Non-specific binding of transferrin and lactoferrin to polystyrene culture tubes: role of the radioligand.

While evaluating the role of iron-binding glycoproteins on the in vitro uptake of 67Ga and 59Fe by tumor cells, it was observed that these radiometals bind to polystyrene culture tubes in the presence of transferrin or lactoferrin. The amount of 67Ga or 59Fe bound to the tube increases with glycoprotein concentrations up to 20 microgram/ml and decreases thereafter. This biphasic response is a reflection of metal-protein affinity and is greatest for 59Fe-lactoferrin. With 125I-labeled transferrin and lactoferrin, the amount of tube-bound radioactivity was inversely dependent on the glycoprotein concentration suggesting that glycoproteins bind to a limited number of binding sites on the tube wall. Approximately 10(13) glycoprotein molecules were bound per tube with an affinity constant of 1.89 X 10(7) 1/M for transferrin and 1.08 X 10(7) l/M for lactoferrin. These sites are not specific since addition of albumin inhibited the binding of radiolabeled glycoproteins to the tube. In the light of these observations, caution is required in interpreting results of cell culture experiments which have not directed attention to protein-plastic interaction.