The influence of unfractionated heparin [average molecular weight (MW) 10,000-15,000 kD] and low-molecular-weight heparin (average MW 400-600 kD) on the plasma kallikrein-kinin and the fibrinolytic system was studied in vitro. Unfractionated heparin added to plasma gave an increase in kalli-krein-like activity with a corresponding decrease of prekalli-krein and functional kallikrein inhibition values. Plasmin-like activity was also increased, but minor changes in plasminogen and no change in antiplasmin values occurred. The changes were less pronounced with low molecular heparin compared with unfractionated heparin. The proteolytic changes were reversed by protamine sulfate but not influenced by the protease inhibitior aprotinin. Gel filtration yielded proteolytic activities able to split the plasma kallikrein substrate S-2305 and, to a minor degree, the plasmin substrate S-2251. The proteolytic activities were not due to complex formation with α2-macroglobulin. We speculate that heparin binds to prekalli-krein to form a heparin-prekallikrein complex which undergoes conformational changes and displays a kallikrein-like activity with the ability to split small synthetic substrates. Whether it is capable to split natural substrates remains unresolved.

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