Some characteristics of collagen-heparin complex

In various forms of purified collagen (powder of insoluble collagen from bovine skin, fibers from rat tail tendons, membrane from bovine gut), carboxyl groups were activated by carbodiimide to allow covalent binding of heparin. Collagen powder and collagen fibers from rat tail tendons were also incubated in a heparin solution under the same reaction conditions but without carbodiimide present to account for other forms of collagen-heparin interaction. It was found that the linkage of heparin to collagen formed in the presence of carbodiimide is stable, as heparin was minimally extractable by 0.2M buffers with a pH ranging from 5 to 9. Collagen powder incubated with heparin in the absence of carbodiimide released heparin almost completely into Tris buffer of pH 9.0. As a consequence of covalent binding of heparin to collagen, the collagen fibers became more stable as shown by their significantly reduced swelling capacity and significantly increased shrinkage temperature. Collagen fibers interacted with heparin in the absence of carbodiimide also showed some stabilization of their structure, which was, however, significantly less than with carbodiimide reaction. By two independent methods it was shown that heparin linked to collagen by a stable bond retains its anticoagulant activity. It is concluded that, in the presence of carbodiimide, heparin covalently binds to collagen thus forming an antithrombogenic surface. At the same time, collagen is crosslinked. Incubation of collagen in the solution of heparin without carbodiimide also stabilizes collagen structure, but to a significantly lesser degree. Such a linkage is unstable as heparin dissociates and is readily extractable into 0.2M Tris buffers with pH 7-9.