PLASMINOGEN ACTIVATION BY LOW MOLECULAR WEIGHT STREPTOKINASE AND FIBRIN EFFECT

The purpose is to study the plasminogen-activating activity of 36 kDa-streptokinase fragment, influences of desAB-fibrin on this process and low molecular weight streptokinase on plasmin catalytic properties as well. The 36 kDa-fragment lacking the 63 N- and 34 C-terminal amino acid residues was obtained by preparative electrophoresis from chymotrypsin hydrolyzate of the native streptokinase. It was shown that 36 kDa streptokinase activates Glu-plasminogen in solution only at high concentrations of reacting components (2•10-7M). Activation process begins after a long lag-period and is in 100 times slower compared with the native streptokinase. Lys-plasminogen, mini-plasminogen (Val442-plasminogen) but not its Glu-form are activated at definitely lower protein concentrations (5•10-8M), while the reaction rate with mini-plasminogen is order of magnitude greater as compared with Lys-plasminogen and is equal to 4,3•10-2 and 5•10-3 o.u./min respectively. DesAB-fibrin increases efficiently the rate of Glu- and Lys- plasminogen activation by 36 kDa-streptokinase and practically has no effect on the rate of mini-plasminogen activation. Low molecular weight streptokinase has no influence on amidase and fibrinolytic activity of plasmin and does not protect the enzyme from inhibitory effect of α2-antiplasmin. In the presence of the streptokinase fragment, plasmin showed no its activator activity towards plasminogen. A conclusion is made, that during plasminogen activation by low molecular weight streptokinase in the presence of desAB fibrin, a certain site of the fibrin molecule acts as N-terminal peptide of the native streptokinase, inducing conformational changes in proenzyme. These changes are necessary for quick complex formation with 36- kDa streptokinase and formation of the active centre in proenzyme molecule by this form of streptokinase