Cleavage of blood coagulation factor XIII and fibrinogen by thrombin during in vitro clotting.

Thrombin cleavage of blood coagulation Factor XIII (a2b2) and fibrinogen was studied during in vitro clotting to determine the physiologic sequence of these events. First, the time course of fibrin formation and cleavage of Factor XIII was measured in platelet-rich plasma. Cleavage of fibrinogen was measured by using a radioimmunoassay for fibrinopeptide A. Conversion of trace amounts of radioiodinated a-chains of 125I-Factor XIII to thrombin-modified a-chains was measured in unreduced 10% sodium dodecyl sulfate-polyacrylamide gels. During spontaneous clotting, a similar percentage of 125I-Factor XIII and fibrinogen was cleaved at each time point. Visible gelation of polymerized fibrin monomer occurred when 24 +/- 8% of fibrinogen was cleaved and 21 +/- 6% of Factor XIII was converted to Factor XIII'. Thrombin cleavage of Factor XIII and fibrinogen was also studied in platelet-poor plasma to which thrombin was added. In order to measure Factor XIIIa activity, fibrin polymerization was completely inhibited by the addition of Gly-Pro-Arg-Pro. Factor XIIIa formation was measured by the incorporation of [3H]putrescine into casein. The concentration of added thrombin required to cleave 50% of fibrinogen and Factor XIII was 0.65 U/ml and 0.35 U/ml, respectively. The rate of cleavage of fibrinogen by thrombin was 43-fold greater than cleavage of Factor XIII. Lower Gly-Pro-Arg-Pro concentrations were used to determine the effects of incompletely inhibiting fibrin polymerization on cleavage of Factor XIII and fibrinogen. Thrombin cleavage of Factor XIII but not fibrinogen was dependent on the extent of fibrin polymerization. The more marked the degree of inhibition of fibrin polymerization, the slower the rate of Factor XIIIa formation. Thus, in platelet-rich plasma, thrombin cleavage of Factor XIII and fibrinogen are closely related events during spontaneous clotting. Furthermore, cleavage of Factor XIII during clotting is enhanced by fibrin polymerization in platelet-poor plasma.