EVALUATION OF THE FACTORS CONTRIBUTING TO FIBRIN-DEPENDENT PLASMINOGEN ACTIVATION

Polymerized fibrin strongly enhances tissue plasminogen activator (tPA )-mediated plasminogen activation, concomitant with exposure of 'fibri n-specific' epitopes at 'A alpha 148-160' and 'gamma 312-324'. To inve stigate which aspects of polymerization are involved in these activiti es, we explored the fibrin polymerization process by evaluating the ab ility of factor XIIIa-crosslinked fibrinogen polymers to expose 'fibri n-specific' epitopes and enhance plasminogen activation. Crosslinked n ormal fibrinogen, fibrinogen with deficient [des B beta 1-42] or defec tive [Birmingham (A alpha R16H)] fibrin 'D:E' assembly sites ('E-A'), or with defective end-to-end self-association sites ('D:D') [Cedar Rap ids (gamma R275C)], exposed both 'fibrin-specific' epitopes and enhanc ed tPA-dependent plasminogen activation, whereas non-crosslinked fibri nogens showed minimal or no such activities. Epitope expression in cro sslinked fibrinogen was retained in the presence of the fibrin E-A sit e peptide homolog, gly-pro-arg-pro (GPRP), which inhibits fibrin D:E a ssociation, except for the A alpha 148-160 epitope in des B beta 1-42 fibrinogen, which was not expressed. Fibrin prepared from crosslinked normal or abnormal fibrinogen, except for the des B beta 1-42 fibrin e pitopes, which were reduced or absent, expressed 'fibrin-specific' epi topes even in the presence of GPRP, which otherwise impairs such expre ssion in non-cross-linked fibrin. Epitope exposure in fibrin prepared from non-crosslinked fibrinogen was nearly normal in Cedar Rapids fibr in (heterozygous D:D defect), but reduced in Birmingham fibrin (hetero zygous E-A defect), nil in des B beta 1-42 fibrin (E-A deficient), and absent in all cases in the presence of GPRP. Ln contrast, plasminogen activation stimulatory activity that had been exposed in crosslinked normal fibrinogen or in crosslinked des B beta 1-42 or Cedar Rapids fi brin, was preserved to a large extent in the presence of GPRP, suggest ing that once enhanced stimulatory activity and epitopes are exposed, they are not completely reversible. The findings indicate that end-to- end intermolecular associations (D:D) are not critical for 'fibrin-spe cific' epitope exposure, but that polymerization brought about in fibr inogen through factor XIIIa crosslinking, or in fibrin through 'D:E' i nteractions, is necessary for 'fibrin-specific' (more correctly, 'poly merization-specific') epitope exposure and enhancement of plasminogen activation.