Summary
We examined in purified systems and in human plasma whether heparin serves as a contact
system activating compound. Purified human factor XII zymogen was not activated by
heparin through an autoactivation mechanism, but was activated in the presence of
purified prekallikrein. Zn2+ (12 εM) did not support autoactivation by heparin. The activation of factor XII and
the contact system by heparin in plasma anticoagulated with citrate or with hirudin
(not chelating ions) was examined by the cleavage of 125I-labeled factor XII and high molecular weight kininogen (HK). Heparin at 1.6 and
16 USP U/ml was not able to produce activation, in contrast to dextran sulfate (20
εg/ml) which supported activation of both factor XII and HK. This study indicates
that heparinized plasma does not support activation of the contact system mediated
through activation of factor XII. It is not expected that heparin anticoagulant therapy
will contribute to activation of the contact system.