Physiological activation of the human fibrinolytic system. Isolation and characterization of human plasminogen variants, Chicago I and Chicago II.

Methods are described for measuring plasminogen levels in human plasma and for studying plasmin generation in plasma with plasminogen activators. In plasma samples from two individuals with histories of deep vein thrombosis, the functional plasminogen values were determined to be below the normal range of 16.7 to 23.8 mg/lOO ml (average of 21.0 mg/lOO ml), and the plasmin generation rates were below normal. Gluplasminogens, isolated from these plasmas, were found to be similar to the native zymogen in their molecular weights and isoelectric forms; they showed slower activation rates, but were 100% activatable to plasmin. Kinetic studies on these variant, or abnormal, plasminogens (I and II) showed: (a) amidase parameters of the equimolar streptokinase complexes of variant Gluplasminogen and derived Lys-plasminogens were similar to the native zymogen l and derived zymogen l streptokinase complexes in their apparent Michaelis constants, but were appreciably lower in their catalytic rate constants; (b) amidase parameters of the native and variant plasmins, and their streptokinase complexes, were identical; (c) activation of native and variant Glu-plasminogens by streptokinase, or the plasmin-derived light(B) chain. streptokinase complex, showed identical catalytic rate constants, but the apparent Michaelis constants of the variant zymogens were appreciably elevated, over 3to &fold with streptokinase and loo-fold with the light(B) chain*streptokinase complex, and a complex curve on a double reciprocal plot was obtained with urokinase; (d) activation of native and variant derived Lys-plasminogens by the same three activators yielded mostly similar apparent Michaelis constants but lower than normal catalytic rate constants with the variant zymogens; (e) activation of an equimolar mixture of native and variant Gluplasminogens by the light(B) chain l streptokinase complex resulted in a lo-fold drop in the apparent Michaelis constant (from about I9 to 1.9 pM compared to 0.15 pM for the native zymogen) while activation by urokinase resulted in kinetic parameters which were identical with those of the native zymogen. These data lead to the conclusion that both the binding properties of activators to the variant plasminogens, and the active sites formed in the plasminogen moiety of the zymogen* streptokinase complexes, have been impaired. Also, interpretation of the data was possible only in terms of a homogeneous population of molecules; thus, the two individuals studied have to be