Factor XII ia a single-chain beta-globulin serine protease with a molecular weight of 80,000 84,000 daltons and a plasma concentration of approximately 30 pg/mL. Proteolytic cleavage of factor XII is mediated by charged surfaces (glass, kaolin, cellite, dextran sulfate, endotoxin, urates, crude collagen, sulfatides), autoactivation, and kallikrein. Prekallikrein, factor XI, factor VII, plasmino...

We have extended earlier studies (Blood 66:204, 1985) of a mechanism of inhibition of factor VIIa/tissue factor activity that requires a plasma component (called herein extrinsic pathway inhibitor or EPI) and factor Xa. An activated peptide release assay using 3H-factor IX as a substrate was used to evaluate inhibition. Increasing the tissue factor concentration from 20% to 40% (vol/vol) overca...