PURIFICATION OF TISSUE FACTOR PATHWAY INHIBITOR (TFPI) FROM RABBIT PLASMA AND CHARACTERIZATION OF ITS DIFFERENCES FROM TFPI ISOLATED FROM HUMAN PLASMA

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor that is thought to function as a natural anticoagulant to regulate tissue factor-induced coagulation (Proc. Natl. Acad. Sci. U.S.A. 88, 708, 1991). TFPI's mechanism of action has been well characterized as a two step reaction in which TFPI combines with factor Xa and subsequently TFPI/factor Xa combines with and effectively neutralizes factor VIIa/tissue factor. In human plasma, TFPI occurs in two major molecular weight forms of approximately 34 and approximately 40 kDa. The 40 kDa form is a heterodimer of TFPI in covalent disulfide linkage to human apolipoprotein A(II). TFPI circulates in human plasma primarily in association with the plasma lipoproteins. We have now isolated and partially characterized TFPI from rabbit plasma and find that, although functionally and immunologically related to TFPI isolated from human plasma, it differs from human TFPI in some of its physical properties. Rabbit TFPI is larger (approximately 45 kDa) and more extensively glycosylated than human TFPI, does not form mixed disulfides with other proteins in plasma, and unlike its human counterpart, does not circulate in plasma associated with lipoproteins.