Bovine factor XII is a single-chain plasma protein that is involved in the early or contact phase of blood coagulation. In the reactions leading to fibrin formation, it is converted to factor XIIa, a serine protease composed of a heavy chain and a light chain held together by a disulfide bond(s). The activation of factor XII is catalyzed by plasma kallikrein which cleaves a single internal arginyl-valine peptide bond in the precursor protein. The activation reaction also requires the presence of a lipid component, such as sulfatide, ganglioside, or stearic acid. Kaolin, in the presence of high molecular weight kininogen, or dextran sulfate can provide a nonphysiological contact surface in this reaction. Other materials from bovine sources, including brain galactoceramide, sphin-gomyelin, phosphatidylcholine, cephalin, aorta proteoglycan, cornea keratan sulfate, vitreous humor hyaluronic acid, submaxillary mucin, and calf skin collagen, were inactive in the activation of factor XII by kallikrein. Factor XIaand plasmin also activated factor XII in the presence of sulfatide but were only 40 and 20% as active as kallikrein, respectively. Other clotting enzymes, including factor IXa, factor Xa, or thrombin, showed no activity in this reaction. The effect of sulfatide on reducing the clotting time of plasma suggests that some lipids, such as sulfatide, may be of importance in the initiation of the coagulation process, while others, such as phospholipids, play an important role in the intermediate phase of blood clotting.
ASJC Scopus subject areas