Vampire bat salivary plasminogen activator exhibits a strict and fastidious requirement for polymeric fibrin as its cofactor, unlike human tissue-type plasminogen activator. A kinetic analysis.

Abstract

The vampire bat salivary plasminogen activator (BatPA) is virtually inactive toward Glu-plasminogen in the absence of a fibrin-like cofactor, unlike human tissue-type plasminogen activator (tPA) (the kcat/Km values were 4 and 470 M-1 s-1, respectively). In the presence of fibrin II, tPA and BatPA activated Glu-plasminogen with comparable catalytic efficiencies (158,000 and 174,000 M-1 s-1, respectively). BatPA's cofactor requirement was partially satisfied by polymeric fibrin I (54,000 M-1 s-1), but monomeric fibrin I was virtually ineffective (970 M-1 s-1). By comparison, a variety of monomeric and polymeric fibrin-like species markedly enhanced tPA-mediated activation of Glu-plasminogen. Fragment X polymer was 2-fold better but 9-fold worse as cofactor for tPA and BatPA, respectively, relative to fibrin II. Fibrinogen, devoid of plasminogen, was a 10-fold better cofactor for tPA than fibrinogen rigorously depleted of plasminogen, Factor XIII, and fibronectin; the enhanced stimulatory effect of the less-purified fibrinogen was apparently due to the presence of Factor XIII. By contrast, the two fibrinogen preparations were equally poor cofactors of BatPA-mediated activation of Glu-plasminogen. BatPA possessed only 23 and 4% of the catalytic efficiencies of tPA and two-chain tPA, respectively, in hydrolyzing the chromogenic substrate Spectrozyme tPA. However in the presence of fibrin II, BatPA and tPA exhibited similar kcat/Km values for the hydrolysis of Spectrozyme tPA. Our data revealed that BatPA, unlike tPA, displayed a strict and fastidious requirement for polymeric fibrin I or II. Consequently, BatPA may preferentially promote plasmin generation during a narrow temporal window of fibrin formation and dissolution.