Contact activation in human plasma is triggered by zinc ion modulation of factor XII (Hageman factor).

Conditions for triggering factor XII to function in the autoactivation reaction in blood coagulation in the presence of different surfaces have been studied using prekallikrein-deficient plasma. Autoactivation was recorded in a chromogenic assay by measuring the amidolytic activity of factor XIIa. The results showed that autoactivation of factor XII was achieved only after modulation of factor XII. This modulation was mediated by Zn2+ and did not require an activating surface. Following modulation, the autoactivation proceeded in the presence of a negatively charged phospholipid or sulphatide, but the sulphatide-mediated autoactivation was inhibited by Zn2+. In the presence of Zn2+ (6.6 mumol/ml plasma), the rate constant for the autoactivation following modulation was calculated to be 3.1 x 10(4) M-1 s-1 and 1.2 x 10(4) M-1 s-1 for the phospholipid and the sulphatide-mediated reactions, respectively. In the absence of Zn2+ no activation was observed in the presence of a negatively charged phospholipid. After removal of Zn2+ by EDTA, the rate constant for the sulphatide-mediated autoactivation increased to 5.7 x 10(4) M-1 s-1 in the presence and 1.0 x 10(5) M-1 s-1 in the absence of a negatively charged phospholipid (phosphatidylinositol phosphate). Dextran sulphate was not able to mediate autoactivation, in either the presence or absence of Zn2+. Aprotinin completely blocked the modulation and/or autoactivation. Soy bean trypsin inhibitor prolonged the period required for autoactivation to start, but had no effect on the autoactivation rate. The main conclusions are that Zn2+ is required to initiate contact activation, modulating factor XII for autoactivation. This modulation does not require the presence of an activating surface.