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

When solutions of poly(ethylene glycol) and dextran are mixed above certain concentrations, they form two immiscible aqueous phases: a poly(ethylene glycol)-rich top phase and a dextranrich bottom phase. These solutions can be buffered and made isotonic. Cells and organelles added to such systems will partition between the two phases and the horizontal interface between them. Depending on the composition and concentration of the polymers and of salts present, partitioning of the cells appears to depend either on membrane charge or on hydrophobic properties of the cell membrane. Furthermore, when a ligand is coupled chemically to either of the polymers, the partition may be affected predominantly by the affinity of membrane receptors for the ligand; this type of separation is termed affinity partition. It is thus possible, by a suitable choice of phase composition and conditions, for the partition of cells to be determined by one of three distinct surface properties of the cells. By measuring the distribution of cells between the phases and the interface in a phase system, an index of these properties can be obtained simply and rapidly. This may then be used to follow changes in the properties of the cells that may arise following treatments with drugs or during cellular differentiation. Cell partition can thus be a useful analytical procedure. In addition, if two cell types differ markedly in their partitioning behaviour, they may readily be separated in a single partition, i.e. one type of cell may partition entirely into the top phase and the other completely into the bottom phase. It is generally necessary, however, to use multiple partitions, termed counter-current distribution, to separate the cells. Use of this technique has enabled preparative separations of cells and subcellular fractionations based on cell-surface properties to be achieved. Furthermore since the use of multiple partitions is a more sensitive method for detecting differences in cell surface properties than is a single partition, the use of counter-current distribution can give information on very subtle changes in the cell surface. Both theoretical considerations and experimental observations have demonstrated that the