Abstract
Actin has been found to bind to plasmin's kringle regions, thereby inhibiting its enzymatic activity in a noncompetitive manner. We, therefore, examined its effect upon the conversion of plasminogen to plasmin by tissue plasminogen activator. Actin stimulated plasmin generation from both Glu- and Lys-plasminogen, lowering the Km for activation of Glu-plasminogen into the low micromolar range. Accelerated plasmin generation did not occur in the presence of epsilon-amino caproic acid or if actin was exposed to acetic anhydride, an agent known to acetylate lysine residues. Actin binds to tissue plasminogen activator (t-Pa) (Kd = 0.55 microM), at least partially via lysine-binding sites. Actin's stimulation of plasmin generation from Glu-plasminogen was inhibited by the addition of aprotinin and was restored by the substitution of plasmin-treated actin, indicating the operation of a plasmin-dependent positive feedback mechanism. Native actin binds to Lys-plasminogen, and promotes its conversion to plasmin even in the presence of aprotinin, indicating that plasmin's cleavage of either actin or plasminogen leads to further plasmin generation. Plasmin-treated actin binds Glu-plasminogen and t-PA simultaneously, thereby raising the local concentration of t-PA and plasminogen. Together, but not separately, actin and t-PA prolong the thrombin time of plasma through the generation of plasmin and fibrinogen degradation products. Actin-stimulated plasmin generation may be responsible for some of the changes found in peripheral blood following tissue injury and sepsis.
Highlights
Actin has been found tobind to plasmin’s kringle regions, thereby inhibiting its enzymatic activity in a noncompetitive manner
Because it seems likely that fibrin clots formed at sites of tissue injury would contain trapped actin filaments,we studied the effects of actin upon clotlysis (21)
These results, we considered whether actin's acceleration of Interaction of Actin with t-PA and Plasminogen-The bind- plasmin generation might depend upon plasmin's initial cleaving of actin to t-PA, Glu-plasminogen, and Lys-plasminogen age of one or more of the primary reactants
Summary
The reaction obeyed Michaelis-Menten kinetics, as demonstratedwhenthedata were examinedwith a Line-. Plasmin was generated in solutions containing t-PA and Glu-progressively lowered the K , for activationbuthadlittle plasminogen only after a prolonged incubation period. Actin was added to solutionsof t-PA and Glu-plasmincause plasmin generation in the absence of t-PA, data not ogen containing either CNBr-cleavefdibrinogen fragments or
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