Nonplasminogen-dependent protease in human plasma

Equal volumes of plasma and 0.3 M K2HPO4, pH 7.4, were mixed, diluted 20-fold, and adjusted to pH 5.2. After incubation at 37 degrees C for 30 min, the euglobulin percipitate, redissolved in 0.1 M K2HPO4, pH 7.4, developed caseinolytic activity (0.05 CTA U/ml). Na2HPO4 or NaCl of similar ionic strength could replace K2HPO4. The pH optimum of the protease was 6.5, activity falling off sharply below pH 6.0 and above 7.4. The proteolytic activity was inhibited by diisopropylphosphofluoridate and by pancreatic trypsin inhibitor, but was not inhibited by soybean trypsin inhibitor. The activity was not due to plasmin, contact activation, or coagulation factors, since it was fully generated in plasminogen-depleted, factors XII, XI, VII deficient, and prekallikrein-deficient plasmas. Purified Cl-esterase was not caseinolytic in our system. Redissolved euglobulin precipitate prepared from normal plasma without salt addition could serve as starting material for the generation of caseinolytic activity, as could serum, indicating that the Hageman factor cofactor and thrombin are not required. The protease had no detectable procoagulant or fibrinolytic activity.