title: Actin accelerates plasmin generation by tissue plasminogen activator.
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.
authors.affiliation: Hematology Unit, Massachusetts General Hospital, Boston.
authors.forename: S E
authors.initials: SE
authors.lastname: Lind
authors.forename: C J
authors.initials: CJ
authors.lastname: Smith
mesh_terms: D000199:Actins / Q000502:physiology*; D000595:Amino Acid Sequence; D000818:Animals; D004591:Electrophoresis, Polyacrylamide Gel; D005341:Fibrinolysin / Q000378:metabolism*; D007700:Kinetics; D008969:Molecular Sequence Data; D011817:Rabbits; D010959:Tissue Plasminogen Activator / Q000378:metabolism*
chemical_list: D000199:Actins; D010959:Tissue Plasminogen Activator; D005341:Fibrinolysin