Behaviour of blood proteins at the interface with procoagulant phospholipids and anticoagulant heparin or polymeric biomaterials: A fluorescence study

Changes in the fluorescence of factors IX and II in the presence of calcium at procoagulant phospholipid interfaces suggest a conformational change of the proteins upon binding. Shifts of transition temperature of phosphatidylcholine—phosphatidylserine mixtures towards those of the pure lecithin component when calcium and factors are added suggest that lateral phase separation does exist. The higher resonance-energy transfer efficiency with charged pyrene-labelled phospholipids leads to the conclusion that within the membrane, factor II and IX binding sites are domains of phosphatidylserine. Calcium-independent phospholipid binding, selective for phosphatidylserine, is well documented and is proposed to be mediated by electrostatic forces between the arginine and lysine residues of the proteins and the negative charges of the phosphatidylserine. The binding of heparin or the synthetic anticoagulant polymer PSSO 2 —Glu with antithrombin or thrombin induces different fluorescence changes in these proteins which traduce local changes in the tryptophan residues of the proteins upon binding. The interaction of heparin with various mixtures of thrombin and antithrombin indicates that heparin does not modify the structure of the preformed thrombin—antithrombin complex, as far as can be seen by spectrofluorimetry.