated twice with 25 mg/kp NDDP in various liposomal formulations on days 12 and 16 after tumor cell inoculation. A significant reduction in the tumor growth rate was observed when NDDP was formulated in PC/Chol/PEG3000-PE liposomes which support both efficient tumor accumulation and enhanced cytotoxicity of liposomal NDDP. On the other hand, NDDP formulated in PC/Chol/GM1 liposomes, which display only a high tumor accumulation, had no effect on the tumor growth rate. Furthermore, NDDP formulated in dimyristoylphosphatidylglycerol (DMPG)-containing liposomes, exhibiting in vitro cytotoxicity comparable to NDDP formulated in PC/Chol/PEG3000-PE liposomes, but showing poor tumor accumulation, was also not effective. These results indicate a potential effectiveness of NDDP formulated in PEG-PE-containing liposomes for therapy of tumors in non-RES organs.

Immunotargeting of liposomes containing lipophilic antitumor prodrugs

Potential therapeutic applications of recently developed liposomes with a reduced affinity to the reticuloendothelial systems and a prolonged circulation time as targeting systems for lipophilic prodrugs were examined. In these studies, liposomes composed of phosphatidylcholine and cholesterol, additionally containing monosialoganglioside (GM1) or polyethylene glycol conjugated to phosphatidylethanolamine (PEG-PE), were used. Three antitumor lipophilic prodrugs, N-trifluoroacetyl-adriamycin-14-valerate (AD32), araC-diphosphate-diglyceride (araCdPdG), and 3',5'-o-dipalmitoyl-5-fluoro-2'-deoxyuridine (dpFUdR), were used to examine the effect of lipophilic prodrug incorporation into long-circulating liposomes and immunoliposomes on their biodistribution in mouse. Biodistribution studies with antibody-free liposomes containing lipophilic prodrugs showed that the activities of GM1 or PEG2000-PE in prolonging the circulation time of liposomes appeared to be preserved in the presence of each of the three lipophilic prodrugs at a drug/lipid molar ratio of 3:97. The effect of lipophilic prodrug incorporation on target binding of immunoliposomes was then examined using a mouse model. Incorporation of AD232 or dpFUdR into immunoliposomes, directed to the normal endothelium, did not affect the targetability of immunoliposomes, suggesting a potential effectiveness of these lipophilic prodrug-containing immunoliposomes in therapy for lung tumors. On the contrary, incorporation of araCdPdG resulted in significantly reduced target binding of immunoliposomes by yet unknown mechanism(s).

Erythrocyte partitioning in dextran-poly(ethylene glycol) aqueous phase systems. Events in phase and cell separation

Early events in the partitioning process which involve characteristic kinetics of cell- and phase-specific interactions and phase separation have been described previously. This paper reports on red cell-phase droplet interactions pertaining at the time of usual phase sampling (i.e., the time at which a clear bulk interface is first apparent) and beyond in cell partitioning and countercurrent distribution experiments. In non-charge-sensitive phase systems close to the critical point, cells can be free or attached to phase droplets. Cells that are free are virtually completely in the top phase, whereas different cell populations that show essentially complete binding to droplets can nevertheless have different partition ratios and be separated, thus reflecting the effects of the difference in the cells' avidity for the phase droplets during the early, elapsed events in partitioning. At higher polymer concentrations (i.e., higher interfacial tensions), the cell populations, completely bound to phase droplets, partition completely to the interface, and consequently cannot be separated. When such systems are made charge-sensitive by the generation of a Donnan potential between the phases or made into affinity systems by the incorporation of PEG ligands (e.g., PEG-palmitate), there is a decrease in the avidity of the cells for phase droplets. The resulting increase in the ratio of free to droplet-bound red cells in the top phase at the time of sampling correlates with an increase in the partition ratio, P, observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat bone marrow erythroid cell fractionation by counter current distribution in non-charge-sensitive two-phase systems

Counter-current distribution in non charge-sensitive aqueous poly(ethylene glycol)-dextran two phase systems allows the fractionation of rat bone marrow cells into two broad cell subpopulations with different distribution coefficients in a relatively short time. Morphological identification and enzymatic studies suggest that erythroid cells are mainly present in the subpopulation with the higher distribution coefficient. The distribution coefficient and, therefore, surface hydrophobicity of these cells, apparently increase in parallel with an increase in their degree of differentiation and maturation.

Affinity-mediated modification of electrical charge on a cell surface: a new approach to the affinity partitioning of biological particles

Polylysine has been covalently bound to human transferrin in a 1:1 molar ratio over a disulfide bond that can be easily split by reducing agents such as dithiothreitol. The association constant for the binding of the transferrin-polylysine derivative to transferrin receptors present on rat erythroblasts and the number of binding sites were identical to the corresponding values found for native transferrin. The incubation of the cells with transferrin-polylysine affected the partitioning of erythroblasts in a charge-sensitive aqueous two-phase system containing Dextran and polyethylene glycol. The polylysine part introduced a nonspecific influence on the partitioning that could be eliminated by preincubation of the cells with an excess of sialic acid. The partition ratio, G, of the erythroblasts changed with a factor of 1.9 for each set of 100,000 polylysine chains attached per cell.

Changes in glycolytic enzyme activities in aging erythrocytes fractionated by counter-current distribution in aqueous polymer two-phase systems

Human and rat erythrocytes were fractionated by counter-current distribution in charge-sensitive dextran/poly(ethylene glycol) two-phase systems. The specific activities of the key glycolytic enzymes (hexokinase, phosphofructokinase and pyruvate kinase) declined along the distribution profiles, although the relative positions of the activity profiles were reversed in the two species. These enzymes maintained their normal response to specific regulatory effectors in all cell fractions. No variations were observed for phosphoglycerate kinase and bisphosphoglycerate mutase activities. Some correlations between enzyme activities (pyruvate kinase/hexokinase, pyruvate kinase/phosphofructokinase, pyruvate kinase/pyruvate kinase plus phosphoglycerate kinase, pyruvate kinase/bisphosphoglycerate mutase and phosphoglycerate kinase/bisphosphoglycerate mutase ratios) were studied in whole erythrocyte populations as well as in cell fractions. These results strongly support the fractionation of human erythrocytes according to cell age, as occurs with rat erythrocytes.

Partition of isomeric dipeptides in poly(ethylene glycol)/magnesium sulfate aqueous two-phase systems

A series of isomeric dipeptides, i.e., those containing identical residues but in different order such as Trp-Gly versus Gly-Trp, was partitioned in a poly(ethylene glycol) (PEG)/magnesium sulfate (MgSO4) aqueous two-phase system. Dipeptides having a more hydrophobic character favored the upper (PEG) phase. Moreover, the partition coefficients for isomeric dipeptides are different, with the partition coefficients for dipeptides containing the more hydrophobic residue in the C-terminal position being, in general, greater than the partition coefficients for corresponding isomers which contain the more hydrophobic residue in the N-terminal position. These observations can be attributed to the different interactions that the isomers have with specific two-phase systems.

Influence of surface hydrophilicity of liposomes on their interaction with plasma protein and clearance from the circulation: studies with poly(ethylene glycol)-coated vesicles

Well-defined liposome systems have previously established the influence of size, surface charge lipid composition and surface ligands, on in vivo fate and behaviour of model compounds entrapped in liposomes. In the present study, preformed liposomes which quantitatively retain aqueous markers were covalenty coupled via dipalmitoylphosphatidyl-ethanolamine, to the hydrophilic polymer, monomethoxypoly(ethylene glycol) (MPEG 5000). Such liposomes retain the coating in the presence of plasma, and appear to adsorb plasma components more slowly than liposomes without the polymer, shown using an aqueous two-phase partitioning technique. MPEG-coupled liposomes were cleared from the blood circulation up to 30% more slowly than liposomes without MPEG after intravenous administration to mice, despite the unmodified liposomes being of a composition and size shown previously to favour achievement of maximum half-life. It is suggested that the polymer acts as a surface barrier to plasma factors which otherwise bind to liposomes in the blood and accelerate vesicle removal.

Settling-time dependence of rat bone marrow cell partition and counter-current distribution in charge-sensitive aqueous two-phase systems. Relationship with the cell partitioning mechanism

Differences in the settling-time dependence of single and multiple cell partitions have been found between heterogeneous (bone marrow cells) and homogeneous (erythrocytes) populations when using charge-sensitive dextran-poly(ethylene glycol) aqueous two-phase systems. The cell populations were partitioned using both single test-tube experiments and multiple thin-layer counter-current distribution. Lengthening the settling time, to favour phase separation, and decreasing the upper phase volume are more effective in fractionation by the counter-current distribution of heterogeneous cell populations than increasing the interfacial tension, although all three were employed to speed phase settling. On the basis of these results, the original cell partitioning mechanism proposed for non-charge-sensitive systems has been extended to charge-sensitive systems.

Detection of surface charge-related properties in model membrane systems by aqueous two-phase partition

Unilamellar vesicles composed of phosphatidylcholine (PC) and either phosphatidic acid (PA) or phosphatidylglycerol (PG) partition to the upper poly(ethylene glycol) (PEG)-rich phase of a charge-sensitive 5%:5% (w/w) PEG 8000/Dextran T-500 phase system containing 10 mM sodium phosphate at pH 7, consistent with the vesicles bearing a net negative charge. When prepared in the presence of a pH gradient (interior acidic), PC/PA vesicles exhibit an increased partition to the top PEG-rich phase, consistent with a redistribution of the PA from the inner to the outer monolayer of the vesicle bilayer. Conversely, when prepared in the presence of a pH gradient (interior basic), PC/PG vesicles exhibit a decreased top-phase partition, consistent with a redistribution of the PG from the outer to the inner monolayer of the vesicle bilayer. Unilamellar vesicles composed of PC and stearylamine partition to the lower dextran-rich phase of a 5%:5% (w/w) PEG 8000/Dextran T-500 phase system containing 10 mM sodium phosphate at pH 8.5, consistent with the vesicles bearing a net positive charge. When prepared in the presence of a pH gradient (interior acidic), conditions under which the stearylamine is trapped on the inner monolayer of the bilayer, the vesicles now partition predominantly to the interface in a manner similar to vesicles composed of PC alone. These results demonstrate that partitioning in aqueous two-phase polymer systems is a sensitive method for monitoring the asymmetry of charged lipids in model membrane systems and also suggests that partitioning in charge-sensitive systems depends only on the physical nature of the exterior surface of the membrane.

Aqueous two-phase polymer partitioning of lipid vesicles of defined size and composition

The kinetics of the partitioning of lipid vesicles containing acidic phospholipids in aqueous two-phase polymer systems are dependent upon the vesicle size; the larger the vesicles, the more readily they absorb to the interfaces between the two polymer phases and hence are cleared from the top phase as phase separation proceeds. The partitioning of neutral lipid vesicles is principally to the bulk interface and is the same in phase systems of both low and high electrostatic potential difference between the two phases (delta psi). The incorporation of negatively charged lipids has two effects upon partition. First, vesicles with negatively charged lipids exhibit increased bottom phase partitioning in phases of low delta psi due to an enhanced wetting of the charged lipids by the lower phase. Second, the presence of a negatively charged group on the vesicle surface results in increased partition to the interface and top phase in phase systems of high delta psi. Differences observed in the partition of vesicles containing various species of negatively charged lipid thus reflect a competition between these two opposing factors.

On the mechanism of growth of cells (Bacillus amyloliquefaciens) in the mixed aqueous two-phase system

The growth of Bacillus amyloliquefaciens in the aqueous two-phase system, made up of polyethylene glycol, dextran, and water, was investigated. Generally, Bacillus partitions in the dextran phase, but the magnitude of the separation depends largely on the overall composition of polymers in the phase system. The kinetics of growth of Bacillus amyloliquefaciens was studied in the polyethylene glycol-rich continuous phase, dextran-rich dispersed phase, and in the mixed phase. From the kinetic data it appears that increasing the overall polymer composition causes the cells to adsorp at the interface. On the other hand, partition measurements indicate that increasing polymer concentrations make the cell partitioning more one-sided. This anomaly is explained by studying the interfacial adsorption of cells via dynamic surface tension measurements.

Partitioning behaviour in aqueous two-phase systems and fractionation by counter-current distribution of chick-embryo erythrocytes with numerical resolution of distribution curves

The partition of chick-embryo and young-chick erythrocytes in dextran-poly-(ethylene glycol) two-phase systems depends on the interfacial tension and electrical potential differences between the phases. Counter-current distribution with charged 5% dextran-poly(ethylene glycol) systems has proved to be an adequate method for the separation of primitive and definitive erythrocytes present in chick embryos when a phase settling time of 20 min is used. The computer-aided numerical resolution of experimental curves has shown the existence of subpopulations which could not have been detected by using conventional methods.

Cell separation by immunoaffinity partitioning with polyethylene glycol-modified protein A in aqueous polymer two-phase systems

Previous work has shown that polyethylene glycol (PEG)-bound antibodies can be used as affinity ligands in PEG-dextran two-phase systems to provide selective partitioning of cells to the PEG-rich phase. In the present work we show that immunoaffinity partitioning can be simplified by use of PEG-modified Protein A which complexes with unmodified antibody and cells and shifts their partitioning into the PEG-rich phase, thus eliminating the need to prepare a PEG-modified antibody for each cell type. In addition, we provide a more rigorous test of the original technique with PEG-bound antibodies by showing that it is effective at shifting the partitioning of either cell type of a mixture of two cell populations.

Pyruvate kinase activity and response to allosteric effectors in rat erythrocytes and reticulocytes fractionated by multiple partitioning in aqueous two-phase systems

Specific activity of pyruvate kinase decreases as the age of rat erythrocytes increases in fractions obtained by counter-current distribution in dextran-polyethylene glycol biphasic systems; the enzyme is inhibited by ATP and activated by fructose-1,6-bisphosphate at low phosphoenol pyruvate concentrations. Specific activity does not change in fractions from greater than 95 per cent-rich reticulocytes (anaemic rats); the enzyme is inhibited by ATP but not activated by fructose-1,6-bisphosphate. These results can be explained on the basis of different pyruvate kinase isozymes and suggest that decrease in activity is not affecting regulatory properties during erythrocytes aging.

Partitioning behaviour of rat bone marrow cells in aqueous two-phase systems. Dependence of cell partition on the interfacial tension and electrical potential difference between the phases

The partitioning of heterogeneous cell populations of rat bone marrow, in comparison with that of homogeneous populations of erythrocytes, has been studied in aqueous two-phase systems containing increasing concentrations of dextran-poly-(ethylene glycol) (to increase the interfacial tension) and/or decreasing phosphate concentrations (to decrease both the electrical potential difference between the phases and the interfacial tension). At any polymer and/or phosphate concentration tested the partition is lower for bone marrow cells, reflecting their lower surface charge and hydrophobicity.

Selective partitioning of conidia of some Penicillium and Aspergillus species in aqueous two-phase systems

Conidia of Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger, Penicillium brevi-compactum, Penicillium frequentans, Penicillium spinulosum, and Penicillium verrucosum var. cyclopium were subjected to partition at varying pH values in an aqueous two-phase system containing charged polyethylene glycol. In the system, the partition behavior of the conidia of the Penicillium species varied when the pH was raised, while the conidia of the Aspergillus species seemed unaffected. P. brevi-compactum was separated from P. verrucosum var. cyclopium after only 10 transfers when subjected to stepwise partitioning. In the same way, 10 transfers were needed to separate P. verrucosum var. cyclopium from a mixture of conidia of three Aspergillus species. The partition behavior was influenced by the culture media used.

Fractionation of bone-marrow cells by counter-current distribution in aqueous polymer two-phase systems. Relation between settling time and the efficiency of separation

The fractionation of heterogeneous populations of rat and human bone-marrow cells has been studied by counter-current distribution in a charged 5% dextran-4% poly(ethylene glycol) two-phase system. The subfractionation into two broad populations has been achieved at a low top/bottom phase volume ratio by increasing (up to 20 min) the settling time allowed for the phases to be separated after each mixing step. No effect of this parameter on a homogeneous population of erythrocytes has been observed. However, heterogeneous cell separations can be improved by exploiting different phase settling times.

Subcellular fractionation of liver organelles from phenobarbital-treated rats by counter-current partition and sucrose gradient centrifugation

Counter-current partition, sucrose density gradient centrifugation and enzymic analysis were used to explore the changes in rat liver organelles induced by phenobarbital. There was a small increase in partition coefficient and marker enzyme activity of the endoplasmic reticulum. The modal density of the marker enzyme showed a significant decrease reflecting the proliferation of the smooth endoplasmic reticulum. The mitochondria showed a bimodal distribution with a small reduction of marker enzyme activity. In contrast, lysosomes and peroxisomes were relatively unaffected by phenobarbital treatment. Phenobarbital caused a small but statistically insignificant increase in gamma-glutamyl transferase activity: density gradient centrifugation studies indicated that the increased activity was predominantly in the biliary canalicular elements. In contrast, cytosolic gamma-glutamyl hydrolase activity was strikingly reduced by phenobarbital treatment.

Heterogeneity in the surface properties of B16 melanoma cells from sublines with differing metastatic potential detected via two-polymer aqueous-phase partition

When mixed in aqueous solution at low concentrations, the neutral polymers dextran and poly(ethylene glycol) (PEG) rapidly form a two-phase system, consisting of a dextran-enriched lower phase and a PEG-enriched upper phase. Two B16 mouse melanoma cell lines, B16-F1 (low lung colonizing capability) and B16-F10 (high lung colonizing capability) were found to partition differentially into the upper phase in a variety of two-phase systems. Upper-phase partition depends primarily on either hydrophilic (i.e., surface charge density) or hydrophobic (i.e., affinity for the hydrocarbon chain of a PEG-fatty acid ester) cell surface properties, depending on the system used. In single-step partition studies, cells of the B16-F10 subline displayed a greater preference than B16-F1 cells for the upper phase in the hydrophilic system and less preference in systems sensitive to hydrophobic properties. Countercurrent distribution (CCD) experiments, performed with [125I]deoxyuridine DNA-labelled cells, were consistent with single-step partition results. These CCD results demonstrated that B16-F10 cells exhibited greater DNA synthesis than B16-F1 cells and that considerable heterogeneity, in both hydrophobic and hydrophilic surface properties, was present in subpopulations of cells of both sublines. The data also showed considerable enrichment of 125I-specific cell activity in certain sections of the distributions, indicating that differences in cellular DNA synthesis are reflected in the surface properties to which partition is sensitive.

The dynamics of phase partition. A study of parameters affecting rat liver organelle partitioning in aqueous two-polymer phase systems

Separation of subcellular organelles by two-phase partition is thought to reflect differential partition of the organelles between the two phases or between one of the phases and the interface. Studies by Fisher and colleagues [Fisher & Walter (1984) Biochim. Biophys. Acta 801, 106-110] suggest that cell separation by phase partition is a dynamic process in which the partition changes with time. This is mainly due to association of the cells with sedimenting droplets of one phase in the bulk of the other. Rat liver organelle partition was studied to determine whether the same dynamic behaviour is observed. Partition was clearly time-dependent during 24 h at unit gravity, and was also affected by altering the volume ratio of the two phases and the duration of phase mixing. These results indicate that, as with cells, the partition of organelles between phases is a dynamic process, and is consistent with the demonstration that organelles adhere to the phase droplet surfaces. Optimization of the volume ratio between phases may lead to significant processing economies. Organelle sedimentation in the upper phase was significantly faster than in the isoosmotic sucrose. Theoretical modelling of apparent organelle sizes indicates that aggregation occurs in the poly(ethylene glycol)-rich upper phase. This phenomenon is likely to limit the use of this technique in organelle separations unless means can be found to decrease aggregation.

Use of a low-speed, iso-density percoll centrifugation method to increase the viability of isolated rat hepatocyte preparations

A simple yet effective method (iso-density percoll centrifugation) has been developed for consistently preparing isolated rat liver parenchymal cells with over 98% initial viability. The method has been applied to cells isolated by a variety of collagenase digestion techniques. This procedure involves the low-speed centrifugation (50 X g) of the initial cell suspension through a percoll medium having a density of 1.06 g/ml and results in the separation of single and viable parenchymal cells from cell aggregates, debris, and nonparenchymal cells. The enriched parenchymal cells have been shown to be superior to untreated cells by a number of criteria including: preparation homogeneity, cell morphology, maintenance of cytochrome P-450, hormonal responsiveness (measured by the induction of tyrosine aminotransferase after treatment with glucagon or dexamethasone, or both), plasma membrane integrity (determined by both trypan blue exclusion and leakage of glutamic-oxaloacetic transaminase), and the DNA repair capability after treatment with benzo[a]pyrene or 2-acetylaminofluorene.

Immuno-affinity partition of cells in aqueous polymer two-phase systems

Poly(ethylene glycol) (PEG) was covalently coupled to IgG antibody preparations directed against human red blood cells. This modification reduces the tendency of the antibody to agglutinate cells and increases its affinity for the upper phase in dextran-PEG aqueous two-phase systems. These effects are related to the molecular weight of the PEG used for modification and to the number of PEG molecules attached to the antibody. Exposure of human red blood cells to PEG-modified antibody causes a substantial and specific increase in cell partition into the PEG-rich phase in a number of PEG-dextran aqueous two-phase systems. Pertinent phase-system parameters were examined. Following a single incubation with PEG-derivatized antibody, a mixture of sheep and human red blood cells was completely separated in 100 min by a 30-transfer countercurrent extraction using a two phase system which normally offers little resolution.

Subcellular distribution of rat pituitary homogenates by poly(ethylene glycol)-dextran countercurrent partitioning

Rat pituitary homogenates were subjected to two phase countercurrent partition in a poly(ethylene glycol)-dextran mixture using a simple apparatus with enhanced gravity to facilitate the phase separations. Assay of the fractions for organelle marker enzymes and prolactin after 17 transfers showed similar distributions for endoplasmic reticulum, lysosomes, prolactin granules and plasma membrane at the lowest dextran concentrations. Increasing the dextran concentrations had a differential effect on the various organelles. Excellent resolution of endoplasmic reticulum from the other organelles was obtained and marked organelle heterogeneity was demonstrated. Two-phase countercurrent partition thus offers an alternative approach to the subcellular fractionation of pituitary homogenates and should prove useful in separating endoplasmic reticulum from plasma membrane and other cell components.

Demonstration of two erythrocyte populations in young chickens by counter-current distribution of 59Fe-labelled cells in dextran-poly(ethylene glycol) two-phase systems

The technique of fractionating cells by a counter-current distribution procedure in dextran-poly(ethylene glycol) biphasic systems was modified by increasing the settling time and decreasing the top/bottom two-phase volume ratio. Two sub-populations of 59Fe-labelled erythrocytes, namely definitive medullar cells and primitive embryonic cells, were present in the blood of young chicks until about day 8 of age. The primitive cells were progressively replaced by definitive red cells as age increased.