The interaction of platelet factor 4 with heparins of different chain length.

The interaction of platelet factor 4 with heparin of varying chain lengths has been investigated by labelling the heparin with fluorescein isothiocyanate and monitoring the change in anisotropy of fluorescence when the protein is added to a solution of the polysaccharide. The shape of the titration curve depends on the Mr of the heparin and chains of Mr greater than 10 000 showed a definite break when the concentration of polysaccharide and protein became equimolar. Evidence is presented to show that most of the fluorescein label is linked to residual serines on the heparin. Similar break-points were observed if total fluorescence or light-scattering was used to monitor the interaction. Unlabelled heparin was used for the latter method. These results together with those obtained in buffer of high ionic strength lead us to propose a model where the heparin is wrapped around the tetrameric protein.