The interaction of heparin with plasma proteins. Demonstration of different binding sites for antithrombin III complexes and antithrombin III.

Affinity chromatography of plasma and serum with the use of heparin conjugated Sepharose has confirmed the existence of two types of protein binding sites. A minor heparin fraction binds free AT III selectively and firmly but not its protease complexes. The complexes bind less firmly to another, much larger heparin fraction together with a select group of the plasma proteins at physiological pH and ionic strength. These included complement proteins (C1q, C2, factor B, properdin, and beta 1H), protease inhibitors (inter-alpha-trypsin inhibitor and C3b inactivator) and cell surface proteins (protein HC and fibronectin) as well as beta-lipoproteins. The results demonstrate that affinity chromatography with heparin-Sepharose is extremely useful as an early preparative step in the isolation of these minor plasma components. The results also indicate that the said proteins can be linked to cell surfaces carrying heparinoids in the intercellular space.