Molecular mechanisms of surface-dependent activation of Hageman factor (Factor XII).

Based on studies using 3H-DFP as an active site titrant, it is suggested that surface binding in itself does not result in the formation of new active sites in the Hageman factor molecule. Moreover, it was found that surface-binding of human and bovine Hageman factor renders these molecules 100 to 1000 times more susceptible to proteolytic activation by kallikrein, plasmin, or other proteases. Initiation of contact activation may involve proteolytic activation of surface-bound Hageman factor by a number of different proteases. Both Hageman factor and prekallikrein react with DFP like weakly active zymogens. Initiation of contact activation may also involve the expression of low intrinsic catalytic activity of these zymogens.