Human Plasmin and Protease Inhibitors: Interaction in a Whole Plasma System

Abstract

As a central event in the human coagulation and fibrinolytic pathways, intravascular proteolysis is normally limited by the presence of numerous circulating protease inhibitors. Plasmin, the fibrinolytic enzyme, is inhibited in vitro by purified preparations of antithrombin, Cl-esterase inhibitor, α1-antitrypsin, α2-niacroglobulin and inter-α-trypsin inhibitor. Using trace amounts of 125I-plasminogen and conventional gel filtration techniques, the elution profile of the labeled zymogen was studied in whole plasma before and after activation with urokinase (UK) or streptokinase (SK). Following activation, the major 125I-plasmin peak shifted to a higher molecular weight fraction of plasma than before activation. Lysine-sepharose affinity chromatography (LSAC), SDS-gel electrophoresis and immunodiffusion studies on the peak radioactive fractions revealed that a small percentage of the 125I-plasmin formed was bound to α2-macroglob-ulin while a majority was complexed to a component in plasma (mol. wt. ~ 60,000) immunologically distinct from the known human antiplasmins. Similar results were obtained when UK or SK activated plasmawas directly subjected to LSAC without prior fractionation. Minor differences were seen when the rate of activation was varied. However, in the presence of heparin, a significant amount of 125I-plasmin was complexed to antithrombin. These results, obtained in a whole plasma system, lend support for the presence of a new antiplasmin which is believed to be the same inhibitor recently described by others.Also, these data suggest that the other antiplasmins may play a minor, yet important role in the regulation of plasmin activity under different physiological conditions.