Unique Pattern of Fibrinogen Cleavage by Human Leukocyte Proteases

Abstract

An extract of human leukocytes was prepared which had marked fibrinogenolytic activity, as determined by its effect upon the thrombin clotting time of plasminogen-free fibrinogen. When incubated with a 1/10 dilution of the extract of 108 leukocytes, fibrinogen became incoagulable within 30 min. The activity of the leukocyte extract was unaffected by 0.2 M epsilon-amino caproic acid and minimally affected by 100 U/ml of trasylol, but it was completely inhibited by 0.1 mg/ml soybean trypsin inhibitor and by 1% plasma. The degradation products of leukocyte protease-treated fibrinogen were clearly different from those of plasmin-treated fibrinogen when examined by polyacrylamide gel electrophoresis. The pattern of cleavage of the amino terminal ends of the Aα and Bβ chains of fibrinogen by the leukocyte extract was determined using radioimmunoassays specific for fibrinopeptides A and B. The fibrinopeptides themselves were not cleaved, but slightly larger dialyzable fragments containing the peptides were cleaved from the amino terminal ends of the Aα and Bβ chains. These larger fragments were clearly distinguished from the fibrinopeptides themselves by immunochemical means. The pattern of release of the fibrinopeptide-containing segments by the leukocyte proteases was different from the patterns described for thrombin, plasmin, trypsin, and certain snake venoms. The results suggest that the specificity of the leukocyte proteases for fibrinogen is unique, and provide a technique for further study of the role of these enzymes in normal and abnormal fibrinogenolysis.