The fibrin(ogen)olytic properties of cathepsin D

Fibrin(ogen) is important for hemostasis and is cleared from sites of vascular injury primarily by the plasminogen activator system. However, there is emerging evidence in plasminogen activator-deficient transgenic mice that non-plasmin pathways may also be important for endogenous fibrinolysis. We have recently described an alternative, plasmin-independent fibrinolytic pathway in activated human monocytes that utilizes the integrin Mac-1 (CD11b/CD18), which directly binds and internalizes fibrin, resulting in its lysosomal degradation. The identity of the lysosomal fibrinolytic enzyme(s) responsible for monocyte/macrophage-mediated fibrinolytic is unknown. Protease inhibitor studies now suggest that an aspartyl protease is responsible for this fibrinolytic activity. We, therefore, examined the fibrinolytic properties of cathepsin D, a lysosomal aspartyl protease, and report that cathepsin D possesses both fibrinogenolytic and fibrinolytic activity. Cathepsin D cleavage of fibrinogen follows Michaelis-Menten kinetics with a Michaelis constant, Km, of 1.5 microM; catalytic rate constant, kcat, of 1.4 x 10(-3) s-1; and catalytic efficiency, kcat/Km, of 9.3 x 10(-4) microM-1 s-1. A pH-activity profile of fibrinogen digestion by cathepsin D demonstrates a pH optimum of 3.5 with 50% residual activity at pH 5.0. Fibrinolysis was assessed by fibrin plate and fibrin clot lysis assays. Cathepsin D possesses significant fibrinolytic activity over a dose range of 100 nM to 10 microM and is able to lyse fibrin, as well as albumin-enriched and albumin/red cell-enriched fibrin clots. Cathepsin D cleaves the alpha-, beta-, and gamma-chains of FGN, generating multiple low-molecular-weight fragments.(ABSTRACT TRUNCATED AT 250 WORDS)