Ligand-receptor interactions in affinity cell partitioning. Studies with transferrin covalently linked to monomethoxypoly(ethylene glycol) and rat reticulocytes.

The partitioning of rat reticulocytes in poly(ethylene glycol) (PEG)-dextran two-phase systems increases into the PEG-rich top phase when the cells are incubated with transferrin covalently modified with monomethoxy-PEG (MPEG-transferrin) prior to partitioning. Two observations support the suggestion that such an increase in top-phase partitioning is due to the specific interaction of the MPEG-transferrin conjugate with the transferrin receptor on the surface of the reticulocyte: first, the MPEG-transferrin conjugate competes with [125I]transferrin for the transferrin receptor on reticulocytes (Ka = 6.28 x 10(6) l mol-1); and second, the MPEG-modified transferrin is unable to change the partitioning of rat erythrocytes, cells lacking the transferrin receptor. This example illustrates the feasibility of manipulating the partitioning of a selected cell population when ligand-receptor interactions are exploited. The increase in the partitioning of the reticulocytes takes place within a narrow range of MPEG-transferrin bound per cell, viz., 10.2-11.3 fg per cell. The latter range corresponds to ca. 80,000-89,000 molecules of MPEG-transferrin bound per cell.