AGGREGATED, CONFORMATIONALLY CHANGED FIBRINOGEN EXPOSES THE STIMULATING SITES FOR T-PA-CATALYZED PLASMINOGEN ACTIVATION

The present paper shows that conformationally changed fibrinogen can e xpose the sites A alpha-(148-160) and gamma-(312-324) involved in stim ulation of the tissue-type plasminogen activator (t-PA)-catalysed plas minogen activation. The exposure of the stimulating sites was determin ed by ELISA using mABs directed to these sites, and was shown to coinc ide with stimulation of t-PA-catalysed plasminogen activation as asses sed in an assay using a chromogenic substrate for plasmin. Gel permeat ion chromatography of fibrinogen conformationally, changed by heat (46 .5 degrees C for 25 min) demonstrated the presence of both aggregated and monomeric fibrinogen. The aggregated fibrinogen, but not the monom eric fibrinogen, had exposed the epitopes A alpha(148-160) and gamma-( 312-324) involved in t-PA-stimulation. Fibrinogen subjected to heat in the presence of 3 mM of the tetrapeptide GPRP neither aggregates nor exposes the rate-enhancing sites. Thus, aggregation and exposure of t- PA-stimulating sites in fibrinogen seem to be related phenomena, and i t is tempting to believe that the exposure of stimulating sites is a c onsequence of the conformational changes that occur during aggregation , or self-association. Fibrin monomers kept in a monomeric state by a final GPRP concentration of 3 mM do not expose the epitopes A alpha-(1 48-160) and gamma-(312-324) involved in t-PA-stimulation, whereas dilu tion of GPRP to a concentration that is no longer anti-polymerizing, r esults in exposure of these sites. Consequently, the exposure of t-PA- stimulating sites in fibrin as well is due to the conformational chang es that occur during self-association.