EFFECT OF PLASMINOGEN AND TISSUE-TYPE PLASMINOGEN-ACTIVATOR ON FIBRIN GEL STRUCTURE

Lys78-plasminogen or tissue-type plasminogen activator (tPA) enhanced the rate of fibrin clot formation, and the optical density of the resulting clot was markedly increased. A similar effect was not observed for Glu1- or Val443-plasminogen. The effect of tPA and Lys78-plasminogen on clot turbidity was reversed by the lysine analogue trans-4 (amino methyl) cyclohexane-1-carboxylic acid (tAMCA). The results suggest that the increase in optical density reflected complex formation and that tAMCA displaced these proteins from the fibrin polymer. Half maximal values K50 = 3-mu-M and K50 = 0.9 mM tAMCA were obtained for Lys78-plasminogen and tPA, respectively. The Lys78-plasminogen displacement curve was superimposable on the curve for tAMCA-induced abolition of fibrin-enhanced tPA activity with Lys78-plasminogen as the substrate. Interaction of plasminogen with plasmin-derived fibrin degradation products was indicated by precipitation of fibrin derivatives induced by addition of Lys78-plasminogen to the dissolved clot. The precipitate was shown to contain fibrin fragments X, Y, and D probably as oligomers. The precipitate was dissolved by tAMCA or by further plasmin degradation. Soluble fibrin degradation products present immediately following clot dissolution effectively enhanced tPA-mediated plasminogen activation. The results are in accordance with a model which suggests that tight binding of Lys78-plasminogen to fibrin depends on assembly of fibrin monomers in a protofibril structure in which D and E domains from different moieties are closely associated and can be bridged by the plasminogen molecule.