PROGRESSIVE CROSS-LINKING OF FIBRIN GAMMA-CHAINS INCREASES RESISTANCETO FIBRINOLYSIS

In the presence of plasma transglutaminase (factor XIIIa) fibrin first undergoes intermolecular covalent cross-linking between its gamma cha ins to create gamma dimers followed by slower cross-linking among its alpha chains to form alpha polymers. Progressive cross-linking of gamm a chain dimers occurs at the slowest rate, resulting in gamma trimers and gamma tetramers (''gamma multimers''). Most studies indicate that cross-linked fibrin clots become resistant to fibrinolysis, but the ba sis for this event is not clear. In this study, we explored the role o f gamma chain multimerization compared with alpha polymerization as ca usal factors in time-dependent development of resistance to fibrinolys is. Fibrin clots prepared from native (intact) fibrinogen were incubat ed for up to 120 h at near physiological ionic strength and a factor X IIIa level approximating that in plasma. These clots were lysed by pla smin at rates that were inversely proportional to the level of gamma m ultimers, which increased progressively with the time of incubation. I n contrast, fibrin cross-linked at high ionic strength (a condition un der which only gamma dimers and alpha polymers form) or fibrin formed in the absence of factor XIII showed no time dependent decrease in lys is rates. Fibrin cross-linked for a fixed time period with increasing amounts of factor XIIIa contained gamma multimer levels that were prop ortional to the factor XIIIa concentration and lysed at rates that wer e inversely proportional to the gamma multimer level. Furthermore, cro ss-linked fibrin formed from fibrinogen fraction I-9, which has limite d potential for alpha polymerization, showed the same reduction in the lysis rate as native cross-linked fibrin. These findings indicate tha t development of resistance to fibrinolysis of cross-linked fibrin is not measurably dependent upon gamma dimer or alpha polymer formation b ut develops solely as a function of gamma multimerization.