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 h ave been studied using prekallikrein-deficient plasma. Autoactivation was recorded in a chromogenic assay by measuring the amidolytic activi ty of factor XIIa. The results showed that autoactivation of factor XI I was achieved only after modulation of factor XII. This modulation wa s mediated by Zn- and did not require an activating surface. Following modulation, the autoactivation proceeded in the presence of a negativ ely charged phospholipid or sulphatide, but the sulphatide-mediated au toactivation was inhibited by Zn2+. In the presence of Zn2+ (6.6 mumol /ml plasma), the rate constant for the autoactivation following modula tion 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, respective ly. In the absence of Zn2+ no activation was observed in the presence of a negatively charged phospholipid. After removal of Zn2+ by EDTA, t he 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 pho sphate). Dextran sulphate was not able to mediate autoactivation, in e ither the presence or absence of Zn2+. Aprotinin completely blocked th e modulation and/or autoactivation. Soy bean trypsin inhibitor prolong ed the period required for autoactivation to start, but had no effect on the autoactivation rate. The main conclusions are that Zn2+ is requ ired to initiate contact activation, modulating factor XII for autoact ivation. This modulation does not require the presence of an activatin g surface.