Effects of Heparin on Proteolytic Activities in Human Plasma

Abstract

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.

The anticoagulant capacity of plasmatic unfractionated heparin decreases at 23°C

“…This contact activation might be favoured at 238C [3]. The procoagulant changes occur especially with UFH and not with LMWH [10]. Cleavage of UFH into LMWH of about 6000 Da might prevent this folding into a molecule with multiple polynegative niches.…”

Inhibition of Intrinsic Thrombin Generation

“…The usual plasmatic unfractionated heparin concentrations in therapeutic anticoagulation are about 10fold higher than the IC50 values observed here in the INCA. The superior anticoagulant capacity of LMWH compared to unfractionated heparin might be due to some contact activating (pre-kallikrein to kallikrein folding) potential of unfractionated heparin, a polynegatively charged molecule of a molecular mass > 15000 Dalton [Kongsgaard et al 1992]. In many clinical situations cells can fragment into phospholipidmicroparticles that activate the intrinsic pathway of hemostasis and are thus of great pathophysiologic importance [Soriano et al 2005;Boulanger et al 2006].…”

Inhibition of Intrinsic Thrombin Generation

“…The usual plasmatic unfractionated heparin concentrations in therapeutic anticoagulation are about 10fold higher than the IC50 values observed here in the INCA. The superior anticoagulant capacity of LMWH compared to unfractionated heparin might be due to some contact activating (pre-kallikrein to kallikrein folding) potential of unfractionated heparin, a polynegatively charged molecule of a molecular mass > 15000 Dalton [Kongsgaard et al 1992]. In many clinical situations cells can fragment into phospholipidmicroparticles that activate the intrinsic pathway of hemostasis and are thus of great pathophysiologic importance [Soriano et al 2005;Boulanger et al 2006].…”