Membrane surface properties other than charge involved in cell separation by partition in polymer, aqueous two-phase systems

Abstract

When aqueous solutions of dextran and of poly-(ethylene glycol) are mixed above certain concentrations, immiscible, liquid two-phase systems are obtained which are useful for separating cells by partition. Some salts partition unequally between the phases, giving rise to an electrostatic potential difference between them. Partition of cells has therefore been thought to depend predominantly on membrane charge. We now report two instances in which membrane charge either does not determine or is not the main determinant of cell partition. (A) Cell partition coefficients in phase systems approaching the critical point (the component concentrations below which a homogeneous solution occurs) increase, even in phase systems in which the phase potential difference is practically zero. Furthermore, in such systems, the partition coefficient of (human) erythrocytes is not reduced by complete removal of sialic acid. (B) Rat and mouse erythrocytes have sizable partition coefficients in a phase system away from the critical point with no potential difference between the phases. Cell surface interaction with the polymers is probably responsible for cell partition in these cases. Partition studies on erythrocytes from nine mammalian species in phases near the critical point with and without electrostatic potential differences reveal major species-specific differences in the membrane charge/noncharge components. A correlation has been found, in phases near the critical point that have essentially no electrostatic potential difference, between partition coefficient and the ratio of poly/monounsaturated fatty acids in the membranes of red cells from different species. Our present results thus provide parameters for the separation of cells by partition in addition to or instead of membrane charge depending on the polymer and salt composition and concentration selected.

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

“…Partition studies on liposomes as model systems to determine the contribution of lipids to the partition of membrane particles indicate that the polar head group plays a dominant role, while the degree of unsaturation is less important (Eriksson & Albertsson, 1978). When a small amount of poly(ethylene glycol) monopalmitate is included in a non-charged phase system away from the critical point, erythrocytes partition into the top phase (Walter et al, 1976b) in a species-dependent manner which is generally similar to that observed for a non-charged phase system that is close to the critical point (Walter et al, 1976a). Since such a molecule will partition into the top phase because it is predominantly 'PEG-like', it is possible to consider (Walter, 1977) that cells which interact with the hydrophobic palmitoyl moiety will be pulled out of the interface into the top phase (hydrophobic affinity partition).…”

“…For phase systems in which the interfacial potential is very low, the partition of cells would appear to depend upon membrane factors other than charge (non-charged membrane components) which qualitatively might be referred to as cell-surface hydrophobicity (Gerson, 1980). Walter et al (1976a) found that, in 'noncharged', low-phosphate/high-NaCl phase systems close to the critical point, some species of erythrocytes (e.g. rat and mouse) showed a high partition into the top phase whereas for other species, e.g.…”

Partition behavior of cells and soluble substances in two‐polymer aqueous phase systems

“…The partition coefficients obtained in this latter case have been shown to have, at least in the case of red blood cells from different species, an excellent correlation to the ratio of their membrane poly/monounsaturated fatty acids as well as to some other membrane lipid components [5].…”

“…Furthermore, in dextran-poly(ethylene glycol) phases closer to the critical point (i.e., in systems of lower polymer concentrations) the partition coefficient of red blood cells from many species (e.g., dog, mouse, rat, rabbit) is the same in NaC1 (ionic strength 0.17) and phosphate (ionic strength 0.22) containing systems [5] clearly indicating that the 'General rule' is not only confined to a selected range of ionic concentrations (and just of phosphate and NaC1 at that), but is also restricted to a narrow range of polymer concentrations.…”

“…The physical properties of the phase systems can, to a great extent, be manipulated by the choice of polymer and ionic composition and concentration [1,3]. In the case of dextranpoly(ethylene glycol) aqueous phases it has been found that by appropriate selection of phase composition one can effect separations based predominantly on charge-associated or lipid-related properties or on biospecific affinity [4][5][6].…”