Trypsin-like activity of serum: Its origin and electrophoretic separation

Abstract

The presence in human serum of proteolytic activity toward N α -carbobenzoxydiglycyl (−)arginine-2-naphthylamide has been established. A method for the determination of this activity under optimal conditions is described. As compared with serum, plasma possesses only a weak activity toward the substrate used. The proteolytic activity of serum studied by the method presented was not affected either by soybean trypsin inhibitor or streptokinase. High concentrations of ϵ-aminocaproic acid caused only a moderate fall in serum activity. The most remarkable feature of the serum proteolytic activity was its rapid decrease during the first hours of contact of serum with clot. Starch gel electrophoresis and Sephadex gel filtration studies resulted in the demonstration of 2 fractions with proteolytic activity in serum, corresponding to α 2-macroglobulin and β-lipoprotein. The α 2-macroglobulin fraction showed the greatest serum proteolytic activity. A weak plasma proteolytic activity was localized in β-lipoprotein. The binding of thrombin, trypsin and plasmin by serum α 2-macroglobulin did not affect their ability to hydrolyse the substrate used. Evidence is presented indicating that serum proteolytic activity studied by means of synthetic amide substrates of arginine originates from blood clotting enzymes, namely thrombin and plasmin, and not from trypsin.